Physics & Astronomy (School of)https://hdl.handle.net/10023/312024-03-28T11:31:40Z2024-03-28T11:31:40ZAdvanced material platforms for holographic applications of photonic metasurfacesBiabanifard Hossein Abadi, Mohammadhttps://hdl.handle.net/10023/295262024-03-19T03:04:35Z2024-06-10T00:00:00ZMetasurface holographic applications such as imaging, sensing, data encryption and biophotonics are promising for more advanced photonic devices. However, most prevailing material platforms are constrained to rigid substrates with transparency windows at λ > 500 nm, where absorption losses become significant at shorter wavelengths. This thesis demonstrates rigid and flexible material platforms to realise holographic metasurfaces functioning at shorter wavelengths across the visible spectrum. In particular, it presents zirconium dioxide metasurfaces as a novel material platform, all-polymeric metasurfaces, and incoherent light metasurfaces. Pillar with top and air-hole meta-atoms configurations, operating in transmission across the visible spectrum, are introduced.
Zirconium dioxide metasurfaces, characterised by a high refractive index, hardness, and biocompatibility, are systematically designed, fabricated, and experimentally measured. These metasurfaces are utilised for holographic image projection and lab-on-chip optical trapping, employing a retrieval algorithm for hologram design. The integration of these lab-on-chip devices has the potential to replace conventional bulky objective lenses, thereby advancing optics integration.
All-polymeric holographic metasurfaces are designed, fabricated, and experimentally characterised using a novel single-material meta-atom design adaptable to various materials. Specifically implemented with SU-8, these metasurfaces are applied to achieve conformable holographic image projection. Their potential for mass-scale production is highlighted by the low fabrication cost and design simplicity, exploiting nanolithography methods.
Metasurfaces designed to operate with incoherent light sources are developed, fabricated, and partially characterised. The design methodology is described. The initial experimental results are rewarding, but further investigations are required to enhance their performance. These findings are promising to extend the applicability of these incoherent metasurfaces for out-of-lab applications.
The advancement of holographic devices, including their materials, operational wavelengths, and excitation methods, will pave the way for advances in the field.
2024-06-10T00:00:00ZBiabanifard Hossein Abadi, MohammadMetasurface holographic applications such as imaging, sensing, data encryption and biophotonics are promising for more advanced photonic devices. However, most prevailing material platforms are constrained to rigid substrates with transparency windows at λ > 500 nm, where absorption losses become significant at shorter wavelengths. This thesis demonstrates rigid and flexible material platforms to realise holographic metasurfaces functioning at shorter wavelengths across the visible spectrum. In particular, it presents zirconium dioxide metasurfaces as a novel material platform, all-polymeric metasurfaces, and incoherent light metasurfaces. Pillar with top and air-hole meta-atoms configurations, operating in transmission across the visible spectrum, are introduced.
Zirconium dioxide metasurfaces, characterised by a high refractive index, hardness, and biocompatibility, are systematically designed, fabricated, and experimentally measured. These metasurfaces are utilised for holographic image projection and lab-on-chip optical trapping, employing a retrieval algorithm for hologram design. The integration of these lab-on-chip devices has the potential to replace conventional bulky objective lenses, thereby advancing optics integration.
All-polymeric holographic metasurfaces are designed, fabricated, and experimentally characterised using a novel single-material meta-atom design adaptable to various materials. Specifically implemented with SU-8, these metasurfaces are applied to achieve conformable holographic image projection. Their potential for mass-scale production is highlighted by the low fabrication cost and design simplicity, exploiting nanolithography methods.
Metasurfaces designed to operate with incoherent light sources are developed, fabricated, and partially characterised. The design methodology is described. The initial experimental results are rewarding, but further investigations are required to enhance their performance. These findings are promising to extend the applicability of these incoherent metasurfaces for out-of-lab applications.
The advancement of holographic devices, including their materials, operational wavelengths, and excitation methods, will pave the way for advances in the field.Holographic metasurfaces for imaging, trapping, sensing, and antennas applicationsXiao, Jianlinghttps://hdl.handle.net/10023/295082024-03-16T03:06:38Z2024-06-10T00:00:00ZHolographic metasurfaces are known for their effective manipulation of light properties. They are the two-dimensional version of bulk metamaterials and are made by artifact subwavelength meta-atoms. Their applications span diverse fields, such as data storage, anti-counterfeiting, optical displays, high numerical aperture lenses, and sensing.
This thesis delves into the exploration of holographic metasurface applications on both rigid and flexible substrates, including imaging, optical trapping, and curvature sensing over a wide wavelength range, from visible to millimeter waves. The design incorporates two types of metallic meta-atoms tailored to the different working wavelengths. This thesis provides detailed insights into the motivation, design, fabrication, and characterization processes.
In this thesis, the implementation of two types of multiplexing is presented: the environment-dependent and the shape-dependent holographic metasurfaces. The former is fabricated on a rigid substrate and allows the generation of two switchable images by varying both the surrounding medium and the incident wavelength. The latter can also generate two switchable images by bending the substrate to target curvatures, either in concave or convex shape. They provide two new ways to encode and generate information through holographic metasurfaces. We have also developed high numerical aperture holographic metasurfaces on rigid substrates, up to 1.2, for on-chip optical trapping applications. We demonstrate the ability to trap both beads and extended objects with higher trapping stiffness greater than 400 pN/µm/W, which is comparable to conventional objectives at the same numerical aperture. Finally, we have developed a holographic metasurface on a flexible substrate for curvature sensing applications that operates in the visible region. Unlike existing solutions, it eliminates the need for pre-calibration and can be mounted on target objects. The sensor consists of two distinct patterned areas that can generate two images: a reference scale and a position indicator. The position of the indicator shifts within the reference scale as the metasurface deforms, providing instant readings of its curvature.
These applications demonstrate the versatility of holographic metasurfaces, paving the way for multi-degree-of-freedom holographic encryption, a new class of multifunctional devices for lab-on-chip trapping applications, and real-time curvature monitoring. In addition, the use of flexible substrates enhances the adaptability of holographic metasurfaces, especially when adapting to non-flat surfaces for out-of-the-lab applications, as the metasurfaces can be transferred to any target object and integrated with other devices.
2024-06-10T00:00:00ZXiao, JianlingHolographic metasurfaces are known for their effective manipulation of light properties. They are the two-dimensional version of bulk metamaterials and are made by artifact subwavelength meta-atoms. Their applications span diverse fields, such as data storage, anti-counterfeiting, optical displays, high numerical aperture lenses, and sensing.
This thesis delves into the exploration of holographic metasurface applications on both rigid and flexible substrates, including imaging, optical trapping, and curvature sensing over a wide wavelength range, from visible to millimeter waves. The design incorporates two types of metallic meta-atoms tailored to the different working wavelengths. This thesis provides detailed insights into the motivation, design, fabrication, and characterization processes.
In this thesis, the implementation of two types of multiplexing is presented: the environment-dependent and the shape-dependent holographic metasurfaces. The former is fabricated on a rigid substrate and allows the generation of two switchable images by varying both the surrounding medium and the incident wavelength. The latter can also generate two switchable images by bending the substrate to target curvatures, either in concave or convex shape. They provide two new ways to encode and generate information through holographic metasurfaces. We have also developed high numerical aperture holographic metasurfaces on rigid substrates, up to 1.2, for on-chip optical trapping applications. We demonstrate the ability to trap both beads and extended objects with higher trapping stiffness greater than 400 pN/µm/W, which is comparable to conventional objectives at the same numerical aperture. Finally, we have developed a holographic metasurface on a flexible substrate for curvature sensing applications that operates in the visible region. Unlike existing solutions, it eliminates the need for pre-calibration and can be mounted on target objects. The sensor consists of two distinct patterned areas that can generate two images: a reference scale and a position indicator. The position of the indicator shifts within the reference scale as the metasurface deforms, providing instant readings of its curvature.
These applications demonstrate the versatility of holographic metasurfaces, paving the way for multi-degree-of-freedom holographic encryption, a new class of multifunctional devices for lab-on-chip trapping applications, and real-time curvature monitoring. In addition, the use of flexible substrates enhances the adaptability of holographic metasurfaces, especially when adapting to non-flat surfaces for out-of-the-lab applications, as the metasurfaces can be transferred to any target object and integrated with other devices.Moving and changing : using cold gas and spatially resolved spectroscopy to understand galaxies as evolving dynamic systemsCampbell, Stephaniehttps://hdl.handle.net/10023/293962024-03-02T03:10:45Z2024-06-10T00:00:00ZGalaxies are vast systems which contain stars, gas, and dust, spiralling under the influence of gravity. On human timescales they appear constant and fixed, however these systems are in motion – moving internally at vast speeds, and evolving through life stages. This work considers how the combination of multiwavelength observations can be used to understand these dynamic systems.
For decades, our observations of the motions of stars in these systems have not matched those expected based on the visible matter which is seen to be present. This has necessitated the inclusion of a dark matter component in our theory of galactic structure. This component cannot be directly observed, however dynamical modelling offers an avenue through which the properties of this can be derived. In this work, I use dynamical modelling to aid in the study of a population of galaxies known as red geysers. These galaxies may offer direct signatures of large scale outflows in galaxies which are needed in our theoretical framework to explain how galaxies stop forming new stars. Through this, more evidence was produced which points towards these galaxies exhibiting large scale outflows. Further, I then use dynamical modelling to explore the dark matter content of galaxies, and develop a novel method which incorporates cold gas observations into the process in order to better constrain the dark matter parameters. This method is shown to improve the modelling outcomes, and is an important proof-of-concept test to highlight the potential for future work using cold gas kinematics in dynamical models.
Galaxies are composed of their stellar populations contained within, and the evolutionary processes we see hinge on how these populations are affected. Some of the most extreme evolution processes are seen in so-called jellyfish galaxies, which are experiencing external environmental pressure as they fall into dense cluster environments. The final section of this work presents evidence of ongoing quenching taking place asymmetrically in a sample of these galaxies, demonstrated by spectral signatures of post-starburst formation histories on their leading edges. Using multiple data sources, I make the argument that this effect is being
caused by the progressive compression and stripping of gas due to the external pressure.
2024-06-10T00:00:00ZCampbell, StephanieGalaxies are vast systems which contain stars, gas, and dust, spiralling under the influence of gravity. On human timescales they appear constant and fixed, however these systems are in motion – moving internally at vast speeds, and evolving through life stages. This work considers how the combination of multiwavelength observations can be used to understand these dynamic systems.
For decades, our observations of the motions of stars in these systems have not matched those expected based on the visible matter which is seen to be present. This has necessitated the inclusion of a dark matter component in our theory of galactic structure. This component cannot be directly observed, however dynamical modelling offers an avenue through which the properties of this can be derived. In this work, I use dynamical modelling to aid in the study of a population of galaxies known as red geysers. These galaxies may offer direct signatures of large scale outflows in galaxies which are needed in our theoretical framework to explain how galaxies stop forming new stars. Through this, more evidence was produced which points towards these galaxies exhibiting large scale outflows. Further, I then use dynamical modelling to explore the dark matter content of galaxies, and develop a novel method which incorporates cold gas observations into the process in order to better constrain the dark matter parameters. This method is shown to improve the modelling outcomes, and is an important proof-of-concept test to highlight the potential for future work using cold gas kinematics in dynamical models.
Galaxies are composed of their stellar populations contained within, and the evolutionary processes we see hinge on how these populations are affected. Some of the most extreme evolution processes are seen in so-called jellyfish galaxies, which are experiencing external environmental pressure as they fall into dense cluster environments. The final section of this work presents evidence of ongoing quenching taking place asymmetrically in a sample of these galaxies, demonstrated by spectral signatures of post-starburst formation histories on their leading edges. Using multiple data sources, I make the argument that this effect is being
caused by the progressive compression and stripping of gas due to the external pressure.From candidate identification to planet characterization : a machine learning approachSchanche, Nicolehttps://hdl.handle.net/10023/293072024-02-22T03:07:04Z2020-12-01T00:00:00ZThis thesis is broken into three main sections tracing the steps of the development of a new framework to search for and characterize planets from the WASP survey. While all methods were developed specifically for the WASP project, the principles are easily transferable to any ground or space based survey. In the first part of the thesis, I discuss the development of two machine learning methods, a Random Forest Classifier and a Convolutional Neural Network, that are able to find new exoplanet candidates from WASP archival data and lightcurves. In preparing the training dataset, I also created a standardized catalog of 1,041 false positives from SuperWASP, the northern component of WASP, that were verified with additional observations from other instruments.
The second part of the thesis begins by discussing the results of the machine learning methods. In the analysis of the resulting probabilities, several patterns began to emerge that indicated the differing predictions between the algorithms carry useful information in itself. This realization sparked the development of a new “stacking” framework where the predictions of a number of different machine learning methods areused as the input to a second-level classifier that makes the final prediction. This method is straightforward to implement and demonstrates an improved performance over any individual classifier, making the stacked approach ideal for future large surveys. I use the model to classify and rank more than 100,000 lightcurves in the WASP archive that do not yet have a disposition associated with them and discuss the candidates that are rated most favourably.
Finally, in part three I discuss what to do once a candidate is confirmed to be a planet. In particular, I describe a new MCMC method that combines the likelihood fits of transit and radial velocity data with prior knowledge from several sources including optical and infrared spectrophotometric measurements and the parallax measurements from Gaia to constrain the stellar parameters. I apply the method to characterize two new hot Jupiter planets found by the WASP collaboration and confirmed with SOPHIE and TESS measurements. WASP-186b is a dense (4.22 ± 0.18MJ, 1.11 ± 0.03RJ ) planet on an eccentric (e=0.33 ± 0.01) 5-day orbit around a mid-F type star. While also in a ~5 day orbit, WASP-187b is puffed up (0.8 ± 0.09MJ, 1.64 ± 0.05RJ ) and orbiting a star that has begun evolving away from the main sequence.
2020-12-01T00:00:00ZSchanche, NicoleThis thesis is broken into three main sections tracing the steps of the development of a new framework to search for and characterize planets from the WASP survey. While all methods were developed specifically for the WASP project, the principles are easily transferable to any ground or space based survey. In the first part of the thesis, I discuss the development of two machine learning methods, a Random Forest Classifier and a Convolutional Neural Network, that are able to find new exoplanet candidates from WASP archival data and lightcurves. In preparing the training dataset, I also created a standardized catalog of 1,041 false positives from SuperWASP, the northern component of WASP, that were verified with additional observations from other instruments.
The second part of the thesis begins by discussing the results of the machine learning methods. In the analysis of the resulting probabilities, several patterns began to emerge that indicated the differing predictions between the algorithms carry useful information in itself. This realization sparked the development of a new “stacking” framework where the predictions of a number of different machine learning methods areused as the input to a second-level classifier that makes the final prediction. This method is straightforward to implement and demonstrates an improved performance over any individual classifier, making the stacked approach ideal for future large surveys. I use the model to classify and rank more than 100,000 lightcurves in the WASP archive that do not yet have a disposition associated with them and discuss the candidates that are rated most favourably.
Finally, in part three I discuss what to do once a candidate is confirmed to be a planet. In particular, I describe a new MCMC method that combines the likelihood fits of transit and radial velocity data with prior knowledge from several sources including optical and infrared spectrophotometric measurements and the parallax measurements from Gaia to constrain the stellar parameters. I apply the method to characterize two new hot Jupiter planets found by the WASP collaboration and confirmed with SOPHIE and TESS measurements. WASP-186b is a dense (4.22 ± 0.18MJ, 1.11 ± 0.03RJ ) planet on an eccentric (e=0.33 ± 0.01) 5-day orbit around a mid-F type star. While also in a ~5 day orbit, WASP-187b is puffed up (0.8 ± 0.09MJ, 1.64 ± 0.05RJ ) and orbiting a star that has begun evolving away from the main sequence.Analogue event horizons in dielectric mediumSingh, Vyomehttps://hdl.handle.net/10023/292642024-02-22T03:01:46Z2021-06-28T00:00:00ZIn this thesis I numerically study an optical pulse travelling in a dielectric medium as an analogue event horizon. A novel numerical method is developed to study the scattering properties of this optical system. Numerical solutions of scattering problems often exhibit instabilities. The staircase approximation, in addition, can cause slow convergence. We present a differential equation for the scattering matrix which solves both of these problems. The new algorithm inherits the numerical stability of the S matrix algorithm and converges faster for a smoothly varying potential than the S matrix algorithm with the staircase approximation. We apply our equation to solve a 1D stationary scattering of plane waves from a non-periodic smoothly varying pulse/scatterer travelling with a constant velocity in a lossless medium. The properties of stability and the convergence of the Riccati matrix equation are demonstrated. Furthermore, we include a relative velocity between the scatterer and the wave medium to generalise the algorithm further where the number of right and left going modes are not equal. The algorithm is applicable for stationary scattering process from arbitrarily shaped smooth scatterers, periodic or non-periodic, even when the scatterer is varying at the scale of wavelengths. This method is used to present numerical results for a sub-femtoseconds optical pulse travelling in bulk silica. We calculate the analogue hawking radiation from the analogue system. The temperature of the hawking radiation is studied systematically with many different profiles of pulses. We find out steepness, intensity and duration of the pulse are most important in producing analogue hawking radiation in these systems. A better numerical and theoretical understanding will make the experiments better suited to detect hawking radiation.
2021-06-28T00:00:00ZSingh, VyomeIn this thesis I numerically study an optical pulse travelling in a dielectric medium as an analogue event horizon. A novel numerical method is developed to study the scattering properties of this optical system. Numerical solutions of scattering problems often exhibit instabilities. The staircase approximation, in addition, can cause slow convergence. We present a differential equation for the scattering matrix which solves both of these problems. The new algorithm inherits the numerical stability of the S matrix algorithm and converges faster for a smoothly varying potential than the S matrix algorithm with the staircase approximation. We apply our equation to solve a 1D stationary scattering of plane waves from a non-periodic smoothly varying pulse/scatterer travelling with a constant velocity in a lossless medium. The properties of stability and the convergence of the Riccati matrix equation are demonstrated. Furthermore, we include a relative velocity between the scatterer and the wave medium to generalise the algorithm further where the number of right and left going modes are not equal. The algorithm is applicable for stationary scattering process from arbitrarily shaped smooth scatterers, periodic or non-periodic, even when the scatterer is varying at the scale of wavelengths. This method is used to present numerical results for a sub-femtoseconds optical pulse travelling in bulk silica. We calculate the analogue hawking radiation from the analogue system. The temperature of the hawking radiation is studied systematically with many different profiles of pulses. We find out steepness, intensity and duration of the pulse are most important in producing analogue hawking radiation in these systems. A better numerical and theoretical understanding will make the experiments better suited to detect hawking radiation.Self-organisation of cold atoms in optical cavitiesStaffini, Maria Laurahttps://hdl.handle.net/10023/292002024-02-14T12:02:10Z2020-07-27T00:00:00ZThe realisation of self-organisation of ultracold atoms in optical cavities, where the light field can couple strongly to the atomic field, paves the way for the observation of phase transitions in which the spatial order is entirely emergent, such as glassiness or supersolidity. This fact, and the flexibility of cavity environment, makes these light-matter systems ideal candidates for quantum simulation. A wide range of physics has already been demonstrated in cavities coupled to ultracold atoms.
In this work, we seek to contribute two additions to the growing toolbox of phase transitions in optical cavities. First, we consider a transversely pumped, single mode cavity containing a Bose-condensed atomic cloud – a system that is well studied, and which undergoes spatial self-organisation. We treat this in an open system formalism, without the two-level approximation or linearised treatment which are common assumptions in literature. Within this complete treatment, we observe a first order phase transition, with associated bistability leading to hysteresis. We also demonstrate that in some parameter range, the system displays chaotic behaviour due to a strange attractor of the dynamics. Both of these observations explain features of experimental data from previous literature.
We then consider the case of a multimode, longitudinally pumped cavity in the confocal geometry, i.e. supporting a degenerate mode family. This is motivated both by the rich physics observed in a multimode confocal cavity in the transversely pumped regime, and by the self-patterning observed in single-mirror experiments. We find that the behaviour of the system is too complex for characterisation, and that the analytic understanding we can gain from weak coupling approximations never holds. We conclude that there is no simple self-organisation in our model.
2020-07-27T00:00:00ZStaffini, Maria LauraThe realisation of self-organisation of ultracold atoms in optical cavities, where the light field can couple strongly to the atomic field, paves the way for the observation of phase transitions in which the spatial order is entirely emergent, such as glassiness or supersolidity. This fact, and the flexibility of cavity environment, makes these light-matter systems ideal candidates for quantum simulation. A wide range of physics has already been demonstrated in cavities coupled to ultracold atoms.
In this work, we seek to contribute two additions to the growing toolbox of phase transitions in optical cavities. First, we consider a transversely pumped, single mode cavity containing a Bose-condensed atomic cloud – a system that is well studied, and which undergoes spatial self-organisation. We treat this in an open system formalism, without the two-level approximation or linearised treatment which are common assumptions in literature. Within this complete treatment, we observe a first order phase transition, with associated bistability leading to hysteresis. We also demonstrate that in some parameter range, the system displays chaotic behaviour due to a strange attractor of the dynamics. Both of these observations explain features of experimental data from previous literature.
We then consider the case of a multimode, longitudinally pumped cavity in the confocal geometry, i.e. supporting a degenerate mode family. This is motivated both by the rich physics observed in a multimode confocal cavity in the transversely pumped regime, and by the self-patterning observed in single-mirror experiments. We find that the behaviour of the system is too complex for characterisation, and that the analytic understanding we can gain from weak coupling approximations never holds. We conclude that there is no simple self-organisation in our model.Methodology development of high sensitivity pulsed EPR and DNPZhao, Yujiehttps://hdl.handle.net/10023/291692024-02-15T03:07:30Z2024-06-10T00:00:00ZDynamic nuclear polarisation (DNP) is a process that transfers electron spin polarisation to nuclei and is a technique that has been widely used to improve nuclear magnetic resonance (NMR) sensitivity. This thesis presents the implementation of a DNP extension to a home-built high power pulsed electron paramagnetic resonance (EPR) spectrometer, HiPER, integrated with an
arbitrary waveform generator (AWG), operating at 94 GHz. It describes the use of high-peak power inverting chirped pulses to increase the polarisation gradient of dipolar-coupled electrons, where static cross-effect DNP enhancements of 340 are achieved within 3 seconds at 65 K with a mixture of 4-amino TEMPO and DNP juice. Several other nitroxide radical polarising agents are also investigated and systematic study into the DNP temperature dependence for different polarising agents reveals the impact of spectral diffusion and the molecular structure of polarising agents.
In addition, for the first time at 94 GHz, a comparative study is made of different coherent pulsed solid-effect DNP schemes including XiX, TPPM, and the adiabatic solid-effect. An ENDOR upgrade to the DNP/EPR spectrometer is also described, and preliminary room-temperature ¹H ENDOR and low-temperature ¹⁹F ENDOR experiments presented, where a combined approach involving shaped pulse excitation and data processing at an intermediate frequency in down-conversion for DC suppression yields a threefold enhancement in SNR. Finally, a brief summary to ongoing projects on magnet shimming, transition metal DNP and DNP characterisation for lithium dendrites is provided.
2024-06-10T00:00:00ZZhao, YujieDynamic nuclear polarisation (DNP) is a process that transfers electron spin polarisation to nuclei and is a technique that has been widely used to improve nuclear magnetic resonance (NMR) sensitivity. This thesis presents the implementation of a DNP extension to a home-built high power pulsed electron paramagnetic resonance (EPR) spectrometer, HiPER, integrated with an
arbitrary waveform generator (AWG), operating at 94 GHz. It describes the use of high-peak power inverting chirped pulses to increase the polarisation gradient of dipolar-coupled electrons, where static cross-effect DNP enhancements of 340 are achieved within 3 seconds at 65 K with a mixture of 4-amino TEMPO and DNP juice. Several other nitroxide radical polarising agents are also investigated and systematic study into the DNP temperature dependence for different polarising agents reveals the impact of spectral diffusion and the molecular structure of polarising agents.
In addition, for the first time at 94 GHz, a comparative study is made of different coherent pulsed solid-effect DNP schemes including XiX, TPPM, and the adiabatic solid-effect. An ENDOR upgrade to the DNP/EPR spectrometer is also described, and preliminary room-temperature ¹H ENDOR and low-temperature ¹⁹F ENDOR experiments presented, where a combined approach involving shaped pulse excitation and data processing at an intermediate frequency in down-conversion for DC suppression yields a threefold enhancement in SNR. Finally, a brief summary to ongoing projects on magnet shimming, transition metal DNP and DNP characterisation for lithium dendrites is provided.Enhanced detection of explosives using organic semiconductorsOgugu, Edward Blessedhttps://hdl.handle.net/10023/291682024-02-15T03:05:47Z2024-06-10T00:00:00ZAbstract redacted
2024-06-10T00:00:00ZOgugu, Edward BlessedAbstract redactedImaging the effects of magnetic proximity coupling on the electronic structure of transition metal dichalcogenidesEdwards, Brendanhttps://hdl.handle.net/10023/290152024-01-19T11:57:20Z2024-06-10T00:00:00ZThe aim of this thesis is to exploit proximity coupling effects to induce and manipulate magnetic states in transition metal dichalcogenides (TMDs), and to investigate the resulting effects on the electronic structure using predominantly photoemission-based techniques. The results presented here focus on three candidate materials that have been suggested to be on the brink of ferromagnetic instabilities, namely NbS₂, VSe₂ and VTe₂.
It has been demonstrated that intercalating magnetic ions between NbS₂ layers leads to the emergence of novel bulk magnetic textures. However, the magnetic phenomena at the termination-dependent surfaces of such intercalated TMDs, where proximity coupling effects with the underlying bulk material will play a crucial role, remain relatively unexplored. Motivated by this, I present a termination-dependent characterisation of the electronic structure of three intercalated TMDs, namely V₁/₃NbS₂, Cr₁/₃NbS₂ and Fe₁/₃NbS₂, obtained by utilising spatially-resolved angle-resolved photoemission spectroscopy (ARPES). Through this, methods to reliably identify unique surface terminations in this class of compounds are demonstrated. Further measurements focusing on the NbS₂-terminated surface of V₁/₃NbS₂ reveal that an RKKY-like interaction between the monolayer-like NbS₂ surface layer and underlying magnetic V intercalates generates a valley-dependent Zeeman spin splitting of the itinerant Nb-derived surface states, exceeding 50 meV. This energy scale is of comparable size to the intrinsic spin–orbit splitting and demonstrates a new route to control valley-spin splittings that provides the largest proximity coupling of a TMD monolayer realised to date.
The thesis concludes by investigating monolayer vanadium dichalcogenides, compounds for which charge density wave (CDW) states may suppress ferromagnetism. Here, I present our progress towards destabilising the CDW order in these compounds by inducing magnetism via proximity coupling to a ferromagnetic substrate. The results include ARPES characterisations of VSe₂ and VTe₂ monolayers, where it is shown that both compounds exhibit metal-insulator transitions in their CDW states. Finally, X-ray magnetic circular dichroism measurements of van der Waals heterostructures consisting of monolayer VTe₂ grown on ferromagnetic Cr₂Te₃ thin films are presented which provide evidence of a magnetic proximity coupling between the substrate and overlayer.
2024-06-10T00:00:00ZEdwards, BrendanThe aim of this thesis is to exploit proximity coupling effects to induce and manipulate magnetic states in transition metal dichalcogenides (TMDs), and to investigate the resulting effects on the electronic structure using predominantly photoemission-based techniques. The results presented here focus on three candidate materials that have been suggested to be on the brink of ferromagnetic instabilities, namely NbS₂, VSe₂ and VTe₂.
It has been demonstrated that intercalating magnetic ions between NbS₂ layers leads to the emergence of novel bulk magnetic textures. However, the magnetic phenomena at the termination-dependent surfaces of such intercalated TMDs, where proximity coupling effects with the underlying bulk material will play a crucial role, remain relatively unexplored. Motivated by this, I present a termination-dependent characterisation of the electronic structure of three intercalated TMDs, namely V₁/₃NbS₂, Cr₁/₃NbS₂ and Fe₁/₃NbS₂, obtained by utilising spatially-resolved angle-resolved photoemission spectroscopy (ARPES). Through this, methods to reliably identify unique surface terminations in this class of compounds are demonstrated. Further measurements focusing on the NbS₂-terminated surface of V₁/₃NbS₂ reveal that an RKKY-like interaction between the monolayer-like NbS₂ surface layer and underlying magnetic V intercalates generates a valley-dependent Zeeman spin splitting of the itinerant Nb-derived surface states, exceeding 50 meV. This energy scale is of comparable size to the intrinsic spin–orbit splitting and demonstrates a new route to control valley-spin splittings that provides the largest proximity coupling of a TMD monolayer realised to date.
The thesis concludes by investigating monolayer vanadium dichalcogenides, compounds for which charge density wave (CDW) states may suppress ferromagnetism. Here, I present our progress towards destabilising the CDW order in these compounds by inducing magnetism via proximity coupling to a ferromagnetic substrate. The results include ARPES characterisations of VSe₂ and VTe₂ monolayers, where it is shown that both compounds exhibit metal-insulator transitions in their CDW states. Finally, X-ray magnetic circular dichroism measurements of van der Waals heterostructures consisting of monolayer VTe₂ grown on ferromagnetic Cr₂Te₃ thin films are presented which provide evidence of a magnetic proximity coupling between the substrate and overlayer.Resonant frequency conversion in photonic crystal fibresPetty, Jackhttps://hdl.handle.net/10023/289742024-01-22T21:59:37Z2024-06-10T00:00:00ZThis thesis is concerned with the frequency conversion process of resonant radiation, whereby a pulse of light in the anomalous dispersion regime of a nonlinear optical fibre sheds light to a resonant dispersive wave in the normal dispersion regime. Using pulses from a Ti:sapphire laser and a photonic crystal fibre (PCF), this mechanism provides efficient, tunable conversion from the near infra-red to visible femtosecond pulses. This promising technology has improved noise properties over the more conventional PCF supercontinuum source, greater tunability than frequency doubling techniques, and a highly portable design requiring only millimetres of fibre. Progress from the literature towards the development of resonant radiation as a practical light source is reviewed, with areas for improvement identified.
A systematic experimental investigation of the effect of incoupled pulse energy, input pulse wavelength and input pulse chirp is performed, expanding the parameter-space from previous works. Particular attention is paid to input pulse chirp, the importance of which has become clear in recent simulations. Using a selection of fused silica solid-core PCFs, extraordinary tunability of the resonant radiation over all visible wavelengths is demonstrated, rivalling the supercontinuum source for visible applications. Unusual regimes of conversion are presented, including generation of multi-peak, very broad, or very narrow spectra. Resonant radiation on the negative branch of the PCF dispersion relation is also demonstrated, in a regime not previously explored.
As a primary novel result, it is shown that optimising input pulse chirp when using ~1nJ incoupled pulse energies boosts the conversion efficiency above the current record for visible resonant radiation. The interpretation and implications of this behaviour are discussed. Temporal measurements of resonant radiation using cross-correlation frequency-resolved optical gating (XFROG) demonstrate a visible pulse duration of 59fs, potentially compressible to even shorter durations.
2024-06-10T00:00:00ZPetty, JackThis thesis is concerned with the frequency conversion process of resonant radiation, whereby a pulse of light in the anomalous dispersion regime of a nonlinear optical fibre sheds light to a resonant dispersive wave in the normal dispersion regime. Using pulses from a Ti:sapphire laser and a photonic crystal fibre (PCF), this mechanism provides efficient, tunable conversion from the near infra-red to visible femtosecond pulses. This promising technology has improved noise properties over the more conventional PCF supercontinuum source, greater tunability than frequency doubling techniques, and a highly portable design requiring only millimetres of fibre. Progress from the literature towards the development of resonant radiation as a practical light source is reviewed, with areas for improvement identified.
A systematic experimental investigation of the effect of incoupled pulse energy, input pulse wavelength and input pulse chirp is performed, expanding the parameter-space from previous works. Particular attention is paid to input pulse chirp, the importance of which has become clear in recent simulations. Using a selection of fused silica solid-core PCFs, extraordinary tunability of the resonant radiation over all visible wavelengths is demonstrated, rivalling the supercontinuum source for visible applications. Unusual regimes of conversion are presented, including generation of multi-peak, very broad, or very narrow spectra. Resonant radiation on the negative branch of the PCF dispersion relation is also demonstrated, in a regime not previously explored.
As a primary novel result, it is shown that optimising input pulse chirp when using ~1nJ incoupled pulse energies boosts the conversion efficiency above the current record for visible resonant radiation. The interpretation and implications of this behaviour are discussed. Temporal measurements of resonant radiation using cross-correlation frequency-resolved optical gating (XFROG) demonstrate a visible pulse duration of 59fs, potentially compressible to even shorter durations.Tracing the chemistry of high-energy processes in planetary atmospheresBarth, Patrickhttps://hdl.handle.net/10023/288752024-01-04T03:07:57Z2024-06-10T00:00:00ZLightning, X-ray & UV radiation, stellar energetic particles, and galactic cosmic rays provide important sources of disequilibrium chemistry in planetary atmospheres and might be responsible for the production of important precursors for the origin of life. A growing number of known exoplanets with a large, diverse range of atmospheres provides new opportunities to explore the effects of these high-energy processes on the chemistry of planetary atmospheres.
By combining X-ray & UV observations of the host star with 3D climate simulations, we study the effect of different types of high-energy radiation on the production of organic and prebiotic molecules in the atmosphere of the hot Jupiter HD 189733 b. We identify ‘fingerprint’ ions for the ionization of the atmosphere by both galactic cosmic rays and stellar energetic particles and find an enhancing effect on the abundance of key organic molecules that are potentially detectable by JWST.
In addition to these types of external high-energy radiation, lightning provides another energy source from within a cloudy atmosphere when charged particles are separated to build up an electric field strong enough for a discharge to develop. Lightning has been suggested to play a role in triggering the occurrence of bio-ready chemical species and nutrients for Earth’s earliest biosphere. We present results from spark discharge experiments in gas mixtures resembling the atmospheres of modern and early Earth. They suggest that lightning-driven nitrogen fixation may have been efficient on early Earth, but measurements of the isotopic composition of the discharge products do not agree with isotope ratios from the sedimentary rock record, which supports the early development of biological nitrogen fixation.
By combining our experimental results with photochemical simulations, we can investigate the effect of lightning on the atmospheric chemistry of terrestrial exoplanets. We find that lightning may be able to mask the ozone features of an oxygen-rich, biotic atmosphere, making it harder to detect the biosphere of such a planet. Similarly, lightning can reduce the concentration of ozone in the anoxic, abiotic atmosphere of a planet orbiting a late M dwarf, thereby reducing the potential for a false-positive life-detection. In summary, the work presented in this thesis provides new constraints for the full characterisation of atmospheric and surface processes on exoplanets and for the interpretation of observations of their atmospheres.
2024-06-10T00:00:00ZBarth, PatrickLightning, X-ray & UV radiation, stellar energetic particles, and galactic cosmic rays provide important sources of disequilibrium chemistry in planetary atmospheres and might be responsible for the production of important precursors for the origin of life. A growing number of known exoplanets with a large, diverse range of atmospheres provides new opportunities to explore the effects of these high-energy processes on the chemistry of planetary atmospheres.
By combining X-ray & UV observations of the host star with 3D climate simulations, we study the effect of different types of high-energy radiation on the production of organic and prebiotic molecules in the atmosphere of the hot Jupiter HD 189733 b. We identify ‘fingerprint’ ions for the ionization of the atmosphere by both galactic cosmic rays and stellar energetic particles and find an enhancing effect on the abundance of key organic molecules that are potentially detectable by JWST.
In addition to these types of external high-energy radiation, lightning provides another energy source from within a cloudy atmosphere when charged particles are separated to build up an electric field strong enough for a discharge to develop. Lightning has been suggested to play a role in triggering the occurrence of bio-ready chemical species and nutrients for Earth’s earliest biosphere. We present results from spark discharge experiments in gas mixtures resembling the atmospheres of modern and early Earth. They suggest that lightning-driven nitrogen fixation may have been efficient on early Earth, but measurements of the isotopic composition of the discharge products do not agree with isotope ratios from the sedimentary rock record, which supports the early development of biological nitrogen fixation.
By combining our experimental results with photochemical simulations, we can investigate the effect of lightning on the atmospheric chemistry of terrestrial exoplanets. We find that lightning may be able to mask the ozone features of an oxygen-rich, biotic atmosphere, making it harder to detect the biosphere of such a planet. Similarly, lightning can reduce the concentration of ozone in the anoxic, abiotic atmosphere of a planet orbiting a late M dwarf, thereby reducing the potential for a false-positive life-detection. In summary, the work presented in this thesis provides new constraints for the full characterisation of atmospheric and surface processes on exoplanets and for the interpretation of observations of their atmospheres.Composite materials with vanishing effective permittivityAkpan, Usenobong Benjaminhttps://hdl.handle.net/10023/286272023-11-25T03:06:58Z2023-11-29T00:00:00ZMy PhD research thesis, entitled "Composite materials with vanishing effective permittivity" is presented here. Epsilon Near Zero (ENZ) Metamaterials (MMs) are photonic media in which the real part of the relative dielectric permittivity is vanishing and are useful for spatial light control such as squeezing and directive emission, enhanced light-matter interaction, sub-wavelength imaging, and nonlinear optics. This thesis demonstrates the use of mixed sub-wavelength metal and dielectric materials, which relies on the effective medium theory approximation to fine-tune the ENZ operation wavelength over wider ranges. First, we synthesized by chemical vapor deposition (CVD) a nonconductive bilayer graphene, and then design and show by a model of a semi-metal graphene/amorphous-germanium (α-Ge) bilayer and multilayer ENZ MM in the mid-infrared (IR) region. We then fabricate and experiment on its bilayer, with a discussion of its dynamic tunability via external control of the graphene’s potential. One intention
is to see how this enables practicable photonic switching at the 10.6µm, where the carrier is the photon and the control mechanism is the back-gated voltage. Another goal is to electro-optically investigate the experimental compatibility of the basic bilayer of graphene/α-Ge structure in a MOSFET-based architecture which holds promise for a holographic imaging application. We found a gate resistance of 5.1MΩ, which met the requirement of a MOSFET device; and graphene’s sheet resistance of 8.9kΩ in our GFET device which is comparable to previously reported values in literature. We found out that graphene/α-Ge FET device lacks electrically compatibility with the MOSFET geometry as it failed to reproduce the usual ambipolar transconductance typical of a graphene/silicon geometry. We found that the optical transmission and reflection visibility of the graphene changes nonlinearly with the gated voltage; and the reflection visibility of graphene with gate voltage decreases when the source voltage is increased. Model results showed that reflection decreases with increasing voltage. Secondly, we design, fabricate and experiment by investigating the coupling loss, due to impedance mismatch, in anti-reflection coated/metal/dielectric, and the absorption loss in metal/dielectric
with gain ENZ composite 1-dimensional anisotropic structures. We found the antireflection coating enhancing the transmission to around 20% and reducing the reflection by approximately 50%, in the ENZ (682nm) region. With the gain-doped dielectric, we tailored the thickness of the Alq₃:DCM dye dielectric in our Ag/Alq₃:DCM multilayer to achieve the ENZ wavelength close to the gain medium central emission wavelength and a reduction of the imaginary permittivity, from which future investigations of stimulated emission effect and other 8 nonlinear properties by all-optical pump-probe mechanism are indispensable. Finally, we design, fabricate, and experiment a graded ENZ composite which finds application as an optical aperture element. We also modeled various standard and ENZ apertures and compared with our experiment, which we observed not to agree completely with the model.
2023-11-29T00:00:00ZAkpan, Usenobong BenjaminMy PhD research thesis, entitled "Composite materials with vanishing effective permittivity" is presented here. Epsilon Near Zero (ENZ) Metamaterials (MMs) are photonic media in which the real part of the relative dielectric permittivity is vanishing and are useful for spatial light control such as squeezing and directive emission, enhanced light-matter interaction, sub-wavelength imaging, and nonlinear optics. This thesis demonstrates the use of mixed sub-wavelength metal and dielectric materials, which relies on the effective medium theory approximation to fine-tune the ENZ operation wavelength over wider ranges. First, we synthesized by chemical vapor deposition (CVD) a nonconductive bilayer graphene, and then design and show by a model of a semi-metal graphene/amorphous-germanium (α-Ge) bilayer and multilayer ENZ MM in the mid-infrared (IR) region. We then fabricate and experiment on its bilayer, with a discussion of its dynamic tunability via external control of the graphene’s potential. One intention
is to see how this enables practicable photonic switching at the 10.6µm, where the carrier is the photon and the control mechanism is the back-gated voltage. Another goal is to electro-optically investigate the experimental compatibility of the basic bilayer of graphene/α-Ge structure in a MOSFET-based architecture which holds promise for a holographic imaging application. We found a gate resistance of 5.1MΩ, which met the requirement of a MOSFET device; and graphene’s sheet resistance of 8.9kΩ in our GFET device which is comparable to previously reported values in literature. We found out that graphene/α-Ge FET device lacks electrically compatibility with the MOSFET geometry as it failed to reproduce the usual ambipolar transconductance typical of a graphene/silicon geometry. We found that the optical transmission and reflection visibility of the graphene changes nonlinearly with the gated voltage; and the reflection visibility of graphene with gate voltage decreases when the source voltage is increased. Model results showed that reflection decreases with increasing voltage. Secondly, we design, fabricate and experiment by investigating the coupling loss, due to impedance mismatch, in anti-reflection coated/metal/dielectric, and the absorption loss in metal/dielectric
with gain ENZ composite 1-dimensional anisotropic structures. We found the antireflection coating enhancing the transmission to around 20% and reducing the reflection by approximately 50%, in the ENZ (682nm) region. With the gain-doped dielectric, we tailored the thickness of the Alq₃:DCM dye dielectric in our Ag/Alq₃:DCM multilayer to achieve the ENZ wavelength close to the gain medium central emission wavelength and a reduction of the imaginary permittivity, from which future investigations of stimulated emission effect and other 8 nonlinear properties by all-optical pump-probe mechanism are indispensable. Finally, we design, fabricate, and experiment a graded ENZ composite which finds application as an optical aperture element. We also modeled various standard and ENZ apertures and compared with our experiment, which we observed not to agree completely with the model.Measurements and modelling of the kinetics of thermally activated delayed fluorescenceDiesing, Stefanhttps://hdl.handle.net/10023/285732023-10-27T02:00:42Z2023-11-29T00:00:00ZEfficient organic light-emitting diodes (OLEDs) enable the commercialisation of energy efficient and high contrast displays, and are being exploited increasingly in telecommunication and biomedical applications. In an efficient OLED all electrically generated singlet and triplet excitons recombine to emit light. In recent years, the exploitation of emitters that show thermally activated delayed fluorescence (TADF) has proven to be a complementary strategy to phosphors that leads to devices of comparable efficiencies without relying on scarce metals in the emitter design.
In this thesis, the kinetics of TADF are explored. First, two families of novel emitters are discussed by investigating their photophysics. One series incorporates a novel pyridazine acceptor whilst the other series makes use of a through-space conjugated donor design using a [2,2]paracyclophane. In both families of emitters, the emitter with the largest donor strength showed efficient triplet up-conversion via TADF in OLEDs.
The kinetics of TADF can be calculated from a bimolecular fit of the transient photo- luminescence (PL). However, in the literature, different assumptions are used to calculate rate constants from these data. Therefore, a three-level model was analysed to understand the limiting cases employed in literature.
The transient PL of TADF is characterised by two decay regimes that stretch over multiple orders of magnitude in time and intensity. This makes recording both regimes in one measurement challenging. To solve this, first improvements and implementations of existing techniques are discussed. In order to remove the need to measure over a large dynamic range of intensities a novel measurement technique was developed which allows the extraction of kinetic parameters of TADF by recording the emission during a step-function excitation.
Finally, a figure of merit based on the TADF kinetics under constant electrical excitation was developed to evaluate new TADF emitters for use in OLED with low efficiency roll-off.
2023-11-29T00:00:00ZDiesing, StefanEfficient organic light-emitting diodes (OLEDs) enable the commercialisation of energy efficient and high contrast displays, and are being exploited increasingly in telecommunication and biomedical applications. In an efficient OLED all electrically generated singlet and triplet excitons recombine to emit light. In recent years, the exploitation of emitters that show thermally activated delayed fluorescence (TADF) has proven to be a complementary strategy to phosphors that leads to devices of comparable efficiencies without relying on scarce metals in the emitter design.
In this thesis, the kinetics of TADF are explored. First, two families of novel emitters are discussed by investigating their photophysics. One series incorporates a novel pyridazine acceptor whilst the other series makes use of a through-space conjugated donor design using a [2,2]paracyclophane. In both families of emitters, the emitter with the largest donor strength showed efficient triplet up-conversion via TADF in OLEDs.
The kinetics of TADF can be calculated from a bimolecular fit of the transient photo- luminescence (PL). However, in the literature, different assumptions are used to calculate rate constants from these data. Therefore, a three-level model was analysed to understand the limiting cases employed in literature.
The transient PL of TADF is characterised by two decay regimes that stretch over multiple orders of magnitude in time and intensity. This makes recording both regimes in one measurement challenging. To solve this, first improvements and implementations of existing techniques are discussed. In order to remove the need to measure over a large dynamic range of intensities a novel measurement technique was developed which allows the extraction of kinetic parameters of TADF by recording the emission during a step-function excitation.
Finally, a figure of merit based on the TADF kinetics under constant electrical excitation was developed to evaluate new TADF emitters for use in OLED with low efficiency roll-off.Imaging surface magnetism in ruthenatesBenedičič, Izidorhttps://hdl.handle.net/10023/285572023-10-24T02:07:22Z2023-11-29T00:00:00ZThe rapid development of technologies used in computers and consumer electronics is founded in modern condensed matter physics. Advances in the physics of lasers, semiconductors and nanofabrication have enabled precise control of charge transport down to the nanoscale. As the use of electronics grows, energy consumption is becoming an increasingly important issue. One possible path to energy‑efficient technologies is to utilise spin transport in magnetic materials. However, the desirable level of control over magnetic transport is yet to be achieved.
Materials with significant electron‑electron interactions, usually called strongly correlated systems, are promising platforms for future technologies. These materials host a range of fascinating phenomena, such as superconductivity, electronic nematicity and various magnetic orders. The ruthenates are a family of materials where many different magnetic ground states are realised. To understand how these magnetic phases emerge and how they can be manipulated, microscopic knowledge of their electronic structure is indispensable.
In this thesis, I present a study of magnetism on surfaces of several compounds in the Ruddlesden Popper series of ruthenates, combining low‑temperature scanning tunnelling microscopy in a vector magnet field with theoretical modelling. I show how tiny structural changes at the surface can lead to dramatic changes in magnetic and electronic properties, such as suppression of superconductivity, metal‑to‑insulator transition and stabilisation of magnetic order. In cases where the surface is magnetic, the interplay between magnetism and spin‑orbit coupling makes the electronic properties very sensitive to an external field.
This work explored the response of ruthenates to a magnetic field, providing insight into the interplay between their electronic and magnetic structures. Building on this knowledge, other control parameters, such as mechanical strain, electric current or light pulses, could be employed to engineer novel magnetic and electronic states. The materials presented in this work are thus prime candidates for developing future spin‑based electronics.
2023-11-29T00:00:00ZBenedičič, IzidorThe rapid development of technologies used in computers and consumer electronics is founded in modern condensed matter physics. Advances in the physics of lasers, semiconductors and nanofabrication have enabled precise control of charge transport down to the nanoscale. As the use of electronics grows, energy consumption is becoming an increasingly important issue. One possible path to energy‑efficient technologies is to utilise spin transport in magnetic materials. However, the desirable level of control over magnetic transport is yet to be achieved.
Materials with significant electron‑electron interactions, usually called strongly correlated systems, are promising platforms for future technologies. These materials host a range of fascinating phenomena, such as superconductivity, electronic nematicity and various magnetic orders. The ruthenates are a family of materials where many different magnetic ground states are realised. To understand how these magnetic phases emerge and how they can be manipulated, microscopic knowledge of their electronic structure is indispensable.
In this thesis, I present a study of magnetism on surfaces of several compounds in the Ruddlesden Popper series of ruthenates, combining low‑temperature scanning tunnelling microscopy in a vector magnet field with theoretical modelling. I show how tiny structural changes at the surface can lead to dramatic changes in magnetic and electronic properties, such as suppression of superconductivity, metal‑to‑insulator transition and stabilisation of magnetic order. In cases where the surface is magnetic, the interplay between magnetism and spin‑orbit coupling makes the electronic properties very sensitive to an external field.
This work explored the response of ruthenates to a magnetic field, providing insight into the interplay between their electronic and magnetic structures. Building on this knowledge, other control parameters, such as mechanical strain, electric current or light pulses, could be employed to engineer novel magnetic and electronic states. The materials presented in this work are thus prime candidates for developing future spin‑based electronics.The formation of high-mass stars and stellar clustersZhang, Suinanhttps://hdl.handle.net/10023/284382023-10-20T08:18:05Z2023-11-29T00:00:00ZThe earliest phase of high-mass star formation has remained a challenging topic. The distinguishing feature between competing theoretical models is the prediction for the high-mass prestellar cores. This thesis presents (sub)arcsecond-resolution interferometric observations in conjunction with synthetic observations at submillimeter wavelengths towards high-mass pre-/proto-stellar objects, for the purpose of characterising the physical and kinematic properties of the early stages of high-mass star formation.
Chapter two showcases deep ALMA 0.82 mm observations (θ ∼ 0.5′′) towards the high-mass prestellar core candidate G11.92-0.61 MM2. Extensive N2H+ (4-3) emission is detected around MM2, displaying complex spectra with multiple velocity components present. Gaussian decomposition and hierarchical clustering are performed to the N₂H⁺ data cube to investigate the kinematics of the N₂H⁺-emitting gas, which reveals a hierarchical system with filamentary substructures showing velocity gradients. The most dominant N₂H⁺ substructure probably traces the accretion flows towards MM2. A mass inflow rate of 2 ×10⁻⁴ ∼ 1.2 ×10⁻³ M⊙ yr⁻¹ is derived with the hypothesis of filamentary accretion flows.
Chapter three presents synthetic 1.3 mm dust continuum images of high-mass star-forming clumps generated with analytic prescriptions and radiative transfer modelling. 432 models with different combinations of stellar masses, separations of sources, and beam sizes are considered. This parametric study predicts that the low-mass objects with masses ≦ 1 M⊙ will not be detected if located ≦ 0.1 pc from a 50 M⊙ protostar in 0.5′′observations.
Chapter four summarises the SMA 1.3 mm observations towards the high-mass protostellar object G34.24+0.13MM. The 1′′ 1.3 mm continuum image reveals that G34.24+0.13MM is a single compact core, with a size of 4700 AU and a mass of 12.5 M⊙. Molecular lines are detected towards the source, possibly indicating ordered motions of the gaseous envelope or unresolved multiplicity. The uniquely high luminosity-mass ratio of G34.24+0.13MM requires future higher-resolution multi-wavelength observations to properly explain.
2023-11-29T00:00:00ZZhang, SuinanThe earliest phase of high-mass star formation has remained a challenging topic. The distinguishing feature between competing theoretical models is the prediction for the high-mass prestellar cores. This thesis presents (sub)arcsecond-resolution interferometric observations in conjunction with synthetic observations at submillimeter wavelengths towards high-mass pre-/proto-stellar objects, for the purpose of characterising the physical and kinematic properties of the early stages of high-mass star formation.
Chapter two showcases deep ALMA 0.82 mm observations (θ ∼ 0.5′′) towards the high-mass prestellar core candidate G11.92-0.61 MM2. Extensive N2H+ (4-3) emission is detected around MM2, displaying complex spectra with multiple velocity components present. Gaussian decomposition and hierarchical clustering are performed to the N₂H⁺ data cube to investigate the kinematics of the N₂H⁺-emitting gas, which reveals a hierarchical system with filamentary substructures showing velocity gradients. The most dominant N₂H⁺ substructure probably traces the accretion flows towards MM2. A mass inflow rate of 2 ×10⁻⁴ ∼ 1.2 ×10⁻³ M⊙ yr⁻¹ is derived with the hypothesis of filamentary accretion flows.
Chapter three presents synthetic 1.3 mm dust continuum images of high-mass star-forming clumps generated with analytic prescriptions and radiative transfer modelling. 432 models with different combinations of stellar masses, separations of sources, and beam sizes are considered. This parametric study predicts that the low-mass objects with masses ≦ 1 M⊙ will not be detected if located ≦ 0.1 pc from a 50 M⊙ protostar in 0.5′′observations.
Chapter four summarises the SMA 1.3 mm observations towards the high-mass protostellar object G34.24+0.13MM. The 1′′ 1.3 mm continuum image reveals that G34.24+0.13MM is a single compact core, with a size of 4700 AU and a mass of 12.5 M⊙. Molecular lines are detected towards the source, possibly indicating ordered motions of the gaseous envelope or unresolved multiplicity. The uniquely high luminosity-mass ratio of G34.24+0.13MM requires future higher-resolution multi-wavelength observations to properly explain.Prominences : the phantom menaceWaugh, Rosiehttps://hdl.handle.net/10023/283412023-10-13T11:08:05Z2023-11-29T00:00:00ZIt has been proposed that slingshot prominences may be a mass and angular momentum loss mechanism for rapidly rotating young stars. Stellar evolution models currently rely only on the wind as the angular momentum loss mechanism and do not include prominences. These models often require more angular momentum to be removed than the wind allows, and prominence ejection may hold at least part of the answer. This thesis aims to investigate the locations of prominence formation through mathematical modelling. The magnetic field structures that could support prominences are investigated, typically using two models: a magnetohydrostatic model and a stability method. The distributions of prominences around the star are calculated and, where possible, compared to observations. In some cases the magnetic field of the star is prescribed to be a simple field such as a dipole or quadrupole, and in others the magnetic field is generated from observations of the surface magnetic field, with the coronal magnetic field reconstructed from this, assuming the field is potential. This work finds that prominences can be formed both within the stellar wind of stars, and within the closed field region. With the magnetohydrostatic model, two classes of prominence are found: those close to the surface of the star that could be analogous to solar prominences, and those at very large distances from the stellar surface. Those in the second category may be ejected from the star and act as a mechanism for removing mass and angular momentum. The removal of mass and angular momentum by prominences was modelled using the stable point method, and it was found that for some stars within the sample of M-dwarfs, the prominences could be a significant angular momentum loss mechanism.
Work here shows that whilst a tilted dipolar field can typically replicate the locations of observed prominences well, using observed prominence locations to infer the tilt of the dipole is not very effective due to the degeneracy of stable point locations. Overall, prominences are likely to be important contributors to the removal of angular momentum and therefore spin down of a star at certain points in its life, whilst they are typically left out of stellar evolution models. They are likely to be very common across young stars, however they are usually only observable when they transit the stellar disc and therefore they will often be missed by observations due to geometric effects. Those that are observed are likely only being partially observed, meaning that mass predictions from observations are underestimates.
2023-11-29T00:00:00ZWaugh, RosieIt has been proposed that slingshot prominences may be a mass and angular momentum loss mechanism for rapidly rotating young stars. Stellar evolution models currently rely only on the wind as the angular momentum loss mechanism and do not include prominences. These models often require more angular momentum to be removed than the wind allows, and prominence ejection may hold at least part of the answer. This thesis aims to investigate the locations of prominence formation through mathematical modelling. The magnetic field structures that could support prominences are investigated, typically using two models: a magnetohydrostatic model and a stability method. The distributions of prominences around the star are calculated and, where possible, compared to observations. In some cases the magnetic field of the star is prescribed to be a simple field such as a dipole or quadrupole, and in others the magnetic field is generated from observations of the surface magnetic field, with the coronal magnetic field reconstructed from this, assuming the field is potential. This work finds that prominences can be formed both within the stellar wind of stars, and within the closed field region. With the magnetohydrostatic model, two classes of prominence are found: those close to the surface of the star that could be analogous to solar prominences, and those at very large distances from the stellar surface. Those in the second category may be ejected from the star and act as a mechanism for removing mass and angular momentum. The removal of mass and angular momentum by prominences was modelled using the stable point method, and it was found that for some stars within the sample of M-dwarfs, the prominences could be a significant angular momentum loss mechanism.
Work here shows that whilst a tilted dipolar field can typically replicate the locations of observed prominences well, using observed prominence locations to infer the tilt of the dipole is not very effective due to the degeneracy of stable point locations. Overall, prominences are likely to be important contributors to the removal of angular momentum and therefore spin down of a star at certain points in its life, whilst they are typically left out of stellar evolution models. They are likely to be very common across young stars, however they are usually only observable when they transit the stellar disc and therefore they will often be missed by observations due to geometric effects. Those that are observed are likely only being partially observed, meaning that mass predictions from observations are underestimates.Learning and interpreting the galaxy-halo connection in cosmic simulationsChittenden, Harry Georgehttps://hdl.handle.net/10023/282642023-09-01T11:56:57Z2023-11-29T00:00:00ZIn modern galactic astronomy, cosmological simulations and observational galaxy surveys work hand in hand, offering valuable insights into the historical evolution of galaxies on both cosmological scales and an individual basis. As dark matter halos constitute a significant portion of the mass in galaxies, clusters, and cosmic structures, they profoundly impact the properties of galaxies. This relationship is known as the galaxy-halo connection.
Galaxies possess a complex nature necessitating computationally intensive modelling. Accurately and consistently modelling galaxy-halo coevolution across all scales thus presents a challenge, and compromises are usually made between simulation size and resolution. However, it is possible to conduct pure dark matter simulations on larger scales, requiring a fraction of the power of complete simulations. As observational surveys expand in size and detail, however, simulations of this magnitude become crucial in supporting their findings, surpassing the limitations of galaxy simulations.
In this thesis, I present a machine learning model which encodes the galaxy-halo connection within a cosmohydrodynamical simulation. This model predicts the star formation and metallicity of galaxies over time, from properties of their halos and cosmic environment. These predictions are used to emulate observational data using spectral synthesis models, and subsequently the model is applied to a large dark matter simulation.
Through these predictions, the model replicates the correlations responsible for galaxy evolution, as well as observable quantities reflecting this galaxy-halo connection, with similar results in dark matter simulations. The model computes accurate galaxy-halo statistics and reveals important physical relationships; specifically, variables associated with halo accretion influence a galaxy's mass and star formation, while environmental variables are linked to its metallicity. While the predictions from dark matter simulations are reasonably accurate, they are affected by the absence of baryonic processes, the resolution of the simulation, and the calculation of halo properties.
2023-11-29T00:00:00ZChittenden, Harry GeorgeIn modern galactic astronomy, cosmological simulations and observational galaxy surveys work hand in hand, offering valuable insights into the historical evolution of galaxies on both cosmological scales and an individual basis. As dark matter halos constitute a significant portion of the mass in galaxies, clusters, and cosmic structures, they profoundly impact the properties of galaxies. This relationship is known as the galaxy-halo connection.
Galaxies possess a complex nature necessitating computationally intensive modelling. Accurately and consistently modelling galaxy-halo coevolution across all scales thus presents a challenge, and compromises are usually made between simulation size and resolution. However, it is possible to conduct pure dark matter simulations on larger scales, requiring a fraction of the power of complete simulations. As observational surveys expand in size and detail, however, simulations of this magnitude become crucial in supporting their findings, surpassing the limitations of galaxy simulations.
In this thesis, I present a machine learning model which encodes the galaxy-halo connection within a cosmohydrodynamical simulation. This model predicts the star formation and metallicity of galaxies over time, from properties of their halos and cosmic environment. These predictions are used to emulate observational data using spectral synthesis models, and subsequently the model is applied to a large dark matter simulation.
Through these predictions, the model replicates the correlations responsible for galaxy evolution, as well as observable quantities reflecting this galaxy-halo connection, with similar results in dark matter simulations. The model computes accurate galaxy-halo statistics and reveals important physical relationships; specifically, variables associated with halo accretion influence a galaxy's mass and star formation, while environmental variables are linked to its metallicity. While the predictions from dark matter simulations are reasonably accurate, they are affected by the absence of baryonic processes, the resolution of the simulation, and the calculation of halo properties.Angle-resolved photoemission studies of uniaxial stress-driven Lifshitz transitions in the bulk and surface electronic structure of Sr₂RuO₄Abarca Morales, Edgarhttps://hdl.handle.net/10023/282202023-09-16T02:07:31Z2023-11-29T00:00:00ZIn experimental condensed matter physics, the utilisation of momentum-resolved probes has proven valuable in disentangling the underpinning effects driving the formation of rich collective states in quantum materials. Furthermore, the ability to tune relevant features in the electronic structure and control the breaking of particular symmetries, comprises a powerful route to stabilise phases and electronic states that are not available naturally to equilibrium chemistry. In this work, I show how one can simultaneously benefit from both approaches, specifically, with the development of a technique combining angle-resolved photoemission spectroscopy (ARPES) with the application of uniaxial stress.
After a thorough discussion of the experimental method, I show its capabilities in the normal state of the unconventional superconductor Sr₂RuO₄, where the application of uniaxial pressure has recently been shown to more than double the transition temperature, leading to a peak in Tc versus strain. We directly visualise how uniaxial stress drives a Lifshitz transition of one of its three Fermi surfaces, which is in close proximity to a van Hove singularity (vHS), and we point to the key role of strain-tuning the vHS to the Fermi level in mediating the peak in Tc. Our measurements also provide stringent constraints for theoretical models of the strain-tuned electronic structure evolution of Sr₂RuO₄.
In the bilayer sister compound Sr₃Ru₂O₇, in-plane rotations of the RuO₆ octahedra and the corresponding doubling of the in-plane unit cell turn the vHS into higher (4th) order. Tuning this extended vHS to the Fermi level with large magnetic fields is thought to drive an exotic nematic state to emerge, which exhibits signatures of quantum criticality. Interestingly, the octahedra rotations that characterise Sr₃Ru₂O₇ are also found in the surface layer of Sr₂RuO₄, potentially making such states accessible also at the surface of the single-layer compound. In this work, I show the evolution of the electronic structure at the surface layer of Sr₂RuO₄ under large in-plane uniaxial stress. From ARPES, we show how the induced strain drives a sequence of Lifshitz transitions, fundamentally reshaping the low-energy electronic structure and the rich spectrum of vHSs that the surface layer of Sr₂RuO₄ hosts. From comparison of tight-binding modelling to our measured dispersions, I show that, surprisingly, the strain is accommodated predominantly by bond-length changes rather than modifications of the octahedral tilt and rotation angles, thus shedding new light on the nature of structural distortions at oxide surfaces, and how targeted control of these can be used to tune density of states singularities to the Fermi level, in turn paving the way to the possible realisation of rich collective states at the surface of Sr₂RuO₄.
2023-11-29T00:00:00ZAbarca Morales, EdgarIn experimental condensed matter physics, the utilisation of momentum-resolved probes has proven valuable in disentangling the underpinning effects driving the formation of rich collective states in quantum materials. Furthermore, the ability to tune relevant features in the electronic structure and control the breaking of particular symmetries, comprises a powerful route to stabilise phases and electronic states that are not available naturally to equilibrium chemistry. In this work, I show how one can simultaneously benefit from both approaches, specifically, with the development of a technique combining angle-resolved photoemission spectroscopy (ARPES) with the application of uniaxial stress.
After a thorough discussion of the experimental method, I show its capabilities in the normal state of the unconventional superconductor Sr₂RuO₄, where the application of uniaxial pressure has recently been shown to more than double the transition temperature, leading to a peak in Tc versus strain. We directly visualise how uniaxial stress drives a Lifshitz transition of one of its three Fermi surfaces, which is in close proximity to a van Hove singularity (vHS), and we point to the key role of strain-tuning the vHS to the Fermi level in mediating the peak in Tc. Our measurements also provide stringent constraints for theoretical models of the strain-tuned electronic structure evolution of Sr₂RuO₄.
In the bilayer sister compound Sr₃Ru₂O₇, in-plane rotations of the RuO₆ octahedra and the corresponding doubling of the in-plane unit cell turn the vHS into higher (4th) order. Tuning this extended vHS to the Fermi level with large magnetic fields is thought to drive an exotic nematic state to emerge, which exhibits signatures of quantum criticality. Interestingly, the octahedra rotations that characterise Sr₃Ru₂O₇ are also found in the surface layer of Sr₂RuO₄, potentially making such states accessible also at the surface of the single-layer compound. In this work, I show the evolution of the electronic structure at the surface layer of Sr₂RuO₄ under large in-plane uniaxial stress. From ARPES, we show how the induced strain drives a sequence of Lifshitz transitions, fundamentally reshaping the low-energy electronic structure and the rich spectrum of vHSs that the surface layer of Sr₂RuO₄ hosts. From comparison of tight-binding modelling to our measured dispersions, I show that, surprisingly, the strain is accommodated predominantly by bond-length changes rather than modifications of the octahedral tilt and rotation angles, thus shedding new light on the nature of structural distortions at oxide surfaces, and how targeted control of these can be used to tune density of states singularities to the Fermi level, in turn paving the way to the possible realisation of rich collective states at the surface of Sr₂RuO₄.An electrically driven organic semiconductor laserGong, Junyihttps://hdl.handle.net/10023/281972023-08-25T10:51:57Z2023-11-29T00:00:00ZThis thesis presents the design and demonstration of an electrically driven organic semiconductor laser. The laser utilized an indirect pumping structure where the current injection section was spatially separated from the gain region.
To realize an electrically driven organic semiconductor laser, a low threshold polymer DFB laser and high brightness OLED were developed separately. Using BBEHP-PPV polymer and a high Q-factor substructured DFB cavity, the optimized optical laser threshold was 92 W/cm². The OLED was carefully designed to maximize the output power density and suppress heat accumulation. A p-i-n OLED on flexible parylene-nanolaminates (PNPN) substrate was designed and fabricated. Under pulsed operation, the OLED showed the highest reported output power density (46 W/cm² at 6.7 kA/cm²), a factor of 30 times improvement on the best deep blue pulsed OLEDs.
An effective way to integrate OLED and the polymer laser was developed where the two sections were held together in optical contact so that the laser could access the enhanced power density.
The integrated device showed a laser threshold current density of 2.83 kA/cm² with a narrow emission peak at 542.2 nm. The emission beam when operated above the threshold had a double lobe beam profile and a strong linear polarization parallel to the grating groove direction. The divergence of the laser was measured to be 2.4 mrad. The operational lifetime of the electrically driven device was 9.57×10⁴ pulses. Overall, the results presented in this thesis show strong evidence that an electrically driven laser was achieved.
An assisted pumping measurement was developed to estimate the irradiance transferred from the OLED to the gain medium. On a different integrated sample, the OLED could transfer about 100 W/cm² of electroluminescence to the polymer laser at 5.2 kA/cm². This is consistent with the simulation of outcoupling efficiency.
2023-11-29T00:00:00ZGong, JunyiThis thesis presents the design and demonstration of an electrically driven organic semiconductor laser. The laser utilized an indirect pumping structure where the current injection section was spatially separated from the gain region.
To realize an electrically driven organic semiconductor laser, a low threshold polymer DFB laser and high brightness OLED were developed separately. Using BBEHP-PPV polymer and a high Q-factor substructured DFB cavity, the optimized optical laser threshold was 92 W/cm². The OLED was carefully designed to maximize the output power density and suppress heat accumulation. A p-i-n OLED on flexible parylene-nanolaminates (PNPN) substrate was designed and fabricated. Under pulsed operation, the OLED showed the highest reported output power density (46 W/cm² at 6.7 kA/cm²), a factor of 30 times improvement on the best deep blue pulsed OLEDs.
An effective way to integrate OLED and the polymer laser was developed where the two sections were held together in optical contact so that the laser could access the enhanced power density.
The integrated device showed a laser threshold current density of 2.83 kA/cm² with a narrow emission peak at 542.2 nm. The emission beam when operated above the threshold had a double lobe beam profile and a strong linear polarization parallel to the grating groove direction. The divergence of the laser was measured to be 2.4 mrad. The operational lifetime of the electrically driven device was 9.57×10⁴ pulses. Overall, the results presented in this thesis show strong evidence that an electrically driven laser was achieved.
An assisted pumping measurement was developed to estimate the irradiance transferred from the OLED to the gain medium. On a different integrated sample, the OLED could transfer about 100 W/cm² of electroluminescence to the polymer laser at 5.2 kA/cm². This is consistent with the simulation of outcoupling efficiency.Determination of conformational changes in the structure of human calmodulin-1 using electron paramagnetic resonance spectroscopyFisher, Stuart Ronanhttps://hdl.handle.net/10023/281252023-08-15T02:01:26Z2023-11-29T00:00:00ZDouble electron electron resonance spectroscopy is a versatile and powerful tool for probing paramagnetic systems such metalloproteins and proteins with conjugated spin labels. Chapter 1 introduces EPR theory and the molecular biological concepts that this work is based on. This lays the groundwork with regard to basic knowledge and principles that the following chapters rely on. Chapter 2 collates the methodology used in all aspects of the work conducted in this body of work so that the techniques used may be critically assessed and the work reproduced under the same conditions in future experimentation. Chapter 3 focuses on the in-silico investigation of human calmodulin1 and applies a combinatorial approach utilising bioinformatic and computational techniques towards site selection for site-directed spin labelling and the elucidation of calmodulin1 conformational states through production of simulated models and predictions for expected DEER distance distributions. This ranges from literature reviews, structural region identification and degree of conservation assessment through to AI generated predicted models for secondary and tertiary structures and spin label modelling across different predicted conformational states and predicted distances. Chapter 4 provides a detailed explanation of the experimental work conducted to produce the variants of interest and the subsequent in-vitro process of data acquisition with details on a case-by-case basis. This focuses on the interactions between calmodulin1, Ca²⁺ and the M13 peptide. Chapter 5 takes a look at a comparative labelling technique that takes advantage of a rigid diHis motif for Cu²⁺ spin labelling in combination with MTSL (R1 when peptide-conjugated) for Cu²⁺NTA-R1 measurements that have been previously conducted in chapter 4 and utilises a different technique for distance measurements, RIDME. This chapter also applies other applications of RIDME to different spin systems, this time measuring distances between HisTag-bound Mn²⁺ and R1, taking advantage of another type of spin labelling technique.
2023-11-29T00:00:00ZFisher, Stuart RonanDouble electron electron resonance spectroscopy is a versatile and powerful tool for probing paramagnetic systems such metalloproteins and proteins with conjugated spin labels. Chapter 1 introduces EPR theory and the molecular biological concepts that this work is based on. This lays the groundwork with regard to basic knowledge and principles that the following chapters rely on. Chapter 2 collates the methodology used in all aspects of the work conducted in this body of work so that the techniques used may be critically assessed and the work reproduced under the same conditions in future experimentation. Chapter 3 focuses on the in-silico investigation of human calmodulin1 and applies a combinatorial approach utilising bioinformatic and computational techniques towards site selection for site-directed spin labelling and the elucidation of calmodulin1 conformational states through production of simulated models and predictions for expected DEER distance distributions. This ranges from literature reviews, structural region identification and degree of conservation assessment through to AI generated predicted models for secondary and tertiary structures and spin label modelling across different predicted conformational states and predicted distances. Chapter 4 provides a detailed explanation of the experimental work conducted to produce the variants of interest and the subsequent in-vitro process of data acquisition with details on a case-by-case basis. This focuses on the interactions between calmodulin1, Ca²⁺ and the M13 peptide. Chapter 5 takes a look at a comparative labelling technique that takes advantage of a rigid diHis motif for Cu²⁺ spin labelling in combination with MTSL (R1 when peptide-conjugated) for Cu²⁺NTA-R1 measurements that have been previously conducted in chapter 4 and utilises a different technique for distance measurements, RIDME. This chapter also applies other applications of RIDME to different spin systems, this time measuring distances between HisTag-bound Mn²⁺ and R1, taking advantage of another type of spin labelling technique.Explosive vapour sensing with organic and hybrid semiconductorsGlackin, James Michael Edwardhttps://hdl.handle.net/10023/280132023-07-25T13:11:44Z2022-06-21T00:00:00ZAbstract redacted
2022-06-21T00:00:00ZGlackin, James Michael EdwardAbstract redactedAtmospheres of rocky exoplanetsHerbort, Oliverhttps://hdl.handle.net/10023/279522023-10-30T16:54:24Z2022-06-13T00:00:00ZThe increasing number of known rocky exoplanets motivates investigations of the diversity of atmospheric and surface composition of these planets. We investigate the link between the composition of the surface, near-crust atmosphere and the lower atmosphere, including the presence of different cloud condensates. This allows working towards inferring the surface composition from clouds and gas species present in the atmosphere. Understanding the diversity of the atmospheric composition provides a further step towards the characterisation of rocky exoplanets.
In this thesis, a fast and simple atmospheric model for the lower atmospheres of rocky exoplanets is presented. A range of different sets of total element abundances is used to investigate the surface composition in contact with the near-crust atmosphere in chemical and phase equilibrium. The atmosphere based on this crust-atmosphere interaction layer is build from bottom-to-top. At every point in the atmosphere, chemical equilibrium is solved and all thermally stable condensates are removed, depleting the atmospheric layers above in the affected elements.
In order to characterise the general atmospheric composition, atmospheric types based on the chemical state of carbon, hydrogen, oxygen, and nitrogen are introduced. In order to further constrain the potential of an atmospheric environment for habitability, different habitability levels are introduced. These take the stability of liquid water as well as the chemical states of carbon, nitrogen, and sulphur into account.
The investigation of the atmosphere-crust interaction layer shows, that the thermal stability of liquid water is only given, if all phyllosilicates (minerals which incorporate OH groups into their lattice structure) have completely formed. The composition of the resulting atmosphere can be categorised into three different atmospheric types. Of special interest is the possibility of the coexistence of CO₂ and CH₄ in chemical equilibrium. The atmospheric type is intrinsic to an atmosphere, as it does not change with the removal of thermally stable condensates in one given atmospheric model.
The atmospheric models reveal a large diversity in thermally stable cloud condensates, which constrain the surface conditions of rocky exoplanets. The presence of water clouds is an integral part of many planetary atmospheres and is independent of the stability of water condensates at the surface. At the water cloud base, we show that reduced gaseous forms of carbon, nitrogen, and sulphur are present, while phosphorus is absent.
2022-06-13T00:00:00ZHerbort, OliverThe increasing number of known rocky exoplanets motivates investigations of the diversity of atmospheric and surface composition of these planets. We investigate the link between the composition of the surface, near-crust atmosphere and the lower atmosphere, including the presence of different cloud condensates. This allows working towards inferring the surface composition from clouds and gas species present in the atmosphere. Understanding the diversity of the atmospheric composition provides a further step towards the characterisation of rocky exoplanets.
In this thesis, a fast and simple atmospheric model for the lower atmospheres of rocky exoplanets is presented. A range of different sets of total element abundances is used to investigate the surface composition in contact with the near-crust atmosphere in chemical and phase equilibrium. The atmosphere based on this crust-atmosphere interaction layer is build from bottom-to-top. At every point in the atmosphere, chemical equilibrium is solved and all thermally stable condensates are removed, depleting the atmospheric layers above in the affected elements.
In order to characterise the general atmospheric composition, atmospheric types based on the chemical state of carbon, hydrogen, oxygen, and nitrogen are introduced. In order to further constrain the potential of an atmospheric environment for habitability, different habitability levels are introduced. These take the stability of liquid water as well as the chemical states of carbon, nitrogen, and sulphur into account.
The investigation of the atmosphere-crust interaction layer shows, that the thermal stability of liquid water is only given, if all phyllosilicates (minerals which incorporate OH groups into their lattice structure) have completely formed. The composition of the resulting atmosphere can be categorised into three different atmospheric types. Of special interest is the possibility of the coexistence of CO₂ and CH₄ in chemical equilibrium. The atmospheric type is intrinsic to an atmosphere, as it does not change with the removal of thermally stable condensates in one given atmospheric model.
The atmospheric models reveal a large diversity in thermally stable cloud condensates, which constrain the surface conditions of rocky exoplanets. The presence of water clouds is an integral part of many planetary atmospheres and is independent of the stability of water condensates at the surface. At the water cloud base, we show that reduced gaseous forms of carbon, nitrogen, and sulphur are present, while phosphorus is absent.Mineral snowflakes on exoplanets and brown dwarfsSamra, Dominic Bartholomew Singhhttps://hdl.handle.net/10023/279322023-07-13T02:07:04Z2023-06-12T00:00:00ZThe diversity of exoplanets and brown dwarfs provides ideal atmospheric laboratories to investigate novel physico-chemical regimes. Furthermore, the atmospheres of exoplanets act as the history books of planetary system. However, as observational data improves, the contributions of cloud particles in exoplanet and brown dwarf atmospheres must be adequately accounted for. Microphysical modelling of cloud formation provides the best method to investigate the potentially observable properties of clouds in these atmospheres. Most observed gas-giant exoplanets have been suggested to host mineral clouds which could form `snowflake-like' structures through condensation and constructive collisions. Cloud particle porosity, size and number density are influenced by constructive and destructive collisions. In this thesis, we expand our kinetic non-equilibrium cloud formation model to explore the effects of non-compact, non-spherical cloud particles on cloud structure and their spectroscopic properties. Additionally, we investigate the effects on clouds of collisional growth and fragmentation. The impact of these affects is assessed on prescribed 1D (Tgas-Pgas) profiles in DRIFT-PHOENIX model atmospheres of brown dwarfs and exoplanets. We utilise Mie theory and effective medium theory to study cloud optical depths, where we additionally represent non-spherical cloud particles with a statistical distribution of hollow spheres. We find that micro-porosity can affect the distribution of cloud particles in an exoplanet atmosphere, and that irregular particle shape impacts the optical depth in the near- and mid-infrared. However, we also find that cloud particle collisions driven by turbulence result in fragmentation of cloud particles for exoplanet atmospheres, which also impacts optical depths in the optical and mid-infrared regions. The global distribution and properties of clouds is also important as observations begin to allow for treating exoplanets in their full 3D nature. We therefore apply a hierarchical approach to global cloud formation modelling. We also apply our 1D cloud formation model to profiles extracted from results of 3D General Circulation Models (GCM) for the gas-giant exoplanet WASP-43b and the ultra-hot Jupiter HAT-P-7b, revealing a dramatic difference in the distribution of clouds between these types of exoplanets as a result of stellar radiation heating the day-side of the ultra-hot planets. This results in an asymmetry in cloud structures for the terminators of WASP-43b and more significantly for HAT-P-7b, observable in the optical depth of the clouds at these points, further complicating retrieval of cloud properties from spectra.
2023-06-12T00:00:00ZSamra, Dominic Bartholomew SinghThe diversity of exoplanets and brown dwarfs provides ideal atmospheric laboratories to investigate novel physico-chemical regimes. Furthermore, the atmospheres of exoplanets act as the history books of planetary system. However, as observational data improves, the contributions of cloud particles in exoplanet and brown dwarf atmospheres must be adequately accounted for. Microphysical modelling of cloud formation provides the best method to investigate the potentially observable properties of clouds in these atmospheres. Most observed gas-giant exoplanets have been suggested to host mineral clouds which could form `snowflake-like' structures through condensation and constructive collisions. Cloud particle porosity, size and number density are influenced by constructive and destructive collisions. In this thesis, we expand our kinetic non-equilibrium cloud formation model to explore the effects of non-compact, non-spherical cloud particles on cloud structure and their spectroscopic properties. Additionally, we investigate the effects on clouds of collisional growth and fragmentation. The impact of these affects is assessed on prescribed 1D (Tgas-Pgas) profiles in DRIFT-PHOENIX model atmospheres of brown dwarfs and exoplanets. We utilise Mie theory and effective medium theory to study cloud optical depths, where we additionally represent non-spherical cloud particles with a statistical distribution of hollow spheres. We find that micro-porosity can affect the distribution of cloud particles in an exoplanet atmosphere, and that irregular particle shape impacts the optical depth in the near- and mid-infrared. However, we also find that cloud particle collisions driven by turbulence result in fragmentation of cloud particles for exoplanet atmospheres, which also impacts optical depths in the optical and mid-infrared regions. The global distribution and properties of clouds is also important as observations begin to allow for treating exoplanets in their full 3D nature. We therefore apply a hierarchical approach to global cloud formation modelling. We also apply our 1D cloud formation model to profiles extracted from results of 3D General Circulation Models (GCM) for the gas-giant exoplanet WASP-43b and the ultra-hot Jupiter HAT-P-7b, revealing a dramatic difference in the distribution of clouds between these types of exoplanets as a result of stellar radiation heating the day-side of the ultra-hot planets. This results in an asymmetry in cloud structures for the terminators of WASP-43b and more significantly for HAT-P-7b, observable in the optical depth of the clouds at these points, further complicating retrieval of cloud properties from spectra.Beyond Markovian dissipation at the nanoscale : towards finding quantum design rules for bio-organic nanodevicesLacroix, Thibauthttps://hdl.handle.net/10023/279182023-07-11T10:43:48Z2023-11-29T00:00:00ZA better understanding of dissipation is crucial for understanding real-world quantum systems. Indeed, all quantum systems experience interactions with an (often) uncontrollable outside environment that can lead to a decay of excited state populations and a loss of quantum coherences. The study of dissipation is timely as the development of next-generation nanoscale quantum technologies is on its way, and the existence of non-trivial quantum effects in biological systems is being seriously investigated. However, descriptions of dissipation in quantum systems are reduced (most of the time) to time-local approaches and (everywhere) to space-local independent environments. These simplifying assumptions do render analytic and numerical calculations possible, yet they get rid of a breadth of physical processes that can alter radically the quantum systems' dynamics. In this thesis, building on a numerically exact tensor networks method, we developed a technique able to handle spatio-temporal correlations between a quantum system and bosonic (i.e. vibrational, electromagnetic, magnons, etc.) environments. With this method we studied the signalling process - a form of information backflow - in quantum systems, and uncovered how it can induce non-trivial dynamics, and be leveraged to populate otherwise inaccessible excited states. We also evidenced the ability of 'non-local' environment reorganisation, induced by system-environment interactions, to radically change the nature of the thermodynamically favoured system ground state. The new phenomenology of physical processes, resulting from considering quantum systems interacting with a common environment, has important consequences for the design of nanodevices as it gives access to new control, sensing and cross-talk mechanisms. In another vein, these results might also give us a new framework to study and interpret (quantum?) effects in the biological realm.
2023-11-29T00:00:00ZLacroix, ThibautA better understanding of dissipation is crucial for understanding real-world quantum systems. Indeed, all quantum systems experience interactions with an (often) uncontrollable outside environment that can lead to a decay of excited state populations and a loss of quantum coherences. The study of dissipation is timely as the development of next-generation nanoscale quantum technologies is on its way, and the existence of non-trivial quantum effects in biological systems is being seriously investigated. However, descriptions of dissipation in quantum systems are reduced (most of the time) to time-local approaches and (everywhere) to space-local independent environments. These simplifying assumptions do render analytic and numerical calculations possible, yet they get rid of a breadth of physical processes that can alter radically the quantum systems' dynamics. In this thesis, building on a numerically exact tensor networks method, we developed a technique able to handle spatio-temporal correlations between a quantum system and bosonic (i.e. vibrational, electromagnetic, magnons, etc.) environments. With this method we studied the signalling process - a form of information backflow - in quantum systems, and uncovered how it can induce non-trivial dynamics, and be leveraged to populate otherwise inaccessible excited states. We also evidenced the ability of 'non-local' environment reorganisation, induced by system-environment interactions, to radically change the nature of the thermodynamically favoured system ground state. The new phenomenology of physical processes, resulting from considering quantum systems interacting with a common environment, has important consequences for the design of nanodevices as it gives access to new control, sensing and cross-talk mechanisms. In another vein, these results might also give us a new framework to study and interpret (quantum?) effects in the biological realm.STM imaging of strongly correlated materials with spin-orbit couplingChakraborti, Dibyashreehttps://hdl.handle.net/10023/278262023-06-29T02:06:41Z2022-06-13T00:00:00ZMost of the properties of electrons in conducting materials that we encounter in our everyday lives can be explained very neatly by considering them to be independent entities residing in the crystal lattice potential, as described by band theory of solids. However, the scenario gets very interesting when inter-electronic interactions and relativistic effects become strong enough, such that they can no longer be treated as small perturbations to the standard band structure. The situation gets even more intriguing for the surface states where the inversion symmetry is naturally lifted, leading to additional spin-orbit coupling.
In thesis I study two quantum materials (PdCoO₂ and PdTe₂) where the interplay of spin orbit coupling (SOC) and inversion symmetry breaking (ISB) give rise to intriguing spin texture of the surface bands, which further leads to spin-selective scattering. Using Scanning Tunneling Microscopy and Spectroscopy, I sought to explore these spin-selective scattering processes by studying the quasi-particle interference (QPI) patterns at different energies in the vicinity of the Fermi-level.
Among the two systems studied in the thesis, the CoO₂ terminated surface of metallic Delafossite PdCoO₂ shows an abnormally large Rashba spin splitting of the surface states. It is a Rashba system where the energy scale associated with the inversion symmetry breaking is much larger than the spin-orbit coupling. The surface states also exhibit signatures of Fermi-liquid like behaviour. Therefore this system allowed me to do a case study of the interplay of strong electronic correlations and spin-orbit coupling and thereby estimate the relative values of various intrinsic length scales in the system, which become significant from the point of view of application in device fabrication. I proceeded to study the influence of quantum confinement on the scattering processes and the related modulations in the local density of states. In course of these experiments, I determined the difference in the local barrier height of polar surface terminations, here the Pd and CoO₂ terminated surfaces of PdCoO₂, which may prove to be useful information for future researchers.
I further went on to investigate the spin selective scattering processes in Dirac semi-metal PdTe₂ from analysis of quasiparticle interference. The surface states in this material have a complicated spin texture and this system also serves as the example case of my study, for the limit where the spin-orbit coupling is the dominant energy scale.
2022-06-13T00:00:00ZChakraborti, DibyashreeMost of the properties of electrons in conducting materials that we encounter in our everyday lives can be explained very neatly by considering them to be independent entities residing in the crystal lattice potential, as described by band theory of solids. However, the scenario gets very interesting when inter-electronic interactions and relativistic effects become strong enough, such that they can no longer be treated as small perturbations to the standard band structure. The situation gets even more intriguing for the surface states where the inversion symmetry is naturally lifted, leading to additional spin-orbit coupling.
In thesis I study two quantum materials (PdCoO₂ and PdTe₂) where the interplay of spin orbit coupling (SOC) and inversion symmetry breaking (ISB) give rise to intriguing spin texture of the surface bands, which further leads to spin-selective scattering. Using Scanning Tunneling Microscopy and Spectroscopy, I sought to explore these spin-selective scattering processes by studying the quasi-particle interference (QPI) patterns at different energies in the vicinity of the Fermi-level.
Among the two systems studied in the thesis, the CoO₂ terminated surface of metallic Delafossite PdCoO₂ shows an abnormally large Rashba spin splitting of the surface states. It is a Rashba system where the energy scale associated with the inversion symmetry breaking is much larger than the spin-orbit coupling. The surface states also exhibit signatures of Fermi-liquid like behaviour. Therefore this system allowed me to do a case study of the interplay of strong electronic correlations and spin-orbit coupling and thereby estimate the relative values of various intrinsic length scales in the system, which become significant from the point of view of application in device fabrication. I proceeded to study the influence of quantum confinement on the scattering processes and the related modulations in the local density of states. In course of these experiments, I determined the difference in the local barrier height of polar surface terminations, here the Pd and CoO₂ terminated surfaces of PdCoO₂, which may prove to be useful information for future researchers.
I further went on to investigate the spin selective scattering processes in Dirac semi-metal PdTe₂ from analysis of quasiparticle interference. The surface states in this material have a complicated spin texture and this system also serves as the example case of my study, for the limit where the spin-orbit coupling is the dominant energy scale.Imaging emergent correlated phases in the strontium ruthenatesDe Almeida Marques, Carolinahttps://hdl.handle.net/10023/278252024-01-19T03:08:26Z2022-06-13T00:00:00ZIn strongly correlated electron materials, charge, spin and orbital degrees of freedom exhibit an intimate relationship, leading to new emergent phases that seemingly break the symmetries of the underlying crystal and are highly sensitive to external stimuli. This is well illustrated in the Ruddlesden-Popper series of the strontium ruthenates, Sr_{n+₁}Ru_nO_{₃n+₁}, where a wide range of properties attributed to such physics can be found, including unconventional superconductivity, quantum criticality, metamagnetic transitions and ferromagnetism. In this thesis, using ultra-low temperature scanning tunneling microscopy, I show a detailed study of the low-energy electronic states at the surface of Sr₂RuO₄, an unconventional superconductor, and Sr₃Ru₂O₇, an itinerant metamagnet associated with quantum criticality. I demonstrate that the increased structural distortions in the surface layer lead to considerable changes in the Fermi surface, allowing the stabilization of new emergent phases beyond those accessible in the bulk.
At the surface of Sr₂RuO₄, we find that the surface reconstruction leads to checkerboard charge order intertwined with nematicity, intimately linked with four van Hove singularities within 5 meV of the Fermi level. Including these orders in a tight-binding model gives excellent agreement with the experiment. By applying a magnetic field, one of the van Hove singularities splits, with one branch extrapolated to reach the Fermi energy at ~32 T, providing a textbook example of tuning towards a Zeeman-driven Lifshitz transition.
Measurements at the surface of Sr₃Ru₂O₇ reveal a magnetic ground state, with substantial anisotropy of the electronic states. With increasing magnetic field, we observe the formation of a stripe order and were able to track a van Hove singularity shift across the Fermi energy. Our measurements establish the surface layer as having a distinct ground state from the bulk, undergoing a magnetic field induced Lifshitz transition at a magnetic field of ~11 T.
2022-06-13T00:00:00ZDe Almeida Marques, CarolinaIn strongly correlated electron materials, charge, spin and orbital degrees of freedom exhibit an intimate relationship, leading to new emergent phases that seemingly break the symmetries of the underlying crystal and are highly sensitive to external stimuli. This is well illustrated in the Ruddlesden-Popper series of the strontium ruthenates, Sr_{n+₁}Ru_nO_{₃n+₁}, where a wide range of properties attributed to such physics can be found, including unconventional superconductivity, quantum criticality, metamagnetic transitions and ferromagnetism. In this thesis, using ultra-low temperature scanning tunneling microscopy, I show a detailed study of the low-energy electronic states at the surface of Sr₂RuO₄, an unconventional superconductor, and Sr₃Ru₂O₇, an itinerant metamagnet associated with quantum criticality. I demonstrate that the increased structural distortions in the surface layer lead to considerable changes in the Fermi surface, allowing the stabilization of new emergent phases beyond those accessible in the bulk.
At the surface of Sr₂RuO₄, we find that the surface reconstruction leads to checkerboard charge order intertwined with nematicity, intimately linked with four van Hove singularities within 5 meV of the Fermi level. Including these orders in a tight-binding model gives excellent agreement with the experiment. By applying a magnetic field, one of the van Hove singularities splits, with one branch extrapolated to reach the Fermi energy at ~32 T, providing a textbook example of tuning towards a Zeeman-driven Lifshitz transition.
Measurements at the surface of Sr₃Ru₂O₇ reveal a magnetic ground state, with substantial anisotropy of the electronic states. With increasing magnetic field, we observe the formation of a stripe order and were able to track a van Hove singularity shift across the Fermi energy. Our measurements establish the surface layer as having a distinct ground state from the bulk, undergoing a magnetic field induced Lifshitz transition at a magnetic field of ~11 T.Investigation of transport regimes in restricted geometries of ultra-pure natural heterostructuresZhakina, Elinahttps://hdl.handle.net/10023/278232024-02-23T03:06:38Z2022-06-13T00:00:00ZThis thesis describes investigations into the origins of the unconventional electrical transport of the non-magnetic delafossite metals PtCoO₂ and PdCoO₂ and the magnetic delafossite metal PdCrO₂ using focused ion beam microstructuring techniques. These compounds are among the highest conductivity materials known, with an extreme purity of up to 1 defect in 120,000 atoms. This remarkable purity, together with the hexagonal Fermi surface, opens the possibility of studying novel regimes of mesoscopic physics. This work is split into two parts. In the first part, I will review the key properties of non-magnetic delafossite metals and the application of focused ion beam microstructuring to transport measurements within low resistivity materials. The related experimental chapter describes an investigation which uses the high energy electron irradiation investigation to probe the effects of a non-circular Fermi surface on the transport within bars and four-terminal, square-shaped junctions inside the ballistic regime. The other studies were concentrated on the magnetic delafossite metal PdCrO₂. I will describe a new method of microstructure preparation which was created for PdCrO₂ transport studies but is widely applicable to other materials. This material obeys the Planckian bound at a wide range of temperatures between 200 K and 500 K. The accompanying experimental chapter details an investigation by high energy electron irradiation of the origin of this behaviour. The new method of mounting microstructures also allows, for the first time, the study of studying unconventional transport regimes in PdCrO₂.
2022-06-13T00:00:00ZZhakina, ElinaThis thesis describes investigations into the origins of the unconventional electrical transport of the non-magnetic delafossite metals PtCoO₂ and PdCoO₂ and the magnetic delafossite metal PdCrO₂ using focused ion beam microstructuring techniques. These compounds are among the highest conductivity materials known, with an extreme purity of up to 1 defect in 120,000 atoms. This remarkable purity, together with the hexagonal Fermi surface, opens the possibility of studying novel regimes of mesoscopic physics. This work is split into two parts. In the first part, I will review the key properties of non-magnetic delafossite metals and the application of focused ion beam microstructuring to transport measurements within low resistivity materials. The related experimental chapter describes an investigation which uses the high energy electron irradiation investigation to probe the effects of a non-circular Fermi surface on the transport within bars and four-terminal, square-shaped junctions inside the ballistic regime. The other studies were concentrated on the magnetic delafossite metal PdCrO₂. I will describe a new method of microstructure preparation which was created for PdCrO₂ transport studies but is widely applicable to other materials. This material obeys the Planckian bound at a wide range of temperatures between 200 K and 500 K. The accompanying experimental chapter details an investigation by high energy electron irradiation of the origin of this behaviour. The new method of mounting microstructures also allows, for the first time, the study of studying unconventional transport regimes in PdCrO₂.Morphology and polymorph control of NbSe₂ monolayers grown via molecular beam epitaxyUnderwood, Kayceehttps://hdl.handle.net/10023/278082023-06-27T02:05:18Z2022-11-29T00:00:00ZAbstract redacted
2022-11-29T00:00:00ZUnderwood, KayceeAbstract redactedPerformant astronomical image processing with PythonHitchcock, Jameshttps://hdl.handle.net/10023/276592023-05-23T02:05:13Z2023-06-12T00:00:00ZImage processing is fundamental to observational astronomy workflows. Astronomers acquire imaging data, and process the imagery to extract useful information. This thesis introduces two new image processing algorithms. The first, PyTorchDIA, is a GPU-accelerated approach to Difference Image Analysis (DIA). The approach is fast, without sacrificing modelling flexibility. It makes use of the Pythonic, PyTorch machine learning framework to accelerate convolution computations on the GPU, and compute gradients of user-specified objective functions with automatic differentiation methods to fit DIA models quickly and accurately. The second algorithm, The Thresher, was designed as a new tool to extracting information from Lucky Imaging (LI) data sets. We adopt a modelling approach which optimises a justifiable, physically motivated likelihood function to return the best estimate of the observed astronomical scene. It does this using all available data, and the more data the model is fit to, the better the signal-to-noise and resolution of the scene estimate. This fundamentally differs from conventional shift-and-add procedures, which typically reject the vast majority of the acquired LI data, as in these approaches, the signal-to-noise of the final coadd is inversely related to its resolution. With an eye to accessibility, integration into workflows and open science, the code for these two algorithms has been open sourced. Lastly, we show how Python image processing applications can be used to realise time-critical, demanding computational challenges in a chapter outlining the results of a novel pilot study to detect the occultations of background stars by small, outer solar system objects with high frame-rate sCMOS cameras.
2023-06-12T00:00:00ZHitchcock, JamesImage processing is fundamental to observational astronomy workflows. Astronomers acquire imaging data, and process the imagery to extract useful information. This thesis introduces two new image processing algorithms. The first, PyTorchDIA, is a GPU-accelerated approach to Difference Image Analysis (DIA). The approach is fast, without sacrificing modelling flexibility. It makes use of the Pythonic, PyTorch machine learning framework to accelerate convolution computations on the GPU, and compute gradients of user-specified objective functions with automatic differentiation methods to fit DIA models quickly and accurately. The second algorithm, The Thresher, was designed as a new tool to extracting information from Lucky Imaging (LI) data sets. We adopt a modelling approach which optimises a justifiable, physically motivated likelihood function to return the best estimate of the observed astronomical scene. It does this using all available data, and the more data the model is fit to, the better the signal-to-noise and resolution of the scene estimate. This fundamentally differs from conventional shift-and-add procedures, which typically reject the vast majority of the acquired LI data, as in these approaches, the signal-to-noise of the final coadd is inversely related to its resolution. With an eye to accessibility, integration into workflows and open science, the code for these two algorithms has been open sourced. Lastly, we show how Python image processing applications can be used to realise time-critical, demanding computational challenges in a chapter outlining the results of a novel pilot study to detect the occultations of background stars by small, outer solar system objects with high frame-rate sCMOS cameras.Light-sheet fluorescence microscopy for biomedicinePoulton, Persephone B.https://hdl.handle.net/10023/276522024-01-19T12:01:48Z2023-06-12T00:00:00ZThe research motivation behind this thesis is to expand the information that can currently be extracted from an ex vivo biological sample via light-sheet fluorescence microscopy (LSFM) by enhancing the achievable resolution at an increased penetration depth. LSFM has already been used considerably for various biomedical research applications. During this thesis, I will explain why and in which contexts LSFM is a favoured technique compared to other currently available biomedical imaging tools. I will show how exotic beam shaping with Airy and Bessel-like beams can be implemented with LSFM to enhance the depth of quality, sub-cellular data available from complex biological samples (increasing the specificity and reach of the illumination to increase contrast in images). I will consider how the use of non-linear fluorescence excitation can improve upon the axial sectioning ability of LSFM with beam shaping in biological samples whilst exploiting the lowered attenuation of near infra-red illumination in biological tissues compared to visible illumination for linear fluorescence excitation. There are always fundamental limits to what we can achieve with optical microscopy. The guiding wish is to get a little bit closer to reaching these absolute physical limits with new, practical microscope designs aimed at medical microscopy applications.
2023-06-12T00:00:00ZPoulton, Persephone B.The research motivation behind this thesis is to expand the information that can currently be extracted from an ex vivo biological sample via light-sheet fluorescence microscopy (LSFM) by enhancing the achievable resolution at an increased penetration depth. LSFM has already been used considerably for various biomedical research applications. During this thesis, I will explain why and in which contexts LSFM is a favoured technique compared to other currently available biomedical imaging tools. I will show how exotic beam shaping with Airy and Bessel-like beams can be implemented with LSFM to enhance the depth of quality, sub-cellular data available from complex biological samples (increasing the specificity and reach of the illumination to increase contrast in images). I will consider how the use of non-linear fluorescence excitation can improve upon the axial sectioning ability of LSFM with beam shaping in biological samples whilst exploiting the lowered attenuation of near infra-red illumination in biological tissues compared to visible illumination for linear fluorescence excitation. There are always fundamental limits to what we can achieve with optical microscopy. The guiding wish is to get a little bit closer to reaching these absolute physical limits with new, practical microscope designs aimed at medical microscopy applications.Unveiling the structure of RNA using EPR spectroscopyPapa, Mariahttps://hdl.handle.net/10023/275932023-06-16T22:20:56Z2023-06-12T00:00:00ZAbstract redacted
2023-06-12T00:00:00ZPapa, MariaAbstract redactedOn speckle patterns : integrating spheres, metrology, and beyondFacchin, Morganhttps://hdl.handle.net/10023/275132023-05-05T09:35:59Z2023-06-12T00:00:00ZSpeckle patterns are grainy intensity patterns resulting from the random interference of light. They often arise from the interaction of light with systems that have a complex structure at the scale of the wavelength. As most ordinary objects have this property, speckle patterns are a somewhat universal phenomenon, although not easily observable under ordinary circumstances due to the lack of coherence of natural light. Since the invention of lasers however, the production of speckle patterns has become extremely simple, and a whole field of optics has emerged from it. Speckle patterns are an excellent tool for metrology. Where intuition says that precise control over all aspects of a setup is required, it is found that introducing some disorder can lead to very powerful techniques, with considerably simpler implementations. In this thesis, we explore new theoretical aspects of speckle patterns and develop new metrology techniques. We pay particular attention to the case of speckle patterns produced by an integrating sphere, which has only recently been used in this field of study. In this geometry, we develop a general model that predicts the amount of change in the resulting speckle, as a result of an arbitrary transformation. This model gives explicit results for various physical effects that can be solved from first principles, such as a variation in wavelength, refractive index, temperature, and position. We use this model in the context of metrology, and improve the state of the art by several orders of magnitude in the case of refractive index variations and displacement. We also explore the relationship between speckle and polarisation, and describe new mathematical techniques for the design of speckle patterns with custom properties (i.e. with maximal and minimal sensitivity to a measurand of interest) using light shaping.
2023-06-12T00:00:00ZFacchin, MorganSpeckle patterns are grainy intensity patterns resulting from the random interference of light. They often arise from the interaction of light with systems that have a complex structure at the scale of the wavelength. As most ordinary objects have this property, speckle patterns are a somewhat universal phenomenon, although not easily observable under ordinary circumstances due to the lack of coherence of natural light. Since the invention of lasers however, the production of speckle patterns has become extremely simple, and a whole field of optics has emerged from it. Speckle patterns are an excellent tool for metrology. Where intuition says that precise control over all aspects of a setup is required, it is found that introducing some disorder can lead to very powerful techniques, with considerably simpler implementations. In this thesis, we explore new theoretical aspects of speckle patterns and develop new metrology techniques. We pay particular attention to the case of speckle patterns produced by an integrating sphere, which has only recently been used in this field of study. In this geometry, we develop a general model that predicts the amount of change in the resulting speckle, as a result of an arbitrary transformation. This model gives explicit results for various physical effects that can be solved from first principles, such as a variation in wavelength, refractive index, temperature, and position. We use this model in the context of metrology, and improve the state of the art by several orders of magnitude in the case of refractive index variations and displacement. We also explore the relationship between speckle and polarisation, and describe new mathematical techniques for the design of speckle patterns with custom properties (i.e. with maximal and minimal sensitivity to a measurand of interest) using light shaping.Pathways towards single-polariton nonlinearity : from ground state exciton-polariton condensates in GaAs to Rydberg exciton-polaritons in Cu₂OOrfanakis, Konstantinoshttps://hdl.handle.net/10023/272472023-04-25T08:18:02Z2023-06-12T00:00:00ZThis work explores several routes towards achieving single-polariton nonlinearity. Exciton-polaritons are part-light part-matter quasiparticles arising from the strong coupling of excitons and cavity photons in micron-sized optical cavities. Owing to their excitonic component, polaritons can be described as "dressed photons" with nonlinear interactions several orders of magnitude larger than in typical optical materials. However, interactions between microcavity polaritons have remained weak, with the single-polariton nonlinearity being much smaller than the cavity linewidth. In this thesis, two approaches are studied as a way of circumventing this limitation and entering the nonlinear regime. The first approach involves narrowing the emission linewidth of a polariton condensate through optical confinement so that the linewidth approaches the interaction constant. The second approach involves exploiting the giant nonlinearities of Rydberg excitons in cuprous oxide, first in nanoparticles to study the effect of quantum confinement and then inside a microcavity to create highly nonlinear Rydberg exciton-polaritons. The main result presented in this thesis are: (1) An optically trapped polariton condensate in a state-of-the-art GaAs-based microcavity approaches but still remains away from the regime of single-polariton nonlinearity. The condensate is characterised by an ultra-narrow linewidth as evidenced by the temporal decay of its coherence. The latter also exhibits an oscillatory behaviour originating from a beating between two condensate modes. (2) Rydberg excitons states are resolved up to principal quantum number n = 6 in the absorption spectrum of clusters of cuprous oxide nanoparticles. Rydberg excitons are also resolved for single nanoparticles; however, the spectrum is dominated by effects inherent to the nanoparticle system, thus hindering the study of the nonlinearity of Rydberg excitons in this quantum-confined structure. (3) Strong coupling between cavity photons and Rydberg excitons can be achieved by embedding a thin cuprous oxide crystal as the active layer of an optical microcavity. Even though the microcavity is below the nonlinear regime for all strongly coupled Rydberg states, non-classical light can be observed by reducing the mode volume and suppressing the phonon-background of cuprous oxide in future microcavities. This thesis is a major step towards realising single-polariton nonlinearity for future quantum applications. The results presented in this work highlight the limitations of traditional GaAs-based semiconductor microcavities while establishing Rydberg polaritons with their huge nonlinearities as a promising route for achieving a scalable, strongly correlated photonic platform.
2023-06-12T00:00:00ZOrfanakis, KonstantinosThis work explores several routes towards achieving single-polariton nonlinearity. Exciton-polaritons are part-light part-matter quasiparticles arising from the strong coupling of excitons and cavity photons in micron-sized optical cavities. Owing to their excitonic component, polaritons can be described as "dressed photons" with nonlinear interactions several orders of magnitude larger than in typical optical materials. However, interactions between microcavity polaritons have remained weak, with the single-polariton nonlinearity being much smaller than the cavity linewidth. In this thesis, two approaches are studied as a way of circumventing this limitation and entering the nonlinear regime. The first approach involves narrowing the emission linewidth of a polariton condensate through optical confinement so that the linewidth approaches the interaction constant. The second approach involves exploiting the giant nonlinearities of Rydberg excitons in cuprous oxide, first in nanoparticles to study the effect of quantum confinement and then inside a microcavity to create highly nonlinear Rydberg exciton-polaritons. The main result presented in this thesis are: (1) An optically trapped polariton condensate in a state-of-the-art GaAs-based microcavity approaches but still remains away from the regime of single-polariton nonlinearity. The condensate is characterised by an ultra-narrow linewidth as evidenced by the temporal decay of its coherence. The latter also exhibits an oscillatory behaviour originating from a beating between two condensate modes. (2) Rydberg excitons states are resolved up to principal quantum number n = 6 in the absorption spectrum of clusters of cuprous oxide nanoparticles. Rydberg excitons are also resolved for single nanoparticles; however, the spectrum is dominated by effects inherent to the nanoparticle system, thus hindering the study of the nonlinearity of Rydberg excitons in this quantum-confined structure. (3) Strong coupling between cavity photons and Rydberg excitons can be achieved by embedding a thin cuprous oxide crystal as the active layer of an optical microcavity. Even though the microcavity is below the nonlinear regime for all strongly coupled Rydberg states, non-classical light can be observed by reducing the mode volume and suppressing the phonon-background of cuprous oxide in future microcavities. This thesis is a major step towards realising single-polariton nonlinearity for future quantum applications. The results presented in this work highlight the limitations of traditional GaAs-based semiconductor microcavities while establishing Rydberg polaritons with their huge nonlinearities as a promising route for achieving a scalable, strongly correlated photonic platform.The application of millimetre-wave radar to the study of the cryosphereHarcourt, William Davidhttps://hdl.handle.net/10023/272352024-02-01T14:21:37Z2023-06-14T00:00:00ZThis thesis develops the technique of millimetre-wave radar at 94 GHz for close-range remote sensing of glaciers and terrestrial snow cover (the cryosphere). The capabilities of 94 GHz radar for cryosphere mapping are demonstrated using the 2ⁿᵈ generation All-weather Volcano Topography Imaging Sensor (AVTIS2), which maps 3D terrain from real-beam scanning.
AVTIS2 acquires 3D point clouds of terrain and a comparison to co-located high density point clouds derived from Terrestrial Laser Scanner (TLS) data showed that AVTIS2 point cloud uncertainties were 1.5 m at 1.5 km and 3 m at 3 km. These values are smaller than other close-range radar systems used to map cryospheric terrain in 3D. Next, the distribution of Normalised Radar Cross Section (σ⁰) values over glacier ice at 94 GHz was found to be −17.0 < σ⁰ < −3.4; σ⁰ₘₑₐₙ = −9.9 dB and followed a log-normal distribution. These values are comparable to other terrain types at 94 GHz such as refrozen snow, wet snow and wet soil, hence glacier surfaces were found to be suitable targets for terrain mapping at 94 GHz.
These fundamental results were used to apply the AVTIS2 94 GHz radar for monitoring snow cover and glacier calving. Field trials in Scotland showed that the transition of wet snow to a refrozen snowpack in response to reductions in air temperature could be detected from changes in 94 GHz σ⁰. Snow hazard features could also be identified, such as post-avalanche debris accumulations which manifest as localised increases in radar backscatter. Next, 3D AVTIS2 data was used to quantify glacier calving rates at the Hansbreen tidewater glacier in Svalbard. A time series of ice cliff morphology derived from the AVTIS2 3D data sets exposed the role of melt undercutting and demonstrated the critical role of the ocean on calving processes in Svalbard.
The high accuracy 3D data acquired from AVTIS2 and the sensitivity of radar backscatter to surface changes has demonstrated the unique capabilities of 94 GHz radar for cryosphere mapping, paving the way forward for new applications and opportunities for close-range remote sensing of the cryosphere.
2023-06-14T00:00:00ZHarcourt, William DavidThis thesis develops the technique of millimetre-wave radar at 94 GHz for close-range remote sensing of glaciers and terrestrial snow cover (the cryosphere). The capabilities of 94 GHz radar for cryosphere mapping are demonstrated using the 2ⁿᵈ generation All-weather Volcano Topography Imaging Sensor (AVTIS2), which maps 3D terrain from real-beam scanning.
AVTIS2 acquires 3D point clouds of terrain and a comparison to co-located high density point clouds derived from Terrestrial Laser Scanner (TLS) data showed that AVTIS2 point cloud uncertainties were 1.5 m at 1.5 km and 3 m at 3 km. These values are smaller than other close-range radar systems used to map cryospheric terrain in 3D. Next, the distribution of Normalised Radar Cross Section (σ⁰) values over glacier ice at 94 GHz was found to be −17.0 < σ⁰ < −3.4; σ⁰ₘₑₐₙ = −9.9 dB and followed a log-normal distribution. These values are comparable to other terrain types at 94 GHz such as refrozen snow, wet snow and wet soil, hence glacier surfaces were found to be suitable targets for terrain mapping at 94 GHz.
These fundamental results were used to apply the AVTIS2 94 GHz radar for monitoring snow cover and glacier calving. Field trials in Scotland showed that the transition of wet snow to a refrozen snowpack in response to reductions in air temperature could be detected from changes in 94 GHz σ⁰. Snow hazard features could also be identified, such as post-avalanche debris accumulations which manifest as localised increases in radar backscatter. Next, 3D AVTIS2 data was used to quantify glacier calving rates at the Hansbreen tidewater glacier in Svalbard. A time series of ice cliff morphology derived from the AVTIS2 3D data sets exposed the role of melt undercutting and demonstrated the critical role of the ocean on calving processes in Svalbard.
The high accuracy 3D data acquired from AVTIS2 and the sensitivity of radar backscatter to surface changes has demonstrated the unique capabilities of 94 GHz radar for cryosphere mapping, paving the way forward for new applications and opportunities for close-range remote sensing of the cryosphere.Effective theories of multimode cavity QEDPalacino, Robertahttps://hdl.handle.net/10023/272342024-03-03T14:31:26Z2022-11-29T00:00:00ZThis thesis focuses on methods for the derivation of effective theories in models describing cold atoms in optical cavities.
Among those models, the Dicke model and its generalisations are among the most representative and studied problems in quantum electrodynamics. It describes the coupling of matter in the form of two-level atoms to a quantised mode of light. The Dicke model has been realised in various physical platforms, such as N-V centers in diamond, molecules coupled to an optical mode, trapped ions,
as well as in superconducting qubits coupled to microwave resonators. Extensions of the Dicke model have also been widely used in modelling exciton-polariton condensation with organic molecules. Another realisation involves laser-driven atoms coupled to light in a dissipative cavity. This thesis will focus on the latter type of physical system. When the coupling between light and matter reaches a critical point, the Dicke model predicts a phase transition to a superradiant state. Moreover, the onset of the superradiant phase transition coincides with the breaking of the discrete Z₂ symmetry in the model. Motivated by experimental advances in the field, there has been a growing interest in the realisation and analysis of generalised Dicke models such as those with continuous symmetry. In these models, the symmetry breaking phase transition is expected to allow for the observation of Goldstone modes. In order for the cavity QED system to be described by a Dicke model with continuous symmetry, one has to consider multimode extensions of the well characterised single mode experiments. Another extension made possible by multimode cavities is associative memory models, where cavity modes mediate interactions between ensembles of atoms. In many body cavity QED models, the Hilbert space dimension grows according to the number of atoms and modes involved, therefore, solving and fully characterising these problems becomes a challenging task. This is why one has to move to effective descriptions in terms of a reduced number of degrees of freedom. The most widely used technique is Redfield theory, an equation of motion describing the dynamics of the slowest part of the system, after elimination of the fast component. In our atom-cavity picture, the slow and fast components of the total system will be the atoms and the cavity light, respectively.
There are cases where Redfield theory is inadequate to accurately capture dynamics and critical behavior. Given the approximations leading to the Redfield equation, this could be considered as the second order of a perturbative expansion in the light-matter coupling. By adopting a diagrammatic method, one could write a Redfield theory beyond its standard second order formula. In this thesis, these techniques will be employed to analyse a two-mode Dicke model with U(1) symmetry and a model of associative memory.
2022-11-29T00:00:00ZPalacino, RobertaThis thesis focuses on methods for the derivation of effective theories in models describing cold atoms in optical cavities.
Among those models, the Dicke model and its generalisations are among the most representative and studied problems in quantum electrodynamics. It describes the coupling of matter in the form of two-level atoms to a quantised mode of light. The Dicke model has been realised in various physical platforms, such as N-V centers in diamond, molecules coupled to an optical mode, trapped ions,
as well as in superconducting qubits coupled to microwave resonators. Extensions of the Dicke model have also been widely used in modelling exciton-polariton condensation with organic molecules. Another realisation involves laser-driven atoms coupled to light in a dissipative cavity. This thesis will focus on the latter type of physical system. When the coupling between light and matter reaches a critical point, the Dicke model predicts a phase transition to a superradiant state. Moreover, the onset of the superradiant phase transition coincides with the breaking of the discrete Z₂ symmetry in the model. Motivated by experimental advances in the field, there has been a growing interest in the realisation and analysis of generalised Dicke models such as those with continuous symmetry. In these models, the symmetry breaking phase transition is expected to allow for the observation of Goldstone modes. In order for the cavity QED system to be described by a Dicke model with continuous symmetry, one has to consider multimode extensions of the well characterised single mode experiments. Another extension made possible by multimode cavities is associative memory models, where cavity modes mediate interactions between ensembles of atoms. In many body cavity QED models, the Hilbert space dimension grows according to the number of atoms and modes involved, therefore, solving and fully characterising these problems becomes a challenging task. This is why one has to move to effective descriptions in terms of a reduced number of degrees of freedom. The most widely used technique is Redfield theory, an equation of motion describing the dynamics of the slowest part of the system, after elimination of the fast component. In our atom-cavity picture, the slow and fast components of the total system will be the atoms and the cavity light, respectively.
There are cases where Redfield theory is inadequate to accurately capture dynamics and critical behavior. Given the approximations leading to the Redfield equation, this could be considered as the second order of a perturbative expansion in the light-matter coupling. By adopting a diagrammatic method, one could write a Redfield theory beyond its standard second order formula. In this thesis, these techniques will be employed to analyse a two-mode Dicke model with U(1) symmetry and a model of associative memory.The role of biological shape in optical and acoustic measurementsBairstow, Fiona Jayhttps://hdl.handle.net/10023/272122023-08-23T21:52:06Z2022-11-29T00:00:00ZThis thesis investigates the role of biological shape in optical and acoustic measurement systems.
Landmark-based geometric morphometrics (GMM) is a method of biological shape quantification
based on the relationship between landmarks placed at anatomical location. GMM enables robust
statistical analysis, comparisons between shapes, and visualisation of shape changes.
This thesis explores the application of GMM with optical imaging for in vivo interpretation
of amphioxus nerve cord shape, and acoustic measurements of Antarctic krill to improve biomass
estimates. Optical imaging was performed using optical coherence tomography (OCT), an in vivo,
label free imaging modality. A longitudinal study of amphioxus using OCT was conducted to
capture the shape of the nerve cord during tail regeneration. Evidence for axial variation in the
shape of the amphioxus nerve cord was found, however this work primarily develops a methodology
and further studies are recommended to draw robust conclusions.
GMM was used to construct a catalogue of realistic krill shapes for use in target strength
models. Target strength is a measure of backscattering efficiency, which scales acoustic density
to biomass. Typically, target strength is modelled with a generic krill shape, and distributions of
length, orientation, and wetmass. This thesis demonstrates the presence of scattering mechanism
influenced by shape and use of a shape catalogue to capture variability in target strength otherwise
neglected.
An error budget for biomass estimates of krill was developed to investigate the influence of
shape and orientation. Orientation was found to have the largest contribution to the measurement
error of biomass. In some cases, narrow orientation distributions resulted in biologically implausible
biomass estimates due to nulls in the relationship between target strength and orientation angle.
These nulls are responsive to changes in shape, suggesting the implementation of a shape catalogue
could improve the accuracy of biomass estimates.
2022-11-29T00:00:00ZBairstow, Fiona JayThis thesis investigates the role of biological shape in optical and acoustic measurement systems.
Landmark-based geometric morphometrics (GMM) is a method of biological shape quantification
based on the relationship between landmarks placed at anatomical location. GMM enables robust
statistical analysis, comparisons between shapes, and visualisation of shape changes.
This thesis explores the application of GMM with optical imaging for in vivo interpretation
of amphioxus nerve cord shape, and acoustic measurements of Antarctic krill to improve biomass
estimates. Optical imaging was performed using optical coherence tomography (OCT), an in vivo,
label free imaging modality. A longitudinal study of amphioxus using OCT was conducted to
capture the shape of the nerve cord during tail regeneration. Evidence for axial variation in the
shape of the amphioxus nerve cord was found, however this work primarily develops a methodology
and further studies are recommended to draw robust conclusions.
GMM was used to construct a catalogue of realistic krill shapes for use in target strength
models. Target strength is a measure of backscattering efficiency, which scales acoustic density
to biomass. Typically, target strength is modelled with a generic krill shape, and distributions of
length, orientation, and wetmass. This thesis demonstrates the presence of scattering mechanism
influenced by shape and use of a shape catalogue to capture variability in target strength otherwise
neglected.
An error budget for biomass estimates of krill was developed to investigate the influence of
shape and orientation. Orientation was found to have the largest contribution to the measurement
error of biomass. In some cases, narrow orientation distributions resulted in biologically implausible
biomass estimates due to nulls in the relationship between target strength and orientation angle.
These nulls are responsive to changes in shape, suggesting the implementation of a shape catalogue
could improve the accuracy of biomass estimates.Title redactedMarwick, Danielhttps://hdl.handle.net/10023/271762023-06-28T21:58:29Z2022-06-13T00:00:00ZAbstract redacted
2022-06-13T00:00:00ZMarwick, DanielAbstract redactedNew developments in orthogonality catastrophe physicsJackson, Conor Georgehttps://hdl.handle.net/10023/271532023-11-30T03:06:24Z2023-06-12T00:00:00ZThe Fermi edge singularity, and related Anderson’s orthogonality catastrophe, has been a touchstone of many body quantum physics for over 50 years. There are, however, a number of facets of this phenomenon that have, up until now, been left largely unexplored in the scientific literature. In this thesis we investigate two of these. Firstly we explore how the orthogonality catastrophe spatially spreads through a system, with particular considerations for the implications for quantum information processing implementations. We find that there is a propagating signal carrying the information about the Fermi edge singularity, but at long times the orthogonality catastrophe reasserts itself, posing a significant obstacle to the transmission of quantum information. We also found an “echo” formed by the interference of multiple Fermi edge singularities at different locations. Secondly we consider the effect of band structure on the Fermi edge singularity. Here we make significant progress in analytically understanding the effect of having a finite band bottom and of band curvature on the Fermi edge singularity. In the course of this we clarify some subtle points about the relationship between energy and time in non-relativistic quantum mechanics, which had been glossed over in the previous literature.
2023-06-12T00:00:00ZJackson, Conor GeorgeThe Fermi edge singularity, and related Anderson’s orthogonality catastrophe, has been a touchstone of many body quantum physics for over 50 years. There are, however, a number of facets of this phenomenon that have, up until now, been left largely unexplored in the scientific literature. In this thesis we investigate two of these. Firstly we explore how the orthogonality catastrophe spatially spreads through a system, with particular considerations for the implications for quantum information processing implementations. We find that there is a propagating signal carrying the information about the Fermi edge singularity, but at long times the orthogonality catastrophe reasserts itself, posing a significant obstacle to the transmission of quantum information. We also found an “echo” formed by the interference of multiple Fermi edge singularities at different locations. Secondly we consider the effect of band structure on the Fermi edge singularity. Here we make significant progress in analytically understanding the effect of having a finite band bottom and of band curvature on the Fermi edge singularity. In the course of this we clarify some subtle points about the relationship between energy and time in non-relativistic quantum mechanics, which had been glossed over in the previous literature.Probabilistic modelling of astrophysical time series : gravitational microlensing and occultation mapping of planets and moonsBartolić, Franhttps://hdl.handle.net/10023/271302023-03-09T17:47:30Z2023-06-12T00:00:00ZProgress in astronomy in the 21st century is contingent on the ability to extract useful information from complex and noisy datasets. This requires modeling the data-generating process – a complex combination of the physical phenomenon of interest and the “noise”. The goal is to create an approximate model that captures the essence of this process and then fit it to the data. This thesis covers the development of new methods and tools for almost all aspects of the data analysis process in two fields of astronomy: gravitational microlensing and occultation/eclipse mapping. In both fields, the objective is to infer the physical properties of exoplanets, stars, or dark compact objects by measuring the brightness variations of a light source as a function of time. Building on recent advancements in statistics, machine learning, and computer science, I developed a new open-source package called caustics for computing the microlensing magnification in single, binary, and triple-lens microlensing events. I also tackled foundational questions on the statistical analysis of single-lens and multiple-lens microlensing events, developing a new paradigm for modeling degenerate single-lens microlensing events and demonstrating the flaws of commonly used methods for analyzing multiple-lens microlensing events. Moreover, I built models for reconstructing two-dimensional emission maps of spherical bodies, exoplanets, and Solar System objects from one-dimensional photometric occultation light curves. Together with collaborators, I developed a novel method for reconstructing spatial maps of volcanic emission on Jupiter’s moon Io from occultation light curves and used the same method for exoplanet eclipse mapping to explore the possibility of detecting weather and climate change on Hot Jupiters using simulated photometric JWST secondary eclipse light curves.
2023-06-12T00:00:00ZBartolić, FranProgress in astronomy in the 21st century is contingent on the ability to extract useful information from complex and noisy datasets. This requires modeling the data-generating process – a complex combination of the physical phenomenon of interest and the “noise”. The goal is to create an approximate model that captures the essence of this process and then fit it to the data. This thesis covers the development of new methods and tools for almost all aspects of the data analysis process in two fields of astronomy: gravitational microlensing and occultation/eclipse mapping. In both fields, the objective is to infer the physical properties of exoplanets, stars, or dark compact objects by measuring the brightness variations of a light source as a function of time. Building on recent advancements in statistics, machine learning, and computer science, I developed a new open-source package called caustics for computing the microlensing magnification in single, binary, and triple-lens microlensing events. I also tackled foundational questions on the statistical analysis of single-lens and multiple-lens microlensing events, developing a new paradigm for modeling degenerate single-lens microlensing events and demonstrating the flaws of commonly used methods for analyzing multiple-lens microlensing events. Moreover, I built models for reconstructing two-dimensional emission maps of spherical bodies, exoplanets, and Solar System objects from one-dimensional photometric occultation light curves. Together with collaborators, I developed a novel method for reconstructing spatial maps of volcanic emission on Jupiter’s moon Io from occultation light curves and used the same method for exoplanet eclipse mapping to explore the possibility of detecting weather and climate change on Hot Jupiters using simulated photometric JWST secondary eclipse light curves.Hybrid ancilla-based quantum computation and emergent Gaussian multipartite entanglementNordgren, Viktor Manuelhttps://hdl.handle.net/10023/270842023-04-27T21:21:31Z2023-06-12T00:00:00ZIn the first half of this thesis, we present two models of ancilla-based quantum computation
(ABQC). Computation in the ABQC models is based on effecting changes on a register through
the interaction with and manipulation of an ancillary system. The two models presented enable
quantum computation through only unitary control of the ancilla – the ancilla-controlled model
(ACQC) – or supplemented by measurements on the ancilla which drive the register transfor-
mations – the ancilla-driven model (ADQC). For each of the models, we work on systems which
couple two continuous variables (CV) or which are hybrid: the register is formed by two-level
systems while the ancilla is a CV degree of freedom.
The initial models are presented using eigenstates of momentum as the ancillas. We move
to a more realistic scenario by modelling the ancillas as finitely squeezed states. We find that
the completely unitary ACQC contains persistent entanglement between register and ancilla in
the finite-squeezing scenario. In the ancilla-driven model, the effect of finite squeezing is to scale
the register state by a real exponential which is inversely proportional to the squeezing in the
ancilla.
In the second part, we cover work on Genuine Gaussian Multipartite Entanglement (Gaussian
GME). We present an algorithm for finding Gaussian states that have GME despite having
all two-state reductions separable. This touches on the idea of entanglement as an emergent
phenomenon. We determine GME via witnesses which probe only a subset of the state. We
therefore referred to them as partially blind witnesses. The algorithm is based on semi-definite
programs (SDPs). Such optimisation schemes can be used to efficiently find an optimal, partially
blind, GME witness for a given CM and vice versa. We then present results of multipartite states
of up to six parties. For the tripartite example, we present two experimental schemes to produce
the state using a circuit of beam-splitters and squeezers.
2023-06-12T00:00:00ZNordgren, Viktor ManuelIn the first half of this thesis, we present two models of ancilla-based quantum computation
(ABQC). Computation in the ABQC models is based on effecting changes on a register through
the interaction with and manipulation of an ancillary system. The two models presented enable
quantum computation through only unitary control of the ancilla – the ancilla-controlled model
(ACQC) – or supplemented by measurements on the ancilla which drive the register transfor-
mations – the ancilla-driven model (ADQC). For each of the models, we work on systems which
couple two continuous variables (CV) or which are hybrid: the register is formed by two-level
systems while the ancilla is a CV degree of freedom.
The initial models are presented using eigenstates of momentum as the ancillas. We move
to a more realistic scenario by modelling the ancillas as finitely squeezed states. We find that
the completely unitary ACQC contains persistent entanglement between register and ancilla in
the finite-squeezing scenario. In the ancilla-driven model, the effect of finite squeezing is to scale
the register state by a real exponential which is inversely proportional to the squeezing in the
ancilla.
In the second part, we cover work on Genuine Gaussian Multipartite Entanglement (Gaussian
GME). We present an algorithm for finding Gaussian states that have GME despite having
all two-state reductions separable. This touches on the idea of entanglement as an emergent
phenomenon. We determine GME via witnesses which probe only a subset of the state. We
therefore referred to them as partially blind witnesses. The algorithm is based on semi-definite
programs (SDPs). Such optimisation schemes can be used to efficiently find an optimal, partially
blind, GME witness for a given CM and vice versa. We then present results of multipartite states
of up to six parties. For the tripartite example, we present two experimental schemes to produce
the state using a circuit of beam-splitters and squeezers.Investigating observational probes of galaxy evolution in observations and simulationsBates, Dominichttps://hdl.handle.net/10023/269222023-02-09T03:05:19Z2022-06-22T00:00:00ZIn this thesis, we perform tests of galaxy evolution using a number of different observational
probes. In chapter 2, we implement a method of obtaining redshift distributions for photometric
samples of galaxies, known as clustering redshifts. We test this on real data from the
Baryon Oscillation Spectroscopic Survey (BOSS), and simulated data from semi-analytic models,
showing that assumptions about the bias evolution of the unknown sample can become
important for some samples of galaxies, particularly at faint magnitudes. We also find that
the choice of clustering scale makes a big difference to the noise in the recovered redshift
distribution. In chapter 3, we apply the clustering redshifts method to data from the Sloan
Digital Sky Survey (SDSS), recovering redshift distributions as a function of colour, allowing
us to compute mass and luminosity functions over large volumes. Little evolution is seen in
our recovered mass function between 0.2 < z < 0.8, implying the most massive galaxies form
most of their mass before z = 0.8. These mass functions are used to produce stellar mass completeness estimates for BOSS, giving a completeness of 80% above M[sub]⋆ > 10[super](11.4) M[sub]☉︎ between 0.2 < z < 0.7, with completeness falling significantly at higher redshifts and lower masses. In
chapter 4, we go on to investigate how well the formation history of a dark matter halo can
be inferred in simulations from the observable properties of a galaxy, finding that applying a
machine learning approach considering multiple properties performs significantly better than
using individual properties. We add errors to parameters, finding that a machine learning approach
still performs best, and finally use this approach to compute formation times for the
GAMA survey. We investigate how formation time changes with environment at fixed mass,
finding signs of assembly bias, with high mass halos in dense environments being younger
than those in under-dense regions, and the trend reversing at lower halo masses, with halos
in dense environments being older.
2022-06-22T00:00:00ZBates, DominicIn this thesis, we perform tests of galaxy evolution using a number of different observational
probes. In chapter 2, we implement a method of obtaining redshift distributions for photometric
samples of galaxies, known as clustering redshifts. We test this on real data from the
Baryon Oscillation Spectroscopic Survey (BOSS), and simulated data from semi-analytic models,
showing that assumptions about the bias evolution of the unknown sample can become
important for some samples of galaxies, particularly at faint magnitudes. We also find that
the choice of clustering scale makes a big difference to the noise in the recovered redshift
distribution. In chapter 3, we apply the clustering redshifts method to data from the Sloan
Digital Sky Survey (SDSS), recovering redshift distributions as a function of colour, allowing
us to compute mass and luminosity functions over large volumes. Little evolution is seen in
our recovered mass function between 0.2 < z < 0.8, implying the most massive galaxies form
most of their mass before z = 0.8. These mass functions are used to produce stellar mass completeness estimates for BOSS, giving a completeness of 80% above M[sub]⋆ > 10[super](11.4) M[sub]☉︎ between 0.2 < z < 0.7, with completeness falling significantly at higher redshifts and lower masses. In
chapter 4, we go on to investigate how well the formation history of a dark matter halo can
be inferred in simulations from the observable properties of a galaxy, finding that applying a
machine learning approach considering multiple properties performs significantly better than
using individual properties. We add errors to parameters, finding that a machine learning approach
still performs best, and finally use this approach to compute formation times for the
GAMA survey. We investigate how formation time changes with environment at fixed mass,
finding signs of assembly bias, with high mass halos in dense environments being younger
than those in under-dense regions, and the trend reversing at lower halo masses, with halos
in dense environments being older.Induced superconductivity in a quantum Hall edge stateMichelsen, Andreas Nicolai Bockhttps://hdl.handle.net/10023/269072023-02-10T09:33:04Z2023-06-12T00:00:00ZIn the search for non-Abelian anyonic zero modes for inherently fault-tolerant quantum computing, the hybridized superconductor - quantum Hall edge system plays an important role. Inspired by recent experimental realizations of this system, we describe it through a microscopic theory based on a BCS superconductor with Rashba spin-orbit coupling and Meissner effect at the surface, which is tunnel-coupled to a spin-polarized integer or fractional quantum Hall edge. By integrating out the superconductor, we arrive at an effective theory of the proximitized edge state and establish a qualitative description of the induced superconductivity. We predict analytical relations between experimentally available parameters and the key parameters of the induced superconductivity, as well as the experimentally relevant transport signatures. Extending the model to the fractional quantum Hall case, we find that both the spin-orbit coupling and the Meissner effect play central roles. The former allows for transport across the interface, while the latter controls the topological phase transition of the induced p-wave pairing in the edge state, allows for particle-hole conversion in transport for weak induced pairing amplitudes, and determines when pairing dominates over fractionalization in the proximitized fractional quantum Hall edge. Further experimental indicators are predicted for the system of a superconductor coupled through a quantum point contact with an integer or fractional quantum Hall edge, with a Pauli blockade which is robust to interactions and fractionalization as a key indicator of induced superconductivity. With these predictions we establish a more solid qualitative understanding of this important system, and advance the field towards the realization of anyonic zero modes.
2023-06-12T00:00:00ZMichelsen, Andreas Nicolai BockIn the search for non-Abelian anyonic zero modes for inherently fault-tolerant quantum computing, the hybridized superconductor - quantum Hall edge system plays an important role. Inspired by recent experimental realizations of this system, we describe it through a microscopic theory based on a BCS superconductor with Rashba spin-orbit coupling and Meissner effect at the surface, which is tunnel-coupled to a spin-polarized integer or fractional quantum Hall edge. By integrating out the superconductor, we arrive at an effective theory of the proximitized edge state and establish a qualitative description of the induced superconductivity. We predict analytical relations between experimentally available parameters and the key parameters of the induced superconductivity, as well as the experimentally relevant transport signatures. Extending the model to the fractional quantum Hall case, we find that both the spin-orbit coupling and the Meissner effect play central roles. The former allows for transport across the interface, while the latter controls the topological phase transition of the induced p-wave pairing in the edge state, allows for particle-hole conversion in transport for weak induced pairing amplitudes, and determines when pairing dominates over fractionalization in the proximitized fractional quantum Hall edge. Further experimental indicators are predicted for the system of a superconductor coupled through a quantum point contact with an integer or fractional quantum Hall edge, with a Pauli blockade which is robust to interactions and fractionalization as a key indicator of induced superconductivity. With these predictions we establish a more solid qualitative understanding of this important system, and advance the field towards the realization of anyonic zero modes.Process tensor networks for non-Markovian open quantum systemsFux, Gerald E.https://hdl.handle.net/10023/269022023-08-18T21:32:00Z2022-11-29T00:00:00ZThe advance of quantum technology relies heavily on an accurate understanding of the unavoidable interactions between quantum systems and their environment. While it is often adequate to account for the environment using approximate time-local (i.e. Markovian) equations of motion, in many scenarios such a description fails, and a more general non-Markovian theory becomes necessary. The failure of Markovian descriptions concerns not only quantitative aspects of the reduced dynamics of a quantum system, but also qualitative and conceptual aspects, such as the failure of the quantum regression formula relating the system's dynamics to its multi-time correlations. Despite considerable progress in recent years, the description and simulation of non-Markovian open quantum systems remains a conceptual and computational challenge. In this thesis we develop a versatile set of numerical methods for non-Markovian open quantum systems by combining the so-called process tensor framework with the numerical power of tensor network methods. The recently introduced process tensor is an alternative approach to open quantum systems and is - unlike the canonical approach based on dynamical maps - well suited for a rigorous characterisation of non-Markovian open quantum systems. We construct and apply process tensors in a matrix product operator form (PT-MPO) to yield a numerically exact, yet efficient representation of non-Markovian open quantum systems, which allows for a variety of practical applications. Building on the PT-MPO we introduce general methods to (1) efficiently find optimal control procedures for non-Markovian open quantum systems, (2) compute the dynamics and multi-time correlations of chains of non-Markovian open quantum systems, and (3) construct a time-translational invariant PT-MPO, which allows efficient computation of steady states even in non-equilibrium non-Markovian scenarios.
2022-11-29T00:00:00ZFux, Gerald E.The advance of quantum technology relies heavily on an accurate understanding of the unavoidable interactions between quantum systems and their environment. While it is often adequate to account for the environment using approximate time-local (i.e. Markovian) equations of motion, in many scenarios such a description fails, and a more general non-Markovian theory becomes necessary. The failure of Markovian descriptions concerns not only quantitative aspects of the reduced dynamics of a quantum system, but also qualitative and conceptual aspects, such as the failure of the quantum regression formula relating the system's dynamics to its multi-time correlations. Despite considerable progress in recent years, the description and simulation of non-Markovian open quantum systems remains a conceptual and computational challenge. In this thesis we develop a versatile set of numerical methods for non-Markovian open quantum systems by combining the so-called process tensor framework with the numerical power of tensor network methods. The recently introduced process tensor is an alternative approach to open quantum systems and is - unlike the canonical approach based on dynamical maps - well suited for a rigorous characterisation of non-Markovian open quantum systems. We construct and apply process tensors in a matrix product operator form (PT-MPO) to yield a numerically exact, yet efficient representation of non-Markovian open quantum systems, which allows for a variety of practical applications. Building on the PT-MPO we introduce general methods to (1) efficiently find optimal control procedures for non-Markovian open quantum systems, (2) compute the dynamics and multi-time correlations of chains of non-Markovian open quantum systems, and (3) construct a time-translational invariant PT-MPO, which allows efficient computation of steady states even in non-equilibrium non-Markovian scenarios.A study of brown dwarf and star formation in NGC 2264Pearson, Samuelhttps://hdl.handle.net/10023/268982023-04-30T21:56:35Z2022-11-29T00:00:00ZBrown dwarfs are substellar objects intermediate between stars and planets that are not massive enough to sustain stable hydrogen fusion (<75Mⱼᵤₚ). They are a natural outcome of the processes that lead to the formation of stars and planets, and we find them in abundance in every environment we have searched with sufficient depth. Brown dwarfs are not rare; we find a brown dwarf for every 2 -- 5 stars, equating to tens of billions across the Milky Way. However, despite their prevalence, the inherent faintness of brown dwarfs makes studying them challenging. Many fundamental questions about these objects therefore remain unanswered, key among them: how do brown dwarfs form?
The overall aim of this thesis has been to work towards this question and better understand the formation of brown dwarfs. To achieve this goal, my research has focused on brown dwarf and star formation in the young cluster, NGC 2264. The aim of studying this cluster was to build the large and well characterised sample of young brown dwarfs needed to answer the outstanding questions in the field of brown dwarf formation. Although NGC 2264 is more distant than the nearest star forming regions, I have shown that it is still accessible for detailed spectroscopic observations of young substellar objects, and crucially has the much larger population that is needed to achieve a robust statistical analysis of the key tracers of brown dwarf formation. I have identified 448 brown dwarf candidates in NGC 2264, based on a combination of NIR colours and additional signatures of youth. I have utilised follow up spectroscopy to show this selection method significantly improves selection efficiency. I find that the star-to-brown dwarf ratio and the slope of the substellar mass function for NGC 2264 are consistent with other young clusters, which points to a uniform IMF across a diverse range of star forming environments. I find no evidence for a variation in the star-to-brown dwarf ratio due to stellar surface density or the presence of OB stars. This rules out brown dwarf formation scenarios that predict strong variations in the relative frequency of brown dwarfs due to environmental conditions.
Using Gaia EDR3 kinematics and photometry, I have selected a clean sample of 664 NGC 2264 stellar cluster members and divided these members into north and south sub-clusters using a K-means clustering algorithm. Through comparison with isochrones I have shown that the average age of the northern sub-cluster is ~1 Myr younger than the southern sub-cluster. Using MIR Spitzer data I have classified the disc morphology of the cluster members, based on the slope of their infrared excess. I have shown that despite the younger age of the northern sub-cluster the disc fraction is approximately the same for both the north and south (~ 40%). I attribute this dearth of northern discs to photoevaporation from S Mon, which contains a massive O7 type star - the only O type star in NGC 2264. I have shown that there is a reduction in the local disc fraction up to 1.5 pc away from S Mon, implying that O type stars have a significant and far reaching impact on the local disc lifetime.
2022-11-29T00:00:00ZPearson, SamuelBrown dwarfs are substellar objects intermediate between stars and planets that are not massive enough to sustain stable hydrogen fusion (<75Mⱼᵤₚ). They are a natural outcome of the processes that lead to the formation of stars and planets, and we find them in abundance in every environment we have searched with sufficient depth. Brown dwarfs are not rare; we find a brown dwarf for every 2 -- 5 stars, equating to tens of billions across the Milky Way. However, despite their prevalence, the inherent faintness of brown dwarfs makes studying them challenging. Many fundamental questions about these objects therefore remain unanswered, key among them: how do brown dwarfs form?
The overall aim of this thesis has been to work towards this question and better understand the formation of brown dwarfs. To achieve this goal, my research has focused on brown dwarf and star formation in the young cluster, NGC 2264. The aim of studying this cluster was to build the large and well characterised sample of young brown dwarfs needed to answer the outstanding questions in the field of brown dwarf formation. Although NGC 2264 is more distant than the nearest star forming regions, I have shown that it is still accessible for detailed spectroscopic observations of young substellar objects, and crucially has the much larger population that is needed to achieve a robust statistical analysis of the key tracers of brown dwarf formation. I have identified 448 brown dwarf candidates in NGC 2264, based on a combination of NIR colours and additional signatures of youth. I have utilised follow up spectroscopy to show this selection method significantly improves selection efficiency. I find that the star-to-brown dwarf ratio and the slope of the substellar mass function for NGC 2264 are consistent with other young clusters, which points to a uniform IMF across a diverse range of star forming environments. I find no evidence for a variation in the star-to-brown dwarf ratio due to stellar surface density or the presence of OB stars. This rules out brown dwarf formation scenarios that predict strong variations in the relative frequency of brown dwarfs due to environmental conditions.
Using Gaia EDR3 kinematics and photometry, I have selected a clean sample of 664 NGC 2264 stellar cluster members and divided these members into north and south sub-clusters using a K-means clustering algorithm. Through comparison with isochrones I have shown that the average age of the northern sub-cluster is ~1 Myr younger than the southern sub-cluster. Using MIR Spitzer data I have classified the disc morphology of the cluster members, based on the slope of their infrared excess. I have shown that despite the younger age of the northern sub-cluster the disc fraction is approximately the same for both the north and south (~ 40%). I attribute this dearth of northern discs to photoevaporation from S Mon, which contains a massive O7 type star - the only O type star in NGC 2264. I have shown that there is a reduction in the local disc fraction up to 1.5 pc away from S Mon, implying that O type stars have a significant and far reaching impact on the local disc lifetime.Title redactedHill, Joseph Samuelhttps://hdl.handle.net/10023/268122023-11-21T03:02:05Z2023-06-12T00:00:00ZAbstract redacted
2023-06-12T00:00:00ZHill, Joseph SamuelAbstract redactedStar formation in the Perseus complex : a tale of seven clustersPavlidou, Tatianahttps://hdl.handle.net/10023/266812023-01-11T03:02:35Z2022-06-13T00:00:00ZFollowing the recent advances in astrometry of Gaia DR2, I use its photometric and kinematic
data to explore the structure of the star forming region associated with the molecular cloud
of Perseus. Apart from the two well-known clusters, IC 348 and NGC 1333, I present five new
clusters, which contain between 30 and 300 members, named Autochthe, Alcaeus, Mestor,
Electryon and Heleus. I construct reliable membership lists for the seven clusters and of the
dispersed population of the complex.
I investigate the youth of individual sources within the clusters, and outside, using a combination
of youth indicators from Gaia, WISE, Spitzer, and Planck data. These give the relative ages of the clusters and yield lists of young sources (τ[sub](age) ≲ 5 Myr). Based on this
work, IC 348 spans ages from 1 Myr up to 5 Myr. Autochthe and NGC 1333 are the youngest
clusters at 1 Myr. These three clusters constitute the star-forming centers of the cloud. Heleus
and Mestor have average ages of about 4 Myr while Alcaeus and Electryon are at about 5 Myr.
NGC 1333, Autochthe and IC 348 have the highest disc fractions (66±14%, 58±30% and 41±7%) while the rest four clusters have fractions below ∼30%.
Finally I construct the system Mass Function (MF) for all seven clusters by determining
the masses of the cluster members using 2MASS photometry. For 0.08 < m/M[sub]☉︎ < 1.4 the MF slopes are in agreement for the seven clusters and range between 0.5 and 1.3. These are by
and large smaller than the Kroupa (2001) slope of 1.3.
The region of Perseus exhibits a bimodal nature in proper motion, spatial distribution, and
age span. This bimodality is supported by a star-formation scenario where the seven clusters
have formed from two kinematically distinct sub-clouds.
2022-06-13T00:00:00ZPavlidou, TatianaFollowing the recent advances in astrometry of Gaia DR2, I use its photometric and kinematic
data to explore the structure of the star forming region associated with the molecular cloud
of Perseus. Apart from the two well-known clusters, IC 348 and NGC 1333, I present five new
clusters, which contain between 30 and 300 members, named Autochthe, Alcaeus, Mestor,
Electryon and Heleus. I construct reliable membership lists for the seven clusters and of the
dispersed population of the complex.
I investigate the youth of individual sources within the clusters, and outside, using a combination
of youth indicators from Gaia, WISE, Spitzer, and Planck data. These give the relative ages of the clusters and yield lists of young sources (τ[sub](age) ≲ 5 Myr). Based on this
work, IC 348 spans ages from 1 Myr up to 5 Myr. Autochthe and NGC 1333 are the youngest
clusters at 1 Myr. These three clusters constitute the star-forming centers of the cloud. Heleus
and Mestor have average ages of about 4 Myr while Alcaeus and Electryon are at about 5 Myr.
NGC 1333, Autochthe and IC 348 have the highest disc fractions (66±14%, 58±30% and 41±7%) while the rest four clusters have fractions below ∼30%.
Finally I construct the system Mass Function (MF) for all seven clusters by determining
the masses of the cluster members using 2MASS photometry. For 0.08 < m/M[sub]☉︎ < 1.4 the MF slopes are in agreement for the seven clusters and range between 0.5 and 1.3. These are by
and large smaller than the Kroupa (2001) slope of 1.3.
The region of Perseus exhibits a bimodal nature in proper motion, spatial distribution, and
age span. This bimodality is supported by a star-formation scenario where the seven clusters
have formed from two kinematically distinct sub-clouds.Manipulation of charge density wave instabilities in Ti-based transition-metal dichalcogenidesAntonelli, Tommasohttps://hdl.handle.net/10023/266722024-02-27T12:09:16Z2023-06-12T00:00:00ZTransition metal dichalcogenides (TMDs) offer a versatile platform to study the interplay of
different electronic interactions underpinning collective quantum phases such as ferromagnetism,
superconductivity and charge density waves (CDW). In particular, TiSe₂ shows an anomalous
CDW instability whose microscopic nature has been debated for ∼50 years, but it still remains
unclear. A similar phase transition has been recently observed in single layer TiTe₂, but not in the
multilayer system opening further controversy on the origin of such lattice instability.
In this thesis, I explore different experimental methods for manipulating the CDW transition
in TiX₂ (X = Se, Te) by means of molecular beam epitaxy and angle resolved photoemission spectroscopy
(ARPES). I report an in-depth photoemission study of the anomalous lattice instability in
single layer TiTe₂ providing the first evidence of an electronic energy gain driving the CDW phase
in the monolayer configuration. The experimental results are rationalised in a minimal model
that ultimately gives a natural explanation for the unusual dimensionality crossover of the CDW
instability in TiTe₂. By chemically substituting Te with Se, 2D ML-TiTe₂ₓSe₂₍₁₋ₓ₎ alloys are synthesised
for the first time, showing a semiconductor (x = 0) to semimetal (x = 1) transition in the
normal phase. The ARPES analysis reported here reveals the continuous evolution of the CDW
instability across the entire alloy series opening a new route to manipulate this collective phase
at the 2D limit. As an alternative approach, I explore the role of remote electronic screening in
manipulating the CDW order in ML-TiSe₂. A V₂O₃ functional substrate able to switch between
metallic (high screening) to insulating (low screening) phase is used for this purpose, although
further optimisations are needed to improve the crystal quality of ML-TiSe₂ deposited on top.
This work brings critical new insights for the understanding of CDW-like states across the
family of group-VI TMDs and to engineer collective electronic states in 2D materials.
2023-06-12T00:00:00ZAntonelli, TommasoTransition metal dichalcogenides (TMDs) offer a versatile platform to study the interplay of
different electronic interactions underpinning collective quantum phases such as ferromagnetism,
superconductivity and charge density waves (CDW). In particular, TiSe₂ shows an anomalous
CDW instability whose microscopic nature has been debated for ∼50 years, but it still remains
unclear. A similar phase transition has been recently observed in single layer TiTe₂, but not in the
multilayer system opening further controversy on the origin of such lattice instability.
In this thesis, I explore different experimental methods for manipulating the CDW transition
in TiX₂ (X = Se, Te) by means of molecular beam epitaxy and angle resolved photoemission spectroscopy
(ARPES). I report an in-depth photoemission study of the anomalous lattice instability in
single layer TiTe₂ providing the first evidence of an electronic energy gain driving the CDW phase
in the monolayer configuration. The experimental results are rationalised in a minimal model
that ultimately gives a natural explanation for the unusual dimensionality crossover of the CDW
instability in TiTe₂. By chemically substituting Te with Se, 2D ML-TiTe₂ₓSe₂₍₁₋ₓ₎ alloys are synthesised
for the first time, showing a semiconductor (x = 0) to semimetal (x = 1) transition in the
normal phase. The ARPES analysis reported here reveals the continuous evolution of the CDW
instability across the entire alloy series opening a new route to manipulate this collective phase
at the 2D limit. As an alternative approach, I explore the role of remote electronic screening in
manipulating the CDW order in ML-TiSe₂. A V₂O₃ functional substrate able to switch between
metallic (high screening) to insulating (low screening) phase is used for this purpose, although
further optimisations are needed to improve the crystal quality of ML-TiSe₂ deposited on top.
This work brings critical new insights for the understanding of CDW-like states across the
family of group-VI TMDs and to engineer collective electronic states in 2D materials.Dirac solitons in general relativity : many-fermion states, singular solutions & the inclusion of a Higgs mechanismLeith, Peter Edward Duncanhttps://hdl.handle.net/10023/265082023-02-17T16:20:44Z2022-11-29T00:00:00ZThe study of gravitationally localised quantum states, in which quantum particles are bound together by their mutual gravitational interaction, has been a topic of considerable research for over 50 years. Stemming from John Wheeler's initial concept of an electromagnetic 'geon', focus quickly converged on scalar fields with the introduction of the objects today referred to as 'boson stars'. It was not until more recently, however, that the fermionic sector was properly addressed by Finster, Smoller & Yau, who successfully constructed the first numerical solutions to the coupled Einstein--Dirac system. The resulting 'particle-like' objects, comprising pairs of neutral fermions, have become known as 'Dirac solitons' or 'Dirac stars', and have been the focus of significantly less study than their bosonic counterparts.
This thesis aims to expand the knowledge of Dirac solitons in a number of ways. First, we conduct a detailed study of the many-fermion system (up to a total of 90 fermions), and interpret the structure of the resulting solutions in terms of a 'self-trapping' effect. We also find somewhat unexpected results for the behaviour of the excited states of these many-fermion solitons. Second, we present particle-like solutions to the minimally-coupled Einstein-Dirac-Higgs system, and show that, in the presence of strong coupling, a mass-scale separation can occur, in which the total mass of the constituent fermions far outweighs the gravitational mass of the state. Finally, we introduce new singular solutions to the Einstein-Dirac system, including the first normalised analytic solution. The properties of these are somewhat unusual, but we show that many are related (and indeed can be smoothly connected) to the non-singular Einstein-Dirac states.
2022-11-29T00:00:00ZLeith, Peter Edward DuncanThe study of gravitationally localised quantum states, in which quantum particles are bound together by their mutual gravitational interaction, has been a topic of considerable research for over 50 years. Stemming from John Wheeler's initial concept of an electromagnetic 'geon', focus quickly converged on scalar fields with the introduction of the objects today referred to as 'boson stars'. It was not until more recently, however, that the fermionic sector was properly addressed by Finster, Smoller & Yau, who successfully constructed the first numerical solutions to the coupled Einstein--Dirac system. The resulting 'particle-like' objects, comprising pairs of neutral fermions, have become known as 'Dirac solitons' or 'Dirac stars', and have been the focus of significantly less study than their bosonic counterparts.
This thesis aims to expand the knowledge of Dirac solitons in a number of ways. First, we conduct a detailed study of the many-fermion system (up to a total of 90 fermions), and interpret the structure of the resulting solutions in terms of a 'self-trapping' effect. We also find somewhat unexpected results for the behaviour of the excited states of these many-fermion solitons. Second, we present particle-like solutions to the minimally-coupled Einstein-Dirac-Higgs system, and show that, in the presence of strong coupling, a mass-scale separation can occur, in which the total mass of the constituent fermions far outweighs the gravitational mass of the state. Finally, we introduce new singular solutions to the Einstein-Dirac system, including the first normalised analytic solution. The properties of these are somewhat unusual, but we show that many are related (and indeed can be smoothly connected) to the non-singular Einstein-Dirac states.Investigating strongly interacting multicritical field theories using numerical conformal bootstrapDowens, Matthewhttps://hdl.handle.net/10023/263822022-11-21T12:16:41Z2022-11-29T00:00:00ZIn strongly correlated quantum systems, conventional techniques for understanding physical behaviour near multicritical points break down. Numerical conformal bootstrap is a promising new method that has proven demonstrably useful in investigating critical phenomena. The method uses the enhanced conformal symmetry exhibited by quantum field theories describing criticality in order to definitively rule out theories based on fundamental symmetry grounds alone. By requiring only that physically plausible conformal field theories (CFTs) satisfy a mathematical self-consistency relation, it allows one to rule out vast amounts of violating CFT data, allowing a rigorous mapping to made of ‘allowed’ CFTs. Such a mapping is powerful due to the concept of universality, whereby the critical behaviour of many disparate physical models is identical and is captured by the same set of critical exponents and the same CFT. This thesis aims to explore the effectiveness of numerical bootstrap. We begin by motivating the bootstrap in a condensed matter context, before reviewing necessary results in the conformal field theory literature and establishing the bootstrap method. A large part of the thesis will be an overview of the working of the method and the approximations made, as well as its applications to simple symmetry groups. We will demonstrate the validity of the method by replicating flagship results, before describing the novel work we have carried out in exploring more complicated global symmetry groups, where more than one fixed point is predicted. Finally, we will conclude and comment on the limitations of the thesis and lines of possible future work.
2022-11-29T00:00:00ZDowens, MatthewIn strongly correlated quantum systems, conventional techniques for understanding physical behaviour near multicritical points break down. Numerical conformal bootstrap is a promising new method that has proven demonstrably useful in investigating critical phenomena. The method uses the enhanced conformal symmetry exhibited by quantum field theories describing criticality in order to definitively rule out theories based on fundamental symmetry grounds alone. By requiring only that physically plausible conformal field theories (CFTs) satisfy a mathematical self-consistency relation, it allows one to rule out vast amounts of violating CFT data, allowing a rigorous mapping to made of ‘allowed’ CFTs. Such a mapping is powerful due to the concept of universality, whereby the critical behaviour of many disparate physical models is identical and is captured by the same set of critical exponents and the same CFT. This thesis aims to explore the effectiveness of numerical bootstrap. We begin by motivating the bootstrap in a condensed matter context, before reviewing necessary results in the conformal field theory literature and establishing the bootstrap method. A large part of the thesis will be an overview of the working of the method and the approximations made, as well as its applications to simple symmetry groups. We will demonstrate the validity of the method by replicating flagship results, before describing the novel work we have carried out in exploring more complicated global symmetry groups, where more than one fixed point is predicted. Finally, we will conclude and comment on the limitations of the thesis and lines of possible future work.Title redactedLian, Chenghttps://hdl.handle.net/10023/260322022-09-17T02:01:01Z2022-11-29T00:00:00Z2022-11-29T00:00:00ZLian, ChengComputational simulations of ultraviolet radiation penetration into human skinBarnard, Isla Rose Maryhttps://hdl.handle.net/10023/244402021-12-01T03:09:38Z2021-11-30T00:00:00ZThis thesis concerns the development of numerical modelling simulations to predict how ultraviolet radiation (UVR) penetrates into human skin in a wavelength dependent manner. UVR has biological effects; for example, UVR causes damage to DNA within skin cells, and these effects are wavelength dependent. A Monte Carlo Radiative Transfer (MCRT) model was developed in order to simulate the transport of UVR from different radiation sources through the upper layers of human skin. Using the results of these simulations, the depth to which different wavelengths of UVR penetrate can be examined, and then resulting biological effects can be predicted.
The research presented here quantifies DNA damage occurring due to sunbed use, investigates the protective effects of melanin and sunscreen, investigates potential novel lamps for psoriasis treatment and examines the safety of UVR sterilisation devices. In addition, research is presented from practical work, evaluating the performance of a handheld UVR meter when used to measure UVR output from commercial sunbeds.
2021-11-30T00:00:00ZBarnard, Isla Rose MaryThis thesis concerns the development of numerical modelling simulations to predict how ultraviolet radiation (UVR) penetrates into human skin in a wavelength dependent manner. UVR has biological effects; for example, UVR causes damage to DNA within skin cells, and these effects are wavelength dependent. A Monte Carlo Radiative Transfer (MCRT) model was developed in order to simulate the transport of UVR from different radiation sources through the upper layers of human skin. Using the results of these simulations, the depth to which different wavelengths of UVR penetrate can be examined, and then resulting biological effects can be predicted.
The research presented here quantifies DNA damage occurring due to sunbed use, investigates the protective effects of melanin and sunscreen, investigates potential novel lamps for psoriasis treatment and examines the safety of UVR sterilisation devices. In addition, research is presented from practical work, evaluating the performance of a handheld UVR meter when used to measure UVR output from commercial sunbeds.Distinguishing standard from modified gravity in the local group and beyondBanik, Indranilhttps://hdl.handle.net/10023/241272023-10-02T11:04:58Z2018-06-27T00:00:00ZThe works in this portfolio test the hypothesis that it is not possible to extrapolate
the Newtonian inverse square law of gravity from Solar System to galaxy scales. In
particular, I look into various tests of Modified Newtonian Dynamics (MOND), which
posits a modification below a very low acceleration threshold. Although discrepancies
with Newtonian dynamics are indeed observed, they can usually be explained by
invoking an appropriate distribution of invisible mass known as dark matter (DM).
This leads to the standard cosmological paradigm, ΛCDM. I consider how it may be
distinguished from MOND using collision velocities of galaxy clusters, which should
sometimes be much faster in MOND. I focus on measuring these velocities more accurately
and conclude that this test ought to be feasible in the near future.
For the time being, I look at the much nearer and more accurately observed
Local Group (LG) of galaxies. Its main constituents the Milky Way (MW) and
Andromeda (M31) should have undergone a past close flyby in MOND but not
in ΛCDM. The fast MW-M31 relative motion around the time of their flyby would
have allowed them to gravitationally slingshot any passing LG dwarf galaxies out at
high speed. I consider whether there is any evidence for such high-velocity galaxies
(HVGs). Several candidates are found in two different ΛCDM models of the LG,
one written by a founding figure of the paradigm. The properties of these HVGs are
similar to what might be expected in MOND, especially their tendency to lie close
to a plane. Being more confident of its validity, I then used MOND to determine the
escape velocity curve of the MW over the distance range 850 kpc, finding reasonable
agreement with the latest observations. I finish by discussing possible future directions
for MOND research.
2018-06-27T00:00:00ZBanik, IndranilThe works in this portfolio test the hypothesis that it is not possible to extrapolate
the Newtonian inverse square law of gravity from Solar System to galaxy scales. In
particular, I look into various tests of Modified Newtonian Dynamics (MOND), which
posits a modification below a very low acceleration threshold. Although discrepancies
with Newtonian dynamics are indeed observed, they can usually be explained by
invoking an appropriate distribution of invisible mass known as dark matter (DM).
This leads to the standard cosmological paradigm, ΛCDM. I consider how it may be
distinguished from MOND using collision velocities of galaxy clusters, which should
sometimes be much faster in MOND. I focus on measuring these velocities more accurately
and conclude that this test ought to be feasible in the near future.
For the time being, I look at the much nearer and more accurately observed
Local Group (LG) of galaxies. Its main constituents the Milky Way (MW) and
Andromeda (M31) should have undergone a past close flyby in MOND but not
in ΛCDM. The fast MW-M31 relative motion around the time of their flyby would
have allowed them to gravitationally slingshot any passing LG dwarf galaxies out at
high speed. I consider whether there is any evidence for such high-velocity galaxies
(HVGs). Several candidates are found in two different ΛCDM models of the LG,
one written by a founding figure of the paradigm. The properties of these HVGs are
similar to what might be expected in MOND, especially their tendency to lie close
to a plane. Being more confident of its validity, I then used MOND to determine the
escape velocity curve of the MW over the distance range 850 kpc, finding reasonable
agreement with the latest observations. I finish by discussing possible future directions
for MOND research.Modelling non-Markovian quantum systems using tensor networksStrathearn, Aidanhttps://hdl.handle.net/10023/240082022-08-29T13:57:22Z2020-07-01T00:00:00ZAccurately modelling the behaviour of quantum systems interacting with the environment is
vital to the development of quantum technology. Open quantum systems are well understood
when the influence of their environment is weak, but the problem of modelling general systems
away from this limit remains difficult. Currently, all approaches to making this problem tractable
require some assumption about the nature of the system and its environment. In this thesis a
general and efficient numerical method for calculating observables of open quantum systems is
presented. The method is to express the exact equations of motion that describe the evolution of
a general open quantum system as a tensor network, whose structure allows for a decomposition in
terms of matrix products that can be efficiently compressed in size. The power and versatility of
the method is demonstrated by using it to study three contrasting models. The first of these is the
Ohmic Spin-Boson model, in which the location of the localisation phase transition is identified by
analysing the dissipative spin dynamics. This requires high precision numerical calculations, which
are shown to be carried out with high efficiency. The second model is that of two spatially separated
spins interacting with the same environment, for which no other exact results are available. Here
the environment is seen to mediate interaction between the spins, which is intuitively found to
be longer ranged for a lower spatial dimension. Finally, an experimentally relevant model of a
driven quantum dot is considered. Calculations of the emission spectrum of the dot reveal complex
interplay between the coherent driving and the dissipative influence of phonons. In particular the
phonon sideband in the spectrum is found to be supressed as the driving of the dot is strengthened.
2020-07-01T00:00:00ZStrathearn, AidanAccurately modelling the behaviour of quantum systems interacting with the environment is
vital to the development of quantum technology. Open quantum systems are well understood
when the influence of their environment is weak, but the problem of modelling general systems
away from this limit remains difficult. Currently, all approaches to making this problem tractable
require some assumption about the nature of the system and its environment. In this thesis a
general and efficient numerical method for calculating observables of open quantum systems is
presented. The method is to express the exact equations of motion that describe the evolution of
a general open quantum system as a tensor network, whose structure allows for a decomposition in
terms of matrix products that can be efficiently compressed in size. The power and versatility of
the method is demonstrated by using it to study three contrasting models. The first of these is the
Ohmic Spin-Boson model, in which the location of the localisation phase transition is identified by
analysing the dissipative spin dynamics. This requires high precision numerical calculations, which
are shown to be carried out with high efficiency. The second model is that of two spatially separated
spins interacting with the same environment, for which no other exact results are available. Here
the environment is seen to mediate interaction between the spins, which is intuitively found to
be longer ranged for a lower spatial dimension. Finally, an experimentally relevant model of a
driven quantum dot is considered. Calculations of the emission spectrum of the dot reveal complex
interplay between the coherent driving and the dissipative influence of phonons. In particular the
phonon sideband in the spectrum is found to be supressed as the driving of the dot is strengthened.What makes a star turn violent? : exploring high-mass star formation and stellar magnetic helicityLund, Kristinhttps://hdl.handle.net/10023/239732021-09-18T09:55:29Z2020-12-01T00:00:00ZHigh-mass stars play an important role in both the dynamical and chemical evolution of the
galaxy, hence how they form has long been a topic of interest. In this thesis I explore accretion
as a possible means of forming high-mass stars. I use a radiation hydrodynamics code to
simulate accretion onto a star which emits ionising radiation. Usually accretion is assumed
to end once ionisation begins, however I find accretion continues through the ionisation front
whilst the ionised region is gravitationally trapped. This extended period of accretion increases
the amount of material added to the star. As most high-mass stars are observed in close binary
systems, next I develop a semi-analytic model to investigate whether accretion onto wide lowmass
binaries can increase the mass of the stars and simultaneously decrease their separation.
The result suggests magnetic braking of the accretion flow is a feasible way of forming the
most massive close binary systems.
The second part of this thesis focuses on the magnetic helicity density of low-mass stars.
There has been growing interest in magnetic helicity as a potential tracer of stellar eruptions,
but measurements based on observations have been limited to the Sun. I present an analytic
expression for the magnetic helicity density across any stellar surface given only observable
quantities. This expression is used to calculate the magnetic helicity density of 52 stars, which
is then plotted against other stellar parameters. I find mostly different behaviours for the
partially and largely convective stars, except when plotting helicity density against toroidal
magnetic energy. In that case the entire stellar sample follows the same power law. Comparing
the Sun to the other stars I find the variation in solar helicity densities across a solar cycle falls
within the scatter of the stellar values.
2020-12-01T00:00:00ZLund, KristinHigh-mass stars play an important role in both the dynamical and chemical evolution of the
galaxy, hence how they form has long been a topic of interest. In this thesis I explore accretion
as a possible means of forming high-mass stars. I use a radiation hydrodynamics code to
simulate accretion onto a star which emits ionising radiation. Usually accretion is assumed
to end once ionisation begins, however I find accretion continues through the ionisation front
whilst the ionised region is gravitationally trapped. This extended period of accretion increases
the amount of material added to the star. As most high-mass stars are observed in close binary
systems, next I develop a semi-analytic model to investigate whether accretion onto wide lowmass
binaries can increase the mass of the stars and simultaneously decrease their separation.
The result suggests magnetic braking of the accretion flow is a feasible way of forming the
most massive close binary systems.
The second part of this thesis focuses on the magnetic helicity density of low-mass stars.
There has been growing interest in magnetic helicity as a potential tracer of stellar eruptions,
but measurements based on observations have been limited to the Sun. I present an analytic
expression for the magnetic helicity density across any stellar surface given only observable
quantities. This expression is used to calculate the magnetic helicity density of 52 stars, which
is then plotted against other stellar parameters. I find mostly different behaviours for the
partially and largely convective stars, except when plotting helicity density against toroidal
magnetic energy. In that case the entire stellar sample follows the same power law. Comparing
the Sun to the other stars I find the variation in solar helicity densities across a solar cycle falls
within the scatter of the stellar values.Optoelectronic measurements of organic and hybrid solar cellsMica, Natalie Annhttps://hdl.handle.net/10023/235382021-09-15T11:45:21Z2021-06-28T00:00:00ZOrganic and hybrid perovskite solar cells offer a cost-effective and efficient alternative to traditional silicon solar technologies. Also, the tunability of these photoactive materials allows for devices to be crafted for bespoke purposes, such as tandem solar cells or for harvesting energy from indoor lighting. The work in this thesis presents the development, understanding, and application of these materials. First, the development of two new small molecule materials for organic solar cell application is discussed. Their solar cell performance is optimised using several techniques to control the organic semiconductor film morphology and changes to the film are characterised using atomic force microscopy. The stability of these new solar cells is then measured in ambient conditions under constant illumination, which is correlated to the film morphology. Then the electron mobility of two high-performing non-fullerene organic materials using a Time of Flight method is measured. This technique not only allowed for the accurate measurement of the electron mobility of these materials, but also provides insight into the disorder of the organic film and the existence of deep-traps within. Lastly, a hybrid perovskite solar cell is optimised for application in visible light communications for simultaneous energy and data harvesting. In this study the devices are measured under various illumination conditions to gain insight into their loss mechanisms, while also optimising them for this specific purpose.
2021-06-28T00:00:00ZMica, Natalie AnnOrganic and hybrid perovskite solar cells offer a cost-effective and efficient alternative to traditional silicon solar technologies. Also, the tunability of these photoactive materials allows for devices to be crafted for bespoke purposes, such as tandem solar cells or for harvesting energy from indoor lighting. The work in this thesis presents the development, understanding, and application of these materials. First, the development of two new small molecule materials for organic solar cell application is discussed. Their solar cell performance is optimised using several techniques to control the organic semiconductor film morphology and changes to the film are characterised using atomic force microscopy. The stability of these new solar cells is then measured in ambient conditions under constant illumination, which is correlated to the film morphology. Then the electron mobility of two high-performing non-fullerene organic materials using a Time of Flight method is measured. This technique not only allowed for the accurate measurement of the electron mobility of these materials, but also provides insight into the disorder of the organic film and the existence of deep-traps within. Lastly, a hybrid perovskite solar cell is optimised for application in visible light communications for simultaneous energy and data harvesting. In this study the devices are measured under various illumination conditions to gain insight into their loss mechanisms, while also optimising them for this specific purpose.New thermally activated delayed fluorescence emitters and room-temperature organic long-persistent luminescenceLi, Wenbohttps://hdl.handle.net/10023/235292024-01-19T03:02:05Z2021-06-28T00:00:00ZOrganic light-emitting diodes (OLEDs) have attracted a lot of attentions because of their high performance in display applications. Organic emitters are still being developed to improve efficiency, colour gamut and sustainability, and thermally activated delayed fluorescence (TADF) materials are widely regarded as one of the most promising next-generation OLED emitters. To date, green TADF emitters have been developed, and the corresponding OLEDs show high light-emitting efficiency and long operation lifetimes. However, the performance of red and blue TADF OLEDs still lag their green-emitting counterparts. For this reason, this work focuses on developing new red and deep-blue TADF materials. A group of red and blue TADF emitters were designed and synthesized. Their photophysical properties and electroluminescence performance were also studied. In addition, it was found that doping some of these new emitters into common host materials, such as 2,8-bis(diphenyl-phosphoryl)dibenzo[b,d]thiophene (PPT), 2,2',2"-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) or poly(methyl methacrylate) (PMMA), can lead to organic long-persistent luminescence (OLPL) lasting for thousands of seconds at room temperature. As traditional room-temperature OLPL materials are based on exciplex emitters, the new OLPL systems discovered here demonstrates that exciplex formation is not required for harvesting OLPL. This enables a wide range of host materials to be used including materials as simple as PMMA. Expanding this concept further, the author developed a method for large-scale PMMA-based OLPL sample fabrication to take full advantage of its low expense. This method gives thick (> 2 mm) and clear OLPL products, and all the required equipment is easily accessed in lab condition. Combining the flexible design of TADF emitters and mature PMMA industry, this work opens the ‘door’ of large-scale, colour tuneable and cost-efficient room-temperature OLPL materials. At the same time, the light-emitting properties and mechanism of these new OLPL emitters were studied, which provides a guideline for further OLPL emitter improvements.
2021-06-28T00:00:00ZLi, WenboOrganic light-emitting diodes (OLEDs) have attracted a lot of attentions because of their high performance in display applications. Organic emitters are still being developed to improve efficiency, colour gamut and sustainability, and thermally activated delayed fluorescence (TADF) materials are widely regarded as one of the most promising next-generation OLED emitters. To date, green TADF emitters have been developed, and the corresponding OLEDs show high light-emitting efficiency and long operation lifetimes. However, the performance of red and blue TADF OLEDs still lag their green-emitting counterparts. For this reason, this work focuses on developing new red and deep-blue TADF materials. A group of red and blue TADF emitters were designed and synthesized. Their photophysical properties and electroluminescence performance were also studied. In addition, it was found that doping some of these new emitters into common host materials, such as 2,8-bis(diphenyl-phosphoryl)dibenzo[b,d]thiophene (PPT), 2,2',2"-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) or poly(methyl methacrylate) (PMMA), can lead to organic long-persistent luminescence (OLPL) lasting for thousands of seconds at room temperature. As traditional room-temperature OLPL materials are based on exciplex emitters, the new OLPL systems discovered here demonstrates that exciplex formation is not required for harvesting OLPL. This enables a wide range of host materials to be used including materials as simple as PMMA. Expanding this concept further, the author developed a method for large-scale PMMA-based OLPL sample fabrication to take full advantage of its low expense. This method gives thick (> 2 mm) and clear OLPL products, and all the required equipment is easily accessed in lab condition. Combining the flexible design of TADF emitters and mature PMMA industry, this work opens the ‘door’ of large-scale, colour tuneable and cost-efficient room-temperature OLPL materials. At the same time, the light-emitting properties and mechanism of these new OLPL emitters were studied, which provides a guideline for further OLPL emitter improvements.Room temperature polariton lasing in organic semiconductorsWei, Mengjiehttps://hdl.handle.net/10023/234442021-09-15T08:22:59Z2021-06-28T00:00:00Z2021-06-28T00:00:00ZWei, MengjieProbing unconventional transport regimes in delafossite metalsMcGuinness, Philippa Helenhttps://hdl.handle.net/10023/234412021-06-29T12:03:12Z2021-06-28T00:00:00ZThis thesis describes investigations into the origins and nature of the remarkable electrical
transport of the delafossite metals PtCoO₂ and PdCoO₂ using focused ion beam based microstructuring
techniques. These compounds are amongst the highest conductivity materials
known, but questions remain regarding the origin of their ultralow resistivity and the effects
of their properties on transport in unconventional regimes such as the ballistic regime.
In the initial introductory chapters, I will review the key properties of both delafossite metals
and the application of focused ion beam microstructuring to transport measurements within
This thesis describes investigations into the origins and nature of the remarkable electrical
transport of the delafossite metals PtCoO₂ and PdCoO₂ using focused ion beam based microstructuring
techniques. These compounds are amongst the highest conductivity materials
known, but questions remain regarding the origin of their ultralow resistivity and the effects
of their properties on transport in unconventional regimes such as the ballistic regime.
In the initial introductory chapters, I will review the key properties of both delafossite metals
and the application of focused ion beam microstructuring to transport measurements within
low resistivity materials.
The experimental findings are split into two chapters. Initially, I will describe an investigation
into the origins of the high conductivity by introducing defects to PtCoO₂ and PdCoO₂
through high energy electron irradiation and observing the changes to the resistivity. These
measurements demonstrate that the ultralow resistivity of the delafossite metals is the result
of an extreme purity of up to 1 defect in 120,000 atoms, rather than backscattering suppression.
In addition, I will report the effects of the defects on the electrical transport more
broadly. Here, by examining the difference before and after irradiation, insight is gained into
the origins of the unconventional magnetotransport of PtCoO₂.
The other study uses PtCoO₂ and PdCoO₂ as test systems for the investigation of the effects
of a non-circular Fermi surface on the transport within four terminal, square-shaped junctions
inside the ballistic regime. These junction devices have been shown to be a sensitive probe
of this regime in other materials, and I will demonstrate that the nearly hexagonal Fermi
surface of the ultrapure delafossite metals results in not only strongly ballistic behaviour,
present at a scale multiple times the mean free path, but also novel phenomena which are
not seen with a circular Fermi surface.
2021-06-28T00:00:00ZMcGuinness, Philippa HelenThis thesis describes investigations into the origins and nature of the remarkable electrical
transport of the delafossite metals PtCoO₂ and PdCoO₂ using focused ion beam based microstructuring
techniques. These compounds are amongst the highest conductivity materials
known, but questions remain regarding the origin of their ultralow resistivity and the effects
of their properties on transport in unconventional regimes such as the ballistic regime.
In the initial introductory chapters, I will review the key properties of both delafossite metals
and the application of focused ion beam microstructuring to transport measurements within
This thesis describes investigations into the origins and nature of the remarkable electrical
transport of the delafossite metals PtCoO₂ and PdCoO₂ using focused ion beam based microstructuring
techniques. These compounds are amongst the highest conductivity materials
known, but questions remain regarding the origin of their ultralow resistivity and the effects
of their properties on transport in unconventional regimes such as the ballistic regime.
In the initial introductory chapters, I will review the key properties of both delafossite metals
and the application of focused ion beam microstructuring to transport measurements within
low resistivity materials.
The experimental findings are split into two chapters. Initially, I will describe an investigation
into the origins of the high conductivity by introducing defects to PtCoO₂ and PdCoO₂
through high energy electron irradiation and observing the changes to the resistivity. These
measurements demonstrate that the ultralow resistivity of the delafossite metals is the result
of an extreme purity of up to 1 defect in 120,000 atoms, rather than backscattering suppression.
In addition, I will report the effects of the defects on the electrical transport more
broadly. Here, by examining the difference before and after irradiation, insight is gained into
the origins of the unconventional magnetotransport of PtCoO₂.
The other study uses PtCoO₂ and PdCoO₂ as test systems for the investigation of the effects
of a non-circular Fermi surface on the transport within four terminal, square-shaped junctions
inside the ballistic regime. These junction devices have been shown to be a sensitive probe
of this regime in other materials, and I will demonstrate that the nearly hexagonal Fermi
surface of the ultrapure delafossite metals results in not only strongly ballistic behaviour,
present at a scale multiple times the mean free path, but also novel phenomena which are
not seen with a circular Fermi surface.Optical micro-resonators for the study and quantification of cellular forcesDalaka, Elenihttps://hdl.handle.net/10023/233552022-12-08T11:39:42Z2020-12-01T00:00:00Z2020-12-01T00:00:00ZDalaka, EleniTitle redactedDeng, Yalihttps://hdl.handle.net/10023/233302024-03-11T11:33:09Z2021-06-28T00:00:00Z2021-06-28T00:00:00ZDeng, YaliDevelopment of continuous-wave pump-enhanced optical parametric oscillators and their application to photo-thermal spectroscopyThomas, Jackhttps://hdl.handle.net/10023/230882021-04-27T10:02:40Z2021-06-28T00:00:00ZThe development of both a spectrally tunable radiation source and a sensitive detection transducer for spectroscopic applications are described in this thesis. The emission source is an optical parametric
oscillator operating in the continuous-wave regime with a pump-enhanced singly-resonant architecture
and involves the separate development of two constituent parts: A continuous-wave pump laser
operating at 1064 nm based on neodymium-doped crystals focused on achieving high power whilst
maintaining single-frequency operation; and a split-ring pump-enhanced optical parametric oscillator
operating from this pump source focused on fine-tuning in mode-hops. This source was capable of
coarsely tuning in the mid-infrared over 3 – 4 μm with an average of 140 mW of power with the ability of
automated mode-hop tuning continuously over 90 cm⁻¹ in 0.07 cm⁻¹ steps. The detection transducer is
based upon photo-thermal interferometric spectroscopy which employs a phase-sensitive method to
detect the heating of a gas sample under radiation from the appropriately tuned source. Utilisation of a
spectrally-independent probe beam permits samples with strong absorption features in the mid/deep-infrared
to be examined whilst utilising the low-cost, high-sensitivity photodetectors in the visible/near-infrared
region. This work implements both a Mach-Zehnder and a Sagnac interferometer, where the
latter holds the potential to minimise the effects of the environment and simplify the associated
experimental steps by removing stabilising electronic circuitry. Combined utilisation of this transducer
alongside the excitation source has demonstrated an ability to detect the presence of ethane down to
100 parts-per-billion. Further development considered the interferometer inherent in photo-thermal
spectroscopy, where this provides an avenue for employing optical squeezing techniques to increase the
ultimate sensitivity of this technique. Construction of a squeezed light generator to use as a probe beam
based on second-order nonlinearity has additionally been attempted in this work and has been shown to
exhibit anti-squeezing characteristic behaviour.
2021-06-28T00:00:00ZThomas, JackThe development of both a spectrally tunable radiation source and a sensitive detection transducer for spectroscopic applications are described in this thesis. The emission source is an optical parametric
oscillator operating in the continuous-wave regime with a pump-enhanced singly-resonant architecture
and involves the separate development of two constituent parts: A continuous-wave pump laser
operating at 1064 nm based on neodymium-doped crystals focused on achieving high power whilst
maintaining single-frequency operation; and a split-ring pump-enhanced optical parametric oscillator
operating from this pump source focused on fine-tuning in mode-hops. This source was capable of
coarsely tuning in the mid-infrared over 3 – 4 μm with an average of 140 mW of power with the ability of
automated mode-hop tuning continuously over 90 cm⁻¹ in 0.07 cm⁻¹ steps. The detection transducer is
based upon photo-thermal interferometric spectroscopy which employs a phase-sensitive method to
detect the heating of a gas sample under radiation from the appropriately tuned source. Utilisation of a
spectrally-independent probe beam permits samples with strong absorption features in the mid/deep-infrared
to be examined whilst utilising the low-cost, high-sensitivity photodetectors in the visible/near-infrared
region. This work implements both a Mach-Zehnder and a Sagnac interferometer, where the
latter holds the potential to minimise the effects of the environment and simplify the associated
experimental steps by removing stabilising electronic circuitry. Combined utilisation of this transducer
alongside the excitation source has demonstrated an ability to detect the presence of ethane down to
100 parts-per-billion. Further development considered the interferometer inherent in photo-thermal
spectroscopy, where this provides an avenue for employing optical squeezing techniques to increase the
ultimate sensitivity of this technique. Construction of a squeezed light generator to use as a probe beam
based on second-order nonlinearity has additionally been attempted in this work and has been shown to
exhibit anti-squeezing characteristic behaviour.Bridging the gap : stellar and gas kinematics from integral field spectroscopy and cosmological simulationsDuckworth, Christopherhttps://hdl.handle.net/10023/230782021-10-13T11:02:16Z2021-06-21T00:00:00ZWe are now in an era of spatially resolved spectroscopic observations for thousands of galaxies. Labelled integral field spectroscopy, a natural product is detailed kinematics of both stars and gas enabling accurate estimators of a galaxy's angular momentum, and hence, kinematic classifications. This presents a significant step forward in how we understand and classify galaxies, previously being reliant on their visual form; morphology. In tandem, significant progress has also been made in theory with realistic, cosmological simulations becoming a reality. These high resolution simulations, coupled with hydrodynamics, have produced populations of thousands of galaxies; replicating a wide variety of properties of actual galaxies in our observable Universe. Both will be crucial in further understanding how galaxies evolve to what we find in the local Universe.
This thesis uses the natural synergy between integral field spectroscopy and cosmological hydrodynamical simulations, to help understand the angular momentum content of galaxies in the local Universe. We investigate the relationship between thousands of galaxies' stellar and gas kinematics using both real and mock observations, to better understand the reliability of such simulations, and, how they can intuit their evolutionary histories.
In this thesis, we investigate how the rotational direction of stars and gas can become decoupled (kinematic misalignment) and how this is related to the galaxy's morphology and the angular momentum content of the surrounding dark matter halo (Chapter 2). We also demonstrate the relationship between kinematic misalignment and stellar angular momentum, and how both are impacted by the role of feedback from supermassive black holes (Chapter 3), and, their large-scale environment. In particular, we investigate how the magnitude and direction of angular momentum in galaxies is modulated by their position within the cosmic web (Chapter 4), and, how kinematic misalignment is related to the assembly history of their dark matter halo (Chapter 5). Finally this thesis investigates the potential to recover the dynamics of dark matter within different large-scale environments (Chapter 6), motivating a novel application of the axisymmetric Jean's equations using whole galaxies as tracers. This work helps lay the foundation for using integral field spectroscopy and cosmological simulations, in tandem, as a powerful tool in building a holistic picture of galaxy evolution.
2021-06-21T00:00:00ZDuckworth, ChristopherWe are now in an era of spatially resolved spectroscopic observations for thousands of galaxies. Labelled integral field spectroscopy, a natural product is detailed kinematics of both stars and gas enabling accurate estimators of a galaxy's angular momentum, and hence, kinematic classifications. This presents a significant step forward in how we understand and classify galaxies, previously being reliant on their visual form; morphology. In tandem, significant progress has also been made in theory with realistic, cosmological simulations becoming a reality. These high resolution simulations, coupled with hydrodynamics, have produced populations of thousands of galaxies; replicating a wide variety of properties of actual galaxies in our observable Universe. Both will be crucial in further understanding how galaxies evolve to what we find in the local Universe.
This thesis uses the natural synergy between integral field spectroscopy and cosmological hydrodynamical simulations, to help understand the angular momentum content of galaxies in the local Universe. We investigate the relationship between thousands of galaxies' stellar and gas kinematics using both real and mock observations, to better understand the reliability of such simulations, and, how they can intuit their evolutionary histories.
In this thesis, we investigate how the rotational direction of stars and gas can become decoupled (kinematic misalignment) and how this is related to the galaxy's morphology and the angular momentum content of the surrounding dark matter halo (Chapter 2). We also demonstrate the relationship between kinematic misalignment and stellar angular momentum, and how both are impacted by the role of feedback from supermassive black holes (Chapter 3), and, their large-scale environment. In particular, we investigate how the magnitude and direction of angular momentum in galaxies is modulated by their position within the cosmic web (Chapter 4), and, how kinematic misalignment is related to the assembly history of their dark matter halo (Chapter 5). Finally this thesis investigates the potential to recover the dynamics of dark matter within different large-scale environments (Chapter 6), motivating a novel application of the axisymmetric Jean's equations using whole galaxies as tracers. This work helps lay the foundation for using integral field spectroscopy and cosmological simulations, in tandem, as a powerful tool in building a holistic picture of galaxy evolution.Semiconductor diode lasers for picosecond pulse generationHughes, David Masonhttps://hdl.handle.net/10023/220402022-04-27T14:58:30Z2002-01-01T00:00:00ZThis thesis describes ultrashort pulse production techniques for semiconductor diode lasers. Three methodologies have been considered: active modelocking, gainswitching and forced Q-switching. The advantages and disadvantages of each technique have been examined. Theoretical assessments of the physical processes governing each procedure were outlined as a basis for the construction of simple yet powerful and adaptable computer simulations. These models were used to predict the temporal behavior of a range of semiconductor devices under broad range of operating conditions.
An actively modelocked external-cavity InGaAsP device was used to implement a novel dual-wavelength laser. Continuous wavelength tuning was demonstrated over a 0.5 - 55nm range with the possibility of multiple wavelength operation.
Multiple-pulse lasers featured several composite external cavity geometries. Pulse frequencies within a tuning range of 6 - 23GHz were generated using this novel modelocked laser system.
The temporal intensity performance of gain-switched DFB and Fabry-Perot laser was assessed and 20 -30ps single feature optical pulses were routinely generated by direct RF modulation of the laser gain. The theoretical model generated optical intensity profiles showing very good agreement with the experimental results Forced Q-switching was implemented for a series of multiple-contact InGaAsP and GaAs diode lasers. Output optical powers were enhanced by the saturable absorption provided by the multi-contact geometry and peak powers of up to lOOmW were demonstrated for symmetrical pulses of-15 - 25ps duration. Pulse timing jitter was considered and distinction made between correlated and uncorrelated timing jitter. Jitter performance was assessed for modelocked, gainswitched and Q-switched regimes. Timing jitter in modelocked lasers was found to be largely correlated (~150fs). Subpicosecond (800fs) jitter was measured at a modulation rate of 2GHz for forced Q-switched multi-contact InGaAsP devices.
2002-01-01T00:00:00ZHughes, David MasonThis thesis describes ultrashort pulse production techniques for semiconductor diode lasers. Three methodologies have been considered: active modelocking, gainswitching and forced Q-switching. The advantages and disadvantages of each technique have been examined. Theoretical assessments of the physical processes governing each procedure were outlined as a basis for the construction of simple yet powerful and adaptable computer simulations. These models were used to predict the temporal behavior of a range of semiconductor devices under broad range of operating conditions.
An actively modelocked external-cavity InGaAsP device was used to implement a novel dual-wavelength laser. Continuous wavelength tuning was demonstrated over a 0.5 - 55nm range with the possibility of multiple wavelength operation.
Multiple-pulse lasers featured several composite external cavity geometries. Pulse frequencies within a tuning range of 6 - 23GHz were generated using this novel modelocked laser system.
The temporal intensity performance of gain-switched DFB and Fabry-Perot laser was assessed and 20 -30ps single feature optical pulses were routinely generated by direct RF modulation of the laser gain. The theoretical model generated optical intensity profiles showing very good agreement with the experimental results Forced Q-switching was implemented for a series of multiple-contact InGaAsP and GaAs diode lasers. Output optical powers were enhanced by the saturable absorption provided by the multi-contact geometry and peak powers of up to lOOmW were demonstrated for symmetrical pulses of-15 - 25ps duration. Pulse timing jitter was considered and distinction made between correlated and uncorrelated timing jitter. Jitter performance was assessed for modelocked, gainswitched and Q-switched regimes. Timing jitter in modelocked lasers was found to be largely correlated (~150fs). Subpicosecond (800fs) jitter was measured at a modulation rate of 2GHz for forced Q-switched multi-contact InGaAsP devices.Quantum interference effects : electromagnetically induced transparency and focusingFulton, David Jameshttps://hdl.handle.net/10023/220352022-04-15T10:59:48Z1997-01-01T00:00:00ZTheoretical and experimental studies of steady state quantum interference effects within an atomic vapour have been carried out. These include the effects of Electromagnetically Induced Transparency (E.I.T.), Electromagnetically Induced Focusing (E.I.F.) and inversionless lasing. Theoretical analysis was carried out using semi-classical density matrix models while experiments employed continuous-wave Ti:sapphire or Dye lasers within atomic rubidium.
An initial study of the properties of E.I.T. was carried out within Doppler broadened rubidium. Coherently induced transparencies were generated in three basic systems: V-type, lambda and cascade. This allowed a theoretical and experimental comparison between the three systems to be made.
The 5S1/2 - 5P3/2 - 5D5/2 cascade system proved the simplest in which to observe E.I.T., providing windows with depths > 90%. Thus, a fundamental study of E.I.T. within this system was carried out. This included the effect of varying optical field parameters and of Zeeman splitting. Highlighted by this analysis was the fact that E.I.T. can provide novel high resolution two-photon spectroscopy techniques.
A unique non-dissipative cross focusing effect on the probe field was observed when the spatial profile's of the optical fields were considered. This was a direct result of the coherent interaction of the coupling field with the rubidium atoms and so was named Electromagnetically Induced Focusing. E.I.F. was shown, both theoretically and experimentally, to vary dramatically with changes in the system's physical parameters.
An examination of the effects of varying the field wavelength's within all three systems was theoretically carried out. This analysis showed that a Doppler broadened medium does not require matched wavelength optical fields in order to observe coherently induced transparency. It also highlighted the relative roles of E.I.T. and Autler-Townes splitting within an induced transparency.
A final study was carried out which theoretically predicts the presence of blue inversionless gain in a system which employed I.R. driving fields within atomic rubidium.
1997-01-01T00:00:00ZFulton, David JamesTheoretical and experimental studies of steady state quantum interference effects within an atomic vapour have been carried out. These include the effects of Electromagnetically Induced Transparency (E.I.T.), Electromagnetically Induced Focusing (E.I.F.) and inversionless lasing. Theoretical analysis was carried out using semi-classical density matrix models while experiments employed continuous-wave Ti:sapphire or Dye lasers within atomic rubidium.
An initial study of the properties of E.I.T. was carried out within Doppler broadened rubidium. Coherently induced transparencies were generated in three basic systems: V-type, lambda and cascade. This allowed a theoretical and experimental comparison between the three systems to be made.
The 5S1/2 - 5P3/2 - 5D5/2 cascade system proved the simplest in which to observe E.I.T., providing windows with depths > 90%. Thus, a fundamental study of E.I.T. within this system was carried out. This included the effect of varying optical field parameters and of Zeeman splitting. Highlighted by this analysis was the fact that E.I.T. can provide novel high resolution two-photon spectroscopy techniques.
A unique non-dissipative cross focusing effect on the probe field was observed when the spatial profile's of the optical fields were considered. This was a direct result of the coherent interaction of the coupling field with the rubidium atoms and so was named Electromagnetically Induced Focusing. E.I.F. was shown, both theoretically and experimentally, to vary dramatically with changes in the system's physical parameters.
An examination of the effects of varying the field wavelength's within all three systems was theoretically carried out. This analysis showed that a Doppler broadened medium does not require matched wavelength optical fields in order to observe coherently induced transparency. It also highlighted the relative roles of E.I.T. and Autler-Townes splitting within an induced transparency.
A final study was carried out which theoretically predicts the presence of blue inversionless gain in a system which employed I.R. driving fields within atomic rubidium.Supersymmetric quantum field theories from induced representationsHartley, Davidhttps://hdl.handle.net/10023/220372022-04-15T11:30:28Z1989-01-01T00:00:00ZThis thesis investigates the application of the method of induced representations in supersymmetric quantum field theories. First, the main features of the theory of induced representations of Lie groups, Lie algebras and Lie superalgebras are presented. The procedure for obtaining irreducible representations in the important case of Lie groups or algebras with an invariant, Abelian subgroup or subalgebra is described. This procedure is then applied to the Poincar6 group for arbitrary dimensions of space-time. The representations obtained are used to construct free quantum fields, without the use of a Lagrangian. The usual characteristics of these fields, such as the field equations, are shown to be consequences of the representation theory. The induced representation procedure for algebras is demonstrated by the construction of the irreducible supermultiplets for the N = 1 Poincaré superalgebra (in those dimensions for which it exists). Again, the construction proceeds from the representations, not from a Lagrangian. Finally, a mixture of the group and algebra versions of the procedure developed in the preceding parts of the thesis is applied to the inhomogeneous orthosymplectic superalgebra. This superalgebra is relevant to BRST covariant quantisation of gauge fields. A consequence of the systematic application of the representation theory is the derivation of the Parisi-Sourlas mechanism in pseudo-Euclidean space.
1989-01-01T00:00:00ZHartley, DavidThis thesis investigates the application of the method of induced representations in supersymmetric quantum field theories. First, the main features of the theory of induced representations of Lie groups, Lie algebras and Lie superalgebras are presented. The procedure for obtaining irreducible representations in the important case of Lie groups or algebras with an invariant, Abelian subgroup or subalgebra is described. This procedure is then applied to the Poincar6 group for arbitrary dimensions of space-time. The representations obtained are used to construct free quantum fields, without the use of a Lagrangian. The usual characteristics of these fields, such as the field equations, are shown to be consequences of the representation theory. The induced representation procedure for algebras is demonstrated by the construction of the irreducible supermultiplets for the N = 1 Poincaré superalgebra (in those dimensions for which it exists). Again, the construction proceeds from the representations, not from a Lagrangian. Finally, a mixture of the group and algebra versions of the procedure developed in the preceding parts of the thesis is applied to the inhomogeneous orthosymplectic superalgebra. This superalgebra is relevant to BRST covariant quantisation of gauge fields. A consequence of the systematic application of the representation theory is the derivation of the Parisi-Sourlas mechanism in pseudo-Euclidean space.Theoretical treatments of phase transitions in systems of polyatomic moleculesBuchanan, David Johnhttps://hdl.handle.net/10023/220322022-04-15T08:29:11Z1971-01-01T00:00:00ZAs is well known, the microscopic and macroscopic properties of a system of molecules may he linked by means of the canonical partition function of statistical mechanics. In principle, a knowledge of the intermolecular forces is sufficient for us to predict the thermodynamic properties but, in practice, the partition function can be evaluated exactly only for some particularly simple systems. For more realistic systems we must resort to approximation methods. Even for monatomic molecules, interacting through a spherically symmetric pair potential, the application of approximation methods can be daunting and consequently little work has been done which takes account of the general polyatomic nature of molecules. In this thesis we attempt to rectify this omission in three different ways. In particular, we shall be concerned with the problem of melting in systems of polyatomic molecules.
The first method is an extension of a theory of melting due to Lennard-Jones and Devonshire and is an attempt at a microscopic treatment of a system where the barriers to molecular re-orientation are of a particularly simple nature. Similar models have been considered by Pople and Karasz and also by Amzel and Becka and our model reduces to these authors' models under appropriate conditions. Secondly we have extended a theory of melting due to Tsuzuki to cover systems of diatomic molecules. This method is semiphenomenological in that a mean field, in which each molecule is situated, is assumed. The results of these two methods are in reasonable agreement with experiment but only after freely adjusting a single parameter in each case. In an attempt to avoid this arbitrariness, the final method is a completely microscopic theory of systems of diatomic and monatomic molecules interacting through a realistic potential.
This method takes advantage of the fact that the Helmholtz free energy can be evaluated exactly for some particularly simple systems and uses these calculations to estimate the free energy of the original system by means of a variational' principle. No freely adjustable parameters arise in this theory and, when this is taken into account, the results for monatomic molecules are extremely good but for diatomic molecules the agreement with experiment is poor, this being due to inaccurate determination of the intermolecular parameters. Finally, we have derived a complementary variational principle and made some suggestions for future work in this field.
1971-01-01T00:00:00ZBuchanan, David JohnAs is well known, the microscopic and macroscopic properties of a system of molecules may he linked by means of the canonical partition function of statistical mechanics. In principle, a knowledge of the intermolecular forces is sufficient for us to predict the thermodynamic properties but, in practice, the partition function can be evaluated exactly only for some particularly simple systems. For more realistic systems we must resort to approximation methods. Even for monatomic molecules, interacting through a spherically symmetric pair potential, the application of approximation methods can be daunting and consequently little work has been done which takes account of the general polyatomic nature of molecules. In this thesis we attempt to rectify this omission in three different ways. In particular, we shall be concerned with the problem of melting in systems of polyatomic molecules.
The first method is an extension of a theory of melting due to Lennard-Jones and Devonshire and is an attempt at a microscopic treatment of a system where the barriers to molecular re-orientation are of a particularly simple nature. Similar models have been considered by Pople and Karasz and also by Amzel and Becka and our model reduces to these authors' models under appropriate conditions. Secondly we have extended a theory of melting due to Tsuzuki to cover systems of diatomic molecules. This method is semiphenomenological in that a mean field, in which each molecule is situated, is assumed. The results of these two methods are in reasonable agreement with experiment but only after freely adjusting a single parameter in each case. In an attempt to avoid this arbitrariness, the final method is a completely microscopic theory of systems of diatomic and monatomic molecules interacting through a realistic potential.
This method takes advantage of the fact that the Helmholtz free energy can be evaluated exactly for some particularly simple systems and uses these calculations to estimate the free energy of the original system by means of a variational' principle. No freely adjustable parameters arise in this theory and, when this is taken into account, the results for monatomic molecules are extremely good but for diatomic molecules the agreement with experiment is poor, this being due to inaccurate determination of the intermolecular parameters. Finally, we have derived a complementary variational principle and made some suggestions for future work in this field.Static Fourier transform spectrometers and laser wavelength metersSteers, Darrenhttps://hdl.handle.net/10023/220272022-04-14T08:03:56Z1999-01-01T00:00:00ZThis thesis reports the development of novel static Fourier transform spectrometers based upon Wallaston prisms and compact detector arrays. The path difference between orthogonal polarisation states of the input light varies smoothly across the aperture of the prism forming an interferogram in the spatial, rather than temporal, domain that is recorded with the detector array. A Fourier trans¬ form of this interferogram gives the spectral distribution of the incident light. The elimination of moving parts from the design makes the recorded interferogram inherently stable.
I have developed an improved spectrometer utilising Wallaston prisms fabricated from materials of opposite sign birefringence. This new instrument has a significantly increased field of view compared with previous Wollaston prism based spectrometers.
Additionally my work has involved the construction and evaluation of spectrometers for operation in the ultraviolet, visible and infrared regions of the spectrum. These spectrometers have been applied to the analysis of gasolines. Important properties such as gasoline brand and octane number have been identified from ultraviolet and near-infrared spectra using principal component analysis. Finally, I adapted the spectrometer design to make a fibre-coupled laser wavelength meter based on a modified Wollaston prism. For a narrow linewidth source a fringe period measurement technique, used as an alternative to the Fourier transform algorithm, obtains precision measurement (1 part in 10^6) of the centre wavelength. The wavelength meter and spectrometers have numerous applications in the held of general laboratory instrumentation and their robust, compact nature makes them particularly suitable where held based operation is required.
1999-01-01T00:00:00ZSteers, DarrenThis thesis reports the development of novel static Fourier transform spectrometers based upon Wallaston prisms and compact detector arrays. The path difference between orthogonal polarisation states of the input light varies smoothly across the aperture of the prism forming an interferogram in the spatial, rather than temporal, domain that is recorded with the detector array. A Fourier trans¬ form of this interferogram gives the spectral distribution of the incident light. The elimination of moving parts from the design makes the recorded interferogram inherently stable.
I have developed an improved spectrometer utilising Wallaston prisms fabricated from materials of opposite sign birefringence. This new instrument has a significantly increased field of view compared with previous Wollaston prism based spectrometers.
Additionally my work has involved the construction and evaluation of spectrometers for operation in the ultraviolet, visible and infrared regions of the spectrum. These spectrometers have been applied to the analysis of gasolines. Important properties such as gasoline brand and octane number have been identified from ultraviolet and near-infrared spectra using principal component analysis. Finally, I adapted the spectrometer design to make a fibre-coupled laser wavelength meter based on a modified Wollaston prism. For a narrow linewidth source a fringe period measurement technique, used as an alternative to the Fourier transform algorithm, obtains precision measurement (1 part in 10^6) of the centre wavelength. The wavelength meter and spectrometers have numerous applications in the held of general laboratory instrumentation and their robust, compact nature makes them particularly suitable where held based operation is required.Spiral waves in accretion discsSteeghs, Daniel Theodore Huberthttps://hdl.handle.net/10023/220232022-04-12T15:57:00Z2000-01-01T00:00:00ZThis thesis discusses observations of the interacting binary IP Pegasi and shows that its accretion disc carries strong, tidally driven spiral arms during outburst. The distribution of line emission across the accretion disc is resolved using Doppler tomography and reveals a two armed spiral pattern in a variety of emission lines. The spirals persist as a strong corotating pattern throughout the outburst maximum for at least 8 days, while the disc radius decreases exponentially. During quiescence, the disc lacks such open spirals but remains non-axisymmetric. Hydrodynamical simulations are used to calculate the properties of thin accretion discs carrying tidal density waves. Predicted emission line properties and simulated image reconstructions reproduce the observations in detail and confirm that the spiral structure observed in IP Pegasi is a tidally driven phenomenon. The IP Pegasi observations provide the first convincing evidence for tidally driven spirals that were proposed over 13 years ago as a possible source of angular momentum transport in accretion discs. The disc simulations also indicate that similar disc structure is expected in other systems carrying large discs such as dwarf novae in outburst and nova-like variables. For small, cool accretion discs, tidally driven spiral waves can be present but are too tightly wrapped to be resolved in the strong emission lines. It is clear that tidally driven waves can severely alter the global dynamics and structure of accretion discs. However, a significant amount of shear viscosity is still needed to switch from a small disc in quiesence to the large disc observed in outburst so that strong open spirals are generated. How much angular momentum can be transported via such waves in comparison to angular momentum dispersal through local viscous processes remains to be quantified by comparing realistic model calculations with present and future observations.
2000-01-01T00:00:00ZSteeghs, Daniel Theodore HubertThis thesis discusses observations of the interacting binary IP Pegasi and shows that its accretion disc carries strong, tidally driven spiral arms during outburst. The distribution of line emission across the accretion disc is resolved using Doppler tomography and reveals a two armed spiral pattern in a variety of emission lines. The spirals persist as a strong corotating pattern throughout the outburst maximum for at least 8 days, while the disc radius decreases exponentially. During quiescence, the disc lacks such open spirals but remains non-axisymmetric. Hydrodynamical simulations are used to calculate the properties of thin accretion discs carrying tidal density waves. Predicted emission line properties and simulated image reconstructions reproduce the observations in detail and confirm that the spiral structure observed in IP Pegasi is a tidally driven phenomenon. The IP Pegasi observations provide the first convincing evidence for tidally driven spirals that were proposed over 13 years ago as a possible source of angular momentum transport in accretion discs. The disc simulations also indicate that similar disc structure is expected in other systems carrying large discs such as dwarf novae in outburst and nova-like variables. For small, cool accretion discs, tidally driven spiral waves can be present but are too tightly wrapped to be resolved in the strong emission lines. It is clear that tidally driven waves can severely alter the global dynamics and structure of accretion discs. However, a significant amount of shear viscosity is still needed to switch from a small disc in quiesence to the large disc observed in outburst so that strong open spirals are generated. How much angular momentum can be transported via such waves in comparison to angular momentum dispersal through local viscous processes remains to be quantified by comparing realistic model calculations with present and future observations.Dissipation in the helium II filmMiller, Daniel B.https://hdl.handle.net/10023/220222022-04-12T15:01:10Z1980-01-01T00:00:00ZThe theories that have been presented to explain the properties of liquid helium are reviewed. The existence of vortex lines and their importance to the dissipation in helium II are emphasised leading to a summary of the theories on dissipation in helium II. Attention is paid to the thermal fluctuation theories and to the resulting radical shift of emphasis on the idea of a critical superfluid velocity. The experimental results of measurements in narrow channels are abstracted and it is shown that support for the theories on vortex line equilibrium has been found and that the ideas from the thermal fluctuation theories can be validated. The properties of the helium II film are summarized; observed changes in flow rates, inertial oscillations occurring via film flow and the application of dissipation theories to film flow are discussed. The experimental system is described. The film flow monitored was initiated through the initial run-in to equilibrium and through changing the chemical potential difference across the experimental beaker by applying a time-dependent voltage across one of the two capacitors that formed the liquid helium reservoirs. A detailed analysis of the accuracy of the experimental results is given.
The measured inertial oscillations were found to be in agreement with theoretical predictions. The flow rates were dominated by flow steps. No pattern in these steps was evident; there was a peak in their distribution at a superfluid velocity of ~25 cm sec⁻¹ and no evidence for either a critical superfluid velocity or forbidden flow rates. No evidence was found to support dissipative functions of the forms dv/dt α f exp(-vb/v) or dv/dt α (v-vₒ)^3/2. Restricting the data to those when the velocity was a monotonic function of the driving force, supported Langer and Reppy’s proposal that vortex lines parallel to the free surface are the dissipative mechanism in helium II film flow.
It is suggested that the very small volume of the annular cylinder formed by the helium film in the region of the beaker where dissipation was dominant may account for the difference between these results and those reported by other researchers.
1980-01-01T00:00:00ZMiller, Daniel B.The theories that have been presented to explain the properties of liquid helium are reviewed. The existence of vortex lines and their importance to the dissipation in helium II are emphasised leading to a summary of the theories on dissipation in helium II. Attention is paid to the thermal fluctuation theories and to the resulting radical shift of emphasis on the idea of a critical superfluid velocity. The experimental results of measurements in narrow channels are abstracted and it is shown that support for the theories on vortex line equilibrium has been found and that the ideas from the thermal fluctuation theories can be validated. The properties of the helium II film are summarized; observed changes in flow rates, inertial oscillations occurring via film flow and the application of dissipation theories to film flow are discussed. The experimental system is described. The film flow monitored was initiated through the initial run-in to equilibrium and through changing the chemical potential difference across the experimental beaker by applying a time-dependent voltage across one of the two capacitors that formed the liquid helium reservoirs. A detailed analysis of the accuracy of the experimental results is given.
The measured inertial oscillations were found to be in agreement with theoretical predictions. The flow rates were dominated by flow steps. No pattern in these steps was evident; there was a peak in their distribution at a superfluid velocity of ~25 cm sec⁻¹ and no evidence for either a critical superfluid velocity or forbidden flow rates. No evidence was found to support dissipative functions of the forms dv/dt α f exp(-vb/v) or dv/dt α (v-vₒ)^3/2. Restricting the data to those when the velocity was a monotonic function of the driving force, supported Langer and Reppy’s proposal that vortex lines parallel to the free surface are the dissipative mechanism in helium II film flow.
It is suggested that the very small volume of the annular cylinder formed by the helium film in the region of the beaker where dissipation was dominant may account for the difference between these results and those reported by other researchers.Continuous-wave optical parametric oscillators pumped by external cavity diode lasersPetridis, Constantinoshttps://hdl.handle.net/10023/220112022-03-28T15:23:20Z2002-01-01T00:00:00ZIn this thesis, high spatial, high spectral quality, ultra stable external cavity diode laser (ECDL) sources and continuous-wave optical parametric oscillators (cw OPOs) are designed, constructed and investigated. The ECDL device that was constructed had an output power of 80 mW, spectral linewidth less than 1 MHz, coarse tuning range of 18 nm, up to 6 GHz continuous mode-hop free tuning, and excellent amplitude and spectral stability, making it suitable to pump various cw OPO systems.
The construction and characterisation of a new and simple electronic circuit is also reported that allowed the ECDL spectrum to be scanned continuously over 80 GHz. This thesis reports the novel direct pump of a doubly resonant (DRO) and a pump enhanced (PE-SRO) continuous wave optical parametric oscillators based on periodically-poled lithium niobate (PPLN) by an ECDL.
The DRO device, based in a 19 mm PPLN crystal, showed a threshold of 15.5 mW and tuning outputs over 1.15-1.25 μm at the signal and 2.3-2.65 μm at the idler. Locked single mode pair operation was sustained for periods of more than 5 minutes.
The PE-SRO was based on a 50 mm long crystal of PPLN with eight gratings equally spaced in period from A=21.0 to 22.4 μm. The device showed a threshold equal to 25- 30 mW. The tuning range at the signal was 1.06-1.19 μm, corresponding to a range of 2.58-3.44 μm at the idler while the pump source was fixed at 812.5 nm.. The ECDL pumped PE-SRO output was locked, using the Pound Drever technique, and showed mode-hop free operation for more than an hour.
2002-01-01T00:00:00ZPetridis, ConstantinosIn this thesis, high spatial, high spectral quality, ultra stable external cavity diode laser (ECDL) sources and continuous-wave optical parametric oscillators (cw OPOs) are designed, constructed and investigated. The ECDL device that was constructed had an output power of 80 mW, spectral linewidth less than 1 MHz, coarse tuning range of 18 nm, up to 6 GHz continuous mode-hop free tuning, and excellent amplitude and spectral stability, making it suitable to pump various cw OPO systems.
The construction and characterisation of a new and simple electronic circuit is also reported that allowed the ECDL spectrum to be scanned continuously over 80 GHz. This thesis reports the novel direct pump of a doubly resonant (DRO) and a pump enhanced (PE-SRO) continuous wave optical parametric oscillators based on periodically-poled lithium niobate (PPLN) by an ECDL.
The DRO device, based in a 19 mm PPLN crystal, showed a threshold of 15.5 mW and tuning outputs over 1.15-1.25 μm at the signal and 2.3-2.65 μm at the idler. Locked single mode pair operation was sustained for periods of more than 5 minutes.
The PE-SRO was based on a 50 mm long crystal of PPLN with eight gratings equally spaced in period from A=21.0 to 22.4 μm. The device showed a threshold equal to 25- 30 mW. The tuning range at the signal was 1.06-1.19 μm, corresponding to a range of 2.58-3.44 μm at the idler while the pump source was fixed at 812.5 nm.. The ECDL pumped PE-SRO output was locked, using the Pound Drever technique, and showed mode-hop free operation for more than an hour.Superselection rules, quantisation by parts and point interactionsTrueman, Colinhttps://hdl.handle.net/10023/220092022-03-24T09:36:13Z2000-01-01T00:00:00ZThis thesis is concerned with the origination and development of mathematical tools, formalisms and idealised physical models, for the analysis of superselection rules and point interactions in quantum theory. We begin by presenting a unified Hilbert space formalism, capable of describing classical, standard quantum and mixed quantum systems in Hilbert space. Superselection rules enable us to construct this unified theory. Incorporating classical systems into a Hilbert space formalism improves our resolution of the boundary between the classical and the quantum, and enables us to compare and contrast a variety of classical, quantum and mixed quantum systems. This comparison yields a new definition of classical systems, as we find the compatibility of a set of observables is no longer a sufficient condition to define a classical system. We find that we can distinguish the three types of systems if we move to discuss their dynamical behaviour. An interesting consequence of casting classical systems into Hilbert space is that we are forced to make a distinction between the operator which generates the dynamics for the system, and the Hamiltonian. We are compelled to make this distinction because we find that the operator which generates the dynamics is not an observable of the system. We also discuss equilibrium and inequilibrium systems and the possibilities of including environmental effects in the formalism. r We develop a novel and rigorous approach to deal with single point interactions in one dimension. We combine the disciplines of selfadjoint extension theory and transfer matrix analysis, to produce a strong analytical quantisation tool. This tool provides a catagorisation of a wide spectrum of possible point interactions in one-dimension. We show that the behaviour of a system with only one single point interaction can be extremely diverse. In particular, new possible behaviours are predicted. Superconducting systems and their inherent superselection rules, can also be admirably handled by this approach. We then move on to analyse a variety of interesting geometries with this method, including quantum dot systems. We find that our new quantisation tool again predicts new behaviour. We present a path space approach to quantum theory, which can be seen to be significantly more intuitive than the standard approach in some cases. The example presented is a ring containing a single point interaction. In this case the path space approach provides an interpretation of flow around the ring and a natural choice for the momentum operator. The path space approach also inherently possesses the required superselection rule when we consider a superconducting system with this geometry. These insights are not available in the traditional approach. Finally we present a discussion of the five distinct origins of superselective behaviour evident in this thesis, and draw conclusions as to whether a definitive origin of this phenomenon may be found in quantum theory.
2000-01-01T00:00:00ZTrueman, ColinThis thesis is concerned with the origination and development of mathematical tools, formalisms and idealised physical models, for the analysis of superselection rules and point interactions in quantum theory. We begin by presenting a unified Hilbert space formalism, capable of describing classical, standard quantum and mixed quantum systems in Hilbert space. Superselection rules enable us to construct this unified theory. Incorporating classical systems into a Hilbert space formalism improves our resolution of the boundary between the classical and the quantum, and enables us to compare and contrast a variety of classical, quantum and mixed quantum systems. This comparison yields a new definition of classical systems, as we find the compatibility of a set of observables is no longer a sufficient condition to define a classical system. We find that we can distinguish the three types of systems if we move to discuss their dynamical behaviour. An interesting consequence of casting classical systems into Hilbert space is that we are forced to make a distinction between the operator which generates the dynamics for the system, and the Hamiltonian. We are compelled to make this distinction because we find that the operator which generates the dynamics is not an observable of the system. We also discuss equilibrium and inequilibrium systems and the possibilities of including environmental effects in the formalism. r We develop a novel and rigorous approach to deal with single point interactions in one dimension. We combine the disciplines of selfadjoint extension theory and transfer matrix analysis, to produce a strong analytical quantisation tool. This tool provides a catagorisation of a wide spectrum of possible point interactions in one-dimension. We show that the behaviour of a system with only one single point interaction can be extremely diverse. In particular, new possible behaviours are predicted. Superconducting systems and their inherent superselection rules, can also be admirably handled by this approach. We then move on to analyse a variety of interesting geometries with this method, including quantum dot systems. We find that our new quantisation tool again predicts new behaviour. We present a path space approach to quantum theory, which can be seen to be significantly more intuitive than the standard approach in some cases. The example presented is a ring containing a single point interaction. In this case the path space approach provides an interpretation of flow around the ring and a natural choice for the momentum operator. The path space approach also inherently possesses the required superselection rule when we consider a superconducting system with this geometry. These insights are not available in the traditional approach. Finally we present a discussion of the five distinct origins of superselective behaviour evident in this thesis, and draw conclusions as to whether a definitive origin of this phenomenon may be found in quantum theory.A paramagnetic resonance spectrometer for the investigation of free radical species in solids at low temperaturesTaylor, Claude Brechinhttps://hdl.handle.net/10023/219982022-03-21T10:24:56Z1961-01-01T00:00:00Z1961-01-01T00:00:00ZTaylor, Claude BrechinHelium II film transferMatheson, Christopher Cameronhttps://hdl.handle.net/10023/219882022-03-09T12:42:20Z1966-01-01T00:00:00ZHelium, the lightest of the noble gases, is the most difficult of all to liquify. Despite this, since the first liquefaction of helium by Kamerlingh Onnes (1908) the properties of the liquid have been extensively investigated. The continued interest in the liquid after almost six decades is due to its unique properties, the most striking is the superfluidity exhibited below 2.19°k. one of the most clear-cut examples of superfluid flow is that of the saturated helium film which is formed on all solid surfaces in contact with the liquid. Although fluid transfer by the film has been the subject of a great many experimental and theoretical scrutinies, many aspects of the phenomenon have remained enigmatic. In particular, the variability of the rate of superfluid transfer through the film under various experimental conditions has never been satisfactorily explained. The present study was undertaken to clarify this situation, and to provide information on which to base a hydrodynamic theory of the film. Part of the research has already been reported (Matheson and Tilley, 1965; Allen and Matheson, 1965).
Both a knowledge of the salient properties of liquid helium and an understanding of the various theories of superfluidity are necessary of the idiosyncrasies of the helium film are to be resolved. It would seem appropriate, therefore, to review these particular facets of the helium problem so that the ensuing investigation of the film can be seen in its proper perspective.
1966-01-01T00:00:00ZMatheson, Christopher CameronHelium, the lightest of the noble gases, is the most difficult of all to liquify. Despite this, since the first liquefaction of helium by Kamerlingh Onnes (1908) the properties of the liquid have been extensively investigated. The continued interest in the liquid after almost six decades is due to its unique properties, the most striking is the superfluidity exhibited below 2.19°k. one of the most clear-cut examples of superfluid flow is that of the saturated helium film which is formed on all solid surfaces in contact with the liquid. Although fluid transfer by the film has been the subject of a great many experimental and theoretical scrutinies, many aspects of the phenomenon have remained enigmatic. In particular, the variability of the rate of superfluid transfer through the film under various experimental conditions has never been satisfactorily explained. The present study was undertaken to clarify this situation, and to provide information on which to base a hydrodynamic theory of the film. Part of the research has already been reported (Matheson and Tilley, 1965; Allen and Matheson, 1965).
Both a knowledge of the salient properties of liquid helium and an understanding of the various theories of superfluidity are necessary of the idiosyncrasies of the helium film are to be resolved. It would seem appropriate, therefore, to review these particular facets of the helium problem so that the ensuing investigation of the film can be seen in its proper perspective.Basic problems in the photographic photometry of nebulaeFraser, Christopher Williamhttps://hdl.handle.net/10023/219822022-03-08T15:32:50Z1965-01-01T00:00:00ZThis thesis is concerned with problems which arise in photographic surface photometry using the Cassegrain Schmidt Telescopes at the University Observatory, St Andrews. Photometric investigations are made of different types of objects with telescopes of different apertures and focal lengths, but with approximately the same focal ratios, in order to study the photometric effects in different types of surface distributions and to assess critically the effect of resolution on the photometry. Basic to the problem is the question of the photographic process with its inherent difficulties and limitations. For this reason, detailed consideration is given to those aspects of the process which are particularly relevant to the problem of extended surface photometry. The question of the effect of resolution arose naturally in the course of the investigation when it was possible to extend the observational programme from the 17-inch to the 37-inch telescope when it was commissioned for observational work. This thesis does not aim to obtain a comprehensive set of parameters to describe the surface distributions of a large number of objects, nor to make theoretical deductions from the data, as the emphasis in the work is primarily on basic problems in surface photometry and techniques of data reduction.
1965-01-01T00:00:00ZFraser, Christopher WilliamThis thesis is concerned with problems which arise in photographic surface photometry using the Cassegrain Schmidt Telescopes at the University Observatory, St Andrews. Photometric investigations are made of different types of objects with telescopes of different apertures and focal lengths, but with approximately the same focal ratios, in order to study the photometric effects in different types of surface distributions and to assess critically the effect of resolution on the photometry. Basic to the problem is the question of the photographic process with its inherent difficulties and limitations. For this reason, detailed consideration is given to those aspects of the process which are particularly relevant to the problem of extended surface photometry. The question of the effect of resolution arose naturally in the course of the investigation when it was possible to extend the observational programme from the 17-inch to the 37-inch telescope when it was commissioned for observational work. This thesis does not aim to obtain a comprehensive set of parameters to describe the surface distributions of a large number of objects, nor to make theoretical deductions from the data, as the emphasis in the work is primarily on basic problems in surface photometry and techniques of data reduction.The helium filmWalker, Cecilia Maryhttps://hdl.handle.net/10023/219612022-03-01T11:27:45Z1962-01-01T00:00:00Z1962-01-01T00:00:00ZWalker, Cecilia MaryNeutron and muon studies of the vortex lattice in anisotropic superconductorsAger, Cameronhttps://hdl.handle.net/10023/219422022-02-24T09:50:18Z2000-01-01T00:00:00Z2000-01-01T00:00:00ZAger, CameronA study of the dissipation which accompanies superfluid flow in liquid helium filmsSaunders, Brian Leonardhttps://hdl.handle.net/10023/219362022-02-23T15:43:17Z1974-01-01T00:00:00ZThe damping of the inertial oscillations of a flowing helium film was found to be well accounted for by a theory put forward by Robinson. In particular this theory predicts that the damping constant should reach a maximum value at a temperature below the lambda point and should decrease at higher temperatures. This behaviour was observed and the values of the damping constant were in good agreement with theory.
The mechanism suggested by Robinson was that the energy of the flowing liquid could be lost in irreversible heat exchange between the beaker and the reservoir causing the decay of the oscillations. It has been shown that, in the case considered here, the heat exchange was dominated by distillation and that this process was well described by the evaporation equation given by Atkins if due allowance was made for the shape of the helium surface.
Earlier attempts by other workers have not found good agreement with Robinson, nor has the maximum in the damping been previously observed. (Hoffer et al have reported the observation of the maximum in experiments carried out independently, at the same time as the work reported here.) The improvement in agreement may have been brought about by the elimination of vibration by the extensive anti-vibration precautions included in the design of the cryostat.
The dissipation accompanying the flow of superfluid at velocities driven by level differences greater than about 30um, that is at velocities greater than those encountered during the inertial oscillations, was measured and compared with two models based on the production of vorticity from thermal fluctuations. The form of the dependence of the flow rate on level difference was found to be similar to that predicted by each of the models but numerical agreement with the characteristic parameters of the theories was poor. In particular the attempt frequency was found to exhibit a dramatic temperature dependence, changing by ten orders of magnitude for a change of the temperature of about 0.5 K! It was shown that mutual annihilation of vorticity could account for this large variation.
If the flow rate is interpreted in terms of these models, then the flow is found to be never strictly frictionless and therefore, a true "critical velocity" was not observed.
1974-01-01T00:00:00ZSaunders, Brian LeonardThe damping of the inertial oscillations of a flowing helium film was found to be well accounted for by a theory put forward by Robinson. In particular this theory predicts that the damping constant should reach a maximum value at a temperature below the lambda point and should decrease at higher temperatures. This behaviour was observed and the values of the damping constant were in good agreement with theory.
The mechanism suggested by Robinson was that the energy of the flowing liquid could be lost in irreversible heat exchange between the beaker and the reservoir causing the decay of the oscillations. It has been shown that, in the case considered here, the heat exchange was dominated by distillation and that this process was well described by the evaporation equation given by Atkins if due allowance was made for the shape of the helium surface.
Earlier attempts by other workers have not found good agreement with Robinson, nor has the maximum in the damping been previously observed. (Hoffer et al have reported the observation of the maximum in experiments carried out independently, at the same time as the work reported here.) The improvement in agreement may have been brought about by the elimination of vibration by the extensive anti-vibration precautions included in the design of the cryostat.
The dissipation accompanying the flow of superfluid at velocities driven by level differences greater than about 30um, that is at velocities greater than those encountered during the inertial oscillations, was measured and compared with two models based on the production of vorticity from thermal fluctuations. The form of the dependence of the flow rate on level difference was found to be similar to that predicted by each of the models but numerical agreement with the characteristic parameters of the theories was poor. In particular the attempt frequency was found to exhibit a dramatic temperature dependence, changing by ten orders of magnitude for a change of the temperature of about 0.5 K! It was shown that mutual annihilation of vorticity could account for this large variation.
If the flow rate is interpreted in terms of these models, then the flow is found to be never strictly frictionless and therefore, a true "critical velocity" was not observed.An experimental investigation of the thermal expansion of alkali halides at low temperaturesJames, Brian Williamhttps://hdl.handle.net/10023/219322022-02-01T12:54:04Z1964-01-01T00:00:00Z1964-01-01T00:00:00ZJames, Brian WilliamA NMR-study on Nd₂CuO₄₋ₓFₓ and building a variable temperature and frequency NMR-probeCtortecka, Berndhttps://hdl.handle.net/10023/219202022-01-31T15:33:55Z1994-01-01T00:00:00ZA study of the Nd₂CuO₄₋ₓFₓ electron-doped high-TC superconductor
has been carried out, using ¹⁹F and ⁶³Cu NMR techniques as well as X-ray
diffraction. In order to do ¹⁹F NMR a spectrometer was set up and a homemade NMR-probe was used, allowing for variable temperature and frequency
studies. Both were extensively tested. Pseudo-single-crystals with identical
grain orientations were obtained at room temperature by an alignment procedure in a strong external field. The alignment quality was confirmed by X-ray
diffraction spectra which showed that the crystallites are aligning with the
c-axis perpendicular to the external field (unlike YBCO and its derivatives).
Extensive angle dependent ⁶³Cu NMR studies were done at room temperature.
The spectra obtained appeared to be unusual, and a hypothesis concerning the
uncommon alignment was formulated. However combining the alignment with
an electric field gradient failed to explain the NMR spectra. Next a model
was developed to explain them. When combining the alignment with a strong
shift anisotropy plus a small or even zero quadrupole interaction, a good agreement with the NMR spectra was found. Fluorine-NMR studies were done on
two out of four samples, finding resonances in both of them. Furthermore a
transferred magnetic field onto the fluorine sites was found and last but not
least a dependency of the applied field strength on the resonance MHz/T could
be established.
1994-01-01T00:00:00ZCtortecka, BerndA study of the Nd₂CuO₄₋ₓFₓ electron-doped high-TC superconductor
has been carried out, using ¹⁹F and ⁶³Cu NMR techniques as well as X-ray
diffraction. In order to do ¹⁹F NMR a spectrometer was set up and a homemade NMR-probe was used, allowing for variable temperature and frequency
studies. Both were extensively tested. Pseudo-single-crystals with identical
grain orientations were obtained at room temperature by an alignment procedure in a strong external field. The alignment quality was confirmed by X-ray
diffraction spectra which showed that the crystallites are aligning with the
c-axis perpendicular to the external field (unlike YBCO and its derivatives).
Extensive angle dependent ⁶³Cu NMR studies were done at room temperature.
The spectra obtained appeared to be unusual, and a hypothesis concerning the
uncommon alignment was formulated. However combining the alignment with
an electric field gradient failed to explain the NMR spectra. Next a model
was developed to explain them. When combining the alignment with a strong
shift anisotropy plus a small or even zero quadrupole interaction, a good agreement with the NMR spectra was found. Fluorine-NMR studies were done on
two out of four samples, finding resonances in both of them. Furthermore a
transferred magnetic field onto the fluorine sites was found and last but not
least a dependency of the applied field strength on the resonance MHz/T could
be established.Compact femtosecond lasers : routes to improved practicality and adaptabilityStormont, Barryhttps://hdl.handle.net/10023/219152022-01-31T10:50:46Z2005-01-01T00:00:00ZIn this thesis, a number of practical femtosecond laser systems are described. Through appropriate selection and design of the intracavity optics used in these lasers, increased adaptability is achieved without making any compromise on the robustness of the final laser design. The laser systems developed during the course of this project made use of Ti: sapphire and Cr: LiSAF as gain media and took full advantage of the optical properties of each. The Ti: sapphire gain medium has been used in the demonstration of a low threshold laser system pumped by an inexpensive, compact, green laser. A design was also assessed where a femtosecond pulse train was produced at a multi-gigahertz pulse repetition frequency. This was made possible through the use of specially engineered intracavity mirrors that were designed as part of this work. Cr: LiSAF crystals when optically pumped with inexpensive red laser diodes are usually regarded as low-gain laser materials. As part of the work presented in this thesis a femtosecond laser based on this material was taken to a pulse repetition rate as high as 1GHz. This approaches the fundamental limit set by the optical properties of the material without the need for stabilisation to prevent damage to the optical components. Also, a scheme whereby the centre wavelength of a femtosecond Cr: LiSAF laser can be tuned without the use of intracavity prisms has been described. This scheme permits the oscillating bandwidth to be controlled thereby allowing a degree of control over the intracavity pulse duration. The research presented in this thesis shows that it is possible to design a laser that is a highly versatile source of ultrashort optical pulses. These concepts and configuration options illustrate that femtosecond lasers can be designed to be highly adaptable tools capable of opening up new avenues of research across science and technology.
2005-01-01T00:00:00ZStormont, BarryIn this thesis, a number of practical femtosecond laser systems are described. Through appropriate selection and design of the intracavity optics used in these lasers, increased adaptability is achieved without making any compromise on the robustness of the final laser design. The laser systems developed during the course of this project made use of Ti: sapphire and Cr: LiSAF as gain media and took full advantage of the optical properties of each. The Ti: sapphire gain medium has been used in the demonstration of a low threshold laser system pumped by an inexpensive, compact, green laser. A design was also assessed where a femtosecond pulse train was produced at a multi-gigahertz pulse repetition frequency. This was made possible through the use of specially engineered intracavity mirrors that were designed as part of this work. Cr: LiSAF crystals when optically pumped with inexpensive red laser diodes are usually regarded as low-gain laser materials. As part of the work presented in this thesis a femtosecond laser based on this material was taken to a pulse repetition rate as high as 1GHz. This approaches the fundamental limit set by the optical properties of the material without the need for stabilisation to prevent damage to the optical components. Also, a scheme whereby the centre wavelength of a femtosecond Cr: LiSAF laser can be tuned without the use of intracavity prisms has been described. This scheme permits the oscillating bandwidth to be controlled thereby allowing a degree of control over the intracavity pulse duration. The research presented in this thesis shows that it is possible to design a laser that is a highly versatile source of ultrashort optical pulses. These concepts and configuration options illustrate that femtosecond lasers can be designed to be highly adaptable tools capable of opening up new avenues of research across science and technology.Comparison of structurally similar organic molecules by X-ray diffractionSharma, Aysel (née Senol)https://hdl.handle.net/10023/219102022-01-13T09:31:01Z1973-01-01T00:00:00Z1973-01-01T00:00:00ZSharma, Aysel (née Senol)Theory of transition metal impurities in semiconductorsO'Neill, A. G.https://hdl.handle.net/10023/219032022-01-07T09:43:46Z1984-01-01T00:00:00ZA new version of crystal field theory is presented in this thesis for transition metal impurities in tetrahedral or octahedral environments. Like the conventional theory it contains only three disposable parameters. Covalency effects have been accounted for by adjustable parameters which are defined by the modification of one-electron d-d electrostatic integrals rather than the modification of one-electron d-orbitals. These parameters may be determined empirically or from first principles theory, thus providing a vital link between theory and experiment.
In the construction of the new theory it is assumed that the d-orbitals of the transition metal impurity remain strongly localised, and only one-centre integrals on the impurity site are considered. The successful application of the theory to describe experimental data is evidence that these approximations are valid for transition metal impurities in II-VI and III-V compounds.
The new theory has been applied to a wide range of tetrahedral systems and the results are compared with previous studies. Several difficulties in the interpretation of experimental data encountered by the conventional crystal field theory have been resolved. A study of trends in the new parameters is also presented, following a discussion of errors in the construction and application of the theory.
A new method is introduced to calculate the valence electron contribution to the crystal field strength by using the pseudopotential method. The critical dependence of the crystal field splitting on the nature of the impurity radial function casts some doubt on the validity of many previous calculations.
1984-01-01T00:00:00ZO'Neill, A. G.A new version of crystal field theory is presented in this thesis for transition metal impurities in tetrahedral or octahedral environments. Like the conventional theory it contains only three disposable parameters. Covalency effects have been accounted for by adjustable parameters which are defined by the modification of one-electron d-d electrostatic integrals rather than the modification of one-electron d-orbitals. These parameters may be determined empirically or from first principles theory, thus providing a vital link between theory and experiment.
In the construction of the new theory it is assumed that the d-orbitals of the transition metal impurity remain strongly localised, and only one-centre integrals on the impurity site are considered. The successful application of the theory to describe experimental data is evidence that these approximations are valid for transition metal impurities in II-VI and III-V compounds.
The new theory has been applied to a wide range of tetrahedral systems and the results are compared with previous studies. Several difficulties in the interpretation of experimental data encountered by the conventional crystal field theory have been resolved. A study of trends in the new parameters is also presented, following a discussion of errors in the construction and application of the theory.
A new method is introduced to calculate the valence electron contribution to the crystal field strength by using the pseudopotential method. The critical dependence of the crystal field splitting on the nature of the impurity radial function casts some doubt on the validity of many previous calculations.Infrared characterisation of semiconductorsThorley, Antony M.https://hdl.handle.net/10023/219022022-01-07T09:25:46Z1990-01-01T00:00:00ZA theoretical and experimental investigation of the optical properties of ion-implanted silicon over the wavelength range 2 μm to 3 cm has been performed. Twelve samples were supplied by Hughes Microelectronics Ltd. and a further two from Plessey Research Caswell. In the near infrared (short wavelength region) plasma edge features are seen for implants of 10¹⁵ions / cm² and above. The feature shape is sensitive to the donor implant distribution and this allows contactless characterisation through the use of a computer model developed in chapters 1 and 2. In the far infrared interference fringes are seen with modulation close to theory and the total transmission is sensitive to implant dose. Millimeter quasi optical results are in close agreement with the theory. The near infrared is limited to doses of 10¹⁵ions / cm² and above for implant distribution measurements, whilst at centimeter wavelengths a 4 x difference between the transmission at 10¹⁵ions / cm² and zero dose should allow an implant as low as 2x10¹⁴/cm² to be identified. At centimeter wavelengths the absolute fit between theory and experiment is not as good as in the millimeter and shorter wavelengths due to standing waves in the experimental arrangement. The possibility exists for tuning out these reflections with matching stubs and, with appropiate modeling, achieving even higher sensitivity in transmission to implant doses below 2x10¹⁴/cm². Level crossing effects in Zeeman split donor states in GaAs were investigated. Ho conclusive evidence for these was found within the limits of sensitivity of the experiments indicating that any amplitude is below 10% of the main transition intensity.
1990-01-01T00:00:00ZThorley, Antony M.A theoretical and experimental investigation of the optical properties of ion-implanted silicon over the wavelength range 2 μm to 3 cm has been performed. Twelve samples were supplied by Hughes Microelectronics Ltd. and a further two from Plessey Research Caswell. In the near infrared (short wavelength region) plasma edge features are seen for implants of 10¹⁵ions / cm² and above. The feature shape is sensitive to the donor implant distribution and this allows contactless characterisation through the use of a computer model developed in chapters 1 and 2. In the far infrared interference fringes are seen with modulation close to theory and the total transmission is sensitive to implant dose. Millimeter quasi optical results are in close agreement with the theory. The near infrared is limited to doses of 10¹⁵ions / cm² and above for implant distribution measurements, whilst at centimeter wavelengths a 4 x difference between the transmission at 10¹⁵ions / cm² and zero dose should allow an implant as low as 2x10¹⁴/cm² to be identified. At centimeter wavelengths the absolute fit between theory and experiment is not as good as in the millimeter and shorter wavelengths due to standing waves in the experimental arrangement. The possibility exists for tuning out these reflections with matching stubs and, with appropiate modeling, achieving even higher sensitivity in transmission to implant doses below 2x10¹⁴/cm². Level crossing effects in Zeeman split donor states in GaAs were investigated. Ho conclusive evidence for these was found within the limits of sensitivity of the experiments indicating that any amplitude is below 10% of the main transition intensity.Novel optical micromanipulation techniques and applications of violet diode lasersCarruthers, Antonia E.https://hdl.handle.net/10023/219002022-01-06T16:24:48Z2005-01-01T00:00:00ZIn this thesis, optical micro-manipulation experiments are described using laser sources spanning the wavelength region 1064 nm to 410 nm. Optical guiding is studied at near infrared wavelengths (780 nm) for comparison of a Gaussian and Bessel beam. The Bessel beam offers good transverse confinement. Extended guiding distances with the Bessel beam are shown with an enhancement factor of 3 over the Gaussian beam. Optical binding is observed in counter-propagating Gaussian beams using infrared light (at 780 nm and 1064 nm). In this geometry, the light-matter interaction induces particles to arrange themselves within a one-dimensional, regularly spaced particle array. A theoretical model is discussed to provide insight into the experimental data. Optical tweezing is performed using newly available short (violet) wavelength laser sources. The optical tweezers' trapping efficiencies were compared with a more traditional infrared source for tweezing. In addition, the violet lasers were used for biological spectroscopy, performing excitation of dyed chromosomes, and green fluorescent protein within cells. Finally, a violet microlensed diode laser at 413 nm is used with a microlensed diode at 662 nm for generating ultraviolet light in a non-linear sum frequency arrangement. This allows spectroscopy of atomic mercury at 254 nm to be performed.
2005-01-01T00:00:00ZCarruthers, Antonia E.In this thesis, optical micro-manipulation experiments are described using laser sources spanning the wavelength region 1064 nm to 410 nm. Optical guiding is studied at near infrared wavelengths (780 nm) for comparison of a Gaussian and Bessel beam. The Bessel beam offers good transverse confinement. Extended guiding distances with the Bessel beam are shown with an enhancement factor of 3 over the Gaussian beam. Optical binding is observed in counter-propagating Gaussian beams using infrared light (at 780 nm and 1064 nm). In this geometry, the light-matter interaction induces particles to arrange themselves within a one-dimensional, regularly spaced particle array. A theoretical model is discussed to provide insight into the experimental data. Optical tweezing is performed using newly available short (violet) wavelength laser sources. The optical tweezers' trapping efficiencies were compared with a more traditional infrared source for tweezing. In addition, the violet lasers were used for biological spectroscopy, performing excitation of dyed chromosomes, and green fluorescent protein within cells. Finally, a violet microlensed diode laser at 413 nm is used with a microlensed diode at 662 nm for generating ultraviolet light in a non-linear sum frequency arrangement. This allows spectroscopy of atomic mercury at 254 nm to be performed.Optical manipulation and the angular momentum of lightO'Neil, Anna T.https://hdl.handle.net/10023/218882021-11-24T12:28:00Z2002-01-01T00:00:00ZThis thesis consists primarily of an investigation into the action of laser light on particles held within optical tweezers. I present work on a dynamic method of measuring both the axial and lateral efficiencies of optical tweezers using Laguerre-Gaussian laser modes. I have developed a new and straight¬ forward way of using an aperture within optical tweezers to rotate particles. I demonstrate a new method for trapping metal particles which is interesting for the insight it gives into the fundamental properties of light. Finally, I show the different ways spin and orbital angular momentum interact with a particle placed off-axis in a Laguerre-Gaussian beam. In addition to the work on optical tweezers I also explain a novel method for producing any type of pure laser mode from a Hermite-Gaussian laser mode. A summary of my conclusions together with some ideas for future work is included.
2002-01-01T00:00:00ZO'Neil, Anna T.This thesis consists primarily of an investigation into the action of laser light on particles held within optical tweezers. I present work on a dynamic method of measuring both the axial and lateral efficiencies of optical tweezers using Laguerre-Gaussian laser modes. I have developed a new and straight¬ forward way of using an aperture within optical tweezers to rotate particles. I demonstrate a new method for trapping metal particles which is interesting for the insight it gives into the fundamental properties of light. Finally, I show the different ways spin and orbital angular momentum interact with a particle placed off-axis in a Laguerre-Gaussian beam. In addition to the work on optical tweezers I also explain a novel method for producing any type of pure laser mode from a Hermite-Gaussian laser mode. A summary of my conclusions together with some ideas for future work is included.The frequency stabilization of gas lasersWallard, Andrew Johnhttps://hdl.handle.net/10023/218762021-11-17T09:12:54Z1972-01-01T00:00:00ZThis thesis contains a description of the design and development of a visible helium-neon laser stabilized to an absorption transition in an intracavity cell of iodine vapour. Two methods of frequency locking the laser have been studied, and a series of experiments has been performed in which the stability and reproducibility of the iodine stabilized laser have been investigated. Chapter 1 discusses the frequency perturbations which might be expected to appear on an unstabilized laser and the steps which may be taken to minimise such perturbations. Three methods of mode selection have been studied in some detail: short lasers, saturated neon absorption and intracavity etalons. These will also be discussed in Chapter 1. The Chapter closes with a discussion of the definition and measurement of frequency stability. The technological features of laser design are described in Chapter 2 together with some of the more important ancillary apparatus involved in these experiments. The Chapter concludes with a discussion of the servosystem techniques involved in frequency locking a laser. Chapter 3 discusses theoretical aspects of the work including the nature of saturated absorption, and the width of the expected saturated absorption peak to which the laser is stabilized; it concludes with an estimate of the laser powers required to achieve saturation. The operation of the laser is outlined in Chapter 4 which points out the advantages of the third derivative locking technique. A short discussion of laser noise problems is incorporated in this Chapter. Finally, Chapter 5 presents results obtained in experiments with two stabilized lasers and investigates the effects of saturated absorption linewidth broadening and shifts introduced by the detection system, the iodine pressure, and the laser power. Measurements of the long term stability have been made, and indicate that the laser frequency has a reproducibility better than 1 part in 10^10. The experiments discussed here indicate that a laser stabilized in this way possesses a stability and reproducibility superior to that of the existing length standard. These properties, together with the optical advantages of a laser source, could make this laser an eminently suitable successor to the existing Krypton lamp standard.
1972-01-01T00:00:00ZWallard, Andrew JohnThis thesis contains a description of the design and development of a visible helium-neon laser stabilized to an absorption transition in an intracavity cell of iodine vapour. Two methods of frequency locking the laser have been studied, and a series of experiments has been performed in which the stability and reproducibility of the iodine stabilized laser have been investigated. Chapter 1 discusses the frequency perturbations which might be expected to appear on an unstabilized laser and the steps which may be taken to minimise such perturbations. Three methods of mode selection have been studied in some detail: short lasers, saturated neon absorption and intracavity etalons. These will also be discussed in Chapter 1. The Chapter closes with a discussion of the definition and measurement of frequency stability. The technological features of laser design are described in Chapter 2 together with some of the more important ancillary apparatus involved in these experiments. The Chapter concludes with a discussion of the servosystem techniques involved in frequency locking a laser. Chapter 3 discusses theoretical aspects of the work including the nature of saturated absorption, and the width of the expected saturated absorption peak to which the laser is stabilized; it concludes with an estimate of the laser powers required to achieve saturation. The operation of the laser is outlined in Chapter 4 which points out the advantages of the third derivative locking technique. A short discussion of laser noise problems is incorporated in this Chapter. Finally, Chapter 5 presents results obtained in experiments with two stabilized lasers and investigates the effects of saturated absorption linewidth broadening and shifts introduced by the detection system, the iodine pressure, and the laser power. Measurements of the long term stability have been made, and indicate that the laser frequency has a reproducibility better than 1 part in 10^10. The experiments discussed here indicate that a laser stabilized in this way possesses a stability and reproducibility superior to that of the existing length standard. These properties, together with the optical advantages of a laser source, could make this laser an eminently suitable successor to the existing Krypton lamp standard.Mechanisms in an argon ion pulsed laserTaylor, Andrew Ramagehttps://hdl.handle.net/10023/218742021-11-16T16:29:51Z1968-01-01T00:00:00ZAlthough many argon ion lasers, both pulsed and D.C., have been built and used, the mechanisms which cause the required population inversion in the gas are still doubtful, and different views are held by different workers in the field. Many possible mechanisms have been proposed, and the problem of deciding which mechanisms are dominant under given operating conditions, still remains.
The first oscillation In an ion laser was observed by Bell (1) o in a pulsed mercury-helium discharge at wavelengths of 6150Å and 5678 Å, which corresponded to two lines of the Hg II spectrum. There followed observations of laser action in ionised krypton (2), xenon (2) and chlorine (3), as well as many other ions, giving a range of wavelengths extending from the near infrared into the ultraviolet regions of the spectrum. The first observation of oscillation in the ionised argon system was made by Bridges (4) using a pulsed discharge when he observed oscillation on 10 wave-lengths in the A II spectrum. The strongest of these were the wavelengths 4880 Å and 5145 Å. Very soon after Gordon et al (5) observed continuous oscillation on 10 wavelengths of the A II spectrum as well as several wavelengths of the Kr II and Xe II spectra. There followed reports on the behaviour of the oscillation in A II under a wide variety of conditions and measurements of the relative gain of the various wavelengths.
Many workers in the argon ion laser field have proposed excitation mechanisms, but the lack of measurements of the fundamental parameters of the system (e.g. electron density, electron temperature etc) have made it difficult to decide which mechanisms are feasible. The problem is that electron densities of the order of 10^-15 cm^-3 are common with electron temperatures in the region of 10^4 to 10^5°K. Probe measurements in this region are notoriously unreliable, while microwave measurements fail completely. An extension of the microwave technique using optical frequencies, to permit penetration of the plasma at high electron densities, provides an estimate of the parameters involved, and line broadening due to the Stark effect can also be used to determine the charged particle density. Using spectroscopic techniques Kitaeva, Osipov and Sobolev (6) have measured electron densities and electron temperatures in an argon ion laser at a current density of about 400 Acm^-2 Measurements from the Doppler broadening of the argon ion spectrum enabled estimates of the ion temperature to be made. There is noinformation available for current densities of about 5000 Acm^-2 and pressures of 30 m torr, so extrapolations must be made from the Russian data to give estimates of the parameters to deduce which mechanisms are important. The behaviour of the system can then be predicted and the predictions compared with observation to deduce which mechanism is dominant.
1968-01-01T00:00:00ZTaylor, Andrew RamageAlthough many argon ion lasers, both pulsed and D.C., have been built and used, the mechanisms which cause the required population inversion in the gas are still doubtful, and different views are held by different workers in the field. Many possible mechanisms have been proposed, and the problem of deciding which mechanisms are dominant under given operating conditions, still remains.
The first oscillation In an ion laser was observed by Bell (1) o in a pulsed mercury-helium discharge at wavelengths of 6150Å and 5678 Å, which corresponded to two lines of the Hg II spectrum. There followed observations of laser action in ionised krypton (2), xenon (2) and chlorine (3), as well as many other ions, giving a range of wavelengths extending from the near infrared into the ultraviolet regions of the spectrum. The first observation of oscillation in the ionised argon system was made by Bridges (4) using a pulsed discharge when he observed oscillation on 10 wave-lengths in the A II spectrum. The strongest of these were the wavelengths 4880 Å and 5145 Å. Very soon after Gordon et al (5) observed continuous oscillation on 10 wavelengths of the A II spectrum as well as several wavelengths of the Kr II and Xe II spectra. There followed reports on the behaviour of the oscillation in A II under a wide variety of conditions and measurements of the relative gain of the various wavelengths.
Many workers in the argon ion laser field have proposed excitation mechanisms, but the lack of measurements of the fundamental parameters of the system (e.g. electron density, electron temperature etc) have made it difficult to decide which mechanisms are feasible. The problem is that electron densities of the order of 10^-15 cm^-3 are common with electron temperatures in the region of 10^4 to 10^5°K. Probe measurements in this region are notoriously unreliable, while microwave measurements fail completely. An extension of the microwave technique using optical frequencies, to permit penetration of the plasma at high electron densities, provides an estimate of the parameters involved, and line broadening due to the Stark effect can also be used to determine the charged particle density. Using spectroscopic techniques Kitaeva, Osipov and Sobolev (6) have measured electron densities and electron temperatures in an argon ion laser at a current density of about 400 Acm^-2 Measurements from the Doppler broadening of the argon ion spectrum enabled estimates of the ion temperature to be made. There is noinformation available for current densities of about 5000 Acm^-2 and pressures of 30 m torr, so extrapolations must be made from the Russian data to give estimates of the parameters to deduce which mechanisms are important. The behaviour of the system can then be predicted and the predictions compared with observation to deduce which mechanism is dominant.Dynamical optical nonlinearities in semiconductor optical amplifiers and quantum cascade lasersGomez-Iglesias, Alvarohttps://hdl.handle.net/10023/218522021-10-27T13:59:28Z2005-01-01T00:00:00ZThe ultrafast dynamics in semiconductor optical amplifiers (SOAs) relevant to all-optical switching have been studied by means of time-resolved spectroscopic experiments on a multiple quantum well InGaAs amplifier. The nonlinear light generation in Quantum Cascade (QC) lasers has also been a subject of research. The switching windows of a TOAD-like three-beam interferometric set-up, measured using 700 fs pulses in the 1.5 μm wavelength region, revealed an ultrafast feature (~ ps) dependent on the pulse energy of the pump beam. These dynamics, and the subsequent reshaping of the switching window edge, are attributed to refractive index changes in the SOA caused by ultrafast carrier heating, consistent with the predictions of a sliced propagation model based on rate equations. Counter-propagating sub-picosecond pulses were used to monitor gain saturation along the SOA waveguide at different wavelengths around the peak gain. The functional form of the spatial dependence of gain saturation is found to depend on pulse energy. These observations are interpreted by combining the optical nonlinearities associated with interband carrier dynamics and carrier heating together and their respective time constants. Aided by the rate-equation model, we show that the amplification of the pump as it propagates along the amplifier may lead to the saturation of a portion of the device and to an effective narrowing of the switching windows of a TOAD (even for pulse energies below 1 pJ). Particular attention is devoted to discussing the limit of full saturation across the entire SOA waveguide. Observation of stimulated electronic Anti-Stokes Raman emission in QC lasers is reported. We present four distinct designs of active regions with enhanced Anti-Stokes Raman nonlinearity. In all four, the pump laser is monolithically integrated with the non-linear region in a two-stack active core within the same waveguide. Stimulated electronic Raman emission was observed in samples of three of these designs. Additionally, for the designs with positive detuning, an incoherent up-conversion signal resulting from optical pumping is detected.
2005-01-01T00:00:00ZGomez-Iglesias, AlvaroThe ultrafast dynamics in semiconductor optical amplifiers (SOAs) relevant to all-optical switching have been studied by means of time-resolved spectroscopic experiments on a multiple quantum well InGaAs amplifier. The nonlinear light generation in Quantum Cascade (QC) lasers has also been a subject of research. The switching windows of a TOAD-like three-beam interferometric set-up, measured using 700 fs pulses in the 1.5 μm wavelength region, revealed an ultrafast feature (~ ps) dependent on the pulse energy of the pump beam. These dynamics, and the subsequent reshaping of the switching window edge, are attributed to refractive index changes in the SOA caused by ultrafast carrier heating, consistent with the predictions of a sliced propagation model based on rate equations. Counter-propagating sub-picosecond pulses were used to monitor gain saturation along the SOA waveguide at different wavelengths around the peak gain. The functional form of the spatial dependence of gain saturation is found to depend on pulse energy. These observations are interpreted by combining the optical nonlinearities associated with interband carrier dynamics and carrier heating together and their respective time constants. Aided by the rate-equation model, we show that the amplification of the pump as it propagates along the amplifier may lead to the saturation of a portion of the device and to an effective narrowing of the switching windows of a TOAD (even for pulse energies below 1 pJ). Particular attention is devoted to discussing the limit of full saturation across the entire SOA waveguide. Observation of stimulated electronic Anti-Stokes Raman emission in QC lasers is reported. We present four distinct designs of active regions with enhanced Anti-Stokes Raman nonlinearity. In all four, the pump laser is monolithically integrated with the non-linear region in a two-stack active core within the same waveguide. Stimulated electronic Raman emission was observed in samples of three of these designs. Additionally, for the designs with positive detuning, an incoherent up-conversion signal resulting from optical pumping is detected.Theory of high power electron cyclotron resonance heatingTaylor, Allan Watsonhttps://hdl.handle.net/10023/218502021-10-27T11:37:07Z1988-01-01T00:00:00ZElectron cyclotron resonance heating has been successfully used on a series of experiments in an attempt to raise plasma temperatures beyond the constraints of the resistive dissipation which occurs with ohmic heating. Recently progress in gyrotron design has allowed for significant increases in applied microwave power and for the first time a free electron laser will generate high power pulsed radio frequency waves in the MTX experiment at Lawrence Livermore Laboratory in 1987. Classically the theory of ECRH has been considered by a Fokker-Planck approach and by a guasilinear approach. Both lead to a diffusion equation in velocity space for the distribution function but as the applied power increases the approximations made in these approaches are likely to become unsatisfactory. Adopting a test particle approach we firstly consider modifications to the velocity space diffusion co-efficient at high powers and then dispense with the diffusion equation completely. We begin by deriving averaged particle equations from a Lagrangian formulation which require less computer processor time to integrate than the exact Lorentz-force equations. These have been incorporated in a particle code to simulate ECRH in a tokamak. The results for this code are compared with analytic expressions derived for a modified diffusion coefficient and a probability function P(v,Δv). We show that for low fields the diffusive form is correct but for higher fields nonlinear effects become important.
1988-01-01T00:00:00ZTaylor, Allan WatsonElectron cyclotron resonance heating has been successfully used on a series of experiments in an attempt to raise plasma temperatures beyond the constraints of the resistive dissipation which occurs with ohmic heating. Recently progress in gyrotron design has allowed for significant increases in applied microwave power and for the first time a free electron laser will generate high power pulsed radio frequency waves in the MTX experiment at Lawrence Livermore Laboratory in 1987. Classically the theory of ECRH has been considered by a Fokker-Planck approach and by a guasilinear approach. Both lead to a diffusion equation in velocity space for the distribution function but as the applied power increases the approximations made in these approaches are likely to become unsatisfactory. Adopting a test particle approach we firstly consider modifications to the velocity space diffusion co-efficient at high powers and then dispense with the diffusion equation completely. We begin by deriving averaged particle equations from a Lagrangian formulation which require less computer processor time to integrate than the exact Lorentz-force equations. These have been incorporated in a particle code to simulate ECRH in a tokamak. The results for this code are compared with analytic expressions derived for a modified diffusion coefficient and a probability function P(v,Δv). We show that for low fields the diffusive form is correct but for higher fields nonlinear effects become important.Continuous-wave, intracavity singly-resonant optical parametric oscillators based upon Nd-doped laser gain mediaCarleton, Alisonhttps://hdl.handle.net/10023/218422021-10-26T14:39:47Z2003-01-01T00:00:00ZThis thesis describes a system which overcomes one of the limitations faced by traditional continuous-wave optical parametric oscillators (cw OPOs). In this approach the high circulating field found within a laser cavity is exploited to reach the high threshold powers required by a singly-resonant oscillator (SRO) by placing the nonlinear crystal inside the cavity, creating an intracavity singly-resonant oscillator (ICSRO). These ICSROs are based upon cw diode-pumped solid-state lasers which, in themselves, are not necessarily widely tunable. However, by placing the OPO internal to the laser cavity means that it now includes a frequency converter and the two outputs produced, called the signal and idler, can be tuned over broad ranges in the mid and far infrared. The inclusion of an OPO internal to the cavity of a laser has a significant impact upon the transient dynamics of the pumping field. Experimental evidence of these effects are presented and an approach by which they can be damped is outlined.
SRO systems based upon periodically-poled LiNbO₃ (PPLN) and periodically-poled RbTiOAsO₄ (PPRTA) pumped internal to diode-pumped Nd:YVO₄ lasers have been demonstrated in both high (15 W) and low (3 W) power regimes and limits to the practical pumping power have been investigated. Extractable idler output powers of 440 mW and 66 mW have been observed for diode pump powers of ~12 W and 3 W respectively. This idler power was shown to be capable of tuning over the ranges 3158 to 4024 nm for PPLN and 3538 to 3404 nm for PPRTA. The spectral quality of the down-converted radiation generated has been studied. The signal (resonated wave) is found to be single frequency and under such conditions the idler wave acquires the linewidth/frequency spectrum of the pump laser. Most applications for such devices require a single frequency output, and techniques to achieve this are discussed along with other refinements that could improve the overall performance of these systems. This should result in devices that are capable of delivering 10s to 100s of mW of continuous wave tunable output from compact geometries. Diode-pumped solid-state laser technology developments, quasi-phase-matched materials and the implementation of the intracavity technique, along with simple, inexpensive components could result in the realisation of straightforward and cost-effective devices.
2003-01-01T00:00:00ZCarleton, AlisonThis thesis describes a system which overcomes one of the limitations faced by traditional continuous-wave optical parametric oscillators (cw OPOs). In this approach the high circulating field found within a laser cavity is exploited to reach the high threshold powers required by a singly-resonant oscillator (SRO) by placing the nonlinear crystal inside the cavity, creating an intracavity singly-resonant oscillator (ICSRO). These ICSROs are based upon cw diode-pumped solid-state lasers which, in themselves, are not necessarily widely tunable. However, by placing the OPO internal to the laser cavity means that it now includes a frequency converter and the two outputs produced, called the signal and idler, can be tuned over broad ranges in the mid and far infrared. The inclusion of an OPO internal to the cavity of a laser has a significant impact upon the transient dynamics of the pumping field. Experimental evidence of these effects are presented and an approach by which they can be damped is outlined.
SRO systems based upon periodically-poled LiNbO₃ (PPLN) and periodically-poled RbTiOAsO₄ (PPRTA) pumped internal to diode-pumped Nd:YVO₄ lasers have been demonstrated in both high (15 W) and low (3 W) power regimes and limits to the practical pumping power have been investigated. Extractable idler output powers of 440 mW and 66 mW have been observed for diode pump powers of ~12 W and 3 W respectively. This idler power was shown to be capable of tuning over the ranges 3158 to 4024 nm for PPLN and 3538 to 3404 nm for PPRTA. The spectral quality of the down-converted radiation generated has been studied. The signal (resonated wave) is found to be single frequency and under such conditions the idler wave acquires the linewidth/frequency spectrum of the pump laser. Most applications for such devices require a single frequency output, and techniques to achieve this are discussed along with other refinements that could improve the overall performance of these systems. This should result in devices that are capable of delivering 10s to 100s of mW of continuous wave tunable output from compact geometries. Diode-pumped solid-state laser technology developments, quasi-phase-matched materials and the implementation of the intracavity technique, along with simple, inexpensive components could result in the realisation of straightforward and cost-effective devices.Electroluminescence and switching phenomena in ZnSe Schottky diodesLivingstone, Alexander Williamhttps://hdl.handle.net/10023/218322021-10-25T11:28:04Z1973-01-01T00:00:00Z1973-01-01T00:00:00ZLivingstone, Alexander WilliamThe physics of microchip lasers : spatial and spectral selectivity mechanismsKemp, Alanhttps://hdl.handle.net/10023/218112021-10-21T13:12:20Z2000-01-01T00:00:00ZThis thesis describes an experimental and theoretical exploration of some of the physics of microchip lasers and micro-lasers, with particular reference to the selection of the transverse and longitudinal modes. The guiding of the transverse mode in Nd:YVO₄ and Nd:YAG microchip lasers is examined at pump powers well above threshold. In Nd:YAG, the guiding is found to be purely thermal. However, in Nd:YVO₄ there is found to be a significant gain-related component to the guiding. This is the first time such an effect has been observed in a continuous wave, four-level microchip laser away from threshold. Modelling of the gain-related and thermal effects gives good agreement with experiment. A Jones matrix formalism is used to understand the polarisation and longitudinal mode structure in frequency doubled microchip lasers and micro-lasers. Such modelling of a frequency doubled microchip lasers gives good agreement with experiment and represents a starting point in understanding the unusual intensity stability properties of these lasers. The Jones matrix model is also applied to the design of single frequency micro-lasers utilising a birefringent filter and having a birefringent gain crystal. This is the first time that birefringent filtering in such lasers has been fully studied. The results of the modelling indicate the importance of the choice of birefringent material in the birefringent filter and its length. The length of the laser cavity and the birefringence of the gain material are also shown to be very important. Utilising the results of this modelling, a single frequency green laser based on Nd:YVO₄ and KTP was designed and built. More than 230 mW of single frequency green was produced in a high quality beam (M-squared less than 1.1) for 1.2 W of laser-diode pump power. Tuning of more than 20 GHz was demonstrated for 0.5 W of laser-diode pump power.
2000-01-01T00:00:00ZKemp, AlanThis thesis describes an experimental and theoretical exploration of some of the physics of microchip lasers and micro-lasers, with particular reference to the selection of the transverse and longitudinal modes. The guiding of the transverse mode in Nd:YVO₄ and Nd:YAG microchip lasers is examined at pump powers well above threshold. In Nd:YAG, the guiding is found to be purely thermal. However, in Nd:YVO₄ there is found to be a significant gain-related component to the guiding. This is the first time such an effect has been observed in a continuous wave, four-level microchip laser away from threshold. Modelling of the gain-related and thermal effects gives good agreement with experiment. A Jones matrix formalism is used to understand the polarisation and longitudinal mode structure in frequency doubled microchip lasers and micro-lasers. Such modelling of a frequency doubled microchip lasers gives good agreement with experiment and represents a starting point in understanding the unusual intensity stability properties of these lasers. The Jones matrix model is also applied to the design of single frequency micro-lasers utilising a birefringent filter and having a birefringent gain crystal. This is the first time that birefringent filtering in such lasers has been fully studied. The results of the modelling indicate the importance of the choice of birefringent material in the birefringent filter and its length. The length of the laser cavity and the birefringence of the gain material are also shown to be very important. Utilising the results of this modelling, a single frequency green laser based on Nd:YVO₄ and KTP was designed and built. More than 230 mW of single frequency green was produced in a high quality beam (M-squared less than 1.1) for 1.2 W of laser-diode pump power. Tuning of more than 20 GHz was demonstrated for 0.5 W of laser-diode pump power.Nuclear magnetic resonance in isopentane at low temperaturesForsyth, Alan G.https://hdl.handle.net/10023/218102021-10-22T02:07:57Z1964-01-01T00:00:00ZNuclear Magnetic Resonance depends for its existence on the fact that most nuclei of the elements exhibit gyromagnetic properties. These properties were first attributed to the nucleus by Pauli (1921) to explain hyperfine structure in atomic spectroscopy. When a sample containing nuclei exhibiting gyromagnetism is placed in a magnetic field, and is irradiated by an appropriate rotating radio-frequency magnetic field, the nuclei can be compelled to reveal their presence, identify themselves, and to describe the nature of their environments. Such experiments can, amongst other things, yield evidence of crystal and molecular structures, hindered molecular motions, and thermal relaxation mechanisms. This thesis is concerned with such effects. Rabi, Millman, Kusch and Zacharias (1939) first applied the resonance method to individual atoms and molecules in atomic beam experiments, and produced transitions between the quantised nuclear magnetic energy levels by a process of absorption or stimulated emission. Gorter had shown that this resonant exchange of energy should not be restricted to molecular beams, but should also be observed in matter in other forms in which internolecular interactions would occur. Gorter (1936), and Gorter and Broer (1942) unsuccessfully attempted to detect energy absorption by ⁷Li nuclei in Li F. It was later shown by Gorter (1931) that the failure was mainly due to the use of unfavourable materials
The first successful experiments of nuclear magnetic resonance in bulk material were made simultaneously and independently by Purcell, Torrey and Pound (1946), and Bloch, Hansen and Packard (1946). The development of the subject since these pioneer experiments has been such that applications have been found in many branches of physics and chemistry. Several companies now manufacture Nuclear Magnetic Resonance spectrometers, and these have achieved equality in importance with infra-red and mass spectrometers in many chemical research laboratories.
1964-01-01T00:00:00ZForsyth, Alan G.Nuclear Magnetic Resonance depends for its existence on the fact that most nuclei of the elements exhibit gyromagnetic properties. These properties were first attributed to the nucleus by Pauli (1921) to explain hyperfine structure in atomic spectroscopy. When a sample containing nuclei exhibiting gyromagnetism is placed in a magnetic field, and is irradiated by an appropriate rotating radio-frequency magnetic field, the nuclei can be compelled to reveal their presence, identify themselves, and to describe the nature of their environments. Such experiments can, amongst other things, yield evidence of crystal and molecular structures, hindered molecular motions, and thermal relaxation mechanisms. This thesis is concerned with such effects. Rabi, Millman, Kusch and Zacharias (1939) first applied the resonance method to individual atoms and molecules in atomic beam experiments, and produced transitions between the quantised nuclear magnetic energy levels by a process of absorption or stimulated emission. Gorter had shown that this resonant exchange of energy should not be restricted to molecular beams, but should also be observed in matter in other forms in which internolecular interactions would occur. Gorter (1936), and Gorter and Broer (1942) unsuccessfully attempted to detect energy absorption by ⁷Li nuclei in Li F. It was later shown by Gorter (1931) that the failure was mainly due to the use of unfavourable materials
The first successful experiments of nuclear magnetic resonance in bulk material were made simultaneously and independently by Purcell, Torrey and Pound (1946), and Bloch, Hansen and Packard (1946). The development of the subject since these pioneer experiments has been such that applications have been found in many branches of physics and chemistry. Several companies now manufacture Nuclear Magnetic Resonance spectrometers, and these have achieved equality in importance with infra-red and mass spectrometers in many chemical research laboratories.Neutron scattering and muon-spin rotation studies of superconducting materialsDrew, Alan J.https://hdl.handle.net/10023/218082021-10-21T10:49:24Z2005-01-01T00:00:00ZSmall angle neutron scattering (SANS), muon spin rotation (μSR) and associated characterisation techniques have been used to measure novel properties of supercon¬ ducting materials. The coexistence of Type-I and Type-II behaviour in LaNiSn has been observed using μSR and the thermodynamic phase boundary has been identified between this behaviour and that of a conventional Type-I superconductor. In a zero field cooled state the existence of a pure Meissner state is observed only at this thermodynamic phase boundary. The magnetic phase diagram of the high temperature superconductor La. ₁.₈₃Sr₀.₁₇ CuO₄ has been investigated. SANS has provided the first microscopic observation of a vortex lattice and the first unambiguous evidence for a field induced hexagonal to square vortex lattice structural transition in the high temperature superconductors. This is supported by μSR measurements, which also yield information on vortex lattice pinning and the melting transition.
A preliminary SANS experiment on La₁.₉Sr₀.₁CuO₄ at low fields suggests a Bragg glass (BG) with nominally a hexagonal structure. μSR has provided unambiguous evidence for a field induced crossover from a BG to a more disordered vortex glass (VG) state and an upper limit on the crossover field is given. This is the first measurement of a disordered VG state on a system of well coupled vortex lines. Furthermore, a study of the evolution of short range order is presented that is of universal significance, as it provides experimental insights into space averaged many particle correlations in bulk systems.
Direct evidence for the coexistence of a spin density wave (SDW) with bulk superconductivity in a Ferromagnetic/Superconducting/Ferromagnetic trilayer has been obtained using low energy μSR. The apparent enhancement of the SDW amplitude in the superconducting state and the π/2 phase shift of one component of the SDW below Tc indicates a profound coupling of these two forms of spin order.
2005-01-01T00:00:00ZDrew, Alan J.Small angle neutron scattering (SANS), muon spin rotation (μSR) and associated characterisation techniques have been used to measure novel properties of supercon¬ ducting materials. The coexistence of Type-I and Type-II behaviour in LaNiSn has been observed using μSR and the thermodynamic phase boundary has been identified between this behaviour and that of a conventional Type-I superconductor. In a zero field cooled state the existence of a pure Meissner state is observed only at this thermodynamic phase boundary. The magnetic phase diagram of the high temperature superconductor La. ₁.₈₃Sr₀.₁₇ CuO₄ has been investigated. SANS has provided the first microscopic observation of a vortex lattice and the first unambiguous evidence for a field induced hexagonal to square vortex lattice structural transition in the high temperature superconductors. This is supported by μSR measurements, which also yield information on vortex lattice pinning and the melting transition.
A preliminary SANS experiment on La₁.₉Sr₀.₁CuO₄ at low fields suggests a Bragg glass (BG) with nominally a hexagonal structure. μSR has provided unambiguous evidence for a field induced crossover from a BG to a more disordered vortex glass (VG) state and an upper limit on the crossover field is given. This is the first measurement of a disordered VG state on a system of well coupled vortex lines. Furthermore, a study of the evolution of short range order is presented that is of universal significance, as it provides experimental insights into space averaged many particle correlations in bulk systems.
Direct evidence for the coexistence of a spin density wave (SDW) with bulk superconductivity in a Ferromagnetic/Superconducting/Ferromagnetic trilayer has been obtained using low energy μSR. The apparent enhancement of the SDW amplitude in the superconducting state and the π/2 phase shift of one component of the SDW below Tc indicates a profound coupling of these two forms of spin order.A μSR study of unconventional superconductorsHillier, Adrianhttps://hdl.handle.net/10023/218022021-10-21T02:00:44Z1999-01-01T00:00:00ZThe problem of finding an appropriate classification scheme for the remarkably diverse range of known superconducting materials is a long standing one. Nevertheless the literature abounds with claims of discoveries of "new classes" of superconductors, although the nature of the "class" is rarely defined explicitly. In cases where the structural, electronic or magnetic properties of the new superconductor are particularly unusual it is tempting to accept such claims: heavy fermions, high Tc cuprates, organic superconductors and buckminsterfullerenes are all examples of superconducting materials which superficially have little in common either with each other or with the more conventional superconducting elements and compounds and might therefore be considered as genuinely belonging to different classes. However the underlying uniformity of the superconducting ground state and its general conformity with the predictions of BCS theory continues to unite an extraordinarily disparate group of superconducting materials. Consequently over the last few decades there have been numerous attempts to provide an empirical framework, based upon the fundamental parameters of the superconducting state, within which superconducting materials can be compared and contrasted, thus enabling classes of superconductor to be unambiguously identified. For example, until the early 1980s a correlation between the superconducting transition temperature, Tc, and the Sommerfeld constant, i.e. the coefficient of the linear electronic specific heat 𝛶, was frequently invoked. A plot of logTc versus log 𝛶 was found to yield an approximately universal curve (see figure 1.1) ¹. However, first the heavy fermion compounds, with enormous 𝛶 s yet low transition temperatures and then, later, the cuprates with modest 𝛶 s but remarkably high Tcs proved to be marked exceptions to this universality, apparently confirming their status as members of new, exotic classes of superconductors.
More recently a rather surprising universal scaling relationship has emerged from systematic transverse field muon spin rotation (μSR) measurements of flux penetration in superconducting systems. Uemura and co-workers ² were the first to recognise the new scaling relationship, observing that for several different members of the family of high temperature cuprate superconductors an initial increase in carrier doping leads to precisely the same linear increase of Tc with the muon spin depolarisation rate, σ. Deviations from linearity, first appearing as a saturation and then a suppression of Tc, only appears at high levels of doping. Remarkably, several Chevrel phase superconductors were also found to follow the same linear relationship, while bismuthates, fullerenes, organic superconductors and heavy fermion compounds exhibit a similar scaling behaviour3,4.
The correlation between Tc and σ observed in μSR studies has suggested a new empirical framework for classifying superconducting materials. In this thesis I shall briefly review the role played by μSR in establishing this new classification scheme, and discuss the underlying physical phenomena responsible for the observed correlations. I shall present some μSR measurements on a range of superconducting families and attempt to interpret the results of these measurements within the proposed classification scheme.
1999-01-01T00:00:00ZHillier, AdrianThe problem of finding an appropriate classification scheme for the remarkably diverse range of known superconducting materials is a long standing one. Nevertheless the literature abounds with claims of discoveries of "new classes" of superconductors, although the nature of the "class" is rarely defined explicitly. In cases where the structural, electronic or magnetic properties of the new superconductor are particularly unusual it is tempting to accept such claims: heavy fermions, high Tc cuprates, organic superconductors and buckminsterfullerenes are all examples of superconducting materials which superficially have little in common either with each other or with the more conventional superconducting elements and compounds and might therefore be considered as genuinely belonging to different classes. However the underlying uniformity of the superconducting ground state and its general conformity with the predictions of BCS theory continues to unite an extraordinarily disparate group of superconducting materials. Consequently over the last few decades there have been numerous attempts to provide an empirical framework, based upon the fundamental parameters of the superconducting state, within which superconducting materials can be compared and contrasted, thus enabling classes of superconductor to be unambiguously identified. For example, until the early 1980s a correlation between the superconducting transition temperature, Tc, and the Sommerfeld constant, i.e. the coefficient of the linear electronic specific heat 𝛶, was frequently invoked. A plot of logTc versus log 𝛶 was found to yield an approximately universal curve (see figure 1.1) ¹. However, first the heavy fermion compounds, with enormous 𝛶 s yet low transition temperatures and then, later, the cuprates with modest 𝛶 s but remarkably high Tcs proved to be marked exceptions to this universality, apparently confirming their status as members of new, exotic classes of superconductors.
More recently a rather surprising universal scaling relationship has emerged from systematic transverse field muon spin rotation (μSR) measurements of flux penetration in superconducting systems. Uemura and co-workers ² were the first to recognise the new scaling relationship, observing that for several different members of the family of high temperature cuprate superconductors an initial increase in carrier doping leads to precisely the same linear increase of Tc with the muon spin depolarisation rate, σ. Deviations from linearity, first appearing as a saturation and then a suppression of Tc, only appears at high levels of doping. Remarkably, several Chevrel phase superconductors were also found to follow the same linear relationship, while bismuthates, fullerenes, organic superconductors and heavy fermion compounds exhibit a similar scaling behaviour3,4.
The correlation between Tc and σ observed in μSR studies has suggested a new empirical framework for classifying superconducting materials. In this thesis I shall briefly review the role played by μSR in establishing this new classification scheme, and discuss the underlying physical phenomena responsible for the observed correlations. I shall present some μSR measurements on a range of superconducting families and attempt to interpret the results of these measurements within the proposed classification scheme.Dynamical backaction effects between localised spins and electronic conductorsMatern, Stephaniehttps://hdl.handle.net/10023/217632023-12-05T03:07:38Z2020-12-01T00:00:00ZIn this thesis we present an investigation of the influence of quantum correlations on a
quantum system’s dynamical behaviour. Our focus is specifically on the time dependence
of quantum spins in an environment of itinerant electrons. This is an archetype for strong
correlation physics, whose dynamical onset is the central correlation effect investigated
in this work.
We derive an analytic result for the time evolution of a single localised quantum spin in
weak contact with conduction electrons. This result is obtained from a detailed analysis
of the pole structure of the Nakajima-Zwanzig equation for the reduced density matrix
in Laplace space. We provide a description of the full time range, from very short times
in which a novel result for non-Markovian behaviour is obtained, to long times in which
we recover the well-known exponential decay expressions. For the short times we show
how the non-Markovian memory effects of the spin’s dynamics arise from the backaction
of coherent electronic particle-hole fluctuations.
As an application of the fast dynamics we propose a cooling protocol going beyond the
paradigm of thermodynamic cycles. The protocol relies on a rapid pump scheme with
a repeated reinitialisation of the fast quantum coherent dynamics, with each repetition
carrying away a small amount of heat from the electronic environment. This protocol is
temperature independent and designed to circumvent a natural bottleneck in standard
demagnetisation cooling due to long relaxation times at low temperatures.
Finally we extend the dynamics to a pair of localised spins coupled through the same
electronic environment, using a self-consistent projection operator framework. In contrast to the conventional RKKY coupling we derive a set of coupled equations including the temporal and spatial correlations. This set becomes finite through a meticulous identification of the electronic fluctuations responsible for the coupled dynamics, allowing for a numerical solution.
2020-12-01T00:00:00ZMatern, StephanieIn this thesis we present an investigation of the influence of quantum correlations on a
quantum system’s dynamical behaviour. Our focus is specifically on the time dependence
of quantum spins in an environment of itinerant electrons. This is an archetype for strong
correlation physics, whose dynamical onset is the central correlation effect investigated
in this work.
We derive an analytic result for the time evolution of a single localised quantum spin in
weak contact with conduction electrons. This result is obtained from a detailed analysis
of the pole structure of the Nakajima-Zwanzig equation for the reduced density matrix
in Laplace space. We provide a description of the full time range, from very short times
in which a novel result for non-Markovian behaviour is obtained, to long times in which
we recover the well-known exponential decay expressions. For the short times we show
how the non-Markovian memory effects of the spin’s dynamics arise from the backaction
of coherent electronic particle-hole fluctuations.
As an application of the fast dynamics we propose a cooling protocol going beyond the
paradigm of thermodynamic cycles. The protocol relies on a rapid pump scheme with
a repeated reinitialisation of the fast quantum coherent dynamics, with each repetition
carrying away a small amount of heat from the electronic environment. This protocol is
temperature independent and designed to circumvent a natural bottleneck in standard
demagnetisation cooling due to long relaxation times at low temperatures.
Finally we extend the dynamics to a pair of localised spins coupled through the same
electronic environment, using a self-consistent projection operator framework. In contrast to the conventional RKKY coupling we derive a set of coupled equations including the temporal and spatial correlations. This set becomes finite through a meticulous identification of the electronic fluctuations responsible for the coupled dynamics, allowing for a numerical solution.Generation and characterisation of ultrashort diode laser pulsesBirkin, David J. L.https://hdl.handle.net/10023/217602021-04-03T02:01:41Z2002-02-08T00:00:00ZThis thesis is concerned with the development of a compact diode laser source
of picosecond optical pulses having enhanced average powers. This is realised
by the application of a large amplitude sinusoidal modulation to a single-contact,
single-mode, narrow stripe, InGaAs/GaAs ridge-waveguide diode laser. The
operational characteristics of the device when in continuous wave and gain-
switched regimes are presented.
In the gain-switched regime, a minimum pulse duration of 30ps is
demonstrated, at average and peak powers up to ≈150mW and ≈1.8mW
respectively. A sonogram technique is employed to determine the sign and
magnitude of the frequency chirp in the optical pulses. On the basis of this
information aperiodic gratings are designed and fabricated in germanosilicate
optical fibres and lithium niobate crystals to realise temporal pulse compression
and efficient second harmonic generation respectively.
The effect of self-injection optical feedback is described, along with the
corresponding realisation in the reduction in the spectral bandwidth of the
optical pulses from ≈11nm to 0.05nm. When the optical feedback is provided
by a standard diffraction grating, a tuning range of 70nm is demonstrated. The
addition of a second grating results in two independently tunable outputs, with
an adjustable spectral separation of up to 53nm.
Bragg gratings are fabricated in the cores of photosensitive germanosilicate
optical fibres. It is demonstrated that when such a fibre is used in an external
cavity configuration, both temporal and spectral compression of the optical
pulses is observed.
Direct frequency conversion of the diode laser output by using quasi-phase
matched crystals of lithium niobate and KTP is demonstrated. High efficiencies
are obtained with a KTP crystal containing a waveguide structure and a Bragg
grating section to provide optical feedback to the diode laser. By this approach
impressively high average second harmonic powers of up to 7.3mW in the blue
spectral region are achieved for this frequency-doubled picosecond diode laser.
2002-02-08T00:00:00ZBirkin, David J. L.This thesis is concerned with the development of a compact diode laser source
of picosecond optical pulses having enhanced average powers. This is realised
by the application of a large amplitude sinusoidal modulation to a single-contact,
single-mode, narrow stripe, InGaAs/GaAs ridge-waveguide diode laser. The
operational characteristics of the device when in continuous wave and gain-
switched regimes are presented.
In the gain-switched regime, a minimum pulse duration of 30ps is
demonstrated, at average and peak powers up to ≈150mW and ≈1.8mW
respectively. A sonogram technique is employed to determine the sign and
magnitude of the frequency chirp in the optical pulses. On the basis of this
information aperiodic gratings are designed and fabricated in germanosilicate
optical fibres and lithium niobate crystals to realise temporal pulse compression
and efficient second harmonic generation respectively.
The effect of self-injection optical feedback is described, along with the
corresponding realisation in the reduction in the spectral bandwidth of the
optical pulses from ≈11nm to 0.05nm. When the optical feedback is provided
by a standard diffraction grating, a tuning range of 70nm is demonstrated. The
addition of a second grating results in two independently tunable outputs, with
an adjustable spectral separation of up to 53nm.
Bragg gratings are fabricated in the cores of photosensitive germanosilicate
optical fibres. It is demonstrated that when such a fibre is used in an external
cavity configuration, both temporal and spectral compression of the optical
pulses is observed.
Direct frequency conversion of the diode laser output by using quasi-phase
matched crystals of lithium niobate and KTP is demonstrated. High efficiencies
are obtained with a KTP crystal containing a waveguide structure and a Bragg
grating section to provide optical feedback to the diode laser. By this approach
impressively high average second harmonic powers of up to 7.3mW in the blue
spectral region are achieved for this frequency-doubled picosecond diode laser.Advanced 3D Monte Carlo algorithms for biophotonic and medical applicationsMcMillan, Lewishttps://hdl.handle.net/10023/215542021-11-30T11:26:57Z2020-07-27T00:00:00ZThe Monte Carlo radiation transfer (MCRT) method can simulate the transport of light through
turbid media. MCRT allows the modelling of multiple anisotropic scattering events, as well as
a range of microphysics such as polarisation and fluorescence. This thesis concerns the development
of several MCRT algorithms to solve various biophotonic and medically-related problems including modelling of tissue ablation and autofluorescent signals. An extension of the MCRT method through a theoretical quasi-wave/particle model is also demonstrated, allowing beam shapes with arbitrary phase profiles to be propagated.
Tissue ablation can be used to treat acne scarring, Rhinophyma, and it can also be used to
help enhance topical drug delivery. Currently the depth of ablation is not easily elucidated from
a given laser or laser power setting. Therefore, a numerical tissue ablation model is developed
using a combination of MCRT, a heat diffusion model, and a numerical tissue damage model to
assess ablation crater depth and thermal damage to the surrounding tissue.
Autofluorescence is the natural fluorescence of biological structures in tissue. Autofluorescence
can be used as a biomarker of several diseases including: cardiovascular diseases,
Alzheimers, and diabetes. However, the origin of the autofluorescence signal is not completely
clear. The effect of tissue optics on the signal, which fluorophores contribute to the signal and by
how much, and how different locations on the body can affect the signal are not well understood.
This thesis presents a study of the effect of tissue optics on the autofluorescent signal. As part of
this study, AmoebaMCRT was created to determine the relative concentrations of fluorophores
for a given autofluorescent signal.
Finally, we developed an extension to the MCRT method which allows the simulation of
quasi-wave/particles. This method relies on the Huygens-Fresnel principle and the tracking
of the phase of each individual photon packet. The extension, φMC, allows the modelling of
complex beams that require the wave properties of light such as arbitrary order Bessel beams
and Gaussian beams. We then use φMC to predict which beam, Bessel or Gaussian, performs
“better" in a highly turbid medium.
2020-07-27T00:00:00ZMcMillan, LewisThe Monte Carlo radiation transfer (MCRT) method can simulate the transport of light through
turbid media. MCRT allows the modelling of multiple anisotropic scattering events, as well as
a range of microphysics such as polarisation and fluorescence. This thesis concerns the development
of several MCRT algorithms to solve various biophotonic and medically-related problems including modelling of tissue ablation and autofluorescent signals. An extension of the MCRT method through a theoretical quasi-wave/particle model is also demonstrated, allowing beam shapes with arbitrary phase profiles to be propagated.
Tissue ablation can be used to treat acne scarring, Rhinophyma, and it can also be used to
help enhance topical drug delivery. Currently the depth of ablation is not easily elucidated from
a given laser or laser power setting. Therefore, a numerical tissue ablation model is developed
using a combination of MCRT, a heat diffusion model, and a numerical tissue damage model to
assess ablation crater depth and thermal damage to the surrounding tissue.
Autofluorescence is the natural fluorescence of biological structures in tissue. Autofluorescence
can be used as a biomarker of several diseases including: cardiovascular diseases,
Alzheimers, and diabetes. However, the origin of the autofluorescence signal is not completely
clear. The effect of tissue optics on the signal, which fluorophores contribute to the signal and by
how much, and how different locations on the body can affect the signal are not well understood.
This thesis presents a study of the effect of tissue optics on the autofluorescent signal. As part of
this study, AmoebaMCRT was created to determine the relative concentrations of fluorophores
for a given autofluorescent signal.
Finally, we developed an extension to the MCRT method which allows the simulation of
quasi-wave/particles. This method relies on the Huygens-Fresnel principle and the tracking
of the phase of each individual photon packet. The extension, φMC, allows the modelling of
complex beams that require the wave properties of light such as arbitrary order Bessel beams
and Gaussian beams. We then use φMC to predict which beam, Bessel or Gaussian, performs
“better" in a highly turbid medium.Mapping galaxy components with kinematics : dynamical models with stellar and gaseous kinematicsYang, Menghttps://hdl.handle.net/10023/214922021-02-24T15:58:29Z2020-12-01T00:00:00ZMapping galaxy mass distribution is an important means of understanding galaxy morphology
and the nature of dark matter haloes, and therefore build up our knowledge of galaxy
formation and evolution. Analytical and numerical dynamical models are powerful methods
to model galaxy mass distributions based on galaxy stellar kinematics, and are strengthened
by including gaseous kinematics.
In this thesis, I start my research with using descriptive analytical models to identify thin-disc
components in galaxies. I kinematically classify the sample into disc-dominated, non-disc-dominated
and disc-free galaxies and measure the dynamical mass for disc-dominated galaxies
to study their mass budget. I find a dichotomy of lenticular galaxies in my classification, which
is explained by the deficiency of molecular gas. Realising the limitation of analytical models,
I continue my research with numerical models. I develop an orbit-based method with stellar
kinematics modelled by the Schwarzschild technique, and cold gas (H I ) assumed as a thin disc
aligned with stars. The application of this method to early-type galaxy NGC 2974 shows that
including extended cold gas kinematics significantly narrows down the measurement of the
dark matter inner slope. I then further generalise this method to be adapted for galaxies with
misaligned stellar and gas discs and apply the generalised method to early-type NGC 3626.
The results suggest that with very extended cold gas discs out to 10 Rₑ, there is a chance to
determine the overall shape of dark matter halo and even distinguish between different halo
models. I finally model a larger sample of galaxies which have two-aperture stellar kinematics
using the Schwarzschild technique to study their orbital components and dark matter profiles
across a wider mass range. The dark matter fraction is typically 40%-70% at 3 Rₑ, and I find
no clear trends between the dark matter fraction and stellar mass.
2020-12-01T00:00:00ZYang, MengMapping galaxy mass distribution is an important means of understanding galaxy morphology
and the nature of dark matter haloes, and therefore build up our knowledge of galaxy
formation and evolution. Analytical and numerical dynamical models are powerful methods
to model galaxy mass distributions based on galaxy stellar kinematics, and are strengthened
by including gaseous kinematics.
In this thesis, I start my research with using descriptive analytical models to identify thin-disc
components in galaxies. I kinematically classify the sample into disc-dominated, non-disc-dominated
and disc-free galaxies and measure the dynamical mass for disc-dominated galaxies
to study their mass budget. I find a dichotomy of lenticular galaxies in my classification, which
is explained by the deficiency of molecular gas. Realising the limitation of analytical models,
I continue my research with numerical models. I develop an orbit-based method with stellar
kinematics modelled by the Schwarzschild technique, and cold gas (H I ) assumed as a thin disc
aligned with stars. The application of this method to early-type galaxy NGC 2974 shows that
including extended cold gas kinematics significantly narrows down the measurement of the
dark matter inner slope. I then further generalise this method to be adapted for galaxies with
misaligned stellar and gas discs and apply the generalised method to early-type NGC 3626.
The results suggest that with very extended cold gas discs out to 10 Rₑ, there is a chance to
determine the overall shape of dark matter halo and even distinguish between different halo
models. I finally model a larger sample of galaxies which have two-aperture stellar kinematics
using the Schwarzschild technique to study their orbital components and dark matter profiles
across a wider mass range. The dark matter fraction is typically 40%-70% at 3 Rₑ, and I find
no clear trends between the dark matter fraction and stellar mass.Nonlinear optical eigenmodes : perturbative approach for classical fields and single photonsDocherty-Walthew, Graeme Scotthttps://hdl.handle.net/10023/214762021-02-23T03:05:54Z2020-12-01T00:00:00ZIn linear optics, the concept of a mode or eigenmode is well established. Often these modes correspond to a set of fields that are mutually orthogonal with intensity profiles that are invariant as they propagate through a given optical system. More generally, using an eigenmode decomposition, one can define a set of orthogonal modes with respect to an optical measure given that is linear in the intensity of the fields or Hermitian in the fields themselves. However, if the intensity of the light is sufficiently large, the dipole response of an optical medium includes nonlinear terms that cause the eigenmode decomposition to break down. In this work, we introduce the eigenmode decomposition in the presence of these nonlinear source terms by introducing small perturbation fields whose interaction is mediated by some high-intensity background field.
Unlike the eigenmodes of linear optics, these novel modes correspond to a set of orthogonal fields that are, in general, distributed across multiple wavelengths. Here, we study the definition and interaction of these eigenmodes for classical electromagnetic fields and multiphoton fields. In the context of classical fields, with our eigenmodes established, we highlight the influence of the high-intensity background field on the symmetry of the eigenmodes. At the multiphoton level, we show that the description of multiphoton fields is simplified by using the propagation eigenmodes while remaining equivalent to the standard approach.
2020-12-01T00:00:00ZDocherty-Walthew, Graeme ScottIn linear optics, the concept of a mode or eigenmode is well established. Often these modes correspond to a set of fields that are mutually orthogonal with intensity profiles that are invariant as they propagate through a given optical system. More generally, using an eigenmode decomposition, one can define a set of orthogonal modes with respect to an optical measure given that is linear in the intensity of the fields or Hermitian in the fields themselves. However, if the intensity of the light is sufficiently large, the dipole response of an optical medium includes nonlinear terms that cause the eigenmode decomposition to break down. In this work, we introduce the eigenmode decomposition in the presence of these nonlinear source terms by introducing small perturbation fields whose interaction is mediated by some high-intensity background field.
Unlike the eigenmodes of linear optics, these novel modes correspond to a set of orthogonal fields that are, in general, distributed across multiple wavelengths. Here, we study the definition and interaction of these eigenmodes for classical electromagnetic fields and multiphoton fields. In the context of classical fields, with our eigenmodes established, we highlight the influence of the high-intensity background field on the symmetry of the eigenmodes. At the multiphoton level, we show that the description of multiphoton fields is simplified by using the propagation eigenmodes while remaining equivalent to the standard approach.Agile quantum cryptography and non-classical state generationThornton, Matthewhttps://hdl.handle.net/10023/213612021-08-14T13:27:35Z2020-12-01T00:00:00ZIn the first half of this Thesis, we introduce a framework of “quantum
cryptographic agility,” which allows for a resource-efficient swap of an
underlying cryptographic protocol. Specifically, we introduce several
schemes which perform the tasks of Digital Signatures and Secret
Sharing. Our first achievement is an investigation of Quantum Digital
Signatures (QDS) over a continuous-variables platform, consisting
of phase-encoded coherent states and heterodyne phase detection.
QDS allows for secure authentication of a classical message, while
guaranteeing message transferability. For the first time, we prove
security of CV QDS in the presence of an eavesdropper on the quantum
channels.
We then introduce a continuous variable (CV) Quantum Secret
Sharing (QSS) protocol. Our security proof allows for classical information
to be split and shared between multiple potentially dishonest
recipients, while retaining security against collective beamsplitter and
entangling-cloner attacks. In the last chapter of this half, we introduce
another QDS scheme which runs over identical hardware setup to
our QSS protocol. We analyse experimental data in which quantum
coherent states were distributed at a rate of 1 GHz, which for QDS
allows us to securely sign a message in less than 0.05 ms.
In the second half of this Thesis we suggest and discuss a deterministic
source of nonclassical light, which we call “PhoG”. Our source is
based on the coherent diffusive photonics, relying on both coherent
and dissipative evolution of the quantum state, and may be realised
in an array of dissipatively-coupled laser-inscribed waveguides in a χ⁽³⁾
glass. We analyse the PhoG device with several analytical and
numerical models and demonstrate that a coherent state input leads
to a bright output state with strong photon-number squeezing. With
minor reconfiguration our system can generate entanglement between
spatially separated modes via a process analogous to four-wave
mixing.
2020-12-01T00:00:00ZThornton, MatthewIn the first half of this Thesis, we introduce a framework of “quantum
cryptographic agility,” which allows for a resource-efficient swap of an
underlying cryptographic protocol. Specifically, we introduce several
schemes which perform the tasks of Digital Signatures and Secret
Sharing. Our first achievement is an investigation of Quantum Digital
Signatures (QDS) over a continuous-variables platform, consisting
of phase-encoded coherent states and heterodyne phase detection.
QDS allows for secure authentication of a classical message, while
guaranteeing message transferability. For the first time, we prove
security of CV QDS in the presence of an eavesdropper on the quantum
channels.
We then introduce a continuous variable (CV) Quantum Secret
Sharing (QSS) protocol. Our security proof allows for classical information
to be split and shared between multiple potentially dishonest
recipients, while retaining security against collective beamsplitter and
entangling-cloner attacks. In the last chapter of this half, we introduce
another QDS scheme which runs over identical hardware setup to
our QSS protocol. We analyse experimental data in which quantum
coherent states were distributed at a rate of 1 GHz, which for QDS
allows us to securely sign a message in less than 0.05 ms.
In the second half of this Thesis we suggest and discuss a deterministic
source of nonclassical light, which we call “PhoG”. Our source is
based on the coherent diffusive photonics, relying on both coherent
and dissipative evolution of the quantum state, and may be realised
in an array of dissipatively-coupled laser-inscribed waveguides in a χ⁽³⁾
glass. We analyse the PhoG device with several analytical and
numerical models and demonstrate that a coherent state input leads
to a bright output state with strong photon-number squeezing. With
minor reconfiguration our system can generate entanglement between
spatially separated modes via a process analogous to four-wave
mixing.Phase transitions in two-dimensional itinerant electron systemsTrott, Matthew J.https://hdl.handle.net/10023/210522020-12-08T03:09:45Z2020-12-01T00:00:00ZThe aim of this thesis it to contribute to three open problems in the theory of itinerant electron systems in two spatial dimensions. Firstly, the mechanism for charge density wave formation in the transition metal dichalcogenides is a debated subject. In this thesis it is shown that charge density wave formation is possible via a purely electronic mechanism in monolayer vanadium diselenide. The competition of superconductivity and density wave formation is taken into account using the renormalisation group. As the Fermi surface is tuned to perfect nesting, a charge density wave phase emerges when the Heisenberg exchange interaction is of the order of the contact Coulomb repulsion. Secondly, the search for materials which exhibit topological superconductivity is ongoing. Possible candidates are strongly spin-orbit-coupled metals. In this thesis a square-lattice Hubbard model with strong Rashba spin-orbit coupling and one of the Fermi surfaces close to a Lifshitz transition is examined. The metal is shown to be generically unstable to the formation of mixed-parity superconductivity with a helical triplet component via a renormalisation group analysis. Thirdly, the breakdown of Fermi liquid theory close to a quantum critical point is still not well understood. In this thesis a functional renormalisation group analysis is presented using a soft frequency cutoff, investigating a general class of Pomeranchuk instabilities with 𝑁[sub]𝑏 flavours of boson. At small 𝑁[sub]𝑏 the theory is characterised by weakly non-Fermi-liquid behaviour of the electrons and 𝑧≈2 dynamics for the order parameter fluctuations. For large 𝑁[sub]𝑏, the theory crosses over to 𝑧≈1 scaling and non-Fermi-liquid behaviour.
2020-12-01T00:00:00ZTrott, Matthew J.The aim of this thesis it to contribute to three open problems in the theory of itinerant electron systems in two spatial dimensions. Firstly, the mechanism for charge density wave formation in the transition metal dichalcogenides is a debated subject. In this thesis it is shown that charge density wave formation is possible via a purely electronic mechanism in monolayer vanadium diselenide. The competition of superconductivity and density wave formation is taken into account using the renormalisation group. As the Fermi surface is tuned to perfect nesting, a charge density wave phase emerges when the Heisenberg exchange interaction is of the order of the contact Coulomb repulsion. Secondly, the search for materials which exhibit topological superconductivity is ongoing. Possible candidates are strongly spin-orbit-coupled metals. In this thesis a square-lattice Hubbard model with strong Rashba spin-orbit coupling and one of the Fermi surfaces close to a Lifshitz transition is examined. The metal is shown to be generically unstable to the formation of mixed-parity superconductivity with a helical triplet component via a renormalisation group analysis. Thirdly, the breakdown of Fermi liquid theory close to a quantum critical point is still not well understood. In this thesis a functional renormalisation group analysis is presented using a soft frequency cutoff, investigating a general class of Pomeranchuk instabilities with 𝑁[sub]𝑏 flavours of boson. At small 𝑁[sub]𝑏 the theory is characterised by weakly non-Fermi-liquid behaviour of the electrons and 𝑧≈2 dynamics for the order parameter fluctuations. For large 𝑁[sub]𝑏, the theory crosses over to 𝑧≈1 scaling and non-Fermi-liquid behaviour.Photonic crystal cavity based optical induced transparencyHu, Changyuhttps://hdl.handle.net/10023/206652021-01-25T15:35:04Z2020-12-01T00:00:00ZNowadays, information technology has been deeply integrated in our daily life. However, within its rapid development, it faces a serious bottleneck due to the prohibitive power consumption and limited transmission bandwidth of electrical interconnects. Silicon photonics introduces a potential solution for information technology based on optical communication. In this field, delay-bandwidth devices offer a high bandwidth optical interconnection and low power consumption for the next generation information communication technology. Through introducing the slow light effect, I can realise time domain control and store the light to achieve a new functional component, which is the optical buffer for optical information processing. The optical buffer allows us to control and store the light, using as the optical information process and transit. However, the current optical buffer devices are limited by high optical loss and the ability to produced tunable group delay of the light. In this thesis, I examine different configurations of the coupled photonic crystal resonator system and then introduce a novel tuneable delay line, based on photonic crystal cavity structures. Through the optical analog to electromagnetically induced transparency (EIT), an EIT-like transmission spectrum has been achieved in coupled photonic crystal cavities. By tuning the phase difference between two coupled resonators and resonance wavelength, I can achieve the desired analog conditions and reach to a maximum group delay of 360 ps. By adding thermal tuning pattern, I have demonstrated a tuning of the group delay of over 120 ps range at a low input power and a maximum delay of 300 ps group delay in coupled photonic crystal cavities system. All devices are with a footprint at only 200 μm², and with integrated compatibles as well. By employing a new vertical coupling technique, a record low loss 15 dB/ns is presented making this system very promising for practical optical information applications.
2020-12-01T00:00:00ZHu, ChangyuNowadays, information technology has been deeply integrated in our daily life. However, within its rapid development, it faces a serious bottleneck due to the prohibitive power consumption and limited transmission bandwidth of electrical interconnects. Silicon photonics introduces a potential solution for information technology based on optical communication. In this field, delay-bandwidth devices offer a high bandwidth optical interconnection and low power consumption for the next generation information communication technology. Through introducing the slow light effect, I can realise time domain control and store the light to achieve a new functional component, which is the optical buffer for optical information processing. The optical buffer allows us to control and store the light, using as the optical information process and transit. However, the current optical buffer devices are limited by high optical loss and the ability to produced tunable group delay of the light. In this thesis, I examine different configurations of the coupled photonic crystal resonator system and then introduce a novel tuneable delay line, based on photonic crystal cavity structures. Through the optical analog to electromagnetically induced transparency (EIT), an EIT-like transmission spectrum has been achieved in coupled photonic crystal cavities. By tuning the phase difference between two coupled resonators and resonance wavelength, I can achieve the desired analog conditions and reach to a maximum group delay of 360 ps. By adding thermal tuning pattern, I have demonstrated a tuning of the group delay of over 120 ps range at a low input power and a maximum delay of 300 ps group delay in coupled photonic crystal cavities system. All devices are with a footprint at only 200 μm², and with integrated compatibles as well. By employing a new vertical coupling technique, a record low loss 15 dB/ns is presented making this system very promising for practical optical information applications.Interplay of spin-orbit coupling and crystal symmetries in the electronic structures of NbGeSb and Ca₃Ru₂O₇Markovic, Igorhttps://hdl.handle.net/10023/205312022-12-03T03:00:42Z2020-07-27T00:00:00ZThis thesis presents the study of electronic structure of two materials with strong spin-orbit coupling using angle-resolved photoemission spectroscopy (ARPES) experiments and density-functional theory (DFT) band calculations. The two materials are NbGeSb and Ca₃Ru₂O₇, which host weak and strong electronic interactions, respectively. While at first glance they seem rather disparate, I will show in both cases how novel phenomena emerge from the interplay of spin-orbit coupling and the crystal symmetries.
In NbGeSb, I combine insights from spin-integrated and spin-resolved ARPES measurements with DFT slab calculations to reveal how band inversion of two pairs of spin-orbit coupled surface states along the edge of the Brillouin zone results in a peculiar crossing structure with two protected and two asymmetrically gapped crossing points. I show how this is caused by the presence of a mirror symmetry line assigning definite mirror parity to orbital and spin angular momentum of the bands. This leads to a low-energy description of the crossing points equivalent to a two-dimensional Weyl equation, establishing them as 2D analogues of Weyl points.
In Ca₃Ru₂O₇, on the other hand, spin-orbit coupling provides a link between the electronic structure, the underlying antiferromagnetic order and the inherent antipolar distortion in the crystal structure. Our results reveal that a known structural and spin reorientation transition is caused by a spin-orbit derived gapping of a large Fermi surface. The hybridisation term couples the magnetic moment direction with the antipolar distortion of the crystal structure, and is only unlocked when the resulting electronic energy gain becomes enough to overcome the cost of spin reorientations.
These findings together highlight the abundance of possibilities for novel phenomena arising from the interplay of spin-orbit coupling and crystal symmetries in quantum materials.
2020-07-27T00:00:00ZMarkovic, IgorThis thesis presents the study of electronic structure of two materials with strong spin-orbit coupling using angle-resolved photoemission spectroscopy (ARPES) experiments and density-functional theory (DFT) band calculations. The two materials are NbGeSb and Ca₃Ru₂O₇, which host weak and strong electronic interactions, respectively. While at first glance they seem rather disparate, I will show in both cases how novel phenomena emerge from the interplay of spin-orbit coupling and the crystal symmetries.
In NbGeSb, I combine insights from spin-integrated and spin-resolved ARPES measurements with DFT slab calculations to reveal how band inversion of two pairs of spin-orbit coupled surface states along the edge of the Brillouin zone results in a peculiar crossing structure with two protected and two asymmetrically gapped crossing points. I show how this is caused by the presence of a mirror symmetry line assigning definite mirror parity to orbital and spin angular momentum of the bands. This leads to a low-energy description of the crossing points equivalent to a two-dimensional Weyl equation, establishing them as 2D analogues of Weyl points.
In Ca₃Ru₂O₇, on the other hand, spin-orbit coupling provides a link between the electronic structure, the underlying antiferromagnetic order and the inherent antipolar distortion in the crystal structure. Our results reveal that a known structural and spin reorientation transition is caused by a spin-orbit derived gapping of a large Fermi surface. The hybridisation term couples the magnetic moment direction with the antipolar distortion of the crystal structure, and is only unlocked when the resulting electronic energy gain becomes enough to overcome the cost of spin reorientations.
These findings together highlight the abundance of possibilities for novel phenomena arising from the interplay of spin-orbit coupling and crystal symmetries in quantum materials.Cool star magnetic field topologies : connecting simulations and observations for solar-like starsLehmann, Lisa Thereshttps://hdl.handle.net/10023/203542022-08-03T02:01:35Z2020-07-27T00:00:00ZGood knowledge about cool star magnetic activity, topology and cycles is crucial to find a
second solar system and to better understand ours. The Zeeman-Doppler-Imaging (ZDI) surveys,
which unveil the stellar magnetic field topology, have now run for long enough to detect
solar-like activity cycles. This is a good point to review what ZDI detects robustly and how to
interpret the resulting ZDI maps. As ZDI only detects the large-scale magnetic field an important
question to answer is: What can we learn from the large-scale field topology about the
small-scale field for solar-like stars?
I connect 3D non-potential flux transport simulations based on the Sun with the observational
ZDI technique. First, I decomposed the magnetic field topology of the simulations
into different length-scales. I discovered that the large-scale field reflects global properties of
the small-scale field emergence for slowly-rotating solar-like stars. Second, I used synthetic
line profiles modelled from the simulations as input for ZDI. I showed that ZDI can recover
the hints of the small-scale flux emergence in the observable large-scale field for slow rotators
but recovers approximately one order of magnitude lower magnetic energy. The maximum
entropy regularisation used in ZDI prevents the correct reconstruction of the magnetic energy
distribution but ZDI can recover the fractions of the different field components reasonably
well.
To examine if ZDI can recover solar-like cycles, I applied ZDI to non-potential flux transport
simulations modelling the solar magnetic field over 15 years. I discovered that the axisymmetric
poloidal fraction and the axi- and non-axisymmetric energy are the best parameters to
track solar-like activity cycles while the averaged large-scale field or the total energy show no
or misleading trends.
2020-07-27T00:00:00ZLehmann, Lisa TheresGood knowledge about cool star magnetic activity, topology and cycles is crucial to find a
second solar system and to better understand ours. The Zeeman-Doppler-Imaging (ZDI) surveys,
which unveil the stellar magnetic field topology, have now run for long enough to detect
solar-like activity cycles. This is a good point to review what ZDI detects robustly and how to
interpret the resulting ZDI maps. As ZDI only detects the large-scale magnetic field an important
question to answer is: What can we learn from the large-scale field topology about the
small-scale field for solar-like stars?
I connect 3D non-potential flux transport simulations based on the Sun with the observational
ZDI technique. First, I decomposed the magnetic field topology of the simulations
into different length-scales. I discovered that the large-scale field reflects global properties of
the small-scale field emergence for slowly-rotating solar-like stars. Second, I used synthetic
line profiles modelled from the simulations as input for ZDI. I showed that ZDI can recover
the hints of the small-scale flux emergence in the observable large-scale field for slow rotators
but recovers approximately one order of magnitude lower magnetic energy. The maximum
entropy regularisation used in ZDI prevents the correct reconstruction of the magnetic energy
distribution but ZDI can recover the fractions of the different field components reasonably
well.
To examine if ZDI can recover solar-like cycles, I applied ZDI to non-potential flux transport
simulations modelling the solar magnetic field over 15 years. I discovered that the axisymmetric
poloidal fraction and the axi- and non-axisymmetric energy are the best parameters to
track solar-like activity cycles while the averaged large-scale field or the total energy show no
or misleading trends.Strong matter-light coupling with organic molecules and inorganic semiconductorsStrashko, Artemhttps://hdl.handle.net/10023/203062021-04-07T15:13:49Z2019-12-03T00:00:00ZThis dissertation studies the effects of strong matter-light coupling on properties
of organic molecules and inorganic semiconductors. The interplay of
complex intramolecular dynamics and strong coupling of a photon to molecular
transitions results in new physics having no counterparts in other systems.
In contrast, low-energy optically active excitations of semiconductors (excitons)
usually do not feature such complexity. However, the combination of
strong electronic correlations and strong matter-light coupling leads to new
physics.
Firstly, the effect of strong coupling between molecular vibrations and
infrared photons on Raman scattering (RS) is considered. This is motivated by
the experiment of Ref. [1] showing up to 10³ enhancement of RS signal under
strong coupling. While the exact analytical results of this dissertation predict
around 100% enhancement of total RS signal, they cannot explain orders of
magnitude enhancement, leaving the question open for further studies.
Next, the effects of strong coupling of an optical photon and a molecular
electronic transition on molecular lasing properties are discussed. Starting
from a microscopic description of a driven-dissipative system, an exact (in the
thermodynamic limit) mean-field solution is developed. It allows to uncover
the mechanism of molecular lasing in the weak and strong coupling regime
and to obtain a non-equilibrium lasing phase diagram.
Finally, a semiconductor with different densities of electrons and holes,
strongly coupled to a microcavity photon, is studied. While finite electron-hole density imbalance is detrimental for excitonic condensation, it may still lead to a condensed state of excitons with finite centre of mass momentum
coexisting with unpaired electrons. On the other hand, due to its low mass,
a photon favours zero center of mass momentum condensation. The variational
mean-field calculations reveal that the interplay of these effects leads
to a variety of novel states with coexisting polariton condensate and unpaired
electrons.
2019-12-03T00:00:00ZStrashko, ArtemThis dissertation studies the effects of strong matter-light coupling on properties
of organic molecules and inorganic semiconductors. The interplay of
complex intramolecular dynamics and strong coupling of a photon to molecular
transitions results in new physics having no counterparts in other systems.
In contrast, low-energy optically active excitations of semiconductors (excitons)
usually do not feature such complexity. However, the combination of
strong electronic correlations and strong matter-light coupling leads to new
physics.
Firstly, the effect of strong coupling between molecular vibrations and
infrared photons on Raman scattering (RS) is considered. This is motivated by
the experiment of Ref. [1] showing up to 10³ enhancement of RS signal under
strong coupling. While the exact analytical results of this dissertation predict
around 100% enhancement of total RS signal, they cannot explain orders of
magnitude enhancement, leaving the question open for further studies.
Next, the effects of strong coupling of an optical photon and a molecular
electronic transition on molecular lasing properties are discussed. Starting
from a microscopic description of a driven-dissipative system, an exact (in the
thermodynamic limit) mean-field solution is developed. It allows to uncover
the mechanism of molecular lasing in the weak and strong coupling regime
and to obtain a non-equilibrium lasing phase diagram.
Finally, a semiconductor with different densities of electrons and holes,
strongly coupled to a microcavity photon, is studied. While finite electron-hole density imbalance is detrimental for excitonic condensation, it may still lead to a condensed state of excitons with finite centre of mass momentum
coexisting with unpaired electrons. On the other hand, due to its low mass,
a photon favours zero center of mass momentum condensation. The variational
mean-field calculations reveal that the interplay of these effects leads
to a variety of novel states with coexisting polariton condensate and unpaired
electrons.Organic semiconductors, diamond and fingerprint recovery : measuring the energy levels of materialsChallinger, Susanna Elisabethhttps://hdl.handle.net/10023/202722023-05-15T16:03:03Z2020-07-27T00:00:00ZOptimisation of the energy levels of materials is crucial for the creation of efficient multi-layer electronic devices. This thesis describes measurements of the energy levels of a variety of different types of materials: from cheap, solution processable, organic semiconductors with potential applications from solar cells to OLED displays, to diamond which is expensive but has excellent thermal properties that could make it ideal for high power RF applications. Additionally, common metal surfaces such as brass and iron are studied for small changes in their energy levels at microscopic scales to reveal invisible fingerprints in an energy level map with potential uses within forensic science and policing. These measurements are performed using Kelvin probe and ambient pressure photoemission spectroscopy non-destructive analysis tools.
A simple ambient pressure photoemission technique is used to study the ionisation energy of different conjugated polymers. These results were in good agreement with literature results and DFT calculations showing that this method offers a useful alternative to vacuum photoemission analysis or cyclic voltammetry.
For the first time, a variety of diamond samples were analysed using combined Kelvin probe and photoemission techniques under ambient conditions. Additionally, hydrogen terminated diamond was investigated under different pressures to study the effect of the presence of a surface water layer on the electron energy level behaviour.
A detailed study of fingerprint recovery using the scanning Kelvin probe (SKP) technique across a variety of metal surfaces and donors was conducted. The longevity of the signal was studied with clear identifiable fingermarks revealed from SKP scans after more than three years ageing. As a non-destructive method, SKP analysis of metal surfaces could provide an important first-stage analysis tool for fingerprint recovery or identification of possible touch DNA areas during the investigation of serious and major crime.
2020-07-27T00:00:00ZChallinger, Susanna ElisabethOptimisation of the energy levels of materials is crucial for the creation of efficient multi-layer electronic devices. This thesis describes measurements of the energy levels of a variety of different types of materials: from cheap, solution processable, organic semiconductors with potential applications from solar cells to OLED displays, to diamond which is expensive but has excellent thermal properties that could make it ideal for high power RF applications. Additionally, common metal surfaces such as brass and iron are studied for small changes in their energy levels at microscopic scales to reveal invisible fingerprints in an energy level map with potential uses within forensic science and policing. These measurements are performed using Kelvin probe and ambient pressure photoemission spectroscopy non-destructive analysis tools.
A simple ambient pressure photoemission technique is used to study the ionisation energy of different conjugated polymers. These results were in good agreement with literature results and DFT calculations showing that this method offers a useful alternative to vacuum photoemission analysis or cyclic voltammetry.
For the first time, a variety of diamond samples were analysed using combined Kelvin probe and photoemission techniques under ambient conditions. Additionally, hydrogen terminated diamond was investigated under different pressures to study the effect of the presence of a surface water layer on the electron energy level behaviour.
A detailed study of fingerprint recovery using the scanning Kelvin probe (SKP) technique across a variety of metal surfaces and donors was conducted. The longevity of the signal was studied with clear identifiable fingermarks revealed from SKP scans after more than three years ageing. As a non-destructive method, SKP analysis of metal surfaces could provide an important first-stage analysis tool for fingerprint recovery or identification of possible touch DNA areas during the investigation of serious and major crime.Femtosecond lasers for datacommunications applicationsLeburn, Christopher Gilmourhttps://hdl.handle.net/10023/200552020-06-09T02:04:11Z2005-11-30T00:00:00ZThe work
presented in this thesis details the development of all-solid-state ultrashort pulsed
lasers suitable for datacommunications applications at either 1300nm or 1550nm. This is
achieved
through the design and construction of three different types of laser system based
on the gain materials Cr⁴⁺:forsterite (chromium-doped magnesium iron silicate) and
Cr⁴⁺:YAG (chromium-doped yttrium aluminium garnet).
A Cr⁴⁺:forsterite based system is the first laser that is presented. This configuration
utilises a
relatively novel GalnNAs semiconductor device to initiate the generation of 130fs
pulses around 1300nm. Although GalnNAs devices have previously been used to generate
pulses of light in the picosecond domain, this is the first time ultrashort pulses have been
achieved in the femtosecond domain. As such, it has been possible to use the results from
this laser system to further the understanding of various dynamics of GalnNAs devices.
An SBR mode-locked Cr⁴⁺:YAG laser system introduces the concept of
Femtosecond pulse generation around 1550nm. This is done in order to lay the necessary
foundations for understanding the motivation and physics behind high pulse repetition
frequency (prf) all-solid state femtosecond lasers suitable for datacommunications
applications. Details are then given for the construction and operation of a simple 3-element
Cr⁴⁺:YAG laser that
generates 70fs pulses at a prf greater than 4GHz. The success of this
system leads to the development of a compact and robust engineered prototype with a
footprint of 215x 106mm².
Integration of the high prf laser systems into novel optical time division
multiplexing/wavelength division multiplexing (OTDM/WDM) based assessments prove
successful with the demonstration of a datacommunications system capable of generating
1.36Tb/s. This still remains to be the only system capable of achieving such a high capacity
from a
single source and demonstrates the ongoing success of femtosecond lasers through
continued research and
development.
2005-11-30T00:00:00ZLeburn, Christopher GilmourThe work
presented in this thesis details the development of all-solid-state ultrashort pulsed
lasers suitable for datacommunications applications at either 1300nm or 1550nm. This is
achieved
through the design and construction of three different types of laser system based
on the gain materials Cr⁴⁺:forsterite (chromium-doped magnesium iron silicate) and
Cr⁴⁺:YAG (chromium-doped yttrium aluminium garnet).
A Cr⁴⁺:forsterite based system is the first laser that is presented. This configuration
utilises a
relatively novel GalnNAs semiconductor device to initiate the generation of 130fs
pulses around 1300nm. Although GalnNAs devices have previously been used to generate
pulses of light in the picosecond domain, this is the first time ultrashort pulses have been
achieved in the femtosecond domain. As such, it has been possible to use the results from
this laser system to further the understanding of various dynamics of GalnNAs devices.
An SBR mode-locked Cr⁴⁺:YAG laser system introduces the concept of
Femtosecond pulse generation around 1550nm. This is done in order to lay the necessary
foundations for understanding the motivation and physics behind high pulse repetition
frequency (prf) all-solid state femtosecond lasers suitable for datacommunications
applications. Details are then given for the construction and operation of a simple 3-element
Cr⁴⁺:YAG laser that
generates 70fs pulses at a prf greater than 4GHz. The success of this
system leads to the development of a compact and robust engineered prototype with a
footprint of 215x 106mm².
Integration of the high prf laser systems into novel optical time division
multiplexing/wavelength division multiplexing (OTDM/WDM) based assessments prove
successful with the demonstration of a datacommunications system capable of generating
1.36Tb/s. This still remains to be the only system capable of achieving such a high capacity
from a
single source and demonstrates the ongoing success of femtosecond lasers through
continued research and
development.Measuring and controlling exciton diffusion, charge generation and charge extraction in organic and hybrid semiconductors for photovoltaic applicationsBlaszczyk, Oskarhttps://hdl.handle.net/10023/199502021-02-09T15:20:57Z2020-06-01T00:00:00ZThe growing demand for energy and the need for renewables as well as the advent of the internet of
things increase the demand for more versatile, efficient, cheap and environmentally friendly solar cells
based on organic and hybrid semiconductors. Understanding and controlling the underlying processes
that govern exciton diffusion, charge generation, charge extraction and domain size is of vital
importance to the efficiency, stability and scalability of these devices.
This thesis examines in detail the exciton and charge carrier behaviour in, and the characteristics of
new materials for organic and hybrid solar cell applications. A particular focus is put on measuring,
understanding and controlling hole extraction from 𝐶𝐻₃𝑁𝐻₃𝑃𝑏𝐼₃ to hole extracting layers and
overcoming the trade-off between exciton harvesting and charge extraction in small molecule bulk
heterojunction organic solar cells. The main method used for this investigation was ultrafast timeresolved
spectroscopy,
specifically
ultrafast
optical
transient
absorption
and
ultrafast
time-resolved
fluorescence
decay
with a
streak
camera.
A study of hole extraction from 𝐶𝐻₃𝑁𝐻₃𝑃𝑏𝐼₃ was carried out for two different hole extracting layers,
the standard PEDOT:PSS polymer used in the inverted p-i-n perovskite solar cells and a new nanoparticle NiO low temperature solution processed thin film. The two extraction layers and the
𝐶𝐻₃𝑁𝐻₃𝑃𝑏𝐼₃ perovskite active layer were first characterized using optical and physical methods such
as UV-Vis spectroscopy and atomic force microscopy as well as air photoemission spectroscopy to
confirm that the same perovskite was grown on top of both PEDOT:PSS and NiO and to investigate
energy level alignment. A new method based on the ultrafast photoluminescence surface quenching
experiment was developed and introduced which allows for the separation of bulk and interfacial
effects on charge extraction from thin films by illuminating the samples from opposite sides. This new
method was used to compare hole extraction from 𝐶𝐻₃𝑁𝐻₃𝑃𝑏𝐼₃ to NiO and PEDOT:PSS. It was found
that NiO shows faster hole extraction from the 300 nm thick perovskite film than PEDOT:PSS on the
time scale of 300 ps, which is independent of charge carrier density in the region of 10¹⁶-10¹⁷ cm⁻³. The interface with PEDOT:PSS was found to severely limit charge extraction rate at charge densities
exceeding 10¹⁶ cm⁻³. Furthermore, the transfer rate was found to decrease with time and to be
dependent on charge density in the region 10¹⁶-10¹⁷ cm⁻³ which we interpreted as charge
accumulation. These findings were confirmed by transient absorption spectroscopy. Hole diffusion
coefficient 𝐷 = 2.2 cm²/s ± 0.4 cm²/s and quenching rate k=3.6 × 10⁵ m/s ±0.2 m/s were
determined in the perovskite film that were independent of charge density. This indicates a band-like
hole transport regime, not observed for solution processed CH₃NH₃PbI₃ films before. Our findings
stress the importance of interface optimization in devices based on perovskite active layers as even in the case of the superior quencher, NiO, there is still room for improvement of the interfacial transfer
rate.
The trade-off between exciton harvesting and charge extraction in small molecule bulk heterojunction
organic solar cells was tackled by employing a post processing method of solvent vapour annealing on
thin films of DR3TBDTT:PC₇₁BM and SMPV1:PC₇₁BM. It was found that as a result of annealing with
carbon disulfide, the UV-Vis absorption spectrum changes which indicates changes to the structure of
the film. It was further revealed, using exciton-exciton annihilation, that the exciton diffusion
coefficient and exciton diffusion length are increased (almost 3-fold for the best case) as a result of
solvent vapour annealing. Furthermore, enhanced device performance after treatment with carbon
disulfide was recorded; this is explained by better charge extraction, an insight revealed by a transient
absorption study. Finally, using an all optical method for domain size determination it was found that
solvent vapour annealing can be used to increase domain size. Normally increased domain size would
have a detrimental effect on device performance due to a loss in the number of excitons which can
reach the donor-acceptor interface, but the increased exciton diffusion length allows us to overcome
this trade-off and achieve better device performance.
2020-06-01T00:00:00ZBlaszczyk, OskarThe growing demand for energy and the need for renewables as well as the advent of the internet of
things increase the demand for more versatile, efficient, cheap and environmentally friendly solar cells
based on organic and hybrid semiconductors. Understanding and controlling the underlying processes
that govern exciton diffusion, charge generation, charge extraction and domain size is of vital
importance to the efficiency, stability and scalability of these devices.
This thesis examines in detail the exciton and charge carrier behaviour in, and the characteristics of
new materials for organic and hybrid solar cell applications. A particular focus is put on measuring,
understanding and controlling hole extraction from 𝐶𝐻₃𝑁𝐻₃𝑃𝑏𝐼₃ to hole extracting layers and
overcoming the trade-off between exciton harvesting and charge extraction in small molecule bulk
heterojunction organic solar cells. The main method used for this investigation was ultrafast timeresolved
spectroscopy,
specifically
ultrafast
optical
transient
absorption
and
ultrafast
time-resolved
fluorescence
decay
with a
streak
camera.
A study of hole extraction from 𝐶𝐻₃𝑁𝐻₃𝑃𝑏𝐼₃ was carried out for two different hole extracting layers,
the standard PEDOT:PSS polymer used in the inverted p-i-n perovskite solar cells and a new nanoparticle NiO low temperature solution processed thin film. The two extraction layers and the
𝐶𝐻₃𝑁𝐻₃𝑃𝑏𝐼₃ perovskite active layer were first characterized using optical and physical methods such
as UV-Vis spectroscopy and atomic force microscopy as well as air photoemission spectroscopy to
confirm that the same perovskite was grown on top of both PEDOT:PSS and NiO and to investigate
energy level alignment. A new method based on the ultrafast photoluminescence surface quenching
experiment was developed and introduced which allows for the separation of bulk and interfacial
effects on charge extraction from thin films by illuminating the samples from opposite sides. This new
method was used to compare hole extraction from 𝐶𝐻₃𝑁𝐻₃𝑃𝑏𝐼₃ to NiO and PEDOT:PSS. It was found
that NiO shows faster hole extraction from the 300 nm thick perovskite film than PEDOT:PSS on the
time scale of 300 ps, which is independent of charge carrier density in the region of 10¹⁶-10¹⁷ cm⁻³. The interface with PEDOT:PSS was found to severely limit charge extraction rate at charge densities
exceeding 10¹⁶ cm⁻³. Furthermore, the transfer rate was found to decrease with time and to be
dependent on charge density in the region 10¹⁶-10¹⁷ cm⁻³ which we interpreted as charge
accumulation. These findings were confirmed by transient absorption spectroscopy. Hole diffusion
coefficient 𝐷 = 2.2 cm²/s ± 0.4 cm²/s and quenching rate k=3.6 × 10⁵ m/s ±0.2 m/s were
determined in the perovskite film that were independent of charge density. This indicates a band-like
hole transport regime, not observed for solution processed CH₃NH₃PbI₃ films before. Our findings
stress the importance of interface optimization in devices based on perovskite active layers as even in the case of the superior quencher, NiO, there is still room for improvement of the interfacial transfer
rate.
The trade-off between exciton harvesting and charge extraction in small molecule bulk heterojunction
organic solar cells was tackled by employing a post processing method of solvent vapour annealing on
thin films of DR3TBDTT:PC₇₁BM and SMPV1:PC₇₁BM. It was found that as a result of annealing with
carbon disulfide, the UV-Vis absorption spectrum changes which indicates changes to the structure of
the film. It was further revealed, using exciton-exciton annihilation, that the exciton diffusion
coefficient and exciton diffusion length are increased (almost 3-fold for the best case) as a result of
solvent vapour annealing. Furthermore, enhanced device performance after treatment with carbon
disulfide was recorded; this is explained by better charge extraction, an insight revealed by a transient
absorption study. Finally, using an all optical method for domain size determination it was found that
solvent vapour annealing can be used to increase domain size. Normally increased domain size would
have a detrimental effect on device performance due to a loss in the number of excitons which can
reach the donor-acceptor interface, but the increased exciton diffusion length allows us to overcome
this trade-off and achieve better device performance.Through-space interactions in thermally activated delayed fluorescent emitters : novel materials and organic light emitting diodesSharma, Nidhihttps://hdl.handle.net/10023/196722021-03-11T11:12:17Z2020-06-22T00:00:00ZThermally activated delayed fluorescence (TADF) has emerged as one of the most promising and efficient approaches realizing highly efficient organic light emitting diodes (OLEDs). This attractive approach utilizes organic emitters that can harvest all the excitons generated during the electroluminescent process to achieve 100% internal quantum efficiency. The TADF mechanism relies on the recruitment of triplet excitons, which occurs through their conversion to singlet excitons through a rapid reverse intersystem crossing (RISC) made possible by a very small singlet-triplet excited state energy difference (ΔE[sub](ST)). Since the first report of an efficient TADF OLED was published in 2012, the field has witnessed a tremendous development over the last 8 years. Many efforts have been devoted in developing an ideal TADF emitter with high photoluminescence quantum yield (Φ[sub](PL)), small ΔE[sub](ST), short delayed electroluminescence lifetime (τ[sub](d)) which should translate to OLEDs with high external quantum efficiencies (EQEs) and better efficiency roll offs. Various design strategies have been proposed, explored and adopted to optimize TADF materials for OLED applications. Throughout the course of this thesis, a design strategy based on “through-space” interactions has been extensively explored to optimize the material and device parameters in TADF. [2.2]Paracyclophane and spiro-conjugated scaffolds are introduced to the design pool of TADF materials and through the careful modulation of through-space interactions both theoretically and experimentally, the photo-physical properties are optimized, and state-of-the-art OLED performances are demonstrated.
2020-06-22T00:00:00ZSharma, NidhiThermally activated delayed fluorescence (TADF) has emerged as one of the most promising and efficient approaches realizing highly efficient organic light emitting diodes (OLEDs). This attractive approach utilizes organic emitters that can harvest all the excitons generated during the electroluminescent process to achieve 100% internal quantum efficiency. The TADF mechanism relies on the recruitment of triplet excitons, which occurs through their conversion to singlet excitons through a rapid reverse intersystem crossing (RISC) made possible by a very small singlet-triplet excited state energy difference (ΔE[sub](ST)). Since the first report of an efficient TADF OLED was published in 2012, the field has witnessed a tremendous development over the last 8 years. Many efforts have been devoted in developing an ideal TADF emitter with high photoluminescence quantum yield (Φ[sub](PL)), small ΔE[sub](ST), short delayed electroluminescence lifetime (τ[sub](d)) which should translate to OLEDs with high external quantum efficiencies (EQEs) and better efficiency roll offs. Various design strategies have been proposed, explored and adopted to optimize TADF materials for OLED applications. Throughout the course of this thesis, a design strategy based on “through-space” interactions has been extensively explored to optimize the material and device parameters in TADF. [2.2]Paracyclophane and spiro-conjugated scaffolds are introduced to the design pool of TADF materials and through the careful modulation of through-space interactions both theoretically and experimentally, the photo-physical properties are optimized, and state-of-the-art OLED performances are demonstrated.Diode-pumped Tm³⁺-doped sesquioxide lasers for ultrashort pulse applications in the 2μm regionStevenson, Neil Kennethhttps://hdl.handle.net/10023/196292021-11-30T11:24:30Z2020-06-22T00:00:00ZThis thesis presents the development of Tm³⁺-doped sesquioxide laser sources in the 2–2.1 μm spectral region. The primary focus of this development has been aimed towards high power diode-pumped mode-locked laser sources capable of femtosecond pulse generation. In addition to this, the early development of a compact and low threshold ultrafast laser inscribed waveguide laser has also been realised.
Continuous wave characterisation of bulk solid-state crystalline Tm:LuScO₃ and ceramic Tm:Lu₂O₃ lasers has been completed using ~795 nm multimode single emitter laser diode pump sources. Average output powers of 660 mW and 901 mW, and emission wavelengths of 2.1 μm and 2.06 μm were achieved from the Tm:LuScO₃ and Tm:Lu₂O₃ lasers, respectively. In addition, both lasers demonstrated smooth and continuous tuning ranges spanning more than 160 nm in the ~2–2.1 μm spectral region.
In the mode-locked regime, pulse durations as short as 170 fs were recorded at an average output power of 113 mW and an emission wavelength of 2094 nm from a diode-pumped mode-locked Tm:LuScO₃ laser through the use of an ion-implanted InGaAsSb quantum-well-based semiconductor saturable absorber mirror. A diode-pumped Tm:Lu₂O₃ laser, utilising the same semiconductor saturable absorber mirror, was able to generate pulses as short as 278 fs at an average output power of 555 mW and a wavelength of 2081 nm through the use of a steeply diving optic axis birefringent filter. This same filter was also used to demonstrate broadly tunable femtosecond pulses in both laser configurations. Subsequent amplification of the ultrashort pulse laser sources realised maximum amplified average output powers of 540 mW and 855 mW, respectively.
The results presented in this thesis demonstrate the potential for diode-pumped Tm³⁺-doped sesquioxide laser sources to be developed into an enabler technology for the advancement of a number of photonics applications and techniques in the mid-infrared region.
2020-06-22T00:00:00ZStevenson, Neil KennethThis thesis presents the development of Tm³⁺-doped sesquioxide laser sources in the 2–2.1 μm spectral region. The primary focus of this development has been aimed towards high power diode-pumped mode-locked laser sources capable of femtosecond pulse generation. In addition to this, the early development of a compact and low threshold ultrafast laser inscribed waveguide laser has also been realised.
Continuous wave characterisation of bulk solid-state crystalline Tm:LuScO₃ and ceramic Tm:Lu₂O₃ lasers has been completed using ~795 nm multimode single emitter laser diode pump sources. Average output powers of 660 mW and 901 mW, and emission wavelengths of 2.1 μm and 2.06 μm were achieved from the Tm:LuScO₃ and Tm:Lu₂O₃ lasers, respectively. In addition, both lasers demonstrated smooth and continuous tuning ranges spanning more than 160 nm in the ~2–2.1 μm spectral region.
In the mode-locked regime, pulse durations as short as 170 fs were recorded at an average output power of 113 mW and an emission wavelength of 2094 nm from a diode-pumped mode-locked Tm:LuScO₃ laser through the use of an ion-implanted InGaAsSb quantum-well-based semiconductor saturable absorber mirror. A diode-pumped Tm:Lu₂O₃ laser, utilising the same semiconductor saturable absorber mirror, was able to generate pulses as short as 278 fs at an average output power of 555 mW and a wavelength of 2081 nm through the use of a steeply diving optic axis birefringent filter. This same filter was also used to demonstrate broadly tunable femtosecond pulses in both laser configurations. Subsequent amplification of the ultrashort pulse laser sources realised maximum amplified average output powers of 540 mW and 855 mW, respectively.
The results presented in this thesis demonstrate the potential for diode-pumped Tm³⁺-doped sesquioxide laser sources to be developed into an enabler technology for the advancement of a number of photonics applications and techniques in the mid-infrared region.Tensor network simulations of open quantum systemsKilda, Dainiushttps://hdl.handle.net/10023/195842023-12-02T03:08:06Z2020-06-22T00:00:00ZThe nonequilibrium effects of dissipation and drive play a key role in an immense variety of nanoscale and mesoscale quantum systems. To understand the behaviour of open quantum systems, we need accurate methods that capture the influence of the environment on the system, while managing the exponentially large Hilbert space required to describe the system. Tensor network algorithms offer an efficient way to approach this challenge. In this thesis, we develop and apply tensor network techniques to study the dynamics and steady states of various open quantum systems.
The first part of the thesis focuses on the driven dissipative many body physics in coupled cavity arrays described by Born-Markov master equations. We extend transfer matrix product operator methods to Liouvillian dynamics, and utilize them to compute dynamical correlation functions and fluorescence spectrum of an infinite coupled cavity array in 1D. We also investigate thermalization, and observe the emergence of a quasi-thermal steady state with a negative effective temperature. In another study, we use infinite projected entangled pair state (iPEPS) methods to compute steady states of coupled cavity lattices in 2D. We find that a straightforward adaptation of iPEPS to Liouvillian dynamics is unstable, contradicting a recent publication in the field.
The second part investigates more general systems involving strong couplings and structured environments that induce non-Markovian dynamics. We develop a powerful time-evolving matrix product operator (TEMPO) algorithm that builds on Feynman-Vernon influence functional formalism, and uses matrix product states (MPS) to represent the temporal non-Markovian correlations efficiently. We apply TEMPO to study the localization phase transition of the spin-boson model and the dynamics of two spatially separated two-level systems coupled to a common environment. Finally, we propose the Toblerone TEMPO algorithm, which extends TEMPO to many-body systems interacting with general bosonic environments.
2020-06-22T00:00:00ZKilda, DainiusThe nonequilibrium effects of dissipation and drive play a key role in an immense variety of nanoscale and mesoscale quantum systems. To understand the behaviour of open quantum systems, we need accurate methods that capture the influence of the environment on the system, while managing the exponentially large Hilbert space required to describe the system. Tensor network algorithms offer an efficient way to approach this challenge. In this thesis, we develop and apply tensor network techniques to study the dynamics and steady states of various open quantum systems.
The first part of the thesis focuses on the driven dissipative many body physics in coupled cavity arrays described by Born-Markov master equations. We extend transfer matrix product operator methods to Liouvillian dynamics, and utilize them to compute dynamical correlation functions and fluorescence spectrum of an infinite coupled cavity array in 1D. We also investigate thermalization, and observe the emergence of a quasi-thermal steady state with a negative effective temperature. In another study, we use infinite projected entangled pair state (iPEPS) methods to compute steady states of coupled cavity lattices in 2D. We find that a straightforward adaptation of iPEPS to Liouvillian dynamics is unstable, contradicting a recent publication in the field.
The second part investigates more general systems involving strong couplings and structured environments that induce non-Markovian dynamics. We develop a powerful time-evolving matrix product operator (TEMPO) algorithm that builds on Feynman-Vernon influence functional formalism, and uses matrix product states (MPS) to represent the temporal non-Markovian correlations efficiently. We apply TEMPO to study the localization phase transition of the spin-boson model and the dynamics of two spatially separated two-level systems coupled to a common environment. Finally, we propose the Toblerone TEMPO algorithm, which extends TEMPO to many-body systems interacting with general bosonic environments.Majorana mediated non-local charge dynamics in topological superconductorsVan Beek, Ian J.https://hdl.handle.net/10023/192482021-04-13T13:32:41Z2019-12-03T00:00:00ZTopology has enjoyed great success as a paradigm for the classification and understanding
of condensed matter outside the framework of spontaneously broken
symmetry. This success is all the more remarkable considering that the impact of
interactions, in particular the Coulomb interaction between electrons, has been neglected
in most analyses. Experience in topologically trivial systems demonstrates
that, beyond simply leading to quantitative modifications, interactions can give rise
to qualitatively new physics in condensed matter. This thesis explores the interplay
between interaction effects and topologically non-trivial states and demonstrates
how this interplay can lead to novel physics which is fundamentally contingent upon
both a system's topological character and interactions.
The prototypical example of a topological state in condensed matter is the Majorana
bound state (MBS). In the work presented here, MBSs are significant because they
lead to non-local fermionic states in superconductors that are bound to near-zero
energy, inside the superconducting gap. The new physics arising from the synergy
of MBSs and electron-electron interactions is illustrated by two examples. A
Majorana-based analogue of the Kondo system is found to exhibit signs of a delocalised
many-body state consisting of electrons from both metallic leads and a
superconducting condensate. The presence of MBSs in a current driven capacitive
Josephson junction enables excitation of the system to a non-equilibrium state and
profoundly affects the overall charge dynamics of the junction.
This thesis offers compelling evidence for the importance of interactions in the context
of topologically non-trivial systems, not only with regard to determining the
topology of the system per se, but also as the means by which new physics is realised.
2019-12-03T00:00:00ZVan Beek, Ian J.Topology has enjoyed great success as a paradigm for the classification and understanding
of condensed matter outside the framework of spontaneously broken
symmetry. This success is all the more remarkable considering that the impact of
interactions, in particular the Coulomb interaction between electrons, has been neglected
in most analyses. Experience in topologically trivial systems demonstrates
that, beyond simply leading to quantitative modifications, interactions can give rise
to qualitatively new physics in condensed matter. This thesis explores the interplay
between interaction effects and topologically non-trivial states and demonstrates
how this interplay can lead to novel physics which is fundamentally contingent upon
both a system's topological character and interactions.
The prototypical example of a topological state in condensed matter is the Majorana
bound state (MBS). In the work presented here, MBSs are significant because they
lead to non-local fermionic states in superconductors that are bound to near-zero
energy, inside the superconducting gap. The new physics arising from the synergy
of MBSs and electron-electron interactions is illustrated by two examples. A
Majorana-based analogue of the Kondo system is found to exhibit signs of a delocalised
many-body state consisting of electrons from both metallic leads and a
superconducting condensate. The presence of MBSs in a current driven capacitive
Josephson junction enables excitation of the system to a non-equilibrium state and
profoundly affects the overall charge dynamics of the junction.
This thesis offers compelling evidence for the importance of interactions in the context
of topologically non-trivial systems, not only with regard to determining the
topology of the system per se, but also as the means by which new physics is realised.New frontiers in strain-tuning : apparatus development, and tuning of the nemacity of FeSe across a wide strain rangeBartlett, Jack Michaelhttps://hdl.handle.net/10023/190582023-04-12T02:07:40Z2019-06-24T00:00:00ZOver the last decade, an ‘iron age’ of superconductivity has challenged the paradigm of
unconventional pairing established by copper-oxide-based materials. Fascinatingly, in
these iron-based compounds superconductivity emerges from a state in which electrons
choose to distinguish between two equivalent directions of the underlying crystalline
axes. The origin of this ‘nematic’ state is highly debated.
This thesis concentrates on FeSe, a material appealing because its nematicity
does not occur in proximity to long-range magnetic order. Although uniaxial strain
couples to nematic order, experiments to date have focused on applying only a small
symmetry-breaking strain. The mechanical properties of FeSe make utilising established
piezoelectric-based apparatus, designed for continuous tuning of large uniaxial strains,
challenging. In this thesis we develop a platform to which samples can be adhered, and
apply large anisotropic strain to FeSe. When of the same symmetry as the nematicity,
and larger than the structural distortion, this applied strain fully constrains the lattice.
We provide a precise set of resistivity measurements across a wide temperature and
strain range, revealing vital new phenomenologies.
We establish the relationship between electrical transport and nematicity across
a large strain range at the structural transition and, by isolating the influence of domain walls, characterise the elastoresistivity for temperatures below this transition.
By tracking the onset of domain formation, we determine the temperature dependence
of the spontaneous structural distortion, and use this to extract the intrinsic resistivity
anisotropy within a single nematic domain. Interestingly, we discover a crossover at ~ 50 K between distinct high- and low-temperature behaviours.
This thesis is also concerned with the development of apparatus for tuning strain.
We conceptualise a new type of stress-controlled cell, which can apply large (up to
8 GPa in compression) uniaxial stresses to microstructured samples – pushing them to
their ultimate mechanical limit.
2019-06-24T00:00:00ZBartlett, Jack MichaelOver the last decade, an ‘iron age’ of superconductivity has challenged the paradigm of
unconventional pairing established by copper-oxide-based materials. Fascinatingly, in
these iron-based compounds superconductivity emerges from a state in which electrons
choose to distinguish between two equivalent directions of the underlying crystalline
axes. The origin of this ‘nematic’ state is highly debated.
This thesis concentrates on FeSe, a material appealing because its nematicity
does not occur in proximity to long-range magnetic order. Although uniaxial strain
couples to nematic order, experiments to date have focused on applying only a small
symmetry-breaking strain. The mechanical properties of FeSe make utilising established
piezoelectric-based apparatus, designed for continuous tuning of large uniaxial strains,
challenging. In this thesis we develop a platform to which samples can be adhered, and
apply large anisotropic strain to FeSe. When of the same symmetry as the nematicity,
and larger than the structural distortion, this applied strain fully constrains the lattice.
We provide a precise set of resistivity measurements across a wide temperature and
strain range, revealing vital new phenomenologies.
We establish the relationship between electrical transport and nematicity across
a large strain range at the structural transition and, by isolating the influence of domain walls, characterise the elastoresistivity for temperatures below this transition.
By tracking the onset of domain formation, we determine the temperature dependence
of the spontaneous structural distortion, and use this to extract the intrinsic resistivity
anisotropy within a single nematic domain. Interestingly, we discover a crossover at ~ 50 K between distinct high- and low-temperature behaviours.
This thesis is also concerned with the development of apparatus for tuning strain.
We conceptualise a new type of stress-controlled cell, which can apply large (up to
8 GPa in compression) uniaxial stresses to microstructured samples – pushing them to
their ultimate mechanical limit.A study of novel superconducting states in hybrid ferromagnetic-superconducting metamaterials and thin film devicesStewart, Rheahttps://hdl.handle.net/10023/189562021-04-20T16:15:43Z2019-12-03T00:00:00ZIn hybrid mesoscopic systems the Meissner response of a superconducting film can be very different from its bulk behaviour. For instance, in normal (N) superconductor (S) bilayers screening can be greatly enhanced depending on the relative material properties and interface conditions. Furthermore, with the addition of ferromagnetic (F) layers comes the possibility of generating a paramagnetic screening response due to spin triplet pairs. Such pairs are produced from the mixing of opposing microscopic orders within S (which conventionally hosts electron pairs of opposite spin and momenta) and F (inside which spin symmetry is broken). The net result is a conversion of a fraction of the pairs to an odd-frequency s-wave triplet state. Any resultant modifications to the screening manifest in the flux profile across a sample which is directly probed using low energy muon spin rotation (LEμSR).
Results of LEμSR experiments involving layered systems comprised of N, S and F elements are presented within chapter 3. A discrepancy between the pre-existing quasiclassical theory and measured flux profiles in the presence of an F layer was observed. A large enhancement to the flux lowering which could not be interpreted within the traditional S/F proximity picture was found. New theoretical developments have since suggested that coupled to direct electronic proximity effects within these systems is an additional electromagnetic component. Chapter 4 presents the results of analysing LEμSR data using a spatial flux profile consistent with new theory. In doing so, the observed anomalous enhancement could be successfully reconciled with the electromagnetic proximity effect. Subsequent experimental work in chapters 4 - 6 sought to test the main predictions of the theory by manipulating S/F interfaces within a variety of different structures. In all tested cases the theory provided an excellent description of experiment suggesting the importance of considering electromagnetic effects within S/F hybrid systems.
2019-12-03T00:00:00ZStewart, RheaIn hybrid mesoscopic systems the Meissner response of a superconducting film can be very different from its bulk behaviour. For instance, in normal (N) superconductor (S) bilayers screening can be greatly enhanced depending on the relative material properties and interface conditions. Furthermore, with the addition of ferromagnetic (F) layers comes the possibility of generating a paramagnetic screening response due to spin triplet pairs. Such pairs are produced from the mixing of opposing microscopic orders within S (which conventionally hosts electron pairs of opposite spin and momenta) and F (inside which spin symmetry is broken). The net result is a conversion of a fraction of the pairs to an odd-frequency s-wave triplet state. Any resultant modifications to the screening manifest in the flux profile across a sample which is directly probed using low energy muon spin rotation (LEμSR).
Results of LEμSR experiments involving layered systems comprised of N, S and F elements are presented within chapter 3. A discrepancy between the pre-existing quasiclassical theory and measured flux profiles in the presence of an F layer was observed. A large enhancement to the flux lowering which could not be interpreted within the traditional S/F proximity picture was found. New theoretical developments have since suggested that coupled to direct electronic proximity effects within these systems is an additional electromagnetic component. Chapter 4 presents the results of analysing LEμSR data using a spatial flux profile consistent with new theory. In doing so, the observed anomalous enhancement could be successfully reconciled with the electromagnetic proximity effect. Subsequent experimental work in chapters 4 - 6 sought to test the main predictions of the theory by manipulating S/F interfaces within a variety of different structures. In all tested cases the theory provided an excellent description of experiment suggesting the importance of considering electromagnetic effects within S/F hybrid systems.Epsilon-near-zero metamaterials for optoelectronic applicationsLi, Xinhttps://hdl.handle.net/10023/189382021-04-08T15:46:08Z2019-12-03T00:00:00ZMy PhD is focused on the design, fabrication and optoelectronic applications of epsilon-near-zero
(ENZ) metamaterials (MMs), which have vanishing real part of the permittivity and
support fascinating optical effects, including light squeezing, sub-wavelength imaging, enhanced
directive emission and enhanced non-resonant optical nonlinearity. Different approaches have
been developed to realise ENZ media, such as tuning the modal dispersion in narrow plasmonic
channels, exploiting the natural dispersion of transparent conductive oxides and creating composite
structures with metals and dielectrics. As a platform with exotic physical properties, ENZ media
also exhibit enormous potentials in combination with tunability and various nanofeatures in the
photonic and plasmonic regimes.
This thesis demonstrates two approaches to achieve the ENZ condition. One method is to stack
metal (Ag) and dielectric (SiO2) layers periodically at sub-wavelength scales. The resulting material
behaves as an effective medium with an average permittivity close to zero, and we show that
this ENZ medium can enhance the emission of quantum dots. This approach generally requires
nanofabrication techniques developed for flat and rigid substrates, for example, the electron beam
evaporation, which are not always applicable to micro- and macroscopic devices with arbitrary
shapes. To surpass these limitations, we design and experimentally demonstrate an optical freestanding and low-loss ENZ membrane in the visible range, by layering polymer (SU-8) and Ag
nano-layers. Additionally, we propose a method to introduce both flexibility and electrical tunability
into ENZ media by replacing the metal layer with a 2D material, graphene, in the multilayer model.
The other way to obtain an ENZ response is using natural materials which operate in proximity
of their plasma frequency, typically here the indium tin oxide (ITO) at the near-infrared
range. The ITO thin films are deposited using radio frequency magnetron sputtering, and their
permittivities are manipulated via controlling fabrication parameters. We succeed in sweeping the
zero-permittivity frequency of ITO media by controlling the gas recipe and deposition temperature
during the sputtering process. To obtain specific optical responses, the ENZ ITO thin films are
designed to be combined with different photonic features, including nanoantenna on microsphere
and nanohelix, associated with a direct fabrication approach based on electron beam induced
deposition (EBID).
Furthermore, this thesis extends the research range by realising the photonic trimming of
quantum emitters via various metallic nanofeatures fabricated directly using EBID method. We
believe that the interaction of ENZ MMs with EBID approach offers an opportunity to create hybrid
ENZ platforms for optoelectronic applications.
2019-12-03T00:00:00ZLi, XinMy PhD is focused on the design, fabrication and optoelectronic applications of epsilon-near-zero
(ENZ) metamaterials (MMs), which have vanishing real part of the permittivity and
support fascinating optical effects, including light squeezing, sub-wavelength imaging, enhanced
directive emission and enhanced non-resonant optical nonlinearity. Different approaches have
been developed to realise ENZ media, such as tuning the modal dispersion in narrow plasmonic
channels, exploiting the natural dispersion of transparent conductive oxides and creating composite
structures with metals and dielectrics. As a platform with exotic physical properties, ENZ media
also exhibit enormous potentials in combination with tunability and various nanofeatures in the
photonic and plasmonic regimes.
This thesis demonstrates two approaches to achieve the ENZ condition. One method is to stack
metal (Ag) and dielectric (SiO2) layers periodically at sub-wavelength scales. The resulting material
behaves as an effective medium with an average permittivity close to zero, and we show that
this ENZ medium can enhance the emission of quantum dots. This approach generally requires
nanofabrication techniques developed for flat and rigid substrates, for example, the electron beam
evaporation, which are not always applicable to micro- and macroscopic devices with arbitrary
shapes. To surpass these limitations, we design and experimentally demonstrate an optical freestanding and low-loss ENZ membrane in the visible range, by layering polymer (SU-8) and Ag
nano-layers. Additionally, we propose a method to introduce both flexibility and electrical tunability
into ENZ media by replacing the metal layer with a 2D material, graphene, in the multilayer model.
The other way to obtain an ENZ response is using natural materials which operate in proximity
of their plasma frequency, typically here the indium tin oxide (ITO) at the near-infrared
range. The ITO thin films are deposited using radio frequency magnetron sputtering, and their
permittivities are manipulated via controlling fabrication parameters. We succeed in sweeping the
zero-permittivity frequency of ITO media by controlling the gas recipe and deposition temperature
during the sputtering process. To obtain specific optical responses, the ENZ ITO thin films are
designed to be combined with different photonic features, including nanoantenna on microsphere
and nanohelix, associated with a direct fabrication approach based on electron beam induced
deposition (EBID).
Furthermore, this thesis extends the research range by realising the photonic trimming of
quantum emitters via various metallic nanofeatures fabricated directly using EBID method. We
believe that the interaction of ENZ MMs with EBID approach offers an opportunity to create hybrid
ENZ platforms for optoelectronic applications.Narrow-linewidth, widely tunable intracavity optical parametric oscillators for mid-infrared spectroscopyHeering, Lea Christinahttps://hdl.handle.net/10023/187772021-03-17T08:59:47Z2019-06-24T00:00:00Z2019-06-24T00:00:00ZHeering, Lea ChristinaTitle redactedTenopala Carmona, Franciscohttps://hdl.handle.net/10023/186232024-03-22T11:09:21Z2019-06-24T00:00:00Z2019-06-24T00:00:00ZTenopala Carmona, FranciscoThe properties of post-starburst galaxies from spectral fittingTaj Aldeen, Laith Talib Hadihttps://hdl.handle.net/10023/185832019-10-01T08:26:39Z2018-12-06T00:00:00ZA bimodality has been seen in the morphology, colour and star-forming properties of galaxies.
Throughout cosmic time, the number of galaxies that are quenched to join the red-sequence
has increased, unlike the number of star forming galaxies. Post-starburst galaxies (PSB) are
suspected to be a transition phase between these two stages.
We tried to answer the questions of the mechanism for this transformation. The goal of
our work is to see whether or not all PSB in the nearby universe are being quenched to join
the red-sequence and by which mechanism. We try to show if there is any difference between
the low and high redshift PSB with regard to the accumulated stellar mass in the last 1 and
1.5 Gyr, the star formation history and the amount of dust within the galaxies. In this work we
fit the observed galaxies with the new STARLIGHT code that can fit spectra with or without
photometric data with a library of synthetic models.
First we investigate 189 PSB candidates in the low redshift Sloan Digital Sky Survey (SDSS)
and 1877 comparison samples of star forming and passive galaxies. Depending on the mass,
low or high, we found more than one pathway and mechanism that may transform the blue
cloud to become red sequence type galaxies. Secondly we use Ultra Deep Survey (UDS) and
the spectroscopic follow-up of the UKIDSS Ultra-Deep Survey (UDSz) data for the PSB galaxies
at 0.9 ≤ z ≤ 1.2. From the fitting of 323 galaxies, we determine more constrained physical
properties of UDSz high redshift galaxies, such as the star formation history, mass fraction
that formed at the time of the burst and the dust contents of the galaxies. We speculate that a
higher gas content in these galaxies could have led to the higher burst.
2018-12-06T00:00:00ZTaj Aldeen, Laith Talib HadiA bimodality has been seen in the morphology, colour and star-forming properties of galaxies.
Throughout cosmic time, the number of galaxies that are quenched to join the red-sequence
has increased, unlike the number of star forming galaxies. Post-starburst galaxies (PSB) are
suspected to be a transition phase between these two stages.
We tried to answer the questions of the mechanism for this transformation. The goal of
our work is to see whether or not all PSB in the nearby universe are being quenched to join
the red-sequence and by which mechanism. We try to show if there is any difference between
the low and high redshift PSB with regard to the accumulated stellar mass in the last 1 and
1.5 Gyr, the star formation history and the amount of dust within the galaxies. In this work we
fit the observed galaxies with the new STARLIGHT code that can fit spectra with or without
photometric data with a library of synthetic models.
First we investigate 189 PSB candidates in the low redshift Sloan Digital Sky Survey (SDSS)
and 1877 comparison samples of star forming and passive galaxies. Depending on the mass,
low or high, we found more than one pathway and mechanism that may transform the blue
cloud to become red sequence type galaxies. Secondly we use Ultra Deep Survey (UDS) and
the spectroscopic follow-up of the UKIDSS Ultra-Deep Survey (UDSz) data for the PSB galaxies
at 0.9 ≤ z ≤ 1.2. From the fitting of 323 galaxies, we determine more constrained physical
properties of UDSz high redshift galaxies, such as the star formation history, mass fraction
that formed at the time of the burst and the dust contents of the galaxies. We speculate that a
higher gas content in these galaxies could have led to the higher burst.Angle resolved photoemission spectroscopy of delafossite metalsSunko, Veronikahttps://hdl.handle.net/10023/185822023-04-12T02:07:06Z2019-06-24T00:00:00ZThis thesis describes the results of angle resolved photoemission spectroscopy (ARPES)
experiments on delafossite oxide metals, and theoretical work explaining the observations.
The main results of the thesis are reported in three chapters, each of which
is dedicated to a different physical observation, as I describe below.
The delafossite metals exhibit extraordinarily high conductivity, motivating a
study of bulk electronic structure. I report measurements of the bulk electronic
structure of the non-magnetic delafossites PdCoO₂ and PtCoO₂. In each, a single
fast band crosses the Fermi level, resulting in a highly two-dimensional Fermi surface
of nearly hexagonal cross-section. The detailed differences between the materials are
described, and so are the possible many-body renormalisations.
Extension of the measurements to the electronic structure of the antiferromagnetic
delafossite metal PdCrO₂ reveals a signal which I realised cannot be explained in
terms of standard pictures of electron behaviour in a periodic potential. Using a
combination of experiment and theory, partly in collaboration with external groups,
we were able to identify its origin as the Kondo-like coupling of itinerant Pd - and
localised Cr - electrons. In doing so we are able to show that that ARPES can be
sensitive to spin-spin correlations.
Furthermore, I report measurements and analysis of the surface states arising
from the transition metal terminated surfaces of PtCoO₂, PdCoO₂ and PdRhO₂.
The states support a large Rashba-like spin-splitting, the energy scale of which
is comparable to the atomic spin-orbit coupling of the relevant transition metal
ion. I show how this arises as a consequence of the large energy scale of inversion
symmetry breaking at the surface, and that this is in turn a consequence of the
unusual structure of the transition metal oxide layer.
2019-06-24T00:00:00ZSunko, VeronikaThis thesis describes the results of angle resolved photoemission spectroscopy (ARPES)
experiments on delafossite oxide metals, and theoretical work explaining the observations.
The main results of the thesis are reported in three chapters, each of which
is dedicated to a different physical observation, as I describe below.
The delafossite metals exhibit extraordinarily high conductivity, motivating a
study of bulk electronic structure. I report measurements of the bulk electronic
structure of the non-magnetic delafossites PdCoO₂ and PtCoO₂. In each, a single
fast band crosses the Fermi level, resulting in a highly two-dimensional Fermi surface
of nearly hexagonal cross-section. The detailed differences between the materials are
described, and so are the possible many-body renormalisations.
Extension of the measurements to the electronic structure of the antiferromagnetic
delafossite metal PdCrO₂ reveals a signal which I realised cannot be explained in
terms of standard pictures of electron behaviour in a periodic potential. Using a
combination of experiment and theory, partly in collaboration with external groups,
we were able to identify its origin as the Kondo-like coupling of itinerant Pd - and
localised Cr - electrons. In doing so we are able to show that that ARPES can be
sensitive to spin-spin correlations.
Furthermore, I report measurements and analysis of the surface states arising
from the transition metal terminated surfaces of PtCoO₂, PdCoO₂ and PdRhO₂.
The states support a large Rashba-like spin-splitting, the energy scale of which
is comparable to the atomic spin-orbit coupling of the relevant transition metal
ion. I show how this arises as a consequence of the large energy scale of inversion
symmetry breaking at the surface, and that this is in turn a consequence of the
unusual structure of the transition metal oxide layer.Title redactedPearson, Scott J.https://hdl.handle.net/10023/185662019-09-26T13:59:57Z2016-06-22T00:00:00Z2016-06-22T00:00:00ZPearson, Scott J.Linear and nonlinear optical properties of silica aerogelFleming, Adam Colinhttps://hdl.handle.net/10023/185572021-03-30T15:23:16Z2019-12-03T00:00:00ZScattering media have traditionally been seen as a hindrance to the controlled transport of
light through media, creating the familiar speckle pattern. However such matter does not cause
the loss of information but instead performs a highly complex deterministic operation on the
incoming flux. Through sculpting the properties of the incoming wavefront, we can unlock the
hidden characteristics of these media, affording us far more degrees of freedom than that which is
available to us in traditional ballistic optics.
These additional degrees of freedom have allowed for the creation of compact sophisticated
optical devices based only on the deterministic nature of light scattering. Such devices include
diffraction-limit-beating lenses, polarimeters, spectrometers, and some which can transmit entire
images through a scattering substance.
Additional degrees of freedom would allow for the creation of even more powerful devices,
in new working regimes. In particular, the application of related techniques where the scattering
material is actively modified is limited.
This thesis is concerned with the use of optothermal nonlinearity in random media as a way to
provide an additional degree of control over light which scatters through it. Specifically, we are
concerned with silica aerogel as a platform for this study.
Silica aerogel is a lightweight skeletal structure of silica fibrils, which results in a material which
is up to 99.98 % by volume. This material exhibits a unique cocktail of properties of use such as near
unitary refractive index, an order of magnitude lower thermal conductivity, and high optothermal
nonlinearity. The latter two of these properties allow for the creation of localised steep thermal
gradients, proportionally affecting the low refractive index significantly. Additionally through
differing fabrication steps, the opacity, and as a result, we can adjust the scattering strength.
In line with the development of light deterministic light scattering techniques in linear media,
we develop through the use of pump-probe setups, a framework for the development of a similar
line of techniques in nonlinear scattering media. We show that we can reversibly control the
far-field propagation of light in weakly scattering silica aerogel. Following this, we show that
nonlinear perturbation can be used to extend and modify the optical memory effect, where slight
adjustments in scattering direction maintain the overall correlation of the scattered profile. Finally,
we measure the nonlinear transmission matrix, a complete description of how any wavefront would
pass through at a particular point in a scattering media, and how that scattering can be modified
through the application of an optothermal nonlinearity.
Extending the tool of scattering media into the nonlinear regime helps pave the way toward the
next set of advances in the field of light scattering control.
2019-12-03T00:00:00ZFleming, Adam ColinScattering media have traditionally been seen as a hindrance to the controlled transport of
light through media, creating the familiar speckle pattern. However such matter does not cause
the loss of information but instead performs a highly complex deterministic operation on the
incoming flux. Through sculpting the properties of the incoming wavefront, we can unlock the
hidden characteristics of these media, affording us far more degrees of freedom than that which is
available to us in traditional ballistic optics.
These additional degrees of freedom have allowed for the creation of compact sophisticated
optical devices based only on the deterministic nature of light scattering. Such devices include
diffraction-limit-beating lenses, polarimeters, spectrometers, and some which can transmit entire
images through a scattering substance.
Additional degrees of freedom would allow for the creation of even more powerful devices,
in new working regimes. In particular, the application of related techniques where the scattering
material is actively modified is limited.
This thesis is concerned with the use of optothermal nonlinearity in random media as a way to
provide an additional degree of control over light which scatters through it. Specifically, we are
concerned with silica aerogel as a platform for this study.
Silica aerogel is a lightweight skeletal structure of silica fibrils, which results in a material which
is up to 99.98 % by volume. This material exhibits a unique cocktail of properties of use such as near
unitary refractive index, an order of magnitude lower thermal conductivity, and high optothermal
nonlinearity. The latter two of these properties allow for the creation of localised steep thermal
gradients, proportionally affecting the low refractive index significantly. Additionally through
differing fabrication steps, the opacity, and as a result, we can adjust the scattering strength.
In line with the development of light deterministic light scattering techniques in linear media,
we develop through the use of pump-probe setups, a framework for the development of a similar
line of techniques in nonlinear scattering media. We show that we can reversibly control the
far-field propagation of light in weakly scattering silica aerogel. Following this, we show that
nonlinear perturbation can be used to extend and modify the optical memory effect, where slight
adjustments in scattering direction maintain the overall correlation of the scattered profile. Finally,
we measure the nonlinear transmission matrix, a complete description of how any wavefront would
pass through at a particular point in a scattering media, and how that scattering can be modified
through the application of an optothermal nonlinearity.
Extending the tool of scattering media into the nonlinear regime helps pave the way toward the
next set of advances in the field of light scattering control.Expanding the scope of electron paramagnetic resonance spectroscopy for structural studies of proteins and peptidesShah, Anokhihttps://hdl.handle.net/10023/184882023-08-21T13:11:48Z2019-12-03T00:00:00ZElectron paramagnetic resonance spectroscopy is a versatile tool for probing structural information about systems with unpaired electrons, in particular, biological systems with metal centres or chemically attached spin labels. This work uses a variety of electron paramagnetic resonance techniques to investigate the ability to measure distances between two spins (radical and metal), as well as characterise the local environment. Distance measurements are used to probe the mechanism of translocation across a membrane, where a change in distance is observed upon ATP cycling, indicating channel movement of the SecYEG:SecA complex. Furthermore, to expand the scope of distance measurements for more complex, cysteine-rich systems, spin labelling regimes are developed and optimised on the test protein myoglobin. Specifically, next generation maleimide spin labels are demonstrated to label and selectively cleave. Myoglobin is further exploited to successfully introduce the unnatural amino acid, dehydroalanine, for selective and orthogonal labelling. The development of the labelling and measurement strategy for the gadolinium-based spin label, [Gd.sTPATCN]-SL, is also shown, where the narrow central transition of the label allows a long phase memory time and increased DEER modulation depth, to give increased measurement sensitivity. In addition, gadolinium(III) distances are used to characterise the binding site of a peptide system for the application of magnetic resonance imaging. The optimisation of measuring inter-gadolinium(III) distances between 2-5 nm at both Q- and W-band is also demonstrated in the corresponding peptide ruler series. The additional benefit of the peptide to act as a metal ruler is further investigated using copper(II), where hyperfine spectroscopy is utilised to successfully confirm the nature of the binding site as all oxygen binding.
2019-12-03T00:00:00ZShah, AnokhiElectron paramagnetic resonance spectroscopy is a versatile tool for probing structural information about systems with unpaired electrons, in particular, biological systems with metal centres or chemically attached spin labels. This work uses a variety of electron paramagnetic resonance techniques to investigate the ability to measure distances between two spins (radical and metal), as well as characterise the local environment. Distance measurements are used to probe the mechanism of translocation across a membrane, where a change in distance is observed upon ATP cycling, indicating channel movement of the SecYEG:SecA complex. Furthermore, to expand the scope of distance measurements for more complex, cysteine-rich systems, spin labelling regimes are developed and optimised on the test protein myoglobin. Specifically, next generation maleimide spin labels are demonstrated to label and selectively cleave. Myoglobin is further exploited to successfully introduce the unnatural amino acid, dehydroalanine, for selective and orthogonal labelling. The development of the labelling and measurement strategy for the gadolinium-based spin label, [Gd.sTPATCN]-SL, is also shown, where the narrow central transition of the label allows a long phase memory time and increased DEER modulation depth, to give increased measurement sensitivity. In addition, gadolinium(III) distances are used to characterise the binding site of a peptide system for the application of magnetic resonance imaging. The optimisation of measuring inter-gadolinium(III) distances between 2-5 nm at both Q- and W-band is also demonstrated in the corresponding peptide ruler series. The additional benefit of the peptide to act as a metal ruler is further investigated using copper(II), where hyperfine spectroscopy is utilised to successfully confirm the nature of the binding site as all oxygen binding.Flexible holographic metasurfacesBurch, Jameshttps://hdl.handle.net/10023/183362021-03-30T13:58:01Z2019-07-22T00:00:00ZMetasurface holography has attracted much attention in recent years because of its practical applications including anti-counterfeiting, sensing, and lensing. However, most metasurface holograms have been constrained to rigid substrates which limit their out-of-the-lab value. Flexible holographic metasurfaces offer increased post-fabrication tunability as they can be bent or stretched, can be conformed to real-world non-flat surfaces, and are compatible with commercially viable roll-to-roll fabrication methods.
This thesis describes how flexible holographic metasurfaces can be fabricated, their utility, and the novel phenomena that arise from them. In particular, it details a hologram retrieval algorithm for non-flat surface topologies, and how the topography determines the resultant symmetry properties of the holographic image. Furthermore, this thesis outlines a concept for replacing the bulky and complicated optics required for light sheet fluorescent microscopy with a simple holographic metasurface illuminated by a collimated beam.
Two plasmonic meta-atom designs, both operating in reflection, are presented here, one for the visible wavelength range using nanorods and Pancharatnam-Berry phase-shifting, and the other for the millimetre wavelength range using c-rings.
Extending the tools available for holography paves-the-way for advances in this field.
2019-07-22T00:00:00ZBurch, JamesMetasurface holography has attracted much attention in recent years because of its practical applications including anti-counterfeiting, sensing, and lensing. However, most metasurface holograms have been constrained to rigid substrates which limit their out-of-the-lab value. Flexible holographic metasurfaces offer increased post-fabrication tunability as they can be bent or stretched, can be conformed to real-world non-flat surfaces, and are compatible with commercially viable roll-to-roll fabrication methods.
This thesis describes how flexible holographic metasurfaces can be fabricated, their utility, and the novel phenomena that arise from them. In particular, it details a hologram retrieval algorithm for non-flat surface topologies, and how the topography determines the resultant symmetry properties of the holographic image. Furthermore, this thesis outlines a concept for replacing the bulky and complicated optics required for light sheet fluorescent microscopy with a simple holographic metasurface illuminated by a collimated beam.
Two plasmonic meta-atom designs, both operating in reflection, are presented here, one for the visible wavelength range using nanorods and Pancharatnam-Berry phase-shifting, and the other for the millimetre wavelength range using c-rings.
Extending the tools available for holography paves-the-way for advances in this field.Portable and versatile cold atom experimentsIreland, Philiphttps://hdl.handle.net/10023/179692021-03-12T16:23:23Z2019-06-24T00:00:00ZThis thesis describes the progress achieved towards two goals. One is the construction of a
compact magneto-optical trap (MOT) apparatus, used both as a standalone undergraduate
laboratory experiment and as part of a public engagement demonstration of cold atom
physics. The setup, which utilises a dichroic atomic vapour laser lock (DAVLL) system
plus homemade sideband generation electronics, is capable of trapping and cooling up to 8x10⁷ ⁸⁵Rb atoms in a standard six-beam laser cooling scheme.
The second goal is the loading of an ultracold ensemble of ⁸⁷Rb atoms into novel
optical trap geometries using a phase-only liquid crystal spatial light modulator (LC SLM)
for applications to atomtronics and quantum simulation. Details of a double vacuum
chamber apparatus, designed to produce the first Bose-Einstein condensates (BECs) at St
Andrews, are provided. The setup incorporates a hybrid trap evaporative cooling scheme
towards quantum degeneracy and reliably produces BECs of 1.9 10⁵ ⁸⁷Rb atoms. Two
computational techniques, developed during the course of this project, are presented with
the aim of creating flexible, smooth holographic optical traps. The first method illuminates
a single SLM with overlapped, co-propagating light beams of different wavelengths to create
a composite optical pattern for atom manipulation. The second uses conjugate gradient
minimisation to create exceptionally high fidelity light patterns in both amplitude and
phase for light in the output plane. These techniques, combined with the implementation
of a light sheet into the experimental setup, are used to trap atoms in both a ring profile
and in a set of narrow waveguides connected with a tunnelling barrier.
2019-06-24T00:00:00ZIreland, PhilipThis thesis describes the progress achieved towards two goals. One is the construction of a
compact magneto-optical trap (MOT) apparatus, used both as a standalone undergraduate
laboratory experiment and as part of a public engagement demonstration of cold atom
physics. The setup, which utilises a dichroic atomic vapour laser lock (DAVLL) system
plus homemade sideband generation electronics, is capable of trapping and cooling up to 8x10⁷ ⁸⁵Rb atoms in a standard six-beam laser cooling scheme.
The second goal is the loading of an ultracold ensemble of ⁸⁷Rb atoms into novel
optical trap geometries using a phase-only liquid crystal spatial light modulator (LC SLM)
for applications to atomtronics and quantum simulation. Details of a double vacuum
chamber apparatus, designed to produce the first Bose-Einstein condensates (BECs) at St
Andrews, are provided. The setup incorporates a hybrid trap evaporative cooling scheme
towards quantum degeneracy and reliably produces BECs of 1.9 10⁵ ⁸⁷Rb atoms. Two
computational techniques, developed during the course of this project, are presented with
the aim of creating flexible, smooth holographic optical traps. The first method illuminates
a single SLM with overlapped, co-propagating light beams of different wavelengths to create
a composite optical pattern for atom manipulation. The second uses conjugate gradient
minimisation to create exceptionally high fidelity light patterns in both amplitude and
phase for light in the output plane. These techniques, combined with the implementation
of a light sheet into the experimental setup, are used to trap atoms in both a ring profile
and in a set of narrow waveguides connected with a tunnelling barrier.Evolution of supermassive black holes : the role of galaxy mergersHewlett, Timothy Paulhttps://hdl.handle.net/10023/179202021-02-23T16:40:20Z2019-06-24T00:00:00ZIn this thesis the connections between galaxy mergers and the growth of supermassive black
holes (SMBHs) are investigated. In chapter 2 we investigate the links between galaxy mergers and active galactic nuclei (AGN) as a function of luminosity, using visual classification of
galaxy images and a new measure of morphological disturbance to identify mergers, testing
the hypothesis that cataclysmic events such as mergers are required to drive the most rapid
periods of accretion. We find no correlation between merger fraction and AGN luminosity, nor
any difference in the total merger fractions of AGN and matched control galaxies. We also
explore the possibility that the role of galaxy mergers evolves through cosmic time over the
redshift range 0.5-2.2, finding a modest evolution, with higher redshift AGN more likely to be
found in mergers than controls with 99% confidence. In chapter 3 we compare samples of simulated (Illustris) galaxies to real observations, exploring the efficacy of different techniques of
merger-identification as a function of merger parameters (mass ratio, time elapsed since the
merger, stellar mass, star formation rate etc.). Detailed analysis of the ability of structural
parameters and visual classification to identify mergers allows for reinterpretation of observational studies in which AGN merger fractions are measured. We find the intrinsic merger
fractions in relevant studies are likely several times higher than reported, with 50 20% of
AGN in the local Universe triggered by galaxy mergers. In chapter 4 AGN are selected in the
optical, infrared and radio to investigate the role of mergers in triggering AGN in different
physical environments. We use structural parameters to estimate merger fractions. It is found
that infrared-selected AGN are more likely to be associated with mergers than their controls,
optically selected AGN are equivalent to controls and radio-selected AGN show fewer signs of
interactions than their controls. Collectively, this is interpreted as evidence that a substantial
fraction of SMBH growth is driven by galaxy mergers, but care needs to be taken to control
for various observational biases, which can hide or dilute real underlying causal relations.
2019-06-24T00:00:00ZHewlett, Timothy PaulIn this thesis the connections between galaxy mergers and the growth of supermassive black
holes (SMBHs) are investigated. In chapter 2 we investigate the links between galaxy mergers and active galactic nuclei (AGN) as a function of luminosity, using visual classification of
galaxy images and a new measure of morphological disturbance to identify mergers, testing
the hypothesis that cataclysmic events such as mergers are required to drive the most rapid
periods of accretion. We find no correlation between merger fraction and AGN luminosity, nor
any difference in the total merger fractions of AGN and matched control galaxies. We also
explore the possibility that the role of galaxy mergers evolves through cosmic time over the
redshift range 0.5-2.2, finding a modest evolution, with higher redshift AGN more likely to be
found in mergers than controls with 99% confidence. In chapter 3 we compare samples of simulated (Illustris) galaxies to real observations, exploring the efficacy of different techniques of
merger-identification as a function of merger parameters (mass ratio, time elapsed since the
merger, stellar mass, star formation rate etc.). Detailed analysis of the ability of structural
parameters and visual classification to identify mergers allows for reinterpretation of observational studies in which AGN merger fractions are measured. We find the intrinsic merger
fractions in relevant studies are likely several times higher than reported, with 50 20% of
AGN in the local Universe triggered by galaxy mergers. In chapter 4 AGN are selected in the
optical, infrared and radio to investigate the role of mergers in triggering AGN in different
physical environments. We use structural parameters to estimate merger fractions. It is found
that infrared-selected AGN are more likely to be associated with mergers than their controls,
optically selected AGN are equivalent to controls and radio-selected AGN show fewer signs of
interactions than their controls. Collectively, this is interpreted as evidence that a substantial
fraction of SMBH growth is driven by galaxy mergers, but care needs to be taken to control
for various observational biases, which can hide or dilute real underlying causal relations.Strong light-matter coupling in organic microcavities : investigating the fundamental principles of strong coupling in strongly disordered materials experimentallyTropf, Laura Christinehttps://hdl.handle.net/10023/178932021-03-05T15:14:36Z2019-06-24T00:00:00ZStrong light–matter coupling gives rise to intriguing possibilities like Bose-Einstein condensation at room temperature. In this context, organic semiconductors are particularly attractive
because they combine large oscillator strengths with high exciton binding energies and thus
readily enable large light–matter coupling strengths up to room temperature. Yet, in these
commonly strongly disordered materials, the applicability of fundamental predictions developed for systems of high order needs to be verified.
Hence, the validity of the theoretically predicted form of the coupling strength and of the
coupled oscillator model is tested for strongly coupled organic microcavities in this thesis.
Experimental investigations of metal-clad microcavities confirm the coupling strength to be
proportional to the electric field with which the excitons interact and to the square root both
of the oscillator strength of the material and of the number of chromophores inside the microcavity. Systematically varying these parameters demonstrates a non-zero threshold for the
onset of the strong coupling regime for the first time, which confirms the applicability of the
coupled oscillator model also for strongly disordered systems.
Moreover, the effect of the coupling strength on the photoluminescence from organic microcavities is investigated. For metal-clad cavities, but not for microcavities with dielectric mirrors, an increase of the luminescence intensity with increasing coupling strength was found.
For the latter system, a systematic study aimed to determine the properties of the cavity and
of the organic material which are crucial for polariton lasing. However, experiments did not
yield polariton lasing, for which two potential reasons are identified: (1) the vanishing of
modes close to resonance and (2) pronounced bimolecular quenching in the studied material.
Since organic microcavities are complex, systematic studies as presented in this thesis are an
important step towards a more profound understanding of organic polaritons.
2019-06-24T00:00:00ZTropf, Laura ChristineStrong light–matter coupling gives rise to intriguing possibilities like Bose-Einstein condensation at room temperature. In this context, organic semiconductors are particularly attractive
because they combine large oscillator strengths with high exciton binding energies and thus
readily enable large light–matter coupling strengths up to room temperature. Yet, in these
commonly strongly disordered materials, the applicability of fundamental predictions developed for systems of high order needs to be verified.
Hence, the validity of the theoretically predicted form of the coupling strength and of the
coupled oscillator model is tested for strongly coupled organic microcavities in this thesis.
Experimental investigations of metal-clad microcavities confirm the coupling strength to be
proportional to the electric field with which the excitons interact and to the square root both
of the oscillator strength of the material and of the number of chromophores inside the microcavity. Systematically varying these parameters demonstrates a non-zero threshold for the
onset of the strong coupling regime for the first time, which confirms the applicability of the
coupled oscillator model also for strongly disordered systems.
Moreover, the effect of the coupling strength on the photoluminescence from organic microcavities is investigated. For metal-clad cavities, but not for microcavities with dielectric mirrors, an increase of the luminescence intensity with increasing coupling strength was found.
For the latter system, a systematic study aimed to determine the properties of the cavity and
of the organic material which are crucial for polariton lasing. However, experiments did not
yield polariton lasing, for which two potential reasons are identified: (1) the vanishing of
modes close to resonance and (2) pronounced bimolecular quenching in the studied material.
Since organic microcavities are complex, systematic studies as presented in this thesis are an
important step towards a more profound understanding of organic polaritons.Non-equilibrium processes and ergodicity-breaking in isolated quantum systemsTaylor, Scott Richardhttps://hdl.handle.net/10023/178672021-03-22T16:03:45Z2019-06-24T00:00:00ZIn this thesis we consider isolated quantum systems influenced by disorder and external driving, and explore aspects of localisation and ergodicity-breaking in three different models.
We first consider the XYZ spin chain in a disordered magnetic field; this system breaks the U(1) symmetry of the XXZ model and therefore violates the conservation of the z component of the total magnetisation.
We identify a quantum phase transition between an ergodic phase and a many-body localised phase at a finite disorder strength that is strongly dependent on the size of the U(1) symmetry-breaking term.
We also investigate the breakdown of thermalisation by studying the matrix elements of local operators in the basis of the system's eigenstates, and we find suggestive evidence of subdiffusive energy transport at disorder strengths preceding the localisation transition.
We then consider a two-level time-dependent quantum system that is influenced by a fluctuating classical noise term.
We show that the noise does not fully randomise the quantum state if the fluctuations of every noise parameter coupled to the system are perfectly correlated; the state of the system instead becomes distributed over a finite subset of the full state space.
Lastly we consider a one-dimensional Anderson-localised system that is affected by randomly fluctuating onsite potentials.
We explore how the spatiotemporal correlations between these onsite potentials affect the delocalisation of a particle initially localised on a single site.
2019-06-24T00:00:00ZTaylor, Scott RichardIn this thesis we consider isolated quantum systems influenced by disorder and external driving, and explore aspects of localisation and ergodicity-breaking in three different models.
We first consider the XYZ spin chain in a disordered magnetic field; this system breaks the U(1) symmetry of the XXZ model and therefore violates the conservation of the z component of the total magnetisation.
We identify a quantum phase transition between an ergodic phase and a many-body localised phase at a finite disorder strength that is strongly dependent on the size of the U(1) symmetry-breaking term.
We also investigate the breakdown of thermalisation by studying the matrix elements of local operators in the basis of the system's eigenstates, and we find suggestive evidence of subdiffusive energy transport at disorder strengths preceding the localisation transition.
We then consider a two-level time-dependent quantum system that is influenced by a fluctuating classical noise term.
We show that the noise does not fully randomise the quantum state if the fluctuations of every noise parameter coupled to the system are perfectly correlated; the state of the system instead becomes distributed over a finite subset of the full state space.
Lastly we consider a one-dimensional Anderson-localised system that is affected by randomly fluctuating onsite potentials.
We explore how the spatiotemporal correlations between these onsite potentials affect the delocalisation of a particle initially localised on a single site.Manipulating anisotropic transport and superconductivity by focused ion beam microstructuringBachmann, Maja Deborahhttps://hdl.handle.net/10023/178662023-04-12T02:07:15Z2019-06-24T00:00:00ZThis thesis presents the results of electrical transport experiments performed on two microstructured quantum materials, namely on the ultra-pure metal PdCoO₂ and on the heavy fermion superconductor CeIrIn₅. Throughout this work, focused ion beam (FIB) microsculpting was utilised to design the investigated devices.
I begin with an introduction to the FIB instrument, with a specific focus on its application for microstructuring transport devices from quantum materials. In particular, our standard fabrication procedure, in which a thin slab of material is extracted from a bulk single crystal for further processing is described in detail, as this approach can be utilised for most metallic compounds. Furthermore, I describe a micro-fabrication process for creating transport devices from platelet-shaped single crystals.
Thereafter I present ballistic transport measurements of the ultra-pure delafossite metal PdCoO₂. By investigating mesoscopic transport bars which are narrower than the electron mean free path (up to 20 μm), I demonstrate that the ballistic transport in PdCoO₂ is strongly anisotropic as a result of the underlying quasi-hexagonal Fermi surface shape. Moreover, I report on the results of transverse electron focusing (TEF) experiments, a technique which directly probes the real space ballistic trajectories of electrons in a magnetic field, which demonstrate the super-geometric focusing effect.
Furthermore, by investigating microstructures of the superconducting heavy fermion compound CeIrIn₅ by means of transport measurements as well as scanning SQUID microscopy in collaboration with external groups, a route to controllably manipulate the local strain in microstructured devices was found. The presented approach is based on exploiting the substrate-induced biaxial strain due to differential thermal contraction, which is spatially tailored by defined FIB cuts. As the superconducting transition in the heavy fermion compound CeIrIn₅ is highly sensitive to strain, the local T[sub]c within the device is controlled via the spatial strain distribution.
2019-06-24T00:00:00ZBachmann, Maja DeborahThis thesis presents the results of electrical transport experiments performed on two microstructured quantum materials, namely on the ultra-pure metal PdCoO₂ and on the heavy fermion superconductor CeIrIn₅. Throughout this work, focused ion beam (FIB) microsculpting was utilised to design the investigated devices.
I begin with an introduction to the FIB instrument, with a specific focus on its application for microstructuring transport devices from quantum materials. In particular, our standard fabrication procedure, in which a thin slab of material is extracted from a bulk single crystal for further processing is described in detail, as this approach can be utilised for most metallic compounds. Furthermore, I describe a micro-fabrication process for creating transport devices from platelet-shaped single crystals.
Thereafter I present ballistic transport measurements of the ultra-pure delafossite metal PdCoO₂. By investigating mesoscopic transport bars which are narrower than the electron mean free path (up to 20 μm), I demonstrate that the ballistic transport in PdCoO₂ is strongly anisotropic as a result of the underlying quasi-hexagonal Fermi surface shape. Moreover, I report on the results of transverse electron focusing (TEF) experiments, a technique which directly probes the real space ballistic trajectories of electrons in a magnetic field, which demonstrate the super-geometric focusing effect.
Furthermore, by investigating microstructures of the superconducting heavy fermion compound CeIrIn₅ by means of transport measurements as well as scanning SQUID microscopy in collaboration with external groups, a route to controllably manipulate the local strain in microstructured devices was found. The presented approach is based on exploiting the substrate-induced biaxial strain due to differential thermal contraction, which is spatially tailored by defined FIB cuts. As the superconducting transition in the heavy fermion compound CeIrIn₅ is highly sensitive to strain, the local T[sub]c within the device is controlled via the spatial strain distribution.The role of large scale flows in molecular cloud formation in spiral galaxiesRamon Fox, Felipe Gerardohttps://hdl.handle.net/10023/177932021-02-16T17:00:58Z2019-06-24T00:00:00ZStar formation begins on the large scales of a galaxy and takes place on the smallest scales. As the interstellar gas flows into a spiral arm, it forms a shock where the change in density, coupled to self-gravity and thermal instabilities, leads to the formation of high density structures where molecular clouds grow. It is important to understand the role of large-scale flows in assembling these clouds. This work explores the gas flows in spiral arms to understand its role on molecular cloud formation comparing between grand-design and flocculent galaxies.
A set of high-resolution smoothed particle hydrodynamics (SPH) simulations are used. One simulation evolves the gas in a potential including a halo, stellar disc, and spiral arms. The second simulation evolves the gas in an N-body stellar disc and bulge within a fixed halo potential. The first and second models are representative of grand-design and flocculent galaxies, respectively. The third simulation is a high-resolution simulation of a region of gas flowing in a spiral arm based on the simulations of Bonnell et al. (2013), which follows in more detail the local cloud dynamics. In the global models, the mass resolution is about 45M⊙ per gas particle and in the spiral simulation, about 0.6M⊙.
The results show that in both the grand-design and flocculent models, the gas is shocked as it flows through an arm. The N-body model shows flow characteristics qualitatively similar to the spiral potential model but with more variations due to the potentials arm-to-arm variations. Clouds are identified using a friends-of-friends algorithm to catalogue clumps above a given density threshold. These have non-negligible streaming motions and their properties are consistent with observed mass-radius and size-velocity dispersion relations.
2019-06-24T00:00:00ZRamon Fox, Felipe GerardoStar formation begins on the large scales of a galaxy and takes place on the smallest scales. As the interstellar gas flows into a spiral arm, it forms a shock where the change in density, coupled to self-gravity and thermal instabilities, leads to the formation of high density structures where molecular clouds grow. It is important to understand the role of large-scale flows in assembling these clouds. This work explores the gas flows in spiral arms to understand its role on molecular cloud formation comparing between grand-design and flocculent galaxies.
A set of high-resolution smoothed particle hydrodynamics (SPH) simulations are used. One simulation evolves the gas in a potential including a halo, stellar disc, and spiral arms. The second simulation evolves the gas in an N-body stellar disc and bulge within a fixed halo potential. The first and second models are representative of grand-design and flocculent galaxies, respectively. The third simulation is a high-resolution simulation of a region of gas flowing in a spiral arm based on the simulations of Bonnell et al. (2013), which follows in more detail the local cloud dynamics. In the global models, the mass resolution is about 45M⊙ per gas particle and in the spiral simulation, about 0.6M⊙.
The results show that in both the grand-design and flocculent models, the gas is shocked as it flows through an arm. The N-body model shows flow characteristics qualitatively similar to the spiral potential model but with more variations due to the potentials arm-to-arm variations. Clouds are identified using a friends-of-friends algorithm to catalogue clumps above a given density threshold. These have non-negligible streaming motions and their properties are consistent with observed mass-radius and size-velocity dispersion relations.Title redactedRobertson, Iainhttps://hdl.handle.net/10023/175232020-03-18T09:54:10Z2016-06-22T00:00:00Z2016-06-22T00:00:00ZRobertson, IainDesigned topological states from hybrid spiral magnet-superconductor heterostructuresCarroll, Christopher Jameshttps://hdl.handle.net/10023/174972022-09-07T02:03:10Z2019-06-24T00:00:00ZIn this thesis a Green’s function based, analytical formalism is developed to describe dense
chains of spiral ordered classical magnetic moments embedded in superconducting substrates.
I demonstrate that an understanding of the dimensional mismatch between the substrate and
impurity chain is crucial to understand the physics at play. Renormalised gap closures are
discovered by exact, analytic solution for a ferromagnetic chain and insights gleaned are used
to understand the spiral ordered case to which it can be smoothly tuned.
I investigate the topological characteristics of this model and find an ambiguity in defining
effective Hamiltonians due to the dimensional embedding of the chain in the substrate. To
aid in resolving this ambiguity I develop formalism which allows one to write down Green’s
functions for bounded, semi infinite systems in terms of more easily attainable, exact solutions
for infinite systems. I derive simple expressions to identify the presence of topological bound
states which agrees with purely one dimensional models and changes suggestively when applied
to the aforementioned two dimensional model.
I then work to extend the formalism to allow the exact solution of two or three parallel
chains, taking into account all possible interactions with the classical magnetic impurities. The
solutions are decoupled so that they can be written in terms of the 𝑇 matrices for a single
chain, with additional levels of resummation. I find that the induced sub-gap spectrum for the
two chain solution completely replaces those from each individual chain, leading to oscillating,
helix like band structures. I use the complicated solution of the three chain solution to inform
a potential ansatz to solve the 𝑁 chain case.
2019-06-24T00:00:00ZCarroll, Christopher JamesIn this thesis a Green’s function based, analytical formalism is developed to describe dense
chains of spiral ordered classical magnetic moments embedded in superconducting substrates.
I demonstrate that an understanding of the dimensional mismatch between the substrate and
impurity chain is crucial to understand the physics at play. Renormalised gap closures are
discovered by exact, analytic solution for a ferromagnetic chain and insights gleaned are used
to understand the spiral ordered case to which it can be smoothly tuned.
I investigate the topological characteristics of this model and find an ambiguity in defining
effective Hamiltonians due to the dimensional embedding of the chain in the substrate. To
aid in resolving this ambiguity I develop formalism which allows one to write down Green’s
functions for bounded, semi infinite systems in terms of more easily attainable, exact solutions
for infinite systems. I derive simple expressions to identify the presence of topological bound
states which agrees with purely one dimensional models and changes suggestively when applied
to the aforementioned two dimensional model.
I then work to extend the formalism to allow the exact solution of two or three parallel
chains, taking into account all possible interactions with the classical magnetic impurities. The
solutions are decoupled so that they can be written in terms of the 𝑇 matrices for a single
chain, with additional levels of resummation. I find that the induced sub-gap spectrum for the
two chain solution completely replaces those from each individual chain, leading to oscillating,
helix like band structures. I use the complicated solution of the three chain solution to inform
a potential ansatz to solve the 𝑁 chain case.Spin- and angle-resolved photoemission study of topological band inversions within a single orbital manifoldClark, Oliver Jonhttps://hdl.handle.net/10023/174582023-09-26T02:07:22Z2019-06-24T00:00:00ZThe isolation of graphene in 2004 and the subsequent characterisation of its many remarkable properties marked the start of an intense and ongoing research effort into other systems hosting so-called Dirac cones within their electronic band structures.
This thesis focuses on two classes of Dirac materials in particular. Dirac semimetals are essentially three-dimensional analogues of graphene, hosting spin-degenerate Dirac cones within their bulk band structure. Topological insulators are characterised by 'topological surface states' which are spin-polarised, surface-localised Dirac cones, essentially enclosing an otherwise insulating material in a highly conductive outer shell.
In both cases, the formation of the Dirac cone requires the crossing of bands within the bulk electronic band structure. In the vast majority of experimental realisations to date, these two bands derive from different atomic and orbital manifolds. Through spin- and angle-resolved photoemission, we establish that Dirac cones arising instead from a single-orbital manifold are both commonplace and advantageous. A mechanism allowing for the simultaneous formation of bulk Dirac cones and topological surface states is introduced, requiring only a discrepancy in bandwidths of a single, crystal field-split orbital manifold along a rotationally-symmetric axis.
Unlike in the conventional cases, the Dirac cones populating the resulting 'topological ladders' are not easily destroyed by changes to the relative energetics of orbital manifolds, caused by deformations to a lattice, for example. Instead, lattice deformations can be used to tune the position along the rotationally-symmetric axis where the crossings occur, but the states themselves are extremely robust. Indeed, we demonstrate that topological ladders of a common origin exist in six transition metal dichalcogenide (TMD) compounds, despite their disparate bulk properties and ground states. Moreover, the underlying mechanism driving the formation of topological ladders is expected to be applicable to many other compound classes, suggesting that single-orbital manifold topological phenomena is prevalent in nature.
2019-06-24T00:00:00ZClark, Oliver JonThe isolation of graphene in 2004 and the subsequent characterisation of its many remarkable properties marked the start of an intense and ongoing research effort into other systems hosting so-called Dirac cones within their electronic band structures.
This thesis focuses on two classes of Dirac materials in particular. Dirac semimetals are essentially three-dimensional analogues of graphene, hosting spin-degenerate Dirac cones within their bulk band structure. Topological insulators are characterised by 'topological surface states' which are spin-polarised, surface-localised Dirac cones, essentially enclosing an otherwise insulating material in a highly conductive outer shell.
In both cases, the formation of the Dirac cone requires the crossing of bands within the bulk electronic band structure. In the vast majority of experimental realisations to date, these two bands derive from different atomic and orbital manifolds. Through spin- and angle-resolved photoemission, we establish that Dirac cones arising instead from a single-orbital manifold are both commonplace and advantageous. A mechanism allowing for the simultaneous formation of bulk Dirac cones and topological surface states is introduced, requiring only a discrepancy in bandwidths of a single, crystal field-split orbital manifold along a rotationally-symmetric axis.
Unlike in the conventional cases, the Dirac cones populating the resulting 'topological ladders' are not easily destroyed by changes to the relative energetics of orbital manifolds, caused by deformations to a lattice, for example. Instead, lattice deformations can be used to tune the position along the rotationally-symmetric axis where the crossings occur, but the states themselves are extremely robust. Indeed, we demonstrate that topological ladders of a common origin exist in six transition metal dichalcogenide (TMD) compounds, despite their disparate bulk properties and ground states. Moreover, the underlying mechanism driving the formation of topological ladders is expected to be applicable to many other compound classes, suggesting that single-orbital manifold topological phenomena is prevalent in nature.Non-equilibrium quantum dynamics : interplay of disorder, interactions and confinementSchulz, Maximilianhttps://hdl.handle.net/10023/173452021-03-01T16:23:34Z2019-06-24T00:00:00ZThe study of collective behaviour in many-body systems often explores fundamentally new ideas absent from the mere constituents of such a system. A paradigmatic model for these studies is the spin-1/2 XXZ chain and its fermionic equivalent. This thesis can be broadly divided into the study of two fundamental aspects of this model. Firstly, we discuss localisation phenomena in one dimensional lattices as often experimentally realised in cold atom systems. Secondly, we investigate how disorder and symmetry influence heat transport in spin chains.
More specifically, in the first part we consider a system of non-interacting fermions in one dimension subject to a single-particle potential consisting of a strong optical lattice, a harmonic trap, and uncorrelated on-site disorder. We investigate a global inhomogeneous quantum quench and present numerical and analytical results for static and dynamical properties. We show that the approach to the non-thermal equilibrium state is extremely slow and that it implies a sensitivity to disorder parametrically stronger than that expected from Anderson localisation.
We also consider the above system in a strong non-uniform electric field. In the non-interacting case, due to Wannier-Stark localisation, the single-particle wave functions are exponentially localised without quenched disorder. We show that this system remains localised in the presence of nearest-neighbour interactions and exhibits physics analogous to models of conventional many-body localisation.
The second part explores the hydrodynamics of the disordered XYZ spin chain. Using time-evolving block decimation on open chains of up to 400 spins attached to thermal baths, we probe the energy transport of this system. Our principal findings are as follows. For weak disorder there is a stable diffusive region that persists up to a critical disorder strength that depends on the XY anisotropy. Then, for disorder strengths above this critical value energy transport becomes increasingly subdiffusive.
2019-06-24T00:00:00ZSchulz, MaximilianThe study of collective behaviour in many-body systems often explores fundamentally new ideas absent from the mere constituents of such a system. A paradigmatic model for these studies is the spin-1/2 XXZ chain and its fermionic equivalent. This thesis can be broadly divided into the study of two fundamental aspects of this model. Firstly, we discuss localisation phenomena in one dimensional lattices as often experimentally realised in cold atom systems. Secondly, we investigate how disorder and symmetry influence heat transport in spin chains.
More specifically, in the first part we consider a system of non-interacting fermions in one dimension subject to a single-particle potential consisting of a strong optical lattice, a harmonic trap, and uncorrelated on-site disorder. We investigate a global inhomogeneous quantum quench and present numerical and analytical results for static and dynamical properties. We show that the approach to the non-thermal equilibrium state is extremely slow and that it implies a sensitivity to disorder parametrically stronger than that expected from Anderson localisation.
We also consider the above system in a strong non-uniform electric field. In the non-interacting case, due to Wannier-Stark localisation, the single-particle wave functions are exponentially localised without quenched disorder. We show that this system remains localised in the presence of nearest-neighbour interactions and exhibits physics analogous to models of conventional many-body localisation.
The second part explores the hydrodynamics of the disordered XYZ spin chain. Using time-evolving block decimation on open chains of up to 400 spins attached to thermal baths, we probe the energy transport of this system. Our principal findings are as follows. For weak disorder there is a stable diffusive region that persists up to a critical disorder strength that depends on the XY anisotropy. Then, for disorder strengths above this critical value energy transport becomes increasingly subdiffusive.Optical techniques for the investigation of a mechanical role for FRMD6/Willin in the Hippo signalling pathwayGoff, Franceshttps://hdl.handle.net/10023/171202023-04-04T02:02:34Z2019-06-24T00:00:00ZThe mammalian hippo signalling pathway controls cell proliferation and apoptosis via transcriptional co-activators YAP and TAZ, and as such is a key regulator of organ and tissue growth. Multiple cellular components converge in this pathway, including the actin cytoskeleton, which is required for YAP/TAZ activity. The precise mechanism by which the mechanical actomyosin network regulates Hippo signalling, however, is unknown.
Optical methods provide a non-invasive way to image and study the biomechanics of cells. In the past two decades, super-resolution fluorescence microscopy techniques that break the diffraction limit of light have come to the fore, enabling visualisation of intracellular detail at the nanoscale level. Optical trapping, on the other hand, allows precise control of micron-sized objects such as cells. Here, super resolution structured illumination microscopy (SIM) and elastic resonator interference stress microscopy (ERISM) were used to investigate a potential role for the FERM-domain protein FRMD6, or Willin, in the mechanical control of the Hippo pathway in a neuronal cell model. A double optical trap was also integrated with the Nikon-SIM with the aim of cell stretching.
Willin expression was shown to modify the morphology, neuronal differentiation, actin cytoskeleton and forces of SH-SY5Y cells. Optical trapping from above the SIM objective, however, was demonstrated to be ineffective for manipulation of adherent cells. The results presented here indicate a function for Willin in the assembly of actin stress fibres that may be the result of an interaction with the Hippo pathway regulator AMOT. Further investigation, for example by direct cell stretching, is required to elucidate the exact role of Willin in the mechanical control of YAP/TAZ.
2019-06-24T00:00:00ZGoff, FrancesThe mammalian hippo signalling pathway controls cell proliferation and apoptosis via transcriptional co-activators YAP and TAZ, and as such is a key regulator of organ and tissue growth. Multiple cellular components converge in this pathway, including the actin cytoskeleton, which is required for YAP/TAZ activity. The precise mechanism by which the mechanical actomyosin network regulates Hippo signalling, however, is unknown.
Optical methods provide a non-invasive way to image and study the biomechanics of cells. In the past two decades, super-resolution fluorescence microscopy techniques that break the diffraction limit of light have come to the fore, enabling visualisation of intracellular detail at the nanoscale level. Optical trapping, on the other hand, allows precise control of micron-sized objects such as cells. Here, super resolution structured illumination microscopy (SIM) and elastic resonator interference stress microscopy (ERISM) were used to investigate a potential role for the FERM-domain protein FRMD6, or Willin, in the mechanical control of the Hippo pathway in a neuronal cell model. A double optical trap was also integrated with the Nikon-SIM with the aim of cell stretching.
Willin expression was shown to modify the morphology, neuronal differentiation, actin cytoskeleton and forces of SH-SY5Y cells. Optical trapping from above the SIM objective, however, was demonstrated to be ineffective for manipulation of adherent cells. The results presented here indicate a function for Willin in the assembly of actin stress fibres that may be the result of an interaction with the Hippo pathway regulator AMOT. Further investigation, for example by direct cell stretching, is required to elucidate the exact role of Willin in the mechanical control of YAP/TAZ.Optoelectronic applications of lead halide perovskitesHarwell, Jonathon R.https://hdl.handle.net/10023/169432020-01-09T03:00:26Z2018-12-06T00:00:00ZHybrid perovskites are a new class of semiconductor which have proven to be an ideal material for
making thin film solar cells. They have the advantages of flexibility, low cost, and easy processing,
whilst achieving efficiencies competitive with monocrystalline silicon. Many of the properties which
make them ideal for solar cells are also applicable to light emitting devices, and there is now
increasing interest in their application for light emitting diodes (LEDs) and lasers. This thesis aims to
use a range of novel spectroscopy techniques to investigate the origin of these favourable
properties, and to exploit these properties to produce high performance distributed feedback lasers.
A detailed understanding of the origins of the excellent properties of hybrid perovskites is of crucial
importance in the search for new variations with improved performance or lowered toxicity. This
thesis uses Kelvin probe, air photoemission, and resonant ultrasound spectroscopy to probe deeply
into the underlying physics of hybrid perovskite single crystals and devices. Using these techniques,
we are able to produce detailed maps of the energy levels in a common perovskite solar cell, and we
also gain strong insight into the underlying strains and instabilities in the perovskite structure that
give rise to their elastic properties.
The strong light emission of hybrid perovskites is then exploited to produce high quality distributed
feedback lasers emitting in the green and infrared part of the spectrum. These lasers are observed to
have superior stability, good thresholds, and many interesting beam parameters owing to their high
refractive index. We explore a wide range of processing methods in order to achieve the lowest
lasing threshold and the best stability. Finally, we investigate the properties of low dimensional
perovskites and investigate their potential in optoelectronic applications.
2018-12-06T00:00:00ZHarwell, Jonathon R.Hybrid perovskites are a new class of semiconductor which have proven to be an ideal material for
making thin film solar cells. They have the advantages of flexibility, low cost, and easy processing,
whilst achieving efficiencies competitive with monocrystalline silicon. Many of the properties which
make them ideal for solar cells are also applicable to light emitting devices, and there is now
increasing interest in their application for light emitting diodes (LEDs) and lasers. This thesis aims to
use a range of novel spectroscopy techniques to investigate the origin of these favourable
properties, and to exploit these properties to produce high performance distributed feedback lasers.
A detailed understanding of the origins of the excellent properties of hybrid perovskites is of crucial
importance in the search for new variations with improved performance or lowered toxicity. This
thesis uses Kelvin probe, air photoemission, and resonant ultrasound spectroscopy to probe deeply
into the underlying physics of hybrid perovskite single crystals and devices. Using these techniques,
we are able to produce detailed maps of the energy levels in a common perovskite solar cell, and we
also gain strong insight into the underlying strains and instabilities in the perovskite structure that
give rise to their elastic properties.
The strong light emission of hybrid perovskites is then exploited to produce high quality distributed
feedback lasers emitting in the green and infrared part of the spectrum. These lasers are observed to
have superior stability, good thresholds, and many interesting beam parameters owing to their high
refractive index. We explore a wide range of processing methods in order to achieve the lowest
lasing threshold and the best stability. Finally, we investigate the properties of low dimensional
perovskites and investigate their potential in optoelectronic applications.Information transfer in open quantum systemsLevi, Elliott Kendrickhttps://hdl.handle.net/10023/166902021-09-23T11:44:27Z2017-06-21T00:00:00ZThis thesis covers open quantum systems and information transfer in the face of
dissipation and disorder through numerical simulation.
In Chapter 3 we present work on an open quantum system comprising a two-level system, single bosonic mode and dissipative environment; we have included
the bosonic mode in the exact system treatment. This model allows us to gain an understanding of an environment’s role in small energy transfer systems. We observe
how the two-level system-mode coupling strength and the spectral density form characterising the environment interplay, affecting the system’s coherent behaviour. We
find strong coupling and a spectral density resonantly peaked on the two-level system oscillation frequency enhances the system’s coherent oscillatory dynamics.
Chapter 4 focusses on a physically motivated study of chain and ladder spin
geometries and their use for entanglement transfer between qubits. We consider a
nitrogen vacancy centre qubit implementation with nitrogen impurity spin-channels
and demonstrate how matrix product operator techniques can be used in simulations
of this physical system. We investigate coupling parameters and environmental decay
rates with respect to transfer efficiency effects. Then, in turn, we simulate the effects
of missing channel spins and disorder in the spin-spin coupling. We conclude by
highlighting where our considered channel geometries outperform each other.
The work in Chapter 5 is an investigation into the feasibility of routing entanglement between distant qubits in 2D spin networks. We no longer consider a physical
implementation, but keep in mind the effects of dissipative environments on entanglement transfer systems. Starting with a single sending qubit-ancilla and multiple
addressable receivers, we show it is possible to target a specific receiver and establish transferred entanglement between it and the sender’s ancilla through eigenstate tunnelling techniques. We proceed to show that eigenstate tunnelling-mediated entanglement transfer can be achieved simultaneously from two senders across one spin network.
2017-06-21T00:00:00ZLevi, Elliott KendrickThis thesis covers open quantum systems and information transfer in the face of
dissipation and disorder through numerical simulation.
In Chapter 3 we present work on an open quantum system comprising a two-level system, single bosonic mode and dissipative environment; we have included
the bosonic mode in the exact system treatment. This model allows us to gain an understanding of an environment’s role in small energy transfer systems. We observe
how the two-level system-mode coupling strength and the spectral density form characterising the environment interplay, affecting the system’s coherent behaviour. We
find strong coupling and a spectral density resonantly peaked on the two-level system oscillation frequency enhances the system’s coherent oscillatory dynamics.
Chapter 4 focusses on a physically motivated study of chain and ladder spin
geometries and their use for entanglement transfer between qubits. We consider a
nitrogen vacancy centre qubit implementation with nitrogen impurity spin-channels
and demonstrate how matrix product operator techniques can be used in simulations
of this physical system. We investigate coupling parameters and environmental decay
rates with respect to transfer efficiency effects. Then, in turn, we simulate the effects
of missing channel spins and disorder in the spin-spin coupling. We conclude by
highlighting where our considered channel geometries outperform each other.
The work in Chapter 5 is an investigation into the feasibility of routing entanglement between distant qubits in 2D spin networks. We no longer consider a physical
implementation, but keep in mind the effects of dissipative environments on entanglement transfer systems. Starting with a single sending qubit-ancilla and multiple
addressable receivers, we show it is possible to target a specific receiver and establish transferred entanglement between it and the sender’s ancilla through eigenstate tunnelling techniques. We proceed to show that eigenstate tunnelling-mediated entanglement transfer can be achieved simultaneously from two senders across one spin network.Heat capacity measurements of Sr₂RuO₄ under uniaxial stressLi, You-Shenghttps://hdl.handle.net/10023/165912023-05-29T15:41:43Z2018-12-06T00:00:00ZThe most-discussed pairing symmetry in Sr₂RuO₄ is chiral p-wave, 𝑝ₓ ± 𝑖𝑝[sub]𝑦, whose degeneracy is protected by the lattice symmetry. When the lattice symmetry is lowered by the application of a symmetry-breaking field, the degeneracy can be lifted, potentially leading to a splitting of the superconducting transition. To lift the degeneracy, the symmetry breaking field used in this study is uniaxial stress. Uniaxial stress generated by a piezo-electric actuator can continuously tune the electronic structure and in situ lower the tetragonal symmetry in Sr₂RuO₄.
Previous studies of magnetic susceptibility and resistivity under uniaxial stress have revealed that there is a strong peak in T[sub]c when the stress is applied along the a-axis of Sr₂RuO₄. In addition, it
has been proposed that the peak in T[sub]c coincides with a van Hove singularity in the band structure, and measurements of 𝐻[sub]𝑐₂ at the maximum T[sub]c indicate the possibility of an even parity condensate for Sr₂RuO₄ at the peak in Tc.
In this thesis, the heat capacity approach is used to study the thermodynamic behavior of Sr₂RuO₄ under uniaxial stress applied along the crystallographic a-axis of Sr₂RuO₄. The first thermodynamic evidence for the peak in T[sub]c is obtained, proving that is a bulk property. However, the experimental data show no clear evidence for splitting of the superconducting transition; only one phase transition can be identified within the experimental resolution. The results impose strong constraints on the existence of a second phase transition, i.e. the size of the second heat capacity jump would be small or the second T[sub]c would have to be very close to the first transition. In addition to these results, I will present heat capacity data from the normal state of Sr₂RuO₄. The experimental results indicate that there is an enhancement of specific heat at the peak in T[sub]c, consistent with the existence of the van Hove singularity. The possibility of even parity superconductivity at the maximum T[sub]c has also been investigated. However, the heat capacity measurements are shown to be relatively insensitive to such a change, so it has not been possible to obtain strong and unambiguous evidence for whether it takes place or not.
2018-12-06T00:00:00ZLi, You-ShengThe most-discussed pairing symmetry in Sr₂RuO₄ is chiral p-wave, 𝑝ₓ ± 𝑖𝑝[sub]𝑦, whose degeneracy is protected by the lattice symmetry. When the lattice symmetry is lowered by the application of a symmetry-breaking field, the degeneracy can be lifted, potentially leading to a splitting of the superconducting transition. To lift the degeneracy, the symmetry breaking field used in this study is uniaxial stress. Uniaxial stress generated by a piezo-electric actuator can continuously tune the electronic structure and in situ lower the tetragonal symmetry in Sr₂RuO₄.
Previous studies of magnetic susceptibility and resistivity under uniaxial stress have revealed that there is a strong peak in T[sub]c when the stress is applied along the a-axis of Sr₂RuO₄. In addition, it
has been proposed that the peak in T[sub]c coincides with a van Hove singularity in the band structure, and measurements of 𝐻[sub]𝑐₂ at the maximum T[sub]c indicate the possibility of an even parity condensate for Sr₂RuO₄ at the peak in Tc.
In this thesis, the heat capacity approach is used to study the thermodynamic behavior of Sr₂RuO₄ under uniaxial stress applied along the crystallographic a-axis of Sr₂RuO₄. The first thermodynamic evidence for the peak in T[sub]c is obtained, proving that is a bulk property. However, the experimental data show no clear evidence for splitting of the superconducting transition; only one phase transition can be identified within the experimental resolution. The results impose strong constraints on the existence of a second phase transition, i.e. the size of the second heat capacity jump would be small or the second T[sub]c would have to be very close to the first transition. In addition to these results, I will present heat capacity data from the normal state of Sr₂RuO₄. The experimental results indicate that there is an enhancement of specific heat at the peak in T[sub]c, consistent with the existence of the van Hove singularity. The possibility of even parity superconductivity at the maximum T[sub]c has also been investigated. However, the heat capacity measurements are shown to be relatively insensitive to such a change, so it has not been possible to obtain strong and unambiguous evidence for whether it takes place or not.Cloudy with a chance of starlight : coupling of smoothed particle hydrodynamics and Monte Carlo radiative transfer for the study of ionising stellar feedbackPetkova, Maya Atanasovahttps://hdl.handle.net/10023/165572019-06-14T13:53:04Z2018-12-06T00:00:00ZIonising radiation is present in a variety of astrophysical problems, and it is particularly important for shaping the process of star formation in molecular clouds, containing hot, high-mass stars. In order to account for the effects of ionising radiation within numerical models of star formation, we need to combine a hydrodynamics method with a radiative transfer method and obtain a radiation hydrodynamics scheme (RHD). In this thesis I achieve live radiation hydrodynamics by coupling the Smoothed Particle Hydrodynamics (SPH) code Phantom with the Monte Carlo Radiative Transfer (MCRT) code CMacIonize. Since SPH is particle-based and MCRT is grid-based, I construct an unstructured, Voronoi grid in order to establish a link between the two codes. In areas with large density gradients, a Voronoi grid based purely on the SPH particle positions achieves insufficient resolution, and therefore I propose a novel algorithm for inserting a small number of additional grid cells to improve the local resolution. Furthermore, the MCRT calculations require the knowledge of an average density for each Voronoi cell. To address this, I develop an analytic density mapping from SPH to a Voronoi grid, by deriving an expression for the integrals of a series of kernel functions over the volume of a random polyhedron. Finally, I demonstrate the validity of the live RHD through the benchmark test of D-type expansion of an H II region, where good agreement is shown with the existing literature. The RHD implementation is then used to perform a proof-of-concept simulation of a collapsing cloud, which produces high-mass stars and is subsequently partially ionised by them. The presented code is a valuable tool for future star formation studies, and it can be used for modelling a broad range of additional astronomical problems involving ionising radiation and hydrodynamics.
2018-12-06T00:00:00ZPetkova, Maya AtanasovaIonising radiation is present in a variety of astrophysical problems, and it is particularly important for shaping the process of star formation in molecular clouds, containing hot, high-mass stars. In order to account for the effects of ionising radiation within numerical models of star formation, we need to combine a hydrodynamics method with a radiative transfer method and obtain a radiation hydrodynamics scheme (RHD). In this thesis I achieve live radiation hydrodynamics by coupling the Smoothed Particle Hydrodynamics (SPH) code Phantom with the Monte Carlo Radiative Transfer (MCRT) code CMacIonize. Since SPH is particle-based and MCRT is grid-based, I construct an unstructured, Voronoi grid in order to establish a link between the two codes. In areas with large density gradients, a Voronoi grid based purely on the SPH particle positions achieves insufficient resolution, and therefore I propose a novel algorithm for inserting a small number of additional grid cells to improve the local resolution. Furthermore, the MCRT calculations require the knowledge of an average density for each Voronoi cell. To address this, I develop an analytic density mapping from SPH to a Voronoi grid, by deriving an expression for the integrals of a series of kernel functions over the volume of a random polyhedron. Finally, I demonstrate the validity of the live RHD through the benchmark test of D-type expansion of an H II region, where good agreement is shown with the existing literature. The RHD implementation is then used to perform a proof-of-concept simulation of a collapsing cloud, which produces high-mass stars and is subsequently partially ionised by them. The presented code is a valuable tool for future star formation studies, and it can be used for modelling a broad range of additional astronomical problems involving ionising radiation and hydrodynamics.Exoplanet transit modelling : three new planet discoveries, and a novel artificial neural network treatment for stellar limb darkeningHay, Kirstinhttps://hdl.handle.net/10023/165012019-04-01T10:09:33Z2018-12-06T00:00:00ZThis first part of this thesis concerns the discovery and parameter determination of three hot Jupiter planets, first detected with by the SuperWASP collaboration, and their planetary nature is confirmed with the modelling of radial velocity measurements and further ground-based transit lightcurves. WASP-92b, WASP-93b and WASP-118b are all hot Jupiters with short orbital periods – 2.17, 2.73 and 4.05 days respectively. The analysis in this thesis finds WASP-92b to have R[sub]p = 1.461 ± 0.077 R[sub]J and M[sub]p = 0.805 ± 0.068 M[sub]J; WASP-93b to have R[sub]p = 1.597 ± 0.077 R[sub]J and M[sub]p = 1.47 ± 0.029 M[sub]J, and WASP-118b to have R[sub]p = 1.440 ± 0.036 R[sub]J and M[sub]p = 0.514 ± 0.020 M[sub]J.
The second part of this thesis presents three novel approaches to modelling the effect of stellar limb darkening when fitting exoplanet transit lightcurves. The first method trains a Gaussian Process to interpolate between pre-calculated limb darkening coefficients for the non-linear limb darkening law. The method uses existing knowledge of the stellar atmosphere parameters as the constraints of the determined limb darkening coefficients for the host star of the transiting exoplanet system.
The second method deploys an artificial neural network to model limb darkening without the requirement of a parametric approximation of the form of the limb profile. The neural network is trained for a specific bandpass directly from the outputs of stellar atmosphere models, allowing predictions to be made for the stellar intensity at a given position on the stellar surface for values of the T[sub]eff , log g and [Fe/H]. The efficacy of the method is demonstrated by accurately fitting a transit lightcurve for the transit of Venus, and for a single transit lightcurve of TRES-2b. The final limb darkening modelling method proposes an adjustment to the neural network model to account for the fact that the stellar radius is not constant across wavelengths. The method also allows the full variation in light at the edge of the star to be modelled by not assuming a sharp boundary at the limb.
2018-12-06T00:00:00ZHay, KirstinThis first part of this thesis concerns the discovery and parameter determination of three hot Jupiter planets, first detected with by the SuperWASP collaboration, and their planetary nature is confirmed with the modelling of radial velocity measurements and further ground-based transit lightcurves. WASP-92b, WASP-93b and WASP-118b are all hot Jupiters with short orbital periods – 2.17, 2.73 and 4.05 days respectively. The analysis in this thesis finds WASP-92b to have R[sub]p = 1.461 ± 0.077 R[sub]J and M[sub]p = 0.805 ± 0.068 M[sub]J; WASP-93b to have R[sub]p = 1.597 ± 0.077 R[sub]J and M[sub]p = 1.47 ± 0.029 M[sub]J, and WASP-118b to have R[sub]p = 1.440 ± 0.036 R[sub]J and M[sub]p = 0.514 ± 0.020 M[sub]J.
The second part of this thesis presents three novel approaches to modelling the effect of stellar limb darkening when fitting exoplanet transit lightcurves. The first method trains a Gaussian Process to interpolate between pre-calculated limb darkening coefficients for the non-linear limb darkening law. The method uses existing knowledge of the stellar atmosphere parameters as the constraints of the determined limb darkening coefficients for the host star of the transiting exoplanet system.
The second method deploys an artificial neural network to model limb darkening without the requirement of a parametric approximation of the form of the limb profile. The neural network is trained for a specific bandpass directly from the outputs of stellar atmosphere models, allowing predictions to be made for the stellar intensity at a given position on the stellar surface for values of the T[sub]eff , log g and [Fe/H]. The efficacy of the method is demonstrated by accurately fitting a transit lightcurve for the transit of Venus, and for a single transit lightcurve of TRES-2b. The final limb darkening modelling method proposes an adjustment to the neural network model to account for the fact that the stellar radius is not constant across wavelengths. The method also allows the full variation in light at the edge of the star to be modelled by not assuming a sharp boundary at the limb.Coherence protection in coupled qubit systemsCammack, Helen Maryhttps://hdl.handle.net/10023/164572019-09-27T11:32:28Z2018-12-06T00:00:00ZDecoherence is a major barrier to the implementation of quantum technologies. Theoretical techniques for understanding decoherence in composite systems have traditionally been focused on systems with distinguishable emission spectra, where measuring the frequency of an emitted photon allows one to determine which process took place. Here the photon contains information about the state of the system.
On the other hand, systems with indistinguishable spectra do not necessarily completely reveal information about the state of the system when a photon is emitted. It can be impossible to say for certain which of two nearly degenerate transitions has occurred just by measuring the photon's frequency. It is then possible to preserve information within the system throughout the decay process.
In this Thesis we show that indistinguishable spectra can lead to protected coherences within one part of a coupled quantum system, even as another part decays. We develop a zero-temperature exact approach for modelling such systems, and compare it to the microscopically derived Born-Markov master equation. This comparison helps us to understand the range of validity of the Markovian approximation. We use this understanding to extend the master equation approach to finite temperature within the Markovian regime, and we compare its high temperature results to a semiclassical model.
We examine the physical conditions required for coherence protection, and remarkably we find that heating the system can improve coherence protection. Similarly, increasing the decay rate of the unprotected part of the coupled system can also enhance the coherence of the protected part. These effects are the results of linewidth broadening and thus greater spectral indistinguishability.
The findings in this Thesis are of interest to both those seeking to engineer hybrid quantum systems and those seeking to develop theoretical techniques for dealing with the decoherence of composite quantum systems.
2018-12-06T00:00:00ZCammack, Helen MaryDecoherence is a major barrier to the implementation of quantum technologies. Theoretical techniques for understanding decoherence in composite systems have traditionally been focused on systems with distinguishable emission spectra, where measuring the frequency of an emitted photon allows one to determine which process took place. Here the photon contains information about the state of the system.
On the other hand, systems with indistinguishable spectra do not necessarily completely reveal information about the state of the system when a photon is emitted. It can be impossible to say for certain which of two nearly degenerate transitions has occurred just by measuring the photon's frequency. It is then possible to preserve information within the system throughout the decay process.
In this Thesis we show that indistinguishable spectra can lead to protected coherences within one part of a coupled quantum system, even as another part decays. We develop a zero-temperature exact approach for modelling such systems, and compare it to the microscopically derived Born-Markov master equation. This comparison helps us to understand the range of validity of the Markovian approximation. We use this understanding to extend the master equation approach to finite temperature within the Markovian regime, and we compare its high temperature results to a semiclassical model.
We examine the physical conditions required for coherence protection, and remarkably we find that heating the system can improve coherence protection. Similarly, increasing the decay rate of the unprotected part of the coupled system can also enhance the coherence of the protected part. These effects are the results of linewidth broadening and thus greater spectral indistinguishability.
The findings in this Thesis are of interest to both those seeking to engineer hybrid quantum systems and those seeking to develop theoretical techniques for dealing with the decoherence of composite quantum systems.Living lasers : lasing from biological and biocompatible soft matterKarl, Markushttps://hdl.handle.net/10023/164002021-03-12T03:06:55Z2018-12-06T00:00:00ZIn recent years, the study of stimulated emission from and by biological systems has gained wide spread attention as a promising technology platform for novel biointegrated laser. However, the photonic properties and the associated physics of many biological laser systems are not yet fully understood and many promising resonator architectures and laser classes have not yet transitioned into the biological world.
In this thesis, we investigate the fundamental photonic properties of lasers based on single biological cells and explore the potential of distributed feedback (DFB) gratings as novel biointegrated laser resonators. We show how the easy and flexible fabrication of DFB resonators helps to realize optofluidic and solid-state biological lasers. Lasing characteristics, such as tunable and single mode emission, are investigated and different applications are explored. Fourier-space emission studies on different biological lasers give insight in to the photonic dispersion relation of the system and the fundamental creation of lasing modes and their confinement in living systems.
The first purely water based optofluidic DFB laser is demonstrated and novel sensing applications are suggested. This device shows low threshold lasing due to an optimized mode shape, which is achieved by a low refractive index substrate and the use of a mixed-order grating. Next, by integrating a high refractive index interlayer on a DFB resonator, a laser device incorporating the novel solid-state biological gain material green fluorescent protein (GFP) is realized. Lastly, we show how the thickness of organic polymer lasers can be reduced to its fundamental limit (< 500 nm) and the resulting membrane like laser devices can be applied to and operated on various body parts to potentially complement biometric identification.
2018-12-06T00:00:00ZKarl, MarkusIn recent years, the study of stimulated emission from and by biological systems has gained wide spread attention as a promising technology platform for novel biointegrated laser. However, the photonic properties and the associated physics of many biological laser systems are not yet fully understood and many promising resonator architectures and laser classes have not yet transitioned into the biological world.
In this thesis, we investigate the fundamental photonic properties of lasers based on single biological cells and explore the potential of distributed feedback (DFB) gratings as novel biointegrated laser resonators. We show how the easy and flexible fabrication of DFB resonators helps to realize optofluidic and solid-state biological lasers. Lasing characteristics, such as tunable and single mode emission, are investigated and different applications are explored. Fourier-space emission studies on different biological lasers give insight in to the photonic dispersion relation of the system and the fundamental creation of lasing modes and their confinement in living systems.
The first purely water based optofluidic DFB laser is demonstrated and novel sensing applications are suggested. This device shows low threshold lasing due to an optimized mode shape, which is achieved by a low refractive index substrate and the use of a mixed-order grating. Next, by integrating a high refractive index interlayer on a DFB resonator, a laser device incorporating the novel solid-state biological gain material green fluorescent protein (GFP) is realized. Lastly, we show how the thickness of organic polymer lasers can be reduced to its fundamental limit (< 500 nm) and the resulting membrane like laser devices can be applied to and operated on various body parts to potentially complement biometric identification.A portable, low-cost system for optical explosive detection based on a CMOS cameraGillanders, RossSamuel, Ifor David WilliamTurnbull, Graham AlexanderCampbell, Iain Angushttps://hdl.handle.net/10023/163452023-04-19T00:41:32Z2016-02-29T00:00:00ZHumanitarian demining requires a variety of methods and instrumentation for effective mine clearance, since a wide range of materials are used in mine manufacturing. However, landmines release vapours over time that can be detected, for example, by sniffer dogs. Optical sensor systems are especially suited to this application due to the potential for lightweight, portable, low-cost systems that nevertheless have fast response times and ppb-level sensitivity to explosive vapours. In this paper we present a system for detection based on a low-cost Raspberry Pi platform with an integrated CMOS camera. The conjugated polymers Super Yellow and Polyfluorene are excited by an LED, and the quenching effect by DNB vapour is monitored by the camera to indicate the presence of explosives. The system shows potential as a user friendly, lightweight platform for explosive vapour sensing.
2016-02-29T00:00:00ZGillanders, RossSamuel, Ifor David WilliamTurnbull, Graham AlexanderCampbell, Iain AngusHumanitarian demining requires a variety of methods and instrumentation for effective mine clearance, since a wide range of materials are used in mine manufacturing. However, landmines release vapours over time that can be detected, for example, by sniffer dogs. Optical sensor systems are especially suited to this application due to the potential for lightweight, portable, low-cost systems that nevertheless have fast response times and ppb-level sensitivity to explosive vapours. In this paper we present a system for detection based on a low-cost Raspberry Pi platform with an integrated CMOS camera. The conjugated polymers Super Yellow and Polyfluorene are excited by an LED, and the quenching effect by DNB vapour is monitored by the camera to indicate the presence of explosives. The system shows potential as a user friendly, lightweight platform for explosive vapour sensing.Ultracold atoms in flexible holographic trapsBowman, Davidhttps://hdl.handle.net/10023/162932019-10-21T10:03:23Z2018-06-27T00:00:00ZThis thesis details the design, construction and characterisation of an ultracold atoms system, developed in conjunction with a flexible optical trapping scheme which utilises a Liquid Crystal Spatial Light Modulator (LC SLM). The ultracold atoms system uses a hybrid trap formed of a quadrupole magnetic field and a focused far-detuned laser beam to form a Bose-Einstein Condensate of 2×10⁵ ⁸⁷Rb atoms. Cold atoms confined in several arbitrary optical trapping geometries are created by overlaying the LC SLM trap on to the hybrid trap, where a simple feedback process using the atomic distribution as a metric is shown to be capable of compensating for optical aberrations.
Two novel methods for creating flexible optical traps with the LC SLM are also detailed, the first of which is a multi-wavelength technique which allows several wavelengths of light to be smoothly shaped and applied to the atoms. The second method uses a computationally-efficient minimisation algorithm to create light patterns which are constrained in both amplitude and phase, where the extra phase constraint was shown to be crucial for controlling propagation effects of the LC SLM trapping beam.
2018-06-27T00:00:00ZBowman, DavidThis thesis details the design, construction and characterisation of an ultracold atoms system, developed in conjunction with a flexible optical trapping scheme which utilises a Liquid Crystal Spatial Light Modulator (LC SLM). The ultracold atoms system uses a hybrid trap formed of a quadrupole magnetic field and a focused far-detuned laser beam to form a Bose-Einstein Condensate of 2×10⁵ ⁸⁷Rb atoms. Cold atoms confined in several arbitrary optical trapping geometries are created by overlaying the LC SLM trap on to the hybrid trap, where a simple feedback process using the atomic distribution as a metric is shown to be capable of compensating for optical aberrations.
Two novel methods for creating flexible optical traps with the LC SLM are also detailed, the first of which is a multi-wavelength technique which allows several wavelengths of light to be smoothly shaped and applied to the atoms. The second method uses a computationally-efficient minimisation algorithm to create light patterns which are constrained in both amplitude and phase, where the extra phase constraint was shown to be crucial for controlling propagation effects of the LC SLM trapping beam.The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey : measuring H(z) and DA(z) at z = 0.57 with clustering wedgesKazin, Eyal A.Sánchez, Ariel G.Cuesta, Antonio J.Beutler, FlorianChuang, Chia-HsunEisenstein, Daniel J.Manera, MarcPadmanabhan, NikhilPercival, Will J.Prada, FranciscoRoss, Ashley J.Seo, Hee-JongTinker, JeremyTojeiro, RitaXu, XiaoyingBrinkmann, J.Joel, BrownsteinNichol, Robert C.Schlegel, David J.Schneider, Donald P.Thomas, Danielhttps://hdl.handle.net/10023/162652022-04-06T14:31:29Z2013-10-11T00:00:00ZWe analyse the 2D correlation function of the Sloan Digital Sky Survey-III Baryon Oscillation Spectroscopic Survey (BOSS) CMASS sample of massive galaxies of the ninth data release to measure cosmic expansion H and the angular diameter distance DA at a mean redshift of 〈z〉 = 0.57. We apply, for the first time, a new correlation function technique called clustering wedges ξΔμ(s). Using a physically motivated model, the anisotropic baryonic acoustic feature in the galaxy sample is detected at a significance level of 4.7σ compared to a featureless model. The baryonic acoustic feature is used to obtain model-independent constraints cz/H/rs = 12.28 ± 0.82 (6.7 percent accuracy) and DA/rs = 9.05 ± 0.27 (3.0 per cent) with a correlation coefficient of −0.5, where rs is the sound horizon scale at the end of the baryonic drag era. We conduct thorough tests on the data and 600 simulated realizations, finding robustness of the results regardless of the details of the analysis method. Combining this with rs constraints from the cosmic microwave background, we obtain H(0.57) = 90.8 ± 6.2 km s−1 Mpc−1 and DA(0.57) = 1386 ± 45 Mpc. We use simulations to forecast results of the final BOSS CMASS data set. We apply the reconstruction technique on the simulations demonstrating that the sharpening of the anisotropic baryonic acoustic feature should improve the detection as well as tighten constraints of H and DA by ∼30 per cent on average.
2013-10-11T00:00:00ZKazin, Eyal A.Sánchez, Ariel G.Cuesta, Antonio J.Beutler, FlorianChuang, Chia-HsunEisenstein, Daniel J.Manera, MarcPadmanabhan, NikhilPercival, Will J.Prada, FranciscoRoss, Ashley J.Seo, Hee-JongTinker, JeremyTojeiro, RitaXu, XiaoyingBrinkmann, J.Joel, BrownsteinNichol, Robert C.Schlegel, David J.Schneider, Donald P.Thomas, DanielWe analyse the 2D correlation function of the Sloan Digital Sky Survey-III Baryon Oscillation Spectroscopic Survey (BOSS) CMASS sample of massive galaxies of the ninth data release to measure cosmic expansion H and the angular diameter distance DA at a mean redshift of 〈z〉 = 0.57. We apply, for the first time, a new correlation function technique called clustering wedges ξΔμ(s). Using a physically motivated model, the anisotropic baryonic acoustic feature in the galaxy sample is detected at a significance level of 4.7σ compared to a featureless model. The baryonic acoustic feature is used to obtain model-independent constraints cz/H/rs = 12.28 ± 0.82 (6.7 percent accuracy) and DA/rs = 9.05 ± 0.27 (3.0 per cent) with a correlation coefficient of −0.5, where rs is the sound horizon scale at the end of the baryonic drag era. We conduct thorough tests on the data and 600 simulated realizations, finding robustness of the results regardless of the details of the analysis method. Combining this with rs constraints from the cosmic microwave background, we obtain H(0.57) = 90.8 ± 6.2 km s−1 Mpc−1 and DA(0.57) = 1386 ± 45 Mpc. We use simulations to forecast results of the final BOSS CMASS data set. We apply the reconstruction technique on the simulations demonstrating that the sharpening of the anisotropic baryonic acoustic feature should improve the detection as well as tighten constraints of H and DA by ∼30 per cent on average.The power spectrum from the angular distribution of galaxies in the CFHTLS-Wide fields at redshift ~0.7Granett, B. R.Guzzo, L.Coupon, J.Arnouts, S.Hudelot, P.Ilbert, O.McCracken, H. J.Mellier, Y.Adami, C.Bel, J.Bolzonella, M.Bottini, D.Cappi, A.Cucciati, O.de la Torre, S.Franzetti, P.Fritz, A.Garilli, B.Iovino, A.Krywult, J.Le Brun, V.Le Fevre, O.Maccagni, D.Malek, K.Marulli, F.Meneux, B.Paioro, L.Polletta, M.Pollo, A.Scodeggio, M.Schlagenhaufer, H.Tasca, L.Tojeiro, R.Vergani, D.Zanichelli, A.https://hdl.handle.net/10023/162422023-04-18T23:46:38Z2012-03-21T00:00:00ZWe measure the real-space galaxy power spectrum on large scales at redshifts 0.5–1.2 using optical colour selected samples from the Canada–France–Hawaii Telescope Legacy Survey. With the redshift distributions measured with a preliminary ∼14 000 spectroscopic redshifts from the VIMOS Public Extragalactic Redshift Survey (VIPERS), we deproject the angular distribution and directly estimate the three-dimensional power spectrum. We use a maximum likelihood estimator that is optimal for a Gaussian random field giving well-defined window functions and error estimates. This measurement presents an initial look at the large-scale structure field probed by the VIPERS. We measure the galaxy bias of the VIPERS-like sample to be bg= 1.38 ± 0.05 (σ8= 0.8) on scales k < 0.2 h Mpc−1 averaged over 0.5 < z < 1.2. We further investigate three photometric redshift slices, and marginalizing over the bias factors while keeping other Λ cold dark matter parameters fixed, we find the matter density Ωm= 0.30 ± 0.06.
2012-03-21T00:00:00ZGranett, B. R.Guzzo, L.Coupon, J.Arnouts, S.Hudelot, P.Ilbert, O.McCracken, H. J.Mellier, Y.Adami, C.Bel, J.Bolzonella, M.Bottini, D.Cappi, A.Cucciati, O.de la Torre, S.Franzetti, P.Fritz, A.Garilli, B.Iovino, A.Krywult, J.Le Brun, V.Le Fevre, O.Maccagni, D.Malek, K.Marulli, F.Meneux, B.Paioro, L.Polletta, M.Pollo, A.Scodeggio, M.Schlagenhaufer, H.Tasca, L.Tojeiro, R.Vergani, D.Zanichelli, A.We measure the real-space galaxy power spectrum on large scales at redshifts 0.5–1.2 using optical colour selected samples from the Canada–France–Hawaii Telescope Legacy Survey. With the redshift distributions measured with a preliminary ∼14 000 spectroscopic redshifts from the VIMOS Public Extragalactic Redshift Survey (VIPERS), we deproject the angular distribution and directly estimate the three-dimensional power spectrum. We use a maximum likelihood estimator that is optimal for a Gaussian random field giving well-defined window functions and error estimates. This measurement presents an initial look at the large-scale structure field probed by the VIPERS. We measure the galaxy bias of the VIPERS-like sample to be bg= 1.38 ± 0.05 (σ8= 0.8) on scales k < 0.2 h Mpc−1 averaged over 0.5 < z < 1.2. We further investigate three photometric redshift slices, and marginalizing over the bias factors while keeping other Λ cold dark matter parameters fixed, we find the matter density Ωm= 0.30 ± 0.06.The progenitors of present-day massive red galaxies up to z ap 0.7 - finding passive galaxies using SDSS-I/II and SDSS-IIITojeiro, R.Percival, W. J.Wake, D. A.Maraston, C.Skibba, R. A.Zehavi, I.Ross, A. J.Brinkmann, J.Conroy, C.Guo, H.Manera, M.Masters, K. L.Pforr, J.Samushia, L.Schneider, D. P.Thomas, D.Weaver, B. A.Bizyaev, D.Brewington, H.Malanushenko, E.Malanushenko, V.Oravetz, D.Pan, K.Shelden, A.Simmons, A.Snedden, S.https://hdl.handle.net/10023/162372023-04-18T23:46:38Z2012-07-21T00:00:00ZWe present a comprehensive study of 250 000 galaxies targeted by the Baryon Oscillation Spectroscopic Survey (BOSS) up to z≈ 0.7 with the specific goal of identifying and characterizing a population of galaxies that has evolved without significant merging. We compute a likelihood that each BOSS galaxy is a progenitor of the luminous red galaxies (LRGs) sample, targeted by SDSS-I/II up z≈ 0.5, by using the fossil record of LRGs and their inferred star formation histories, metallicity histories and dust content. We determine merger rates, luminosity growth rates and the evolution of the large-scale clustering between the two surveys, and we investigate the effect of using different stellar population synthesis models in our conclusions. We demonstrate that our sample is slowly evolving (of the order of 2 ± 1.5 per cent Gyr−1 by merging) by computing the change in weighted luminosity-per-galaxy between the two samples, and that this result is robust to our choice of stellar population models. Our conclusions refer to the bright and massive end of the galaxy population, with Mi0.55≲−22 and M*≳ 1011.2 M⊙, corresponding roughly to 95 and 40 per cent of the LRGs and BOSS galaxy populations, respectively. Our analysis further shows that any possible excess of flux in BOSS galaxies, when compared to LRGs, from potentially unresolved targets at z≈ 0.55 must be less than 1 per cent in the r0.55 band (approximately equivalent to the g band in the rest frame of galaxies at z= 0.55). When weighting the BOSS galaxies based on the predicted properties of the LRGs, and restricting the analysis to the reddest BOSS galaxies, we find an evolution of the large-scale clustering that is consistent with dynamical passive evolution, assuming a standard cosmology. We conclude that our likelihoods give a weighted sample that is as clean and as close to passive evolution (in dynamical terms, i.e. no or negligible merging) as possible, and that is optimal for cosmological studies.
2012-07-21T00:00:00ZTojeiro, R.Percival, W. J.Wake, D. A.Maraston, C.Skibba, R. A.Zehavi, I.Ross, A. J.Brinkmann, J.Conroy, C.Guo, H.Manera, M.Masters, K. L.Pforr, J.Samushia, L.Schneider, D. P.Thomas, D.Weaver, B. A.Bizyaev, D.Brewington, H.Malanushenko, E.Malanushenko, V.Oravetz, D.Pan, K.Shelden, A.Simmons, A.Snedden, S.We present a comprehensive study of 250 000 galaxies targeted by the Baryon Oscillation Spectroscopic Survey (BOSS) up to z≈ 0.7 with the specific goal of identifying and characterizing a population of galaxies that has evolved without significant merging. We compute a likelihood that each BOSS galaxy is a progenitor of the luminous red galaxies (LRGs) sample, targeted by SDSS-I/II up z≈ 0.5, by using the fossil record of LRGs and their inferred star formation histories, metallicity histories and dust content. We determine merger rates, luminosity growth rates and the evolution of the large-scale clustering between the two surveys, and we investigate the effect of using different stellar population synthesis models in our conclusions. We demonstrate that our sample is slowly evolving (of the order of 2 ± 1.5 per cent Gyr−1 by merging) by computing the change in weighted luminosity-per-galaxy between the two samples, and that this result is robust to our choice of stellar population models. Our conclusions refer to the bright and massive end of the galaxy population, with Mi0.55≲−22 and M*≳ 1011.2 M⊙, corresponding roughly to 95 and 40 per cent of the LRGs and BOSS galaxy populations, respectively. Our analysis further shows that any possible excess of flux in BOSS galaxies, when compared to LRGs, from potentially unresolved targets at z≈ 0.55 must be less than 1 per cent in the r0.55 band (approximately equivalent to the g band in the rest frame of galaxies at z= 0.55). When weighting the BOSS galaxies based on the predicted properties of the LRGs, and restricting the analysis to the reddest BOSS galaxies, we find an evolution of the large-scale clustering that is consistent with dynamical passive evolution, assuming a standard cosmology. We conclude that our likelihoods give a weighted sample that is as clean and as close to passive evolution (in dynamical terms, i.e. no or negligible merging) as possible, and that is optimal for cosmological studies.Exoplanet transits as the foundation of an interstellar communications networkForgan, Duncan H.https://hdl.handle.net/10023/160132023-04-25T23:52:30Z2018-03-12T00:00:00ZTwo fundamental problems for extraterrestrial intelligences (ETIs) attempting to establish interstellar communication are timing and energy consumption. Humanity's study of exoplanets via their transit across the host star highlights a means of solving both problems. An ETI ‘A’ can communicate with ETI ‘B’ if B is observing transiting planets in A's star system, either by building structures to produce artificial transits observable by B, or by emitting signals at B during transit, at significantly lower energy consumption than typical electromagnetic transmission schemes. This can produce a network of interconnected civilizations, establishing contact via observing each other's transits. Assuming that civilizations reside in a Galactic Habitable Zone (GHZ), I conduct Monte Carlo Realization simulations of the establishment and growth of this network, and analyse its properties in the context of graph theory. I find that at any instant, only a few civilizations are correctly aligned to communicate via transits. However, we should expect the true network to be cumulative, where a ‘handshake’ connection at any time guarantees connection in the future via e.g. electromagnetic signals. In all our simulations, the cumulative network connects all civilizations together in a complete network. If civilizations share knowledge of their network connections, the network can be fully complete on timescales of order a hundred thousand years. Once established, this network can connect any two civilizations either directly, or via intermediate civilizations, with a path much less than the dimensions of the GHZ.
The author gratefully acknowledges support from the ECOGAL project, grant agreement 291227, funded by the European Research Council under ERC-2011-ADG, and the STFC grant ST/J001422/1.
2018-03-12T00:00:00ZForgan, Duncan H.Two fundamental problems for extraterrestrial intelligences (ETIs) attempting to establish interstellar communication are timing and energy consumption. Humanity's study of exoplanets via their transit across the host star highlights a means of solving both problems. An ETI ‘A’ can communicate with ETI ‘B’ if B is observing transiting planets in A's star system, either by building structures to produce artificial transits observable by B, or by emitting signals at B during transit, at significantly lower energy consumption than typical electromagnetic transmission schemes. This can produce a network of interconnected civilizations, establishing contact via observing each other's transits. Assuming that civilizations reside in a Galactic Habitable Zone (GHZ), I conduct Monte Carlo Realization simulations of the establishment and growth of this network, and analyse its properties in the context of graph theory. I find that at any instant, only a few civilizations are correctly aligned to communicate via transits. However, we should expect the true network to be cumulative, where a ‘handshake’ connection at any time guarantees connection in the future via e.g. electromagnetic signals. In all our simulations, the cumulative network connects all civilizations together in a complete network. If civilizations share knowledge of their network connections, the network can be fully complete on timescales of order a hundred thousand years. Once established, this network can connect any two civilizations either directly, or via intermediate civilizations, with a path much less than the dimensions of the GHZ.The dark and luminous structure of early-type galaxies : observational dynamics and stellar populationsBoardman, Nicholas Fraserhttps://hdl.handle.net/10023/159792019-04-01T10:07:51Z2018-06-27T00:00:00ZLenticular and elliptical galaxies, collectively referred to as "early-type galaxies" (ETGs), are
commonly thought to represent the end-points of galaxy evolution. Lying in the red sequence
of galaxies, these objects are defined by their mostly old stellar populations and by their "red
and dead" appearance in optical observations. Much progress in understanding these objects
has been made with integral-field spectroscopy in recent years, with results repeatedly pointing
to a link between early-type galaxies and high-redshift spiral galaxies. However, the exact
nature of this link remains unclear, with a wide variety of evolution scenarios likely required
to fully explain the range of observed early-type galaxy properties.
In my study, I analysed observations of twelve early-type galaxies taken with the Mitchell
Integral-Field Spectrograph at McDonald Observatory, Texas. These galaxies have previously
been found to contain detectable quantities of neutral hydrogen gas, with ten out of the twelve
displaying large-scale hydrogen disks. I extracted line-of-sight kinematics of the stellar and
ionised gas components of these galaxies, and I used various modelling approaches to constrain
their stellar population parameters as well as their three-dimensional mass structure in terms
of both dark and visible components. An important feature of this study is the wide field of
view of the spectroscopic observations, which reach beyond two half-light radii for almost all
of the sample; this remains rare for integral-field unit (IFU) studies of ETGs, and so sets this
study apart from most earlier works. The gas-rich nature of the sample is likewise novel. I find
all aspects of my analysis to yield a consistent view of these galaxies’ evolution, in which one or
more gaseous interaction events served to shape them into their observed forms. I find these
galaxies to contain low dark matter fractions on average within the inner half-light radius, and
I also find mass modelling to favour near-isothermal total density profiles over much of the
sample.
2018-06-27T00:00:00ZBoardman, Nicholas FraserLenticular and elliptical galaxies, collectively referred to as "early-type galaxies" (ETGs), are
commonly thought to represent the end-points of galaxy evolution. Lying in the red sequence
of galaxies, these objects are defined by their mostly old stellar populations and by their "red
and dead" appearance in optical observations. Much progress in understanding these objects
has been made with integral-field spectroscopy in recent years, with results repeatedly pointing
to a link between early-type galaxies and high-redshift spiral galaxies. However, the exact
nature of this link remains unclear, with a wide variety of evolution scenarios likely required
to fully explain the range of observed early-type galaxy properties.
In my study, I analysed observations of twelve early-type galaxies taken with the Mitchell
Integral-Field Spectrograph at McDonald Observatory, Texas. These galaxies have previously
been found to contain detectable quantities of neutral hydrogen gas, with ten out of the twelve
displaying large-scale hydrogen disks. I extracted line-of-sight kinematics of the stellar and
ionised gas components of these galaxies, and I used various modelling approaches to constrain
their stellar population parameters as well as their three-dimensional mass structure in terms
of both dark and visible components. An important feature of this study is the wide field of
view of the spectroscopic observations, which reach beyond two half-light radii for almost all
of the sample; this remains rare for integral-field unit (IFU) studies of ETGs, and so sets this
study apart from most earlier works. The gas-rich nature of the sample is likewise novel. I find
all aspects of my analysis to yield a consistent view of these galaxies’ evolution, in which one or
more gaseous interaction events served to shape them into their observed forms. I find these
galaxies to contain low dark matter fractions on average within the inner half-light radius, and
I also find mass modelling to favour near-isothermal total density profiles over much of the
sample.Enabling technologies for high performance millimetre and aub-millimetre wave radarRobertson, Duncan A.https://hdl.handle.net/10023/159692023-04-19T00:43:07Z2018-06-20T00:00:00ZThe successful realisation of high performance millimetre and sub-millimetre wave radars requires key enabling technologies, many of which are not yet commercially available. This paper illustrates some of the key enabling technologies developed to address radar system requirements including chirp generation, feedhorns, duplexing and non-mechanical beam steering. The type of high performance radar system which can be achieved using these technologies is illustrated with the examples of the ‘T-220’ 94 GHz FMCW Doppler radar used for high sensitivity target and clutter phenomenology studies and the ‘CONSORTIS’ 340 GHz 3D imaging radar developed for concealed object detection as required for next generation aviation security screening.
2018-06-20T00:00:00ZRobertson, Duncan A.The successful realisation of high performance millimetre and sub-millimetre wave radars requires key enabling technologies, many of which are not yet commercially available. This paper illustrates some of the key enabling technologies developed to address radar system requirements including chirp generation, feedhorns, duplexing and non-mechanical beam steering. The type of high performance radar system which can be achieved using these technologies is illustrated with the examples of the ‘T-220’ 94 GHz FMCW Doppler radar used for high sensitivity target and clutter phenomenology studies and the ‘CONSORTIS’ 340 GHz 3D imaging radar developed for concealed object detection as required for next generation aviation security screening.Magnetic resonance spectroscopy of organic photovoltaic cellsThomson, Stuart A. J.https://hdl.handle.net/10023/159252021-04-21T02:06:10Z2017-01-01T00:00:00ZOrganic photovoltaics (OPV) have the potential advantages of low-cost, flexibility and
high throughput production. However, at present their efficiency is lower than other thin
film technologies and they are susceptible to degradation which limits cell lifetimes.
Magnetic resonance spectroscopy is a powerful technique to study the key processes
involved in the operation of OPV cells. In this thesis a range of electron paramagnetic
resonance (EPR) methods are used to investigate the processes which influence cell
efficiencies.
The understanding of degradation pathways and how they influence cell performance is
important if OPV cells are to reach commercialisation. The efficiency of PTB7:PC₇₁BM
cells is severely reduced when exposed to ambient atmosphere during processing.
Current-voltage analysis was combined with EDMR spectroscopy to investigate the
source of this performance loss. This investigation revealed that exposure of
PTB7:PC₇₁BM films to the solvent additive DIO and ambient atmosphere leads to
electron trap formation on the PC₇₁BM which acts as a recombination centre.
Using time resolved EPR spectroscopy the variation of charge separation across blends
of the DTS family of small molecule electron donors with PC₆₁BM is investigated.
Charge separation is found to be slowest in the [1,2,5]thiadiazolo[3,4-c]pyridine blend.
This slower separation is accompanied by a higher population of triplet excitons formed
by back electron transfer. This finding demonstrates that back electron transfer is a loss
mechanism in these molecular systems when charge separation is slow.
The EPR signatures of negative polarons on two high efficiency non-fullerene acceptors,
ITIC and IDTBR, are identified using multifrequency light induced EPR spectroscopy.
The polaron signatures of ITIC and IDTBR were found to overlap with polarons on P3HT
at all three microwave frequencies. Using multifrequency simulations the negative
polaron signatures and g-tensors of ITIC and IDTBR were determined for the first time.
2017-01-01T00:00:00ZThomson, Stuart A. J.Organic photovoltaics (OPV) have the potential advantages of low-cost, flexibility and
high throughput production. However, at present their efficiency is lower than other thin
film technologies and they are susceptible to degradation which limits cell lifetimes.
Magnetic resonance spectroscopy is a powerful technique to study the key processes
involved in the operation of OPV cells. In this thesis a range of electron paramagnetic
resonance (EPR) methods are used to investigate the processes which influence cell
efficiencies.
The understanding of degradation pathways and how they influence cell performance is
important if OPV cells are to reach commercialisation. The efficiency of PTB7:PC₇₁BM
cells is severely reduced when exposed to ambient atmosphere during processing.
Current-voltage analysis was combined with EDMR spectroscopy to investigate the
source of this performance loss. This investigation revealed that exposure of
PTB7:PC₇₁BM films to the solvent additive DIO and ambient atmosphere leads to
electron trap formation on the PC₇₁BM which acts as a recombination centre.
Using time resolved EPR spectroscopy the variation of charge separation across blends
of the DTS family of small molecule electron donors with PC₆₁BM is investigated.
Charge separation is found to be slowest in the [1,2,5]thiadiazolo[3,4-c]pyridine blend.
This slower separation is accompanied by a higher population of triplet excitons formed
by back electron transfer. This finding demonstrates that back electron transfer is a loss
mechanism in these molecular systems when charge separation is slow.
The EPR signatures of negative polarons on two high efficiency non-fullerene acceptors,
ITIC and IDTBR, are identified using multifrequency light induced EPR spectroscopy.
The polaron signatures of ITIC and IDTBR were found to overlap with polarons on P3HT
at all three microwave frequencies. Using multifrequency simulations the negative
polaron signatures and g-tensors of ITIC and IDTBR were determined for the first time.Many-Body Interactions and
Spin-Splitting of the Electronic
Structure in WSe2 and EuORiley, Jonathon M.https://hdl.handle.net/10023/157072023-03-20T16:53:52Z2017-01-01T00:00:00ZThe ability to create and control spin-polarised electronic states in solids is vital for realising all-electrical manipulation of spins for the next generation of electronic
devices. Such devices are likely to rely upon the injection of a spin-current
into a semiconducting material whereupon the current is manipulated for the
transfer of data. Two such materials that have been proposed for spin injection
and manipulation are EuO, a ferromagnetic semiconductor, and WSe2, a member
of the layered transition metal dichalcogenides, respectively. By directly probing
the electronic band structure using angle-resolved photoemission spectroscopy,
ARPES, and its spin-resolved analogue, SARPES, a "hidden" spin-polarisaion is
observed in bulk WSe2 due to the local breaking of inversion symmetry that locks
the spin to the valley- and layer-pseudospins. By chemical gating, the layerdegeneracy is broken which induces a spin-splitting of the valence bands and
the formation of a 2DEG. Increasing the carrier density of the 2DEG is shown to
result in a lowering of the chemical potential, providing direct spectroscopic evidence for many body effects due to electron-electron interactions. Whereas WSe2
hosts spin-polarised states due to local inversion-symmetry breaking, ferromagnetism
induces spin-polarised valence and conduction band states in Gd-doped
EuO that break time-reversal symmetry. Using ARPES, the full 3D dispersion
of the conduction bands is observed for the first time and the temperature dependence
of the spin-split conduction band is analysed. By varying the Gd concentration,
the carrier density is varied, revealing two kinds of satellite features
due to electron-phonon and electron-plasmon interactions in the low temperature
phase. Thus, despite differences in dimensionality, structure and symmetries,
both materials show evidence of electron interactions which are directly
measurable by ARPES and are seen to host spin-split electronic states that can be
manipulated via external conditions.
2017-01-01T00:00:00ZRiley, Jonathon M.The ability to create and control spin-polarised electronic states in solids is vital for realising all-electrical manipulation of spins for the next generation of electronic
devices. Such devices are likely to rely upon the injection of a spin-current
into a semiconducting material whereupon the current is manipulated for the
transfer of data. Two such materials that have been proposed for spin injection
and manipulation are EuO, a ferromagnetic semiconductor, and WSe2, a member
of the layered transition metal dichalcogenides, respectively. By directly probing
the electronic band structure using angle-resolved photoemission spectroscopy,
ARPES, and its spin-resolved analogue, SARPES, a "hidden" spin-polarisaion is
observed in bulk WSe2 due to the local breaking of inversion symmetry that locks
the spin to the valley- and layer-pseudospins. By chemical gating, the layerdegeneracy is broken which induces a spin-splitting of the valence bands and
the formation of a 2DEG. Increasing the carrier density of the 2DEG is shown to
result in a lowering of the chemical potential, providing direct spectroscopic evidence for many body effects due to electron-electron interactions. Whereas WSe2
hosts spin-polarised states due to local inversion-symmetry breaking, ferromagnetism
induces spin-polarised valence and conduction band states in Gd-doped
EuO that break time-reversal symmetry. Using ARPES, the full 3D dispersion
of the conduction bands is observed for the first time and the temperature dependence
of the spin-split conduction band is analysed. By varying the Gd concentration,
the carrier density is varied, revealing two kinds of satellite features
due to electron-phonon and electron-plasmon interactions in the low temperature
phase. Thus, despite differences in dimensionality, structure and symmetries,
both materials show evidence of electron interactions which are directly
measurable by ARPES and are seen to host spin-split electronic states that can be
manipulated via external conditions.Title redactedStevenson, James Robert Younghttps://hdl.handle.net/10023/156912019-07-25T08:38:39Z2016-01-01T00:00:00Z2016-01-01T00:00:00ZStevenson, James Robert YoungTitle redactedMorawska, Paulina Olgahttps://hdl.handle.net/10023/156812019-10-02T15:26:44Z2016-01-01T00:00:00Z2016-01-01T00:00:00ZMorawska, Paulina OlgaThe evolution of galaxy structures from a Bayesian perspectiveArgyle, Joshua J.https://hdl.handle.net/10023/156472019-04-01T10:05:09Z2017-01-01T00:00:00ZGalaxy structures in the local Universe are the result of an evolution spanning billions of years.
The diversity in morphologies observed is due to mechanisms that could either be from external
interactions or internal processes. The hierarchical merging of two massive galaxies has long been
thought to give rise to pressure-supported spherical structures, including elliptical galaxies and classical bulges. On the other hand, isolated galaxies may evolve at a much slower pace with the accretion of gas forming flattened rotationally-supported discs. Secular evolution could also result in newly formed internal structures, such as bars or discy-bulges. The first step in understanding the complex pathways that formed these monolithic beasts, we need to robustly measure their structures.
This thesis investigates how the structure of galaxies have evolved over the last seven Gyrs. In
the first part I present a new Bayesian Markov Chain Monte Carlo (MCMC) two-dimensional (2D)
photometric decompositions algorithm called PHI. The purpose of the algorithm is to decompose a
galaxy’s light profile into the various components that make-up the structure. By implementing a
three level method, PHI is able to obtain a full understanding of the parameter space overcoming
many of the major issues previous codes have struggled with.
The second part of the thesis describes the generation of synthetic galaxy images which are used
to test the robustness of PHIand to also highlight the cosmological and instrumental effects that may
bias the outcomes. We also present is a performance test for the Bayesian application to bulge-disc
decompositions of galaxies using images from the Sloan Digital Sky Survey (SDSS), also included is
the parameter estimation and model comparison method using the Bayesian Information Criteria.
In the third part of this thesis we show how the use of Hierarchical Bayesian Models (HBM)
can be used to describe structural scaling relations in the local Universe. A constant piece-wise
representation fully captures the underlying nature of the sample. This leads to the analysis of
many structural scaling relations, one being the positive trend between the effective radius of the
bulge and the Sérsic index.
Lastly, a study investigating the structural evolution of galaxies within the COSMOS field in the
redshift range 0.5 < z < 1.25 is presented. The flexible nature of the HBM allows for a detailed
description of the build-up of galaxy structures in the local Universe.
2017-01-01T00:00:00ZArgyle, Joshua J.Galaxy structures in the local Universe are the result of an evolution spanning billions of years.
The diversity in morphologies observed is due to mechanisms that could either be from external
interactions or internal processes. The hierarchical merging of two massive galaxies has long been
thought to give rise to pressure-supported spherical structures, including elliptical galaxies and classical bulges. On the other hand, isolated galaxies may evolve at a much slower pace with the accretion of gas forming flattened rotationally-supported discs. Secular evolution could also result in newly formed internal structures, such as bars or discy-bulges. The first step in understanding the complex pathways that formed these monolithic beasts, we need to robustly measure their structures.
This thesis investigates how the structure of galaxies have evolved over the last seven Gyrs. In
the first part I present a new Bayesian Markov Chain Monte Carlo (MCMC) two-dimensional (2D)
photometric decompositions algorithm called PHI. The purpose of the algorithm is to decompose a
galaxy’s light profile into the various components that make-up the structure. By implementing a
three level method, PHI is able to obtain a full understanding of the parameter space overcoming
many of the major issues previous codes have struggled with.
The second part of the thesis describes the generation of synthetic galaxy images which are used
to test the robustness of PHIand to also highlight the cosmological and instrumental effects that may
bias the outcomes. We also present is a performance test for the Bayesian application to bulge-disc
decompositions of galaxies using images from the Sloan Digital Sky Survey (SDSS), also included is
the parameter estimation and model comparison method using the Bayesian Information Criteria.
In the third part of this thesis we show how the use of Hierarchical Bayesian Models (HBM)
can be used to describe structural scaling relations in the local Universe. A constant piece-wise
representation fully captures the underlying nature of the sample. This leads to the analysis of
many structural scaling relations, one being the positive trend between the effective radius of the
bulge and the Sérsic index.
Lastly, a study investigating the structural evolution of galaxies within the COSMOS field in the
redshift range 0.5 < z < 1.25 is presented. The flexible nature of the HBM allows for a detailed
description of the build-up of galaxy structures in the local Universe.Continuous-wave, singly-resonant optical parametric oscillators pumped internal to Nd:YVO₄ lasersStothard, David James Markhttps://hdl.handle.net/10023/155792019-04-01T10:07:33Z2002-01-01T00:00:00ZThe advent of new quasi phase matched materials and high spectral/spatial quality pump
sources has led to a renaissance in the development of continuous-wave optical
parametric oscillators for the coherent generation of broadly tunable light in the mid infrared spectral region. This thesis describes a novel technique which overcomes the
threshold constraints of the singly resonant oscillator (SRO) and stability constraints of
the doubly resonant oscillator (DRO) traditionally associated with these devices by
placing a singly resonant optical parametric oscillator in the high circulating field found
within the cavity of a laser: the intracavity optical parametric oscillator.
An SRO based upon the nonlinear material periodically poled LiNb0₃ (PPLN) operating
internal to an all solid state, 1W diode pumped Nd:YV0₄ mini-laser is demonstrated and
characterised. This system exhibits SRO threshold at only 330mW of external diode
pump power, and produced a total of 70mW of extractable idler at 1W diode pump
power. Through multi-parameter tuning of the poled nonlinear material we demonstrate
broad tuning of the non-resonant idler through the spectrally important range 3.1 - 4μm.
Novel cavity design desensitises the system to the effects of thermal lensing in the
nonlinear medium, resulting in stable spatial and mean power outputs. The short term
pump field stability is characterised by intensity modulation brought about by the onset of
relaxation oscillations ; a consequence of placing the SRO within the cavity of the pump
laser.
A comparative study of SRO's based upon PPLN and the new material periodically poled
RbTi0As0₄ (PPRTA) pumped internal to a high power fibre coupled diode pumped
Nd:YV0₄ laser cavity is undertaken and presented. We see that although the nonlinearity
and interaction length of the PPRTA is smaller than that of PPLN, the system based upon
PPRTA outperforms or is at least comparable with that based upon PPLN in every respect
with the exception of idler tuning range. We attribute this to the reduced sensitivity of this
material to the effect of thermal lensing. Up to 440mW of extracted idler was produced
by each system. The reduction of interferometric feedback of the pump field by the signal
cavity mirror was found to eliminate the onset of relaxation oscillations in the case of
PPRTA but not PPLN, due to thermal air currents dominating the triggering process in
iii
this system. Recently published Sellmeier equations and temperature derivatives for
PPRTA are compared with the experimentally observed temperature tuning behaviour.
The inclusion of an optical parametric oscillator within the cavity of the pump laser
impacts significantly upon the transient dynamics of the pump laser in which it resides.
We show experimental evidence of this effect and outline a strategy to minimise the
effects of relaxation oscillations in the context of a simple numerical model which shall
be derived.
Possible future avenues of research are discussed in the context of the results and
conclusions obtained over the course of this research program.
2002-01-01T00:00:00ZStothard, David James MarkThe advent of new quasi phase matched materials and high spectral/spatial quality pump
sources has led to a renaissance in the development of continuous-wave optical
parametric oscillators for the coherent generation of broadly tunable light in the mid infrared spectral region. This thesis describes a novel technique which overcomes the
threshold constraints of the singly resonant oscillator (SRO) and stability constraints of
the doubly resonant oscillator (DRO) traditionally associated with these devices by
placing a singly resonant optical parametric oscillator in the high circulating field found
within the cavity of a laser: the intracavity optical parametric oscillator.
An SRO based upon the nonlinear material periodically poled LiNb0₃ (PPLN) operating
internal to an all solid state, 1W diode pumped Nd:YV0₄ mini-laser is demonstrated and
characterised. This system exhibits SRO threshold at only 330mW of external diode
pump power, and produced a total of 70mW of extractable idler at 1W diode pump
power. Through multi-parameter tuning of the poled nonlinear material we demonstrate
broad tuning of the non-resonant idler through the spectrally important range 3.1 - 4μm.
Novel cavity design desensitises the system to the effects of thermal lensing in the
nonlinear medium, resulting in stable spatial and mean power outputs. The short term
pump field stability is characterised by intensity modulation brought about by the onset of
relaxation oscillations ; a consequence of placing the SRO within the cavity of the pump
laser.
A comparative study of SRO's based upon PPLN and the new material periodically poled
RbTi0As0₄ (PPRTA) pumped internal to a high power fibre coupled diode pumped
Nd:YV0₄ laser cavity is undertaken and presented. We see that although the nonlinearity
and interaction length of the PPRTA is smaller than that of PPLN, the system based upon
PPRTA outperforms or is at least comparable with that based upon PPLN in every respect
with the exception of idler tuning range. We attribute this to the reduced sensitivity of this
material to the effect of thermal lensing. Up to 440mW of extracted idler was produced
by each system. The reduction of interferometric feedback of the pump field by the signal
cavity mirror was found to eliminate the onset of relaxation oscillations in the case of
PPRTA but not PPLN, due to thermal air currents dominating the triggering process in
iii
this system. Recently published Sellmeier equations and temperature derivatives for
PPRTA are compared with the experimentally observed temperature tuning behaviour.
The inclusion of an optical parametric oscillator within the cavity of the pump laser
impacts significantly upon the transient dynamics of the pump laser in which it resides.
We show experimental evidence of this effect and outline a strategy to minimise the
effects of relaxation oscillations in the context of a simple numerical model which shall
be derived.
Possible future avenues of research are discussed in the context of the results and
conclusions obtained over the course of this research program.Uniaxial stress technique and investigations into correlated electron systemsBarber, Mark E.https://hdl.handle.net/10023/154292019-06-06T02:00:27Z2017-06-21T00:00:00ZIn the repertoire of an experimental condensed matter physicist, the ability to tune continuously through features in the electronic structure and to selectively break point-group symmetries are both valuable techniques. The experimental technique at the heart of this dissertation, uniaxial stress, can do both such things.
The thesis will start with a thorough discussion of our new technique, which was continually developed over the course of this work, presenting both its unique capabilities and also some guidance on the best working practices, before moving on to describe results obtained on two different strongly correlated electron materials.
The first, Sr₂RuO₄, is an unconventional superconductor, whose order parameter has long been speculated to be odd-parity. Of interest to us is the close proximity of one of its three Fermi surfaces to a Van Hove singularity (VHs). Our results strongly suggest that we have been able to traverse the VHs, inducing a topological Lifshitz transition. T[sub]c is enhanced by a factor ~2.3 and measurements of H[sub](c2) open the possibility that optimally strained Sr₂RuO₄ has an even-parity, rather than odd-parity, order parameter. Measurements of the normal state properties show that quasiparticle scattering is increased across all the bands and in all directions, and effects of quantum criticality are observed around the suspected Lifshitz transition.
Sr₃Ru₂O₇ has a metamagnetic quantum critical endpoint, which in highly pure samples is masked by a novel phase. Weak in-plane magnetic fields are well-known to induce strong resistive anisotropy in the novel phase, leading to speculation that a spontaneous, electronically driven lowering of symmetry occurs. Using magnetic susceptibility and resistivity measurements we can show that in-plane anisotropic strain also reveals the strong susceptibility to electronic anisotropy. However, the phase diagram that these pressure measurements reveal is consistent only with large but finite susceptibility, and not with spontaneous symmetry reduction.
2017-06-21T00:00:00ZBarber, Mark E.In the repertoire of an experimental condensed matter physicist, the ability to tune continuously through features in the electronic structure and to selectively break point-group symmetries are both valuable techniques. The experimental technique at the heart of this dissertation, uniaxial stress, can do both such things.
The thesis will start with a thorough discussion of our new technique, which was continually developed over the course of this work, presenting both its unique capabilities and also some guidance on the best working practices, before moving on to describe results obtained on two different strongly correlated electron materials.
The first, Sr₂RuO₄, is an unconventional superconductor, whose order parameter has long been speculated to be odd-parity. Of interest to us is the close proximity of one of its three Fermi surfaces to a Van Hove singularity (VHs). Our results strongly suggest that we have been able to traverse the VHs, inducing a topological Lifshitz transition. T[sub]c is enhanced by a factor ~2.3 and measurements of H[sub](c2) open the possibility that optimally strained Sr₂RuO₄ has an even-parity, rather than odd-parity, order parameter. Measurements of the normal state properties show that quasiparticle scattering is increased across all the bands and in all directions, and effects of quantum criticality are observed around the suspected Lifshitz transition.
Sr₃Ru₂O₇ has a metamagnetic quantum critical endpoint, which in highly pure samples is masked by a novel phase. Weak in-plane magnetic fields are well-known to induce strong resistive anisotropy in the novel phase, leading to speculation that a spontaneous, electronically driven lowering of symmetry occurs. Using magnetic susceptibility and resistivity measurements we can show that in-plane anisotropic strain also reveals the strong susceptibility to electronic anisotropy. However, the phase diagram that these pressure measurements reveal is consistent only with large but finite susceptibility, and not with spontaneous symmetry reduction.Novel scanning techniques for CCD image capture and displayEmberson, M.D. (Matthew David)https://hdl.handle.net/10023/150392019-04-01T10:05:28Z1995-07-01T00:00:00ZThis work details two investigations into image capture, taken from the fields of x-ray and laser research, and also details two scanning systems: a wire surface generator and a video security device. Firstly a camera system is described that can display images, digitize them and provide real time false shading. This camera is shown to have a linear intensity response and to have a maximum saturation level below the digitizing range. Some example outputs are then illustrated. The ability to irradiate CCDs with direct X-ray radiation is also investigated. A camera is developed that vertically integrates such images and is shown to give an increase in the processing speed of existing equipment and to reduce experiment times by a factor of 388. Taking this idea further, a fast one dimensional camera is developed. This camera couples laser pulses onto a CCDs via a fibre optic faceplate and a 25 mum slit. Unusual scanning techniques are used to achieve image storage within the sensor itself and a method for correcting dark current and other errors is proposed. Next a mechanism for displaying wire surface representations of intensity) images is investigated. Results obtained from real time, hidden line removing hardware are illustrated, along with improved algorithms for shaded surface generation. This is then developed into a security device protecting VDUs from radio based surveillance. This is achieved by randomizing the display order of raster lines along with a hardware solution for random sequence generation. Finally the generation of Uniformly distributed random numbers is achieved by processing readings from a digitized. Normally distributed voltage source. The effects of this processing are investigated and an analysis of the underlying theory is used to determine an optimal setting for the gain stage.
1995-07-01T00:00:00ZEmberson, M.D. (Matthew David)This work details two investigations into image capture, taken from the fields of x-ray and laser research, and also details two scanning systems: a wire surface generator and a video security device. Firstly a camera system is described that can display images, digitize them and provide real time false shading. This camera is shown to have a linear intensity response and to have a maximum saturation level below the digitizing range. Some example outputs are then illustrated. The ability to irradiate CCDs with direct X-ray radiation is also investigated. A camera is developed that vertically integrates such images and is shown to give an increase in the processing speed of existing equipment and to reduce experiment times by a factor of 388. Taking this idea further, a fast one dimensional camera is developed. This camera couples laser pulses onto a CCDs via a fibre optic faceplate and a 25 mum slit. Unusual scanning techniques are used to achieve image storage within the sensor itself and a method for correcting dark current and other errors is proposed. Next a mechanism for displaying wire surface representations of intensity) images is investigated. Results obtained from real time, hidden line removing hardware are illustrated, along with improved algorithms for shaded surface generation. This is then developed into a security device protecting VDUs from radio based surveillance. This is achieved by randomizing the display order of raster lines along with a hardware solution for random sequence generation. Finally the generation of Uniformly distributed random numbers is achieved by processing readings from a digitized. Normally distributed voltage source. The effects of this processing are investigated and an analysis of the underlying theory is used to determine an optimal setting for the gain stage.Novel solid-state millimetre-wave sourcesRobertson, Malcolm R.https://hdl.handle.net/10023/150382019-04-01T10:07:12Z1993-07-01T00:00:00ZA study of some principal solid-state millimetre-wave sources was carried out, using mainly quasi-optical techniques developed at St. Andrews. A review of the theory of operation of Gunn oscillators was undertaken, and a number of wideband tunable Gunn oscillators were built, incorporating Gunn diodes from several different manufacturers. Characterisation of their frequency range, power output, bias tuning, and frequency stability were measured. Effects such as bias oscillations and frequency jumps were also noted, and the iterative techniques required to build an oscillator to a certain specification were presented. Frequency multipliers are widely used to extend the frequency range of solid-state oscillators, and the techniques used to design frequency multipliers were studied. The use of non-linear capacitors (varactor diodes), and non-linear resistors (varistors and resonant-tunneling diodes) was considered in some detail. An experimental doubler block was designed and built, and doubling using both varactors and varistors was measured and evaluated. A number of resonant-tunneling double-barrier diodes, or quantum-well devices, was made available through collaboration with Nottingham University. The tunneling process in these devices is inherently fast, making these devices suitable for high-frequency operation as detectors, oscillators, multipliers and mixers. The theory of operation of double-barrier diodes was studied, and methods of evaluating their maximum oscillation frequency were compared. Several different devices were mounted in a whisker-contacted waveguide circuit, and oscillations at W-band were measured. A whisker-contacted multiplier block was designed and built, and zero-bias tripling to 254GHz was observed. Resonant-tunneling diodes were also shown to be capable of acting as self-oscillating mixers at W-band. Effects such as injection locking and chaotic oscillations were measured. A new class of noise source, the chaotic or semi-chaotic noise source, was considered as a future device. Its potential applications, and its advantages over conventional noise sources, was discussed.
1993-07-01T00:00:00ZRobertson, Malcolm R.A study of some principal solid-state millimetre-wave sources was carried out, using mainly quasi-optical techniques developed at St. Andrews. A review of the theory of operation of Gunn oscillators was undertaken, and a number of wideband tunable Gunn oscillators were built, incorporating Gunn diodes from several different manufacturers. Characterisation of their frequency range, power output, bias tuning, and frequency stability were measured. Effects such as bias oscillations and frequency jumps were also noted, and the iterative techniques required to build an oscillator to a certain specification were presented. Frequency multipliers are widely used to extend the frequency range of solid-state oscillators, and the techniques used to design frequency multipliers were studied. The use of non-linear capacitors (varactor diodes), and non-linear resistors (varistors and resonant-tunneling diodes) was considered in some detail. An experimental doubler block was designed and built, and doubling using both varactors and varistors was measured and evaluated. A number of resonant-tunneling double-barrier diodes, or quantum-well devices, was made available through collaboration with Nottingham University. The tunneling process in these devices is inherently fast, making these devices suitable for high-frequency operation as detectors, oscillators, multipliers and mixers. The theory of operation of double-barrier diodes was studied, and methods of evaluating their maximum oscillation frequency were compared. Several different devices were mounted in a whisker-contacted waveguide circuit, and oscillations at W-band were measured. A whisker-contacted multiplier block was designed and built, and zero-bias tripling to 254GHz was observed. Resonant-tunneling diodes were also shown to be capable of acting as self-oscillating mixers at W-band. Effects such as injection locking and chaotic oscillations were measured. A new class of noise source, the chaotic or semi-chaotic noise source, was considered as a future device. Its potential applications, and its advantages over conventional noise sources, was discussed.Linear and nonlinear optical pulse characterisationFinch, Andrewhttps://hdl.handle.net/10023/150372019-04-01T10:06:56Z1989-07-01T00:00:00ZDevelopmental work on the generation and measurement of ultrashort pulses has been performed. A colliding pulse, passively mode-locked (CPM) ring dye laser has been investigated by spectral analysis and the nonlinear technique of second harmonic generation autocorrelation. Two systems for the intracavity compensation of group velocity dispersion (GVD) have been experimentally compared in the CPM laser. Initially one scheme, utilising Gires-Toumois interferometers, has achieved pulse durations of 64 fs. A second technique employing a four-prism sequence within the cavity gave typical pulse durations of -40 fs and focussing adjustments within the jets achieved durations as short as 19 fs for the first time. A realtime interferometric autocorrelator was constructed and detailed theoretical work has been performed to model the resultant fringe resolved autocorrelations as a function of pulseshape and frequency chirp. Spectral and autocorrelation analysis of the CPM laser led to the inference that the laser pulse intensity profiles were distinctly asymmetric. The main sources of frequency chirp within the laser cavity were assessed in order to find possible explanations for this type of laser behaviour. The linear pulse measurement technique employing synchroscan streak cameras was also critically assessed in terms of the available temporal resolutions as a function of phase noise in the RF deflection signal. Two streak tube designs, the Photochron II and the Photochron IV, have been experimentally compared employing the CPM laser as a test pulse source. Optimisation of the synchronisation circuitry has allowed the notable achievement of a temporal resolution of 0.93ps for the Photochron IV streak camera. A computer-interfaced readout system incorporating a charge coupled device (CCD) sensor has been developed which allows the recording of synchroscan streak events with a digitisation accuracy up to 12 bits. Preliminary experimentation was also performed to investigate the feasibility of incorporating a electron sensitive CCD structure within the envelope of the streak camera. It is intended that such a streak camera will be incorporated in a spaceborne laser ranging system and a theoretical assessment of the expected instrument performance has been performed. The above investigations have direct relevance to other types of ultrashort pulse sources and their application in optical communications, time-resolved spectroscopy and ultrafast electrooptic sampling.
1989-07-01T00:00:00ZFinch, AndrewDevelopmental work on the generation and measurement of ultrashort pulses has been performed. A colliding pulse, passively mode-locked (CPM) ring dye laser has been investigated by spectral analysis and the nonlinear technique of second harmonic generation autocorrelation. Two systems for the intracavity compensation of group velocity dispersion (GVD) have been experimentally compared in the CPM laser. Initially one scheme, utilising Gires-Toumois interferometers, has achieved pulse durations of 64 fs. A second technique employing a four-prism sequence within the cavity gave typical pulse durations of -40 fs and focussing adjustments within the jets achieved durations as short as 19 fs for the first time. A realtime interferometric autocorrelator was constructed and detailed theoretical work has been performed to model the resultant fringe resolved autocorrelations as a function of pulseshape and frequency chirp. Spectral and autocorrelation analysis of the CPM laser led to the inference that the laser pulse intensity profiles were distinctly asymmetric. The main sources of frequency chirp within the laser cavity were assessed in order to find possible explanations for this type of laser behaviour. The linear pulse measurement technique employing synchroscan streak cameras was also critically assessed in terms of the available temporal resolutions as a function of phase noise in the RF deflection signal. Two streak tube designs, the Photochron II and the Photochron IV, have been experimentally compared employing the CPM laser as a test pulse source. Optimisation of the synchronisation circuitry has allowed the notable achievement of a temporal resolution of 0.93ps for the Photochron IV streak camera. A computer-interfaced readout system incorporating a charge coupled device (CCD) sensor has been developed which allows the recording of synchroscan streak events with a digitisation accuracy up to 12 bits. Preliminary experimentation was also performed to investigate the feasibility of incorporating a electron sensitive CCD structure within the envelope of the streak camera. It is intended that such a streak camera will be incorporated in a spaceborne laser ranging system and a theoretical assessment of the expected instrument performance has been performed. The above investigations have direct relevance to other types of ultrashort pulse sources and their application in optical communications, time-resolved spectroscopy and ultrafast electrooptic sampling.Design and evaluation of ultrafast electron-optical streak and framing camerasLiu, Yuepinghttps://hdl.handle.net/10023/150362019-04-01T10:06:43Z1993-07-01T00:00:00ZThe assessment of the performance of ultrafast electron-optical cameras operating in single-shot, synchroscan streak modes and framing mode has been accomplished and it has provided a better understanding of the limiting factors in the performance of these systems. The characterisation of a Photochron IV streak camera in synchroscan operation has been carried out and this has indicated that apart from the limiting time resolution of the streak tube itself, the amplitude and phase noise in laser sources and electronic circuitry also represent significant limitations to the overall system performance. The simulation of space-charge effects in the single-shot operation of streak cameras has demonstrated that space-charge effects are the key factor that limits the dynamic range. The use of travelling-wave deflection systems has been shown to offer a high deflection sensitivity over a broad frequency range. To achieve femtosecond time resolutions from both synchroscan and single-shot streak cameras, the design of a Photochron V streak tube having optimised electron-optical focusing and deflection systems has been discussed. Analyses have indicated that a limiting time resolution of 250 fs is to be expected. Preliminary tests on an experimental version of a Photochron V tube operating in a synchroscan mode have shown a recorded pulse duration of 3.8 ps. Quantitative theoretical studies of output temporal responses of this tube for different input temporal profiles have indicated that it can reproduce temporal structures of input signals with high fidelity for both synchroscan and single-shot operations in a timescale of around Ips. The analysis of the Picoframe type of framing cameras has led to the refinements of the dynamic performance. For the future development, proposals have been made for the designs of a new streak tube having high dynamic range and a new framing tube having frame times less than 10 ps with potential applications in medical diagnosis.
1993-07-01T00:00:00ZLiu, YuepingThe assessment of the performance of ultrafast electron-optical cameras operating in single-shot, synchroscan streak modes and framing mode has been accomplished and it has provided a better understanding of the limiting factors in the performance of these systems. The characterisation of a Photochron IV streak camera in synchroscan operation has been carried out and this has indicated that apart from the limiting time resolution of the streak tube itself, the amplitude and phase noise in laser sources and electronic circuitry also represent significant limitations to the overall system performance. The simulation of space-charge effects in the single-shot operation of streak cameras has demonstrated that space-charge effects are the key factor that limits the dynamic range. The use of travelling-wave deflection systems has been shown to offer a high deflection sensitivity over a broad frequency range. To achieve femtosecond time resolutions from both synchroscan and single-shot streak cameras, the design of a Photochron V streak tube having optimised electron-optical focusing and deflection systems has been discussed. Analyses have indicated that a limiting time resolution of 250 fs is to be expected. Preliminary tests on an experimental version of a Photochron V tube operating in a synchroscan mode have shown a recorded pulse duration of 3.8 ps. Quantitative theoretical studies of output temporal responses of this tube for different input temporal profiles have indicated that it can reproduce temporal structures of input signals with high fidelity for both synchroscan and single-shot operations in a timescale of around Ips. The analysis of the Picoframe type of framing cameras has led to the refinements of the dynamic performance. For the future development, proposals have been made for the designs of a new streak tube having high dynamic range and a new framing tube having frame times less than 10 ps with potential applications in medical diagnosis.Radiofrequency analysis using optical signal processingDawber, W. N.https://hdl.handle.net/10023/150352019-04-01T10:04:20Z1991-07-01T00:00:00ZThe basic form of conventional electronic and acoustooptic radiofrequency spectrum analysers is described. The advantages and disadvantages of the various systems are discussed with particular reference to radar signal processing in a hostile environment. Acoustooptic interaction is described using electromagnetic wave theory and also in terms of particle dynamics. A discussion of the various factors which effect Bragg-cell performance is presented, together with experimental results from the characterisation of acoustooptic cells. Coherent light detection is described when used in conjunction with a Bragg-cell spectrum analyser. Using this approach the dynamic range of the device may be dramatically increased. A novel approach is described which uses optical fibres in the Fourier transform plane and fusion spliced couplers to combine the signal and local oscillator beams. Experimental results are presented using single-mode fibres. Improvements in diffraction efficiency, reduced material intermodulation and increased frequency resolution are possible in an acoustooptic spectrum analyser if a Bragg-cell with a long transducer is used. However this leads to reduced instantaneous bandwidth in a conventional configuration. Two new approaches are described which allow a long transducer to be used without loss of bandwidth. An analysis of Bragg-cell diffraction within active and passive resonant optical cavities shows the diffraction efficiency per watt of a Bragg-cell may be increased by orders of magnitude by placing it within a passive cavity. Various cavity configurations are analysed and experimental results are given. A temporal analysis of light diffracted from radiofrequency pulses within an acoustooptic Bragg-cell is presented. Experimental evidence backs up the theory, which shows a possible means of eliminating the "Rabbit's Ears" phenomenon. Conventional acoustooptic Bragg cells have bandwidths limited by the acoustic losses in the crystals used for the cells and impedance matching of the transducer to the driver and crystal. Commercial cells are available with bandwidths of several gigahertz. Many applications require significantly larger bandwidths than are offered by conventional Bragg cells. We describe a new kind of diffraction cell with a potential bandwidth in excess of fifty gigahertz. The theory of operation and an example design are presented. A novel ultra-high data rate optical communication link is described. This makes use of the temporal distribution produced by light diffracted from radiofrequency pulses within a Bragg-cell. Also a covert, free-space link is described. A two channel system is demonstrated using acoustooptic cells.
1991-07-01T00:00:00ZDawber, W. N.The basic form of conventional electronic and acoustooptic radiofrequency spectrum analysers is described. The advantages and disadvantages of the various systems are discussed with particular reference to radar signal processing in a hostile environment. Acoustooptic interaction is described using electromagnetic wave theory and also in terms of particle dynamics. A discussion of the various factors which effect Bragg-cell performance is presented, together with experimental results from the characterisation of acoustooptic cells. Coherent light detection is described when used in conjunction with a Bragg-cell spectrum analyser. Using this approach the dynamic range of the device may be dramatically increased. A novel approach is described which uses optical fibres in the Fourier transform plane and fusion spliced couplers to combine the signal and local oscillator beams. Experimental results are presented using single-mode fibres. Improvements in diffraction efficiency, reduced material intermodulation and increased frequency resolution are possible in an acoustooptic spectrum analyser if a Bragg-cell with a long transducer is used. However this leads to reduced instantaneous bandwidth in a conventional configuration. Two new approaches are described which allow a long transducer to be used without loss of bandwidth. An analysis of Bragg-cell diffraction within active and passive resonant optical cavities shows the diffraction efficiency per watt of a Bragg-cell may be increased by orders of magnitude by placing it within a passive cavity. Various cavity configurations are analysed and experimental results are given. A temporal analysis of light diffracted from radiofrequency pulses within an acoustooptic Bragg-cell is presented. Experimental evidence backs up the theory, which shows a possible means of eliminating the "Rabbit's Ears" phenomenon. Conventional acoustooptic Bragg cells have bandwidths limited by the acoustic losses in the crystals used for the cells and impedance matching of the transducer to the driver and crystal. Commercial cells are available with bandwidths of several gigahertz. Many applications require significantly larger bandwidths than are offered by conventional Bragg cells. We describe a new kind of diffraction cell with a potential bandwidth in excess of fifty gigahertz. The theory of operation and an example design are presented. A novel ultra-high data rate optical communication link is described. This makes use of the temporal distribution produced by light diffracted from radiofrequency pulses within a Bragg-cell. Also a covert, free-space link is described. A two channel system is demonstrated using acoustooptic cells.Ultrafast electron-optical visible / X-ray-sensitivity streak and framing camerasWalker, David R.https://hdl.handle.net/10023/150332020-07-15T11:24:34Z1990-07-01T00:00:00ZIn this thesis the development of ultrafast electron-optical streak and framing cameras having radiation sensitivities ranging from the visible to soft X-ray are discussed. A framing camera incorporating a vacuum demountable image tube with ultraviolet / soft X-ray sensitivity has been demonstrated to be capable of providing multiple, temporally separated, two-dimensional images with picosecond image exposure times under various operating conditions. Experimental evidence has been presented to show that this camera system can provide up to four high quality temporally separated images with an exposure time of 230 ps (FWHM) and inter-frame times of ~1ns under UV illumination. In the two-frame operation with soft X-ray illumination (generated using a laser produced plasma) image exposure times of as short as 100 ps (FWHM) and inter-frame times of 400 ps have been achieved. The dynamic spatial resolution of the camera has been shown to be ~8 lp/mm and ~5 lp/mm for the UV and soft X-ray sensitive devices respectively. A visible-sensitivity electron-optical single-shot streak camera possessing a novel travelling-wave deflection structure has been experimentally evaluated using a mode-locked cw ring dye laser. The limiting temporal resolution for this has been measured to be 300 fs and the merits of the travelling-wave deflection structure have been discussed. The implementation of this type of deflector geometry has also been demonstrated in conjunction with the vacuum demountable framing camera system. Computer aided design techniques have been utilised to further optimise the electron-optical framing tube configuration, and modifications have been proposed to enable shorter frame periods to be obtained while maintaining the dynamic spatial resolution. Results from preliminary evaluations of this design using a vacuum demountable UV-sensitive system are included. A novel streak camera design has also been proposed in which very high electrostatic photocathode extraction fields (up to 12 kV/mm) may be employed without danger of structural damage arising from electrostatic breakdown. This has been achieved by the removal of the usual mesh electrode placed in close proximity to the photocathode. Preliminary evaluations of a vacuum demountable UV-sensitive version of this camera geometry have been achieved which demonstrate a static spatial resolution of 80 lp/mm (when referred to the photocathode).
1990-07-01T00:00:00ZWalker, David R.In this thesis the development of ultrafast electron-optical streak and framing cameras having radiation sensitivities ranging from the visible to soft X-ray are discussed. A framing camera incorporating a vacuum demountable image tube with ultraviolet / soft X-ray sensitivity has been demonstrated to be capable of providing multiple, temporally separated, two-dimensional images with picosecond image exposure times under various operating conditions. Experimental evidence has been presented to show that this camera system can provide up to four high quality temporally separated images with an exposure time of 230 ps (FWHM) and inter-frame times of ~1ns under UV illumination. In the two-frame operation with soft X-ray illumination (generated using a laser produced plasma) image exposure times of as short as 100 ps (FWHM) and inter-frame times of 400 ps have been achieved. The dynamic spatial resolution of the camera has been shown to be ~8 lp/mm and ~5 lp/mm for the UV and soft X-ray sensitive devices respectively. A visible-sensitivity electron-optical single-shot streak camera possessing a novel travelling-wave deflection structure has been experimentally evaluated using a mode-locked cw ring dye laser. The limiting temporal resolution for this has been measured to be 300 fs and the merits of the travelling-wave deflection structure have been discussed. The implementation of this type of deflector geometry has also been demonstrated in conjunction with the vacuum demountable framing camera system. Computer aided design techniques have been utilised to further optimise the electron-optical framing tube configuration, and modifications have been proposed to enable shorter frame periods to be obtained while maintaining the dynamic spatial resolution. Results from preliminary evaluations of this design using a vacuum demountable UV-sensitive system are included. A novel streak camera design has also been proposed in which very high electrostatic photocathode extraction fields (up to 12 kV/mm) may be employed without danger of structural damage arising from electrostatic breakdown. This has been achieved by the removal of the usual mesh electrode placed in close proximity to the photocathode. Preliminary evaluations of a vacuum demountable UV-sensitive version of this camera geometry have been achieved which demonstrate a static spatial resolution of 80 lp/mm (when referred to the photocathode).An all-solid-state optical parametric oscillator for the infraredTerry, Jonathan A.C.https://hdl.handle.net/10023/150322019-04-01T10:10:48Z1994-07-01T00:00:00ZA low threshold, efficient optical parametric oscillator (OPO) based on the material Potassium Titanyl Phosphate (KTP) and pumped by a diode-laser-pumped, Q-switched Nd:YLF laser has been demonstrated and investigated. This all-solid-state device was operated in a non-critical phase match (NCPM) geometry converting the 1 mum pump light to output wavelengths of 1.54 and 3.28 mum, and has potential as an 'eyesafe' laser source with scaling to higher powers. A major contributing factor to the success of this work was the extension of the steady state theory of the singly resonant OPO to include the build-up time effects that are dominant in the pulsed regime. A number of diode pumped lasers were constructed, allowing a comparison to be made between side- and end-pumping geometries, and also between the materials Nd:YAG and Nd:YLF. The end-pumping geometry in conjunction with the higher absorption and longer upper state lifetime in Nd:YLF made it the design of choice for the case of low pump pulse energies (~ 12 mJ at 797 nm). Anamorphic expansion of the laser mode in the plane parallel to the diode laser junction was employed to achieve TEM00 operation of this laser. Subsequent Q-switching with a polariser and LiNbO3 Pockels cell combination produced 2.2 mJ at 1.047 mum in an 18 ns pulse. Investigation of the dynamic loss of the Q-switch (which is due to the elasto-optic effect) allowed improvement of laser performance. The established model for a pulsed singly resonant OPO which describes the case for a plane-plane resonator was inappropriate in this work and so the steady state focused beam theory was extended to include time dependence. Fair agreement was found between the computer model and the experimental results, where the effects of pump and signal focusing, and output coupling were investigated. The high conversion efficiency of 30% for converting the 1 mum pump light to the eyesafe wavelength of 1.54 mum is superior to the present alternative source of the Er:glass laser. Pump energy thresholds of less than 0.5 mJ were obtained, along with internal conversions approaching 50 %. An empirical relation describing pump depletion was derived which showed good agreement with experiment. A high resolution investigation of the spectral properties of the OPO identified the roles of resonant reflection and doubly resonant behaviour on the mode structure of the output. The former suggests a way in which single mode operation could be achieved without the use of additional intracavity elements, or a seeding source.
1994-07-01T00:00:00ZTerry, Jonathan A.C.A low threshold, efficient optical parametric oscillator (OPO) based on the material Potassium Titanyl Phosphate (KTP) and pumped by a diode-laser-pumped, Q-switched Nd:YLF laser has been demonstrated and investigated. This all-solid-state device was operated in a non-critical phase match (NCPM) geometry converting the 1 mum pump light to output wavelengths of 1.54 and 3.28 mum, and has potential as an 'eyesafe' laser source with scaling to higher powers. A major contributing factor to the success of this work was the extension of the steady state theory of the singly resonant OPO to include the build-up time effects that are dominant in the pulsed regime. A number of diode pumped lasers were constructed, allowing a comparison to be made between side- and end-pumping geometries, and also between the materials Nd:YAG and Nd:YLF. The end-pumping geometry in conjunction with the higher absorption and longer upper state lifetime in Nd:YLF made it the design of choice for the case of low pump pulse energies (~ 12 mJ at 797 nm). Anamorphic expansion of the laser mode in the plane parallel to the diode laser junction was employed to achieve TEM00 operation of this laser. Subsequent Q-switching with a polariser and LiNbO3 Pockels cell combination produced 2.2 mJ at 1.047 mum in an 18 ns pulse. Investigation of the dynamic loss of the Q-switch (which is due to the elasto-optic effect) allowed improvement of laser performance. The established model for a pulsed singly resonant OPO which describes the case for a plane-plane resonator was inappropriate in this work and so the steady state focused beam theory was extended to include time dependence. Fair agreement was found between the computer model and the experimental results, where the effects of pump and signal focusing, and output coupling were investigated. The high conversion efficiency of 30% for converting the 1 mum pump light to the eyesafe wavelength of 1.54 mum is superior to the present alternative source of the Er:glass laser. Pump energy thresholds of less than 0.5 mJ were obtained, along with internal conversions approaching 50 %. An empirical relation describing pump depletion was derived which showed good agreement with experiment. A high resolution investigation of the spectral properties of the OPO identified the roles of resonant reflection and doubly resonant behaviour on the mode structure of the output. The former suggests a way in which single mode operation could be achieved without the use of additional intracavity elements, or a seeding source.Oxygen and its control over structural and electronic properties of Pb based 1212 superconductorsMitchell, Brian Jameshttps://hdl.handle.net/10023/150312019-04-01T10:09:57Z1998-02-01T00:00:00ZThe phase stability, electronic properties and crystal structure of the Pb based 1212 cuprate superconductor (Pb[1+x]/2Cu[1-x]/2)Sr2(Y1-xCax)Cu2O7+/-s were studied using a number of techniques. Since synthetic difficulties limited study of these materials, sol-gel techniques incorporating mixed metal nitrate precursors were applied. Novel synthesis techniques improved reaction rate but did not entirely overcome synthesis difficulties. The effects of Ca content; reaction temperature, and oxygen content in controlling product phase formation and phase stability under synthesis conditions were investigated. X-ray and neutron diffraction were used to probe structural disorder within samples as Ca levels increased and sample oxygen content was varied. The subsequent Rietveld refinements revealed that the levels of cation site disorder increase in line with increasing Ca levels. Under oxidising conditions, annealing temperature and thus oxygen content of the sample plays a critical role in controlling superconducting properties. Optimal Tc and superconducting volume are only achieved over the oxygen content range 6.99-7.00. Low pO2 annealing conditions were used to control sample oxygen content; however, these were not significantly better than standard techniques. Low pO2 environments facilitate the removal of oxygen from samples and allowed the study of phase stability. It was possible to show that decomposition of the 1212 phase leads to formation of new superconducting phases that might well account for contentious literature claims. EXAFS and NMR measurements show that on a local scale, the structure of the 1212 phase rocksalt layer is quite different to the average structure generated from diffraction measurements. The Pb atoms tend to form a PbO4+x molecular type cluster, in which x can increase up to 0.5 to accommodate the inclusion of an excess 0.1 oxygens into the structure. Solid-state NMR measurements reveal in Ca containing samples that up to 20% of the Y within the structure is dimerised.
1998-02-01T00:00:00ZMitchell, Brian JamesThe phase stability, electronic properties and crystal structure of the Pb based 1212 cuprate superconductor (Pb[1+x]/2Cu[1-x]/2)Sr2(Y1-xCax)Cu2O7+/-s were studied using a number of techniques. Since synthetic difficulties limited study of these materials, sol-gel techniques incorporating mixed metal nitrate precursors were applied. Novel synthesis techniques improved reaction rate but did not entirely overcome synthesis difficulties. The effects of Ca content; reaction temperature, and oxygen content in controlling product phase formation and phase stability under synthesis conditions were investigated. X-ray and neutron diffraction were used to probe structural disorder within samples as Ca levels increased and sample oxygen content was varied. The subsequent Rietveld refinements revealed that the levels of cation site disorder increase in line with increasing Ca levels. Under oxidising conditions, annealing temperature and thus oxygen content of the sample plays a critical role in controlling superconducting properties. Optimal Tc and superconducting volume are only achieved over the oxygen content range 6.99-7.00. Low pO2 annealing conditions were used to control sample oxygen content; however, these were not significantly better than standard techniques. Low pO2 environments facilitate the removal of oxygen from samples and allowed the study of phase stability. It was possible to show that decomposition of the 1212 phase leads to formation of new superconducting phases that might well account for contentious literature claims. EXAFS and NMR measurements show that on a local scale, the structure of the 1212 phase rocksalt layer is quite different to the average structure generated from diffraction measurements. The Pb atoms tend to form a PbO4+x molecular type cluster, in which x can increase up to 0.5 to accommodate the inclusion of an excess 0.1 oxygens into the structure. Solid-state NMR measurements reveal in Ca containing samples that up to 20% of the Y within the structure is dimerised.High spatial and spectral quality diode-laser-based pump sources for solid-state lasers and optical parametric oscillatorsLindsay, Ian D.https://hdl.handle.net/10023/150302019-04-01T10:04:43Z1999-09-01T00:00:00ZIn this thesis the use of high spatial- and spectral-quality diode-laser pump sources for solid-state lasers and continuous-wave optical parametric oscillators (cw OPOs) is investigated. While diode lasers are potentially attractive, compact, low-cost pump sources for solid-state lasers and cw OPOs, the difficulty in obtaining moderate output powers, while retaining high spatial beam quality and spectral purity, often limits the potential of such lasers in these applications. Techniques for obtaining high-power, high spatial- and spectral-quality output from diode lasers are reviewed and the design, development and characterisation of an injection-locked broad-area diode-laser system is described. This system produced output powers of ≈400mW in a near-diffraction-limited beam (M2≈w1.3) and with a spectral width of <30MHz. The injection-locked system was used as the pump source for a quasi-three-level 946-nm Nd:YAG laser. End-pumped solid-state lasers of this type can offer potentially efficient, low-threshold operation if a near-diffraction limited pump source is used allowing optimal overlap with the laser mode. A model, including pump beam quality effects, is developed for such lasers and used to highlight the advantages of a near- diffraction-limited pump source, especially in the case of the 946-nm Nd:YAG transition which suffers from low gain and significant reabsorption losses. A 946-nm Nd: YAG laser pumped by the injection-locked system is described, yielding cw output powers up to 120mW with a 46% slope efficiency, performance comparable to Ti:sapphire- or dye-laser pumping, and 27ns Q-switched pulses having peak powers of 180W. 50W, 20ns pulses at 473nm were obtained by second-harmonic generation in KNbOs. The performance and relative merits of various cw OPO configurations, in the context of diode-laser pumping, are discussed and the development of a doubly- resonant OPO (DRO) based on periodically-poled lithium niobate is described. When pumped by the injection-locked system, this device showed a threshold of 25mW and .tuning of the outputs over 1.15- 1.25 mum at the signal and 2.3-2.65 mum at the idler was obtained by variation of crystal temperature, PPLN grating period and pump wavelength. When pumping with a 100mW single-mode diode laser, a 15mW OPO threshold was observed while retaining a similar tuning range. This represented the first demonstration of a cw DRO directly pumped by a single-mode diode laser. The achievement of such spectral coverage while pumping with this source points to the potential of such systems as compact, tunable sources in the near-to mid-infrared.
1999-09-01T00:00:00ZLindsay, Ian D.In this thesis the use of high spatial- and spectral-quality diode-laser pump sources for solid-state lasers and continuous-wave optical parametric oscillators (cw OPOs) is investigated. While diode lasers are potentially attractive, compact, low-cost pump sources for solid-state lasers and cw OPOs, the difficulty in obtaining moderate output powers, while retaining high spatial beam quality and spectral purity, often limits the potential of such lasers in these applications. Techniques for obtaining high-power, high spatial- and spectral-quality output from diode lasers are reviewed and the design, development and characterisation of an injection-locked broad-area diode-laser system is described. This system produced output powers of ≈400mW in a near-diffraction-limited beam (M2≈w1.3) and with a spectral width of <30MHz. The injection-locked system was used as the pump source for a quasi-three-level 946-nm Nd:YAG laser. End-pumped solid-state lasers of this type can offer potentially efficient, low-threshold operation if a near-diffraction limited pump source is used allowing optimal overlap with the laser mode. A model, including pump beam quality effects, is developed for such lasers and used to highlight the advantages of a near- diffraction-limited pump source, especially in the case of the 946-nm Nd:YAG transition which suffers from low gain and significant reabsorption losses. A 946-nm Nd: YAG laser pumped by the injection-locked system is described, yielding cw output powers up to 120mW with a 46% slope efficiency, performance comparable to Ti:sapphire- or dye-laser pumping, and 27ns Q-switched pulses having peak powers of 180W. 50W, 20ns pulses at 473nm were obtained by second-harmonic generation in KNbOs. The performance and relative merits of various cw OPO configurations, in the context of diode-laser pumping, are discussed and the development of a doubly- resonant OPO (DRO) based on periodically-poled lithium niobate is described. When pumped by the injection-locked system, this device showed a threshold of 25mW and .tuning of the outputs over 1.15- 1.25 mum at the signal and 2.3-2.65 mum at the idler was obtained by variation of crystal temperature, PPLN grating period and pump wavelength. When pumping with a 100mW single-mode diode laser, a 15mW OPO threshold was observed while retaining a similar tuning range. This represented the first demonstration of a cw DRO directly pumped by a single-mode diode laser. The achievement of such spectral coverage while pumping with this source points to the potential of such systems as compact, tunable sources in the near-to mid-infrared.Studies of optical parametric oscillators for the ultraviolet and visible spectral regionsHenderson, Angushttps://hdl.handle.net/10023/149512019-04-01T10:10:07Z1993-07-01T00:00:00ZThe work described herein concerns the characterisation and development of optical parametric oscillators (OPOs) tunable in the ultraviolet, visible and near infrared regions. These devices were pumped by the 308nm output from line-narrowed Xenon Chloride excimer lasers of pulse energy up to 150mJ. The behaviour of Type 2 phase-matched Urea, and Type 1 phase-matched Barium Borate OPOs in terms of oscillation threshold and conversion efficiency, has been explored. The detrimental effects of pump beam walkoff on the threshold of the critically phase-matched Barium Borate OPO have been quantified. It was found that minimum 17ns pulse energies of 5mJ were required to reach threshold in a device based on a crystal of 20mm length. By contrast, noncritically phase-matched Urea OPOs using crystal lengths of 25mm were operated with as little as 0.6mJ pump energy. A deterioration in performance was observed in both cases with decreasing pump beam waist. Maximum pump depletions of 72% and 64% were observed in Urea and BBO respectively. The useful output from the urea device reached 65%, while higher absorption/scattering losses meant that the useful fraction in BBO was very much lower. Two different types of noncollinear phase-matching were studied in the BBO-OPO. The first recorded observation of operation of a Type 1 OPO at crystal angles beyond the degenerate wavelength point was made. The output took the form of two concentric rings and was attributed to simultaneous singly and doubly resonant operation. Finally, single longitudinal mode operation of the BBO-OPO was demonstrated using a dispersive cavity arrangement. The widely varying inherent linewidth of the device required that different strategies be adopted over different wavelength ranges. Encouraging performance in terms of threshold was observed using the dispersive cavity, and the feasibility of using this device as a low-power first stage for an oscillator/amplifier set-up was studied.
1993-07-01T00:00:00ZHenderson, AngusThe work described herein concerns the characterisation and development of optical parametric oscillators (OPOs) tunable in the ultraviolet, visible and near infrared regions. These devices were pumped by the 308nm output from line-narrowed Xenon Chloride excimer lasers of pulse energy up to 150mJ. The behaviour of Type 2 phase-matched Urea, and Type 1 phase-matched Barium Borate OPOs in terms of oscillation threshold and conversion efficiency, has been explored. The detrimental effects of pump beam walkoff on the threshold of the critically phase-matched Barium Borate OPO have been quantified. It was found that minimum 17ns pulse energies of 5mJ were required to reach threshold in a device based on a crystal of 20mm length. By contrast, noncritically phase-matched Urea OPOs using crystal lengths of 25mm were operated with as little as 0.6mJ pump energy. A deterioration in performance was observed in both cases with decreasing pump beam waist. Maximum pump depletions of 72% and 64% were observed in Urea and BBO respectively. The useful output from the urea device reached 65%, while higher absorption/scattering losses meant that the useful fraction in BBO was very much lower. Two different types of noncollinear phase-matching were studied in the BBO-OPO. The first recorded observation of operation of a Type 1 OPO at crystal angles beyond the degenerate wavelength point was made. The output took the form of two concentric rings and was attributed to simultaneous singly and doubly resonant operation. Finally, single longitudinal mode operation of the BBO-OPO was demonstrated using a dispersive cavity arrangement. The widely varying inherent linewidth of the device required that different strategies be adopted over different wavelength ranges. Encouraging performance in terms of threshold was observed using the dispersive cavity, and the feasibility of using this device as a low-power first stage for an oscillator/amplifier set-up was studied.Continuously frequency-tunable CW optical parametric oscillators and their application to spectroscopyGibson, Graham Martinhttps://hdl.handle.net/10023/149502019-04-01T10:02:50Z1999-05-01T00:00:00ZThis thesis describes the development and applications of single-frequency, continuously tunable, continuous-wave (cw), optical parametric oscillators (OPOs). Two doubly-resonant OPOs (DROs) are presented, one providing tunable light around 1?m, the other specifically designed as a spectroscopic source for methane near 1649nm. Once stabilised, the frequency-selective nature of the DRO ensures operation on a single mode-pair without the need for additional intracavity frequency-selective components. Both DROs are smoothly tunable by smoothly tuning the pump laser. The 1mum DRO is based on a bulk KTP crystal cut for near-degenerate, type-II, critical phase-matching (theta= 90°, ϕ = 37°). Angle tuning the crystal provides coarse tuning of the output frequencies over a range of ~50nm. Small perturbations to the OPO cavity is sufficient to cause a systematic mode-hop and provides a method of tuning across the phase-matching bandwidth (~0.5THz). This DRO is demonstrated as a spectroscopic source by recording the absorption spectrum of cesium molecules near 1050nm. The DRO as a potentially compact source of tunable light is demonstrated by using a frequency-doubled microchip laser as the pump source. The output consists of a single pair of signal and idler modes even when using a multilongitudinal-mode pump laser. Smooth tuning of the output frequencies is achieved by temperature tuning the pump laser. The 1.65mum DRO is based on periodically poled KTiOPO4 (PPKTP). The suitability of PPKTP for cw OPOs was first assessed by a difference frequency generation experiment from which the effective d33 coefficient was estimated to be ~5mum/V. The idler wavelength is coarsely tuned at a rate of 0.73nm/°C by varying the crystal temperature. A combination of computer modelling and experimental observation is used to study the dynamic behaviour of a DRO. The numerical model calculates the time required for the OPO to build-up from the parametric fluorescence and is in excellent agreement with experimental observations.
1999-05-01T00:00:00ZGibson, Graham MartinThis thesis describes the development and applications of single-frequency, continuously tunable, continuous-wave (cw), optical parametric oscillators (OPOs). Two doubly-resonant OPOs (DROs) are presented, one providing tunable light around 1?m, the other specifically designed as a spectroscopic source for methane near 1649nm. Once stabilised, the frequency-selective nature of the DRO ensures operation on a single mode-pair without the need for additional intracavity frequency-selective components. Both DROs are smoothly tunable by smoothly tuning the pump laser. The 1mum DRO is based on a bulk KTP crystal cut for near-degenerate, type-II, critical phase-matching (theta= 90°, ϕ = 37°). Angle tuning the crystal provides coarse tuning of the output frequencies over a range of ~50nm. Small perturbations to the OPO cavity is sufficient to cause a systematic mode-hop and provides a method of tuning across the phase-matching bandwidth (~0.5THz). This DRO is demonstrated as a spectroscopic source by recording the absorption spectrum of cesium molecules near 1050nm. The DRO as a potentially compact source of tunable light is demonstrated by using a frequency-doubled microchip laser as the pump source. The output consists of a single pair of signal and idler modes even when using a multilongitudinal-mode pump laser. Smooth tuning of the output frequencies is achieved by temperature tuning the pump laser. The 1.65mum DRO is based on periodically poled KTiOPO4 (PPKTP). The suitability of PPKTP for cw OPOs was first assessed by a difference frequency generation experiment from which the effective d33 coefficient was estimated to be ~5mum/V. The idler wavelength is coarsely tuned at a rate of 0.73nm/°C by varying the crystal temperature. A combination of computer modelling and experimental observation is used to study the dynamic behaviour of a DRO. The numerical model calculates the time required for the OPO to build-up from the parametric fluorescence and is in excellent agreement with experimental observations.Applications of nonlinear optics to the development of all-solid-state sources of tunable light.Tang, Yanhttps://hdl.handle.net/10023/149492019-04-01T10:06:03Z1997-06-01T00:00:00ZThis thesis describes the development of singly-resonant optical parametric oscillators (OPOs) based on the nonlinear material KTP (potassium titanyl phosphate), and used to provide tunable light in the infrared, with low oscillation threshold and high efficiency. Further, the generation of tunable red light by the frequency mixing of the signal wave from the OPO with the pump wave in a non-critical temperature phase-matched lithium triborate crystal (LBO) is reported. We believe this is the first demonstration of such an application of LBO. Two diode-pumped solid-state lasers were used as the pump sources. One was an electro-optically Q-switched Nd:YLF laser which provided high peak power (~600 kW) pulses; and the other one was an acousto-optically Q-switched slab-geometry Nd:YLF laser which provided high repetition rate (1~10 kHz) a.nd low peak power(<30 kW) pulses. A second version of the acousto-optically Q-switched slab-geometry Nd:YLF laser was designed and constructed with improvements in the pump module and cooling system so as to be much more compact and easier to control. In the first stage of this work, two theoretical models were constructed. One was a model for pump threshold of singly-resonant OPOs for the case of focused Gaussian beams, and was based on Guha's theory. The second one was a model for conversion efficiency of singly-resonant OPOs, for the case of plane waves with pump depletion, and was derived from the coupled wave equations. In the second stage of this work, the effects of beam focusing and Poynting vector walk-off on pump threshold and conversion efficiency for OPOs were extensively studied theoretically and experimentally. Experimental results were found to be in good agreement with theory. The high pump threshold of the critically phase-matched KTP OPO led to several other pump configurations being considered, including intracavity OPOs, cylindrical focusing, and donble-pass of the pump. As a result of the KTP OPO study, very low pump thresholds were achieved in both non-critical phase-matched (NCPM) and critical phase-matched (CPM) KTP OPOs by using long crystal in both intracavity OPOs and the double-pass-pump configuration. Maximum external conversion efficiency from pump to signal was demonstrated to be 37% for the NCPM OPO and 40% for the CPM OPO. The signal wavelength tuning ranges were observed to be 1.54-1.56 mum from the NCPM KTP OPO, and 1.58-1.8 mum from the CPM KTP OPO. In the final stage of this work, the temperature phase-matching properties of LBO were investigated with the use of our measured thermo-optical coefficients of LBO. A particularly interesting result of the investigation is the possibility of sum-frequency generation in non-critically phase-matched LBO with temperature tuning giving considerable wavelength ranges for both the type I and type II geometries. Experimentally, we demonstrated tunalile red light generation by sum-frequency mixing of the 1 mum pump wave and the signal wave of the KTP OPO with an over all conversion efficiency of more than 13%. The wavelength tuning range was observed to 0.62-0.65 mum. The effects of beam focusing on the conversion efficiency for sum-frequency generation were analysed theoretically, and several opportunities for further improvement were shown clearly from this analysis.
1997-06-01T00:00:00ZTang, YanThis thesis describes the development of singly-resonant optical parametric oscillators (OPOs) based on the nonlinear material KTP (potassium titanyl phosphate), and used to provide tunable light in the infrared, with low oscillation threshold and high efficiency. Further, the generation of tunable red light by the frequency mixing of the signal wave from the OPO with the pump wave in a non-critical temperature phase-matched lithium triborate crystal (LBO) is reported. We believe this is the first demonstration of such an application of LBO. Two diode-pumped solid-state lasers were used as the pump sources. One was an electro-optically Q-switched Nd:YLF laser which provided high peak power (~600 kW) pulses; and the other one was an acousto-optically Q-switched slab-geometry Nd:YLF laser which provided high repetition rate (1~10 kHz) a.nd low peak power(<30 kW) pulses. A second version of the acousto-optically Q-switched slab-geometry Nd:YLF laser was designed and constructed with improvements in the pump module and cooling system so as to be much more compact and easier to control. In the first stage of this work, two theoretical models were constructed. One was a model for pump threshold of singly-resonant OPOs for the case of focused Gaussian beams, and was based on Guha's theory. The second one was a model for conversion efficiency of singly-resonant OPOs, for the case of plane waves with pump depletion, and was derived from the coupled wave equations. In the second stage of this work, the effects of beam focusing and Poynting vector walk-off on pump threshold and conversion efficiency for OPOs were extensively studied theoretically and experimentally. Experimental results were found to be in good agreement with theory. The high pump threshold of the critically phase-matched KTP OPO led to several other pump configurations being considered, including intracavity OPOs, cylindrical focusing, and donble-pass of the pump. As a result of the KTP OPO study, very low pump thresholds were achieved in both non-critical phase-matched (NCPM) and critical phase-matched (CPM) KTP OPOs by using long crystal in both intracavity OPOs and the double-pass-pump configuration. Maximum external conversion efficiency from pump to signal was demonstrated to be 37% for the NCPM OPO and 40% for the CPM OPO. The signal wavelength tuning ranges were observed to be 1.54-1.56 mum from the NCPM KTP OPO, and 1.58-1.8 mum from the CPM KTP OPO. In the final stage of this work, the temperature phase-matching properties of LBO were investigated with the use of our measured thermo-optical coefficients of LBO. A particularly interesting result of the investigation is the possibility of sum-frequency generation in non-critically phase-matched LBO with temperature tuning giving considerable wavelength ranges for both the type I and type II geometries. Experimentally, we demonstrated tunalile red light generation by sum-frequency mixing of the 1 mum pump wave and the signal wave of the KTP OPO with an over all conversion efficiency of more than 13%. The wavelength tuning range was observed to 0.62-0.65 mum. The effects of beam focusing on the conversion efficiency for sum-frequency generation were analysed theoretically, and several opportunities for further improvement were shown clearly from this analysis.Novel configurations for pulsed optical parametric oscillators and their pump sourcesRae, Cameron Francishttps://hdl.handle.net/10023/149482019-04-01T10:05:23Z1998-05-01T00:00:00ZThe development of all-solid-state, diode-laser pumped neodymium (Nd) lasers and optical parametric oscillators (OPOs) is described, which realise practical sources of coherent radiation with a high degree of frequency agility, are efficient, reliable and potentially compact. A comparison of various neodymium doped host materials reveals yttrium lithium fluoride (YLF) to be an appropriate replacement for the more widely known host yttrium aluminium garnet (YAG) in diode-laser pumped devices. The development of an end-pumped Nd:YLF laser that utilises a 12-mJ, 60W, quasi-CW diode-laser bar is initially described. Multilongitudinal-mode, TEM00 pulse energies of greater than 2 mJ have been observed, with corresponding peak output powers in excess of 118 kW. The incorporation of a novel pre-lase Q-switching technique has realised single-longitudinal-mode peak powers in excess of 90 kW continuing to be achieved. Further, the development of a more powerful end- pumped Nd:YLF laser utilising 2, 3-bar diode-laser arrays, each providing 72-mJ of pump energy is described. In this case, Q-switched, multilongitudinal-mode, TEM00 pulse energies of greater than 11 mJ are reported, with the clear potential for increasing this to greater than 20 mJ, based on measured fixed-Q pulse energies of greater than 30 mJ. Complementing the development of these diode-laser pumped solid-state lasers is the development of optical parametric oscillators based on the nonlinear materials lithium triborate (LBO) and beta-barium borate (?-BBO). Pumped by the frequency up-converted (third harmonic) output of the mid laser, such optical parametric oscillators introduce extensive frequency agility spanning a spectral range from the deep blue (0.4 mum) to the mid-infrared (2.5 mum). Initially, the development of an LBO based device is reported, which in a type I critical phase- match (CPM) geometry has a measured oscillation threshold of <0.3 mJ, when pumped by the frequency tripled output of the 144-mJ diode-laser pumped Nd:YLF laser at 0.349 mum. Observed pump depletions are as high as 35%. A similar CPM geometry is reported in beta-BBO, in this case pumped by the frequency tripled and amplified output of a diode-laser pumped Nd:YAG laser at 0.355 mum. This is a more energetic device with thresholds of >5 mJ, but through the introduction of interferometric, dispersive and injection seeding techniques made to operate on a single axial mode. Near transform limited linewidths are reported in devices which continue to have modest pump thresholds and broad tunability. The parametric generation of broad spectral bandwidths (polychromatic) by the use of suitable phase-matching geometries is also reported. Greater than 100 nm simultaneous bandwidth in the visible spectrum is generated in a collimated signal-wave from a novel, noncollinear phase-matching geometry in a beta-BBO optical parametric oscillator, which is pumped by the collimated output of frequency tripled diode-laser pumped Nd:YAG laser. The device is demonstrated to be efficient, having a similar pump threshold and efficiency to that of the well known collinear phase-matched tunable device, and to continue to encompass a degree of tunability allowing the large simultaneous bandwidth to be tuned across the entire visible spectrum. Dispersive cavity tuning of the optical parametric oscillator by the use of a Littrow-mounted grating or acousto-optic tuning filter, with a static crystal and pump configuration, is also described.
1998-05-01T00:00:00ZRae, Cameron FrancisThe development of all-solid-state, diode-laser pumped neodymium (Nd) lasers and optical parametric oscillators (OPOs) is described, which realise practical sources of coherent radiation with a high degree of frequency agility, are efficient, reliable and potentially compact. A comparison of various neodymium doped host materials reveals yttrium lithium fluoride (YLF) to be an appropriate replacement for the more widely known host yttrium aluminium garnet (YAG) in diode-laser pumped devices. The development of an end-pumped Nd:YLF laser that utilises a 12-mJ, 60W, quasi-CW diode-laser bar is initially described. Multilongitudinal-mode, TEM00 pulse energies of greater than 2 mJ have been observed, with corresponding peak output powers in excess of 118 kW. The incorporation of a novel pre-lase Q-switching technique has realised single-longitudinal-mode peak powers in excess of 90 kW continuing to be achieved. Further, the development of a more powerful end- pumped Nd:YLF laser utilising 2, 3-bar diode-laser arrays, each providing 72-mJ of pump energy is described. In this case, Q-switched, multilongitudinal-mode, TEM00 pulse energies of greater than 11 mJ are reported, with the clear potential for increasing this to greater than 20 mJ, based on measured fixed-Q pulse energies of greater than 30 mJ. Complementing the development of these diode-laser pumped solid-state lasers is the development of optical parametric oscillators based on the nonlinear materials lithium triborate (LBO) and beta-barium borate (?-BBO). Pumped by the frequency up-converted (third harmonic) output of the mid laser, such optical parametric oscillators introduce extensive frequency agility spanning a spectral range from the deep blue (0.4 mum) to the mid-infrared (2.5 mum). Initially, the development of an LBO based device is reported, which in a type I critical phase- match (CPM) geometry has a measured oscillation threshold of <0.3 mJ, when pumped by the frequency tripled output of the 144-mJ diode-laser pumped Nd:YLF laser at 0.349 mum. Observed pump depletions are as high as 35%. A similar CPM geometry is reported in beta-BBO, in this case pumped by the frequency tripled and amplified output of a diode-laser pumped Nd:YAG laser at 0.355 mum. This is a more energetic device with thresholds of >5 mJ, but through the introduction of interferometric, dispersive and injection seeding techniques made to operate on a single axial mode. Near transform limited linewidths are reported in devices which continue to have modest pump thresholds and broad tunability. The parametric generation of broad spectral bandwidths (polychromatic) by the use of suitable phase-matching geometries is also reported. Greater than 100 nm simultaneous bandwidth in the visible spectrum is generated in a collimated signal-wave from a novel, noncollinear phase-matching geometry in a beta-BBO optical parametric oscillator, which is pumped by the collimated output of frequency tripled diode-laser pumped Nd:YAG laser. The device is demonstrated to be efficient, having a similar pump threshold and efficiency to that of the well known collinear phase-matched tunable device, and to continue to encompass a degree of tunability allowing the large simultaneous bandwidth to be tuned across the entire visible spectrum. Dispersive cavity tuning of the optical parametric oscillator by the use of a Littrow-mounted grating or acousto-optic tuning filter, with a static crystal and pump configuration, is also described.Femtosecond optical parametric oscillators in the mid-infraredPenman, Zoe E.https://hdl.handle.net/10023/149472019-04-01T10:04:30Z1999-04-01T00:00:00ZThe work described in this thesis is concerned with the development of self-modelocked Ti:sapphire lasers and femtosecond optical parametric oscillators based on periodically-poled rubidium titanyl arsenate and periodically-poled lithium niobate and operating in the near and mid-infrared. In Chapter 1 the theory of ultrashort pulse generation is explained with regard to the Ti:sapphire laser. The optical properties of Ti:sapphire are discussed along with the principles of laser oscillation and pulse generation. The techniques used to modelock the lasers used in the experimental work, which follows, are also considered. The second part of the chapter deals with typical measurement techniques for characterising femtosecond optical pulses from a laser or an OPO, including a detailed explanation of second harmonic generation autocorrelation. Chapter 1 concludes with a thorough description of frequency-resolved optical gating, the newest of these pulse characterisation techniques. In Chapter 2 the subject of nonlinear optics and the properties of nonlinear optical materials are discussed. Phasematching in nonlinear optical materials is explained along with the principle techniques for achieving this, including birefringent phasematching and quasi-phasematching. A review of techniques for periodically- poling nonlinear optical crystals is also given. The chapter concludes with a section on the optical effects of group velocity dispersion and self-phase modulation, that influence the output from an ultrashort pulse laser or OPO and describes methods for second and third-order dispersion compensation. Chapter 2 concludes the theory required to explain the experiments described in Chapters 3, 4, 5 and 6. Chapter 3 describes the operation and characterisation of two different Ti:sapphire laser systems involving different methods of dispersion compensation. The first laser produces 100 fs duration self-modelocked laser pulses and dispersion compensation is achieved by including a pair of prisms in the cavity. This laser system is discussed further in Chapter 5, where it is operated in conjunction with a Spectra Physics Millennia, as the pump source for an all-solid-state femtosecond OPO based on periodically-poled lithium niobate. A second laser system is described in Chapter 3, which produces self-modelocked pulses of ~15 fs duration and dispersion compensation is achieved by including chirped multilayer dielectric mirrors in the cavity. The subject matter that Chapter 4 is concerned with includes the operation and characterisation of a femtosecond OPO based on PPRTA. Ti:sapphire pump wavelength tuning and cavity-length tuning of the OPO are shown to produce wavelengths throughout the range 1.060 mum to 1.225 mum in the signal and 2.67 jam to 4.5 mum in the idler, with average output powers as high as 120 mW in the signal and 105 mW in the idler output. The effects of photorefractive damage are minimal and consequently this offers the possibility of room-temperature operation of the PPRTA- based OPO. Chapter 5 is concerned with the generation of longer idler wavelengths, in the region of 5 mum, from an all-solid-state OPO based on periodically-poled lithium niobate. The approach used with the PPRTA-based OPO is extended to PPLN and in Chapter 5, results are presented which show that the use of an all-solid-state Ti:sapphire pump source in combination with a PPLN-based OPO represents a robust source of high- repetition-rate femtosecond pulses in the mid-infrared at wavelengths out to ~5 mum. Significantly higher output powers in the signal and idler than previously reported are also measured. In Chapter 6 a similar PPLN-based OPO is described, with modifications to the cavity elements, to reduce the output pulse duration of the OPO. This system is pumped by a sub- 20 fs Ti:sapphire laser. A pulse duration of 175 fs is recorded for the signal at a wavelength of 1.07 mum. Output powers of 28 mW for the signal at 1.07 mum and 6.8 mW for the idler at 2.7 mum are also measured. The tuning range for the signal extends from 1.045 mum to 1.190 mum, and for the idler, extends from 2.57 mum to 3.67 mum.
1999-04-01T00:00:00ZPenman, Zoe E.The work described in this thesis is concerned with the development of self-modelocked Ti:sapphire lasers and femtosecond optical parametric oscillators based on periodically-poled rubidium titanyl arsenate and periodically-poled lithium niobate and operating in the near and mid-infrared. In Chapter 1 the theory of ultrashort pulse generation is explained with regard to the Ti:sapphire laser. The optical properties of Ti:sapphire are discussed along with the principles of laser oscillation and pulse generation. The techniques used to modelock the lasers used in the experimental work, which follows, are also considered. The second part of the chapter deals with typical measurement techniques for characterising femtosecond optical pulses from a laser or an OPO, including a detailed explanation of second harmonic generation autocorrelation. Chapter 1 concludes with a thorough description of frequency-resolved optical gating, the newest of these pulse characterisation techniques. In Chapter 2 the subject of nonlinear optics and the properties of nonlinear optical materials are discussed. Phasematching in nonlinear optical materials is explained along with the principle techniques for achieving this, including birefringent phasematching and quasi-phasematching. A review of techniques for periodically- poling nonlinear optical crystals is also given. The chapter concludes with a section on the optical effects of group velocity dispersion and self-phase modulation, that influence the output from an ultrashort pulse laser or OPO and describes methods for second and third-order dispersion compensation. Chapter 2 concludes the theory required to explain the experiments described in Chapters 3, 4, 5 and 6. Chapter 3 describes the operation and characterisation of two different Ti:sapphire laser systems involving different methods of dispersion compensation. The first laser produces 100 fs duration self-modelocked laser pulses and dispersion compensation is achieved by including a pair of prisms in the cavity. This laser system is discussed further in Chapter 5, where it is operated in conjunction with a Spectra Physics Millennia, as the pump source for an all-solid-state femtosecond OPO based on periodically-poled lithium niobate. A second laser system is described in Chapter 3, which produces self-modelocked pulses of ~15 fs duration and dispersion compensation is achieved by including chirped multilayer dielectric mirrors in the cavity. The subject matter that Chapter 4 is concerned with includes the operation and characterisation of a femtosecond OPO based on PPRTA. Ti:sapphire pump wavelength tuning and cavity-length tuning of the OPO are shown to produce wavelengths throughout the range 1.060 mum to 1.225 mum in the signal and 2.67 jam to 4.5 mum in the idler, with average output powers as high as 120 mW in the signal and 105 mW in the idler output. The effects of photorefractive damage are minimal and consequently this offers the possibility of room-temperature operation of the PPRTA- based OPO. Chapter 5 is concerned with the generation of longer idler wavelengths, in the region of 5 mum, from an all-solid-state OPO based on periodically-poled lithium niobate. The approach used with the PPRTA-based OPO is extended to PPLN and in Chapter 5, results are presented which show that the use of an all-solid-state Ti:sapphire pump source in combination with a PPLN-based OPO represents a robust source of high- repetition-rate femtosecond pulses in the mid-infrared at wavelengths out to ~5 mum. Significantly higher output powers in the signal and idler than previously reported are also measured. In Chapter 6 a similar PPLN-based OPO is described, with modifications to the cavity elements, to reduce the output pulse duration of the OPO. This system is pumped by a sub- 20 fs Ti:sapphire laser. A pulse duration of 175 fs is recorded for the signal at a wavelength of 1.07 mum. Output powers of 28 mW for the signal at 1.07 mum and 6.8 mW for the idler at 2.7 mum are also measured. The tuning range for the signal extends from 1.045 mum to 1.190 mum, and for the idler, extends from 2.57 mum to 3.67 mum.Femtosecond optical parametric oscillators for the mid-infraredMcGowan, Cathrinehttps://hdl.handle.net/10023/149462019-04-01T10:02:25Z1998-02-01T00:00:00ZThe research presented in this thesis is concerned with the generation and characterisation of femtosecond pulses in the near and mid-infrared spectral regions. The three optical parametric oscillators which were constructed were synchronously- pumped by a self-modelocked femtosecond Ti:sapphire laser. Noncollinear critical birefringent phasematching was used in an oscillator based on KTiOAsO4, which was tunable from 1.03 to 1.2 mum and 2.51 to 4.1 mum by varying the crystal angle. The mid-infrared pulses were sub-100 fs, and essentially free from frequency chirp. With appropriate dispersion compensation the near-infrared signal pulses were temporally compressed to 69 fs. Theoretical models of noncollinear phasematching were derived and the results agreed closely with experiment. A novel optical parametric oscillator design based on a semi-monolithic noncritically phasematched RbTiOAsO4 crystal was implemented. This unique cavity configuration allowed independent focussing of the pump and signal beams within the crystal. It facilitated a reduction in cavity length to bring the signal pulse repetition rate into synchronism with the second (172 MHz) and fourth (344 MHz) harmonics of the pump pulse repetition frequency. Extraction efficiencies as high as 55% were observed. Quasi-phasematched femtosecond optical parametric oscillation was demonstrated in periodically poled lithium niobate. This device offered extensive tunability, covering 0.975 to 1.54 mum in the signal branch and 1.67 to 4.55 mum in the idler branch, from a combination of grating, pump wavelength and cavity length tuning. A theoretical model indicated that a very broad gain bandwidth allowed the wide tuning range. An attractively low oscillation threshold of 45 mW was recorded, and a visible output of 70 mW at 540 nm was observed, caused by simultaneously phasematched frequency-doubling of the signal output. The pulses from the Ti:sapphire laser and from the optical parametric oscillators were characterised by autocorrelation and frequency-resolved optical gating techniques. A highly advantageous autocorrelator arrangement based on quadratic nonlinearity in light-emitting diodes and photodiodes was demonstrated, and a novel second harmonic generation frequency-resolved optical gating system allowed real-time monitoring of pulsed outputs and complete characterisation of the intensity and phase of pulses.
1998-02-01T00:00:00ZMcGowan, CathrineThe research presented in this thesis is concerned with the generation and characterisation of femtosecond pulses in the near and mid-infrared spectral regions. The three optical parametric oscillators which were constructed were synchronously- pumped by a self-modelocked femtosecond Ti:sapphire laser. Noncollinear critical birefringent phasematching was used in an oscillator based on KTiOAsO4, which was tunable from 1.03 to 1.2 mum and 2.51 to 4.1 mum by varying the crystal angle. The mid-infrared pulses were sub-100 fs, and essentially free from frequency chirp. With appropriate dispersion compensation the near-infrared signal pulses were temporally compressed to 69 fs. Theoretical models of noncollinear phasematching were derived and the results agreed closely with experiment. A novel optical parametric oscillator design based on a semi-monolithic noncritically phasematched RbTiOAsO4 crystal was implemented. This unique cavity configuration allowed independent focussing of the pump and signal beams within the crystal. It facilitated a reduction in cavity length to bring the signal pulse repetition rate into synchronism with the second (172 MHz) and fourth (344 MHz) harmonics of the pump pulse repetition frequency. Extraction efficiencies as high as 55% were observed. Quasi-phasematched femtosecond optical parametric oscillation was demonstrated in periodically poled lithium niobate. This device offered extensive tunability, covering 0.975 to 1.54 mum in the signal branch and 1.67 to 4.55 mum in the idler branch, from a combination of grating, pump wavelength and cavity length tuning. A theoretical model indicated that a very broad gain bandwidth allowed the wide tuning range. An attractively low oscillation threshold of 45 mW was recorded, and a visible output of 70 mW at 540 nm was observed, caused by simultaneously phasematched frequency-doubling of the signal output. The pulses from the Ti:sapphire laser and from the optical parametric oscillators were characterised by autocorrelation and frequency-resolved optical gating techniques. A highly advantageous autocorrelator arrangement based on quadratic nonlinearity in light-emitting diodes and photodiodes was demonstrated, and a novel second harmonic generation frequency-resolved optical gating system allowed real-time monitoring of pulsed outputs and complete characterisation of the intensity and phase of pulses.High-power, high-repetition-rate picosecond optical parametric oscillators for the visible to mid-infraredFrench, Stevenhttps://hdl.handle.net/10023/148902019-04-01T10:05:58Z1997-07-01T00:00:00ZThis thesis describes the design, configuration and operation of picosecond optical parametric oscillators (OPOs) tunable from the visible to mid infrared. These systems were based on the materials LiB3O5 (LBO) and KTiOAsO4 (KTA), and were pumped by a self-mode-locked Ti:sapphire laser at a repetition rate of 81 MHz. The initial design of the picosecond parametric oscillator was based on a 16 mm long crystal of LBO. This system produces transform-limited signal pulses with durations of -720 fs. Total average output powers of up to 90 mW over a signal (idler) tuning range of 1.374-1.530 mum (1.676-1.828 mum) have been generated at 1.3 times the 900 mW threshold. The system performance was improved by the use of a new LBO crystal of length 30 mm. This system was continuously tunable from 1.160 to 2.185 mum. Up to 690 mW of output power has been generated for 2 W of input pump power at 5 times threshold. For this output power a depletion of 52 % was achieved with a corresponding external extraction efficiency of 34.5 %. Picosecond pulse generation in the visible by external single-pass frequency- doubling of the LBO OPO to provide picosecond pulses in the 584-771 nm range has been demonstrated. Conversion efficiencies as high as 18 % have been demonstrated, with output powers in excess of 65 mW being measured, when utilising a combination of type I and type II temperature-tuned non-critical phase-matching in LBO. The pulse width of the second harmonic was in the region of 840-880 fs. A further new source of tunable high-repetition-rate picosecond pulses for the visible has also been demonstrated, which is based on an internally-doubled, Ti:sapphire-pumped OPO that uses temperature-tuned LBO both as the OPO and SHG crystal. Oscillation has been obtained for an input pump power of 700 mW with output powers in excess of 320 mW being generated, representing conversion efficiencies of as much as 16 %. The system is continuously tunable from 584 to 771 nm and can provide transform-limited visible pulses with durations of 840-880 fs across the available range. The ability to tune beyond wavelengths of 2.5 mum was also required. To this end a new source of tunable picosecond pulses for the near - to mid - infrared has been developed which is based on the material KTA. Oscillation has been obtained for input pump powers as low as 230 mW. The system produces total output powers in excess of 403 mW with conversion efficiencies of 31 % at 5.2 times threshold. Transform-limited signal (idler) pulses of 1.02 (2.9) ps have been generated over the tuning range 1.139-1.281 (2.377-3.160) mum.
1997-07-01T00:00:00ZFrench, StevenThis thesis describes the design, configuration and operation of picosecond optical parametric oscillators (OPOs) tunable from the visible to mid infrared. These systems were based on the materials LiB3O5 (LBO) and KTiOAsO4 (KTA), and were pumped by a self-mode-locked Ti:sapphire laser at a repetition rate of 81 MHz. The initial design of the picosecond parametric oscillator was based on a 16 mm long crystal of LBO. This system produces transform-limited signal pulses with durations of -720 fs. Total average output powers of up to 90 mW over a signal (idler) tuning range of 1.374-1.530 mum (1.676-1.828 mum) have been generated at 1.3 times the 900 mW threshold. The system performance was improved by the use of a new LBO crystal of length 30 mm. This system was continuously tunable from 1.160 to 2.185 mum. Up to 690 mW of output power has been generated for 2 W of input pump power at 5 times threshold. For this output power a depletion of 52 % was achieved with a corresponding external extraction efficiency of 34.5 %. Picosecond pulse generation in the visible by external single-pass frequency- doubling of the LBO OPO to provide picosecond pulses in the 584-771 nm range has been demonstrated. Conversion efficiencies as high as 18 % have been demonstrated, with output powers in excess of 65 mW being measured, when utilising a combination of type I and type II temperature-tuned non-critical phase-matching in LBO. The pulse width of the second harmonic was in the region of 840-880 fs. A further new source of tunable high-repetition-rate picosecond pulses for the visible has also been demonstrated, which is based on an internally-doubled, Ti:sapphire-pumped OPO that uses temperature-tuned LBO both as the OPO and SHG crystal. Oscillation has been obtained for an input pump power of 700 mW with output powers in excess of 320 mW being generated, representing conversion efficiencies of as much as 16 %. The system is continuously tunable from 584 to 771 nm and can provide transform-limited visible pulses with durations of 840-880 fs across the available range. The ability to tune beyond wavelengths of 2.5 mum was also required. To this end a new source of tunable picosecond pulses for the near - to mid - infrared has been developed which is based on the material KTA. Oscillation has been obtained for input pump powers as low as 230 mW. The system produces total output powers in excess of 403 mW with conversion efficiencies of 31 % at 5.2 times threshold. Transform-limited signal (idler) pulses of 1.02 (2.9) ps have been generated over the tuning range 1.139-1.281 (2.377-3.160) mum.UV pumped holosteric optical parametric oscillatorCui, Yonghttps://hdl.handle.net/10023/148892019-12-09T14:27:31Z1993-07-01T00:00:00ZThe all-solid-state (or "holosteric") optical parametric oscillator has resulted from the recent development of diode-laser-pumped solid-state lasers and from recent advancements in new optically nonlinear materials. As a result, all-solid-state sources of widely tunable (ultraviolet - visible - near infrared) coherent radiation are now possible by using the radiation from diode-laser-pumped solid-state lasers, either directly or after frequency conversion, to pump optical parametric oscillators. Such devices can be made compact, efficient and reliable. The work described in this thesis explores the feasibility of obtaining widely tunable radiation from such devices, with particular reference to low threshold, high efficiency operation, so requiring only modest energies (1 mJ in ultraviolet) from the pump source. In particular, a frequency tripled or frequency quadruped Nd:YAG laser pumped by pulsed, GaAlAs diode laser bars has been used as the pump source, and lithium triborate has been used as the nonlinear medium in the optical parametric oscillator. Two geometries of lithium triborate crystals have been investigated as the nonlinear medium. One was cut for a type II non-critical phase matching geometry, while the other was cut for a type I critical phase matching geometry. The oscillator cavities were designed for optimum focusing and mode matching aiming for operation with a low pump energy through the use of tightly focused pump radiation. The ultraviolet pump source was based on a Q-switched diode-laser-pumped Nd:YAG laser which generated pulses at 1.064 mum with energy 10 mJ and duration around 10 ns. These were then frequency up-converted to the UV at 355 nm or 266 nm, so as to be suitable for pumping the parametric oscillators. Generally, an overall conversion efficiency from 1.064 mum to 355 nm of >30% was obtained using the nonlinear materials potassium titanyl phosphate and lithium triborate for second harmonic generation and sum-frequency mixing respectively. For conversion to 266 nm, an overall efficiency of > 18 % was obtained using the nonlinear materials KTP and BBO for two step second harmonic generation. In the experimental investigations of the all-solid-state lithium triborate optical parametric oscillator pumped at 355 nm a low oscillation threshold was obtained in the type II non-critical phase matching geometry (around 0.2 mJ) and pump depletions of 50 % were obtained at around six times threshold. This device could be temperature tuned (20 - 200 °C) from 457 to 481 nm (signal wave) and 1.6 to 1.35 mum (idler wave). Optimised focusing conditions corresponding to the theory of Guha et al were approached in the type I phase matching geometry, and despite the existence of a 1° walkoff angle, the minimum oscillation threshold was measured to be around 0.3 mJ. Generally, pump depletions of about 35 % were obtained, at around four times threshold. These devices could be angle tuned (through crystal internal angle 14°) from 457 to 666 nm (signal wave) and 1.6 mum to 768 nm (idler wave). (The whole of the range 420 nm to 2.3 mum could be covered with such a device given additional mirror sets). The all-solid-state type II geometry lithium triborate optical parametric oscillator was also pumped at 266 nm, when it was temperature tunable (20 - 200 °C) from 306 to 314 nm (signal wave) and 2.03 to 1.75 mum (idler wave). Pump depletions of 25 % were demonstrated with this device at pump energies of four times threshold. In addition to the above experimental investigations, the thesis addresses the issues of (i) choice of nonlinear material for optical parametric oscillators in terms of appropriate figures of merit, and (ii) optimisation of pump and resonated wave focusing parameters. Reviews of the appropriate theoretical background to phase matching geometries and optical parametric interaction are included.
1993-07-01T00:00:00ZCui, YongThe all-solid-state (or "holosteric") optical parametric oscillator has resulted from the recent development of diode-laser-pumped solid-state lasers and from recent advancements in new optically nonlinear materials. As a result, all-solid-state sources of widely tunable (ultraviolet - visible - near infrared) coherent radiation are now possible by using the radiation from diode-laser-pumped solid-state lasers, either directly or after frequency conversion, to pump optical parametric oscillators. Such devices can be made compact, efficient and reliable. The work described in this thesis explores the feasibility of obtaining widely tunable radiation from such devices, with particular reference to low threshold, high efficiency operation, so requiring only modest energies (1 mJ in ultraviolet) from the pump source. In particular, a frequency tripled or frequency quadruped Nd:YAG laser pumped by pulsed, GaAlAs diode laser bars has been used as the pump source, and lithium triborate has been used as the nonlinear medium in the optical parametric oscillator. Two geometries of lithium triborate crystals have been investigated as the nonlinear medium. One was cut for a type II non-critical phase matching geometry, while the other was cut for a type I critical phase matching geometry. The oscillator cavities were designed for optimum focusing and mode matching aiming for operation with a low pump energy through the use of tightly focused pump radiation. The ultraviolet pump source was based on a Q-switched diode-laser-pumped Nd:YAG laser which generated pulses at 1.064 mum with energy 10 mJ and duration around 10 ns. These were then frequency up-converted to the UV at 355 nm or 266 nm, so as to be suitable for pumping the parametric oscillators. Generally, an overall conversion efficiency from 1.064 mum to 355 nm of >30% was obtained using the nonlinear materials potassium titanyl phosphate and lithium triborate for second harmonic generation and sum-frequency mixing respectively. For conversion to 266 nm, an overall efficiency of > 18 % was obtained using the nonlinear materials KTP and BBO for two step second harmonic generation. In the experimental investigations of the all-solid-state lithium triborate optical parametric oscillator pumped at 355 nm a low oscillation threshold was obtained in the type II non-critical phase matching geometry (around 0.2 mJ) and pump depletions of 50 % were obtained at around six times threshold. This device could be temperature tuned (20 - 200 °C) from 457 to 481 nm (signal wave) and 1.6 to 1.35 mum (idler wave). Optimised focusing conditions corresponding to the theory of Guha et al were approached in the type I phase matching geometry, and despite the existence of a 1° walkoff angle, the minimum oscillation threshold was measured to be around 0.3 mJ. Generally, pump depletions of about 35 % were obtained, at around four times threshold. These devices could be angle tuned (through crystal internal angle 14°) from 457 to 666 nm (signal wave) and 1.6 mum to 768 nm (idler wave). (The whole of the range 420 nm to 2.3 mum could be covered with such a device given additional mirror sets). The all-solid-state type II geometry lithium triborate optical parametric oscillator was also pumped at 266 nm, when it was temperature tunable (20 - 200 °C) from 306 to 314 nm (signal wave) and 2.03 to 1.75 mum (idler wave). Pump depletions of 25 % were demonstrated with this device at pump energies of four times threshold. In addition to the above experimental investigations, the thesis addresses the issues of (i) choice of nonlinear material for optical parametric oscillators in terms of appropriate figures of merit, and (ii) optimisation of pump and resonated wave focusing parameters. Reviews of the appropriate theoretical background to phase matching geometries and optical parametric interaction are included.Electrical characteristics of n-InSb crystals used for sub-mm wave emission and detectionSmith, Darrell I.https://hdl.handle.net/10023/148882019-04-01T10:05:01Z1993-07-01T00:00:00ZA detailed study of the electrical characteristics of high purity n-InSb has been under taken. The main aim was to determine the effect of the electrical characteristics on the emission and detection properties of n-InSb. Several samples of high purity n-InSb, with different electrical properties, were used. Calculations of the individual and combined effects of the relaxation and excitation mechanisms on the samples were made. Lattice temperatures of 1.5, 4.2 & 77 Kelvins were considered. The electrical properties predicted, agree well with previous results. The variation of current density with electric field was measured at lattice temperatures of 1.5 & 4.2 K, from which the conductivity was calculated. The carrier concentrations of the crystals were measured and the mobilities were then calculated. At an electric field of 0.25 Vcm-1, a sharp rise in conductivity was observed in all samples and attributed to a transition between energy relaxation mechanisms. At an electric field of 20 Vcm-1, one sample showed a sharp rise in conductivity, which was shown (by Hall measurements) to be due to the ionisation of deep donors. At an electric field of 80 Vcm-1, a large increase in conductivity with an associated hysteresis was observed. Reasons for both are suggested. The hysteresis was also observed in the variation of the crystal resistance with lattice temperature. The implications of this work on the possibilities for a tuneable sub-mm wave source or laser has been discussed.
1993-07-01T00:00:00ZSmith, Darrell I.A detailed study of the electrical characteristics of high purity n-InSb has been under taken. The main aim was to determine the effect of the electrical characteristics on the emission and detection properties of n-InSb. Several samples of high purity n-InSb, with different electrical properties, were used. Calculations of the individual and combined effects of the relaxation and excitation mechanisms on the samples were made. Lattice temperatures of 1.5, 4.2 & 77 Kelvins were considered. The electrical properties predicted, agree well with previous results. The variation of current density with electric field was measured at lattice temperatures of 1.5 & 4.2 K, from which the conductivity was calculated. The carrier concentrations of the crystals were measured and the mobilities were then calculated. At an electric field of 0.25 Vcm-1, a sharp rise in conductivity was observed in all samples and attributed to a transition between energy relaxation mechanisms. At an electric field of 20 Vcm-1, one sample showed a sharp rise in conductivity, which was shown (by Hall measurements) to be due to the ionisation of deep donors. At an electric field of 80 Vcm-1, a large increase in conductivity with an associated hysteresis was observed. Reasons for both are suggested. The hysteresis was also observed in the variation of the crystal resistance with lattice temperature. The implications of this work on the possibilities for a tuneable sub-mm wave source or laser has been discussed.An analysis of the performance characteristics of continuous-wave optical parametric oscillatorsColville, F.G. (FinlayG.)https://hdl.handle.net/10023/148872019-04-01T10:04:01Z1995-07-01T00:00:00ZThis thesis gives a description of studies relating to the development of continuous-wave (cw) optical parametric oscillators (OPOs) and their application to schemes that require high-precision, narrow-linewidth, and frequency-tunable radiation. There are three separate aims to the work presented within this thesis. First, the requirements on pump sources, nonlinear materials, OPO cavity resonances and phase-matching geometries are analysed with a view to operating cw OPOs with stability above threshold. Second, the results of four distinct experiments are outlined, compared to theory, and discussed within the general context of cw OPO development. Third, this thesis is the first comprehensive review of the above-mentioned performance characteristics of cw OPOs, and focuses on their role as optical frequency dividers within frequency synthesis chains spanning the optical spectrum. The modelling sections highlight the importance of cavity resonances in cw OPOs when evaluating pump power thresholds, conversion efficiencies, and mode-selection properties. Simultaneous signal and idler cavity resonances are shown to be critical when relying upon cw laser sources to reach OPO threshold powers. Such arrangements require the use of stable pump lasers and servo-locked OPO cavity lengths to maintain this doubleresonance condition. There is an in-depth analysis of OPO cavity geometries that can generate frequency-stable and continuously-tunable outputs. The selection of nonlinear materials for cw OPOs is also considered with regard to providing signal and idler frequencies in integral-related frequency ratios, thereby satisfying an important requirement within optical frequency division techniques. Four specific experiments were designed to address many of the issues raised within the modelling sections. These experiments realized the following novel arrangements; the use of lithium triborate as a gain material within cw OPOs; a cw OPO which used a pump source operating in the ultraviolet spectral region; the highest frequency output from a cw OPO; a cw OPO which used a tunable pump source operating in the near infra-red spectral region; multiple parameter pump / OPO coarse frequency tuning; a non- degenerate type II cw OPO phase-matching geometry; and a dual-cavity doubly-resonant cw OPO. Other notable features of the experimental designs included stabilized single-frequency output from a single-cavity cw OPO geometry, continuous frequency tuning from a dual-cavity, doubly-resonant OPO resonator, and the general characteristics of low pump power thresholds and moderate conversion efficiencies. An important feature discussed in detail throughout the thesis is the comparison between type I and type II phase-matching geometries. These two cases give rise to different polarization states for the signal and idler fields within OPO cavities. Type II phase-matching geometries are shown, both in theory and experiments, to be preferable to equivalent type I geometries, when considering stable OPO operation, fine frequency tuning, and multiple cavity oscillators. This is so because type II phase-matching geometries, in general, provide significantly different signal and idler refractive indices which in turn yield a considerable mis-match in the signal and idler free spectral ranges. Subsequently this relaxes the stability requirements within single-cavity doubly-resonant OPOs, and allows for polarization separation to form dual-cavity resonators which are vital to the effective operation of cw OPOs within metrology and spectroscopy. The work contained in this thesis forms an integral part of current research in cw OPOs, a field presently enjoying its most productive and prosperous period. The potential incorporation of cw OPOs within frequency synthesis chains is shown to be dependent on the further development of pump lasers and nonlinear materials. In the short term, the actual use of cw OPOs is assessed in relation to more convenient and widespread techniques for converting, comparing, and measuring absolute frequencies.
1995-07-01T00:00:00ZColville, F.G. (FinlayG.)This thesis gives a description of studies relating to the development of continuous-wave (cw) optical parametric oscillators (OPOs) and their application to schemes that require high-precision, narrow-linewidth, and frequency-tunable radiation. There are three separate aims to the work presented within this thesis. First, the requirements on pump sources, nonlinear materials, OPO cavity resonances and phase-matching geometries are analysed with a view to operating cw OPOs with stability above threshold. Second, the results of four distinct experiments are outlined, compared to theory, and discussed within the general context of cw OPO development. Third, this thesis is the first comprehensive review of the above-mentioned performance characteristics of cw OPOs, and focuses on their role as optical frequency dividers within frequency synthesis chains spanning the optical spectrum. The modelling sections highlight the importance of cavity resonances in cw OPOs when evaluating pump power thresholds, conversion efficiencies, and mode-selection properties. Simultaneous signal and idler cavity resonances are shown to be critical when relying upon cw laser sources to reach OPO threshold powers. Such arrangements require the use of stable pump lasers and servo-locked OPO cavity lengths to maintain this doubleresonance condition. There is an in-depth analysis of OPO cavity geometries that can generate frequency-stable and continuously-tunable outputs. The selection of nonlinear materials for cw OPOs is also considered with regard to providing signal and idler frequencies in integral-related frequency ratios, thereby satisfying an important requirement within optical frequency division techniques. Four specific experiments were designed to address many of the issues raised within the modelling sections. These experiments realized the following novel arrangements; the use of lithium triborate as a gain material within cw OPOs; a cw OPO which used a pump source operating in the ultraviolet spectral region; the highest frequency output from a cw OPO; a cw OPO which used a tunable pump source operating in the near infra-red spectral region; multiple parameter pump / OPO coarse frequency tuning; a non- degenerate type II cw OPO phase-matching geometry; and a dual-cavity doubly-resonant cw OPO. Other notable features of the experimental designs included stabilized single-frequency output from a single-cavity cw OPO geometry, continuous frequency tuning from a dual-cavity, doubly-resonant OPO resonator, and the general characteristics of low pump power thresholds and moderate conversion efficiencies. An important feature discussed in detail throughout the thesis is the comparison between type I and type II phase-matching geometries. These two cases give rise to different polarization states for the signal and idler fields within OPO cavities. Type II phase-matching geometries are shown, both in theory and experiments, to be preferable to equivalent type I geometries, when considering stable OPO operation, fine frequency tuning, and multiple cavity oscillators. This is so because type II phase-matching geometries, in general, provide significantly different signal and idler refractive indices which in turn yield a considerable mis-match in the signal and idler free spectral ranges. Subsequently this relaxes the stability requirements within single-cavity doubly-resonant OPOs, and allows for polarization separation to form dual-cavity resonators which are vital to the effective operation of cw OPOs within metrology and spectroscopy. The work contained in this thesis forms an integral part of current research in cw OPOs, a field presently enjoying its most productive and prosperous period. The potential incorporation of cw OPOs within frequency synthesis chains is shown to be dependent on the further development of pump lasers and nonlinear materials. In the short term, the actual use of cw OPOs is assessed in relation to more convenient and widespread techniques for converting, comparing, and measuring absolute frequencies.Continuously frequency-tunable CW optical parametric oscillators and their application to spectroscopyGibson, Graham Martinhttps://hdl.handle.net/10023/148862018-07-26T09:23:12Z1999-05-01T00:00:00ZWithin the past few years, renewed interest has been generated in the field of continuous wave optical parametric oscillators. This has been due in part, to the wider range of nonlinear materials now available, and also to the improvements being made in the pump sources for these devices. OPO resonator design has also been stimulated and the use of monolithic / semi-monolithic cavities which display high levels of passive stability has been realised. The work to be described herein was prompted by a desire to develop a doubly resonant optical parametric oscillator with frequency stable, high integer ratio signal-idler tunable outputs pumped in the green spectral region. Given a pump source constrained to be in the green spectral region, the nonlinear material of choice for the optical parametric oscillator (OPO) was lithium triborate (LBO). The target device for this work was a tunable, type I LBO doubly resonant oscillator (DRO) with a signal: idler frequency output ratio of 3 : 1. This configuration necessitates a high operating temperature which, it was predicted, would lead to problems with the nonlinear OPO crystal coatings. LBO, as will be discussed later, suffers from an anisotropic thermal expansion which leads it to expand on heating in two crystal directions but to contract in the third crystal direction. This has in the past led to problems with the dielectric coatings applied to the surfaces of the crystal whereby the coatings and the crystal become separated upon temperature cycling. The solution to this problem developed within this thesis, involves the use of index-matching fluid to attach cavity mirrors directly onto the faces of the nonlinear material. The cavity mirrors, constructed from a substrate which exhibits an isotropic thermal expansion, could then be dielectrically coated and temperature cycled successfully. The index-matching fluid would provide the contact necessary to maintain high finesse optical cavities. The first device to be built in this way was a type EL, LBO DRO which operated successfully with signal and idler output wavelengths of 946 and 1126 nm respectively. Other devices were also investigated during this time, including the target device, and they are discussed later within this thesis. However, the first device developed displayed frequency stability characteristics far in excess of what had been expected theoretically and a thermal feedback mechanism was proposed to explain its behaviour. This device was presented at The Conference on Lasers and Electro-Optics (C.L.E.O.) in 1995 and a discussion with a fellow delegate brought to my attention an independent proposal of a similar thermal feedback mechanism used to explain the enhanced frequency stability within other parametric devices. A period of work was then undertaken to prove the existence of this mechanism and to characterise it fully. The thermal feedback mechanism was subsequently computer modelled successfully and a solid-state laser was designed and built in order to probe this mechanism experimentally. Whilst technical problems were encountered which prevented a full characterisation of the said mechanism, evidence for it was found in the micro-operating characteristics of such a solid-state laser pumped, type II, pseudo-monolithic DRO.
1999-05-01T00:00:00ZGibson, Graham MartinWithin the past few years, renewed interest has been generated in the field of continuous wave optical parametric oscillators. This has been due in part, to the wider range of nonlinear materials now available, and also to the improvements being made in the pump sources for these devices. OPO resonator design has also been stimulated and the use of monolithic / semi-monolithic cavities which display high levels of passive stability has been realised. The work to be described herein was prompted by a desire to develop a doubly resonant optical parametric oscillator with frequency stable, high integer ratio signal-idler tunable outputs pumped in the green spectral region. Given a pump source constrained to be in the green spectral region, the nonlinear material of choice for the optical parametric oscillator (OPO) was lithium triborate (LBO). The target device for this work was a tunable, type I LBO doubly resonant oscillator (DRO) with a signal: idler frequency output ratio of 3 : 1. This configuration necessitates a high operating temperature which, it was predicted, would lead to problems with the nonlinear OPO crystal coatings. LBO, as will be discussed later, suffers from an anisotropic thermal expansion which leads it to expand on heating in two crystal directions but to contract in the third crystal direction. This has in the past led to problems with the dielectric coatings applied to the surfaces of the crystal whereby the coatings and the crystal become separated upon temperature cycling. The solution to this problem developed within this thesis, involves the use of index-matching fluid to attach cavity mirrors directly onto the faces of the nonlinear material. The cavity mirrors, constructed from a substrate which exhibits an isotropic thermal expansion, could then be dielectrically coated and temperature cycled successfully. The index-matching fluid would provide the contact necessary to maintain high finesse optical cavities. The first device to be built in this way was a type EL, LBO DRO which operated successfully with signal and idler output wavelengths of 946 and 1126 nm respectively. Other devices were also investigated during this time, including the target device, and they are discussed later within this thesis. However, the first device developed displayed frequency stability characteristics far in excess of what had been expected theoretically and a thermal feedback mechanism was proposed to explain its behaviour. This device was presented at The Conference on Lasers and Electro-Optics (C.L.E.O.) in 1995 and a discussion with a fellow delegate brought to my attention an independent proposal of a similar thermal feedback mechanism used to explain the enhanced frequency stability within other parametric devices. A period of work was then undertaken to prove the existence of this mechanism and to characterise it fully. The thermal feedback mechanism was subsequently computer modelled successfully and a solid-state laser was designed and built in order to probe this mechanism experimentally. Whilst technical problems were encountered which prevented a full characterisation of the said mechanism, evidence for it was found in the micro-operating characteristics of such a solid-state laser pumped, type II, pseudo-monolithic DRO.Novel nonlinear techniques for femtosecond pulse generation in the visible and near-infraredReid, Derryck T.https://hdl.handle.net/10023/148852019-04-01T10:03:17Z1995-07-01T00:00:00ZThe work presented in this thesis describes the design, configuration and operation of femtosecond optical parametric oscillators based on the materials KTiOPO4 (KTP) and RbTiOAsO4 (RTA) and pumped by a self- modelocked Ti:sapphire laser. The alignment of the pump laser is detailed and thermal effects in the Ti:sapphire rod are examined in the context of a general technique which optimises modelocked performance at any pump power. A KTP-based femtosecond parametric oscillator is described which produces 400-fs-duration signal pulses at an average output power of 150 mW when operated in the absence of group-velocity dispersion- compensation. With intracavity dispersion-compensation, the oscillator produces 40-fs-duration pulses with an average power of 50 mW. Tuning is demonstrated from 1.12 - 1.25 mum in the signal wave and from 2.5 - 3.0 mum in the idler wave by changing only the pump-laser wavelength. Using a novel idler-feedback arrangement, reductions in the oscillation threshold and increases in the signal output power of 10 % are described. Soliton generation in the oscillator is achieved when the net cavity dispersion is positive and results show good agreement with theory. An oscillator using RTA is demonstrated which achieves conversion efficiencies exceeding 30 % and has an operating threshold of only 50 mW. Average signal powers of 100 mW and 185 mW are extracted from the oscillators with and without dispersion-compensation respectively. The corresponding pulse durations are 67 fs and 980 fs and tunability in the signal and idler waves from 1.23 - 1.34 mum and 2.10 - 2.43 mum is demonstrated. Visible output from 620 - 660 nm is obtained by intracavity- doubling and powers of up to 170 mW are measured. These results suggest that RTA has a higher nonlinear coefficient than KTP.
1995-07-01T00:00:00ZReid, Derryck T.The work presented in this thesis describes the design, configuration and operation of femtosecond optical parametric oscillators based on the materials KTiOPO4 (KTP) and RbTiOAsO4 (RTA) and pumped by a self- modelocked Ti:sapphire laser. The alignment of the pump laser is detailed and thermal effects in the Ti:sapphire rod are examined in the context of a general technique which optimises modelocked performance at any pump power. A KTP-based femtosecond parametric oscillator is described which produces 400-fs-duration signal pulses at an average output power of 150 mW when operated in the absence of group-velocity dispersion- compensation. With intracavity dispersion-compensation, the oscillator produces 40-fs-duration pulses with an average power of 50 mW. Tuning is demonstrated from 1.12 - 1.25 mum in the signal wave and from 2.5 - 3.0 mum in the idler wave by changing only the pump-laser wavelength. Using a novel idler-feedback arrangement, reductions in the oscillation threshold and increases in the signal output power of 10 % are described. Soliton generation in the oscillator is achieved when the net cavity dispersion is positive and results show good agreement with theory. An oscillator using RTA is demonstrated which achieves conversion efficiencies exceeding 30 % and has an operating threshold of only 50 mW. Average signal powers of 100 mW and 185 mW are extracted from the oscillators with and without dispersion-compensation respectively. The corresponding pulse durations are 67 fs and 980 fs and tunability in the signal and idler waves from 1.23 - 1.34 mum and 2.10 - 2.43 mum is demonstrated. Visible output from 620 - 660 nm is obtained by intracavity- doubling and powers of up to 170 mW are measured. These results suggest that RTA has a higher nonlinear coefficient than KTP.Optical spanners and improved optical tweezersSimpson, Neil B.https://hdl.handle.net/10023/148842019-04-01T10:04:02Z1998-03-01T00:00:00ZThis thesis describes the experimental and theoretical work that investigated the transfer of orbital angular momentum from light to matter. This was achieved by combining two established areas of laser physics which were "optical tweezers" and Laguerre-Gaussian laser modes. The optical tweezers are essentially a tightly focussed laser beam from a high numerical aperture microscope objective lens, which traps particles in three dimensions just below the beam focus. By incorporating a Laguerre- Gaussian laser mode into the tweezers system, the trapping efficiency was doubled. These improved optical tweezers have been successfully demonstrated both theoretically and experimentally. In addition to the spin angular momentum which is associated with the polarisation state, the Laguerre-Gaussian laser modes also possess orbital angular momentum. The "optical spanners" utilised this property by transferring orbital angular momentum from the laser beam to the trapped particle, causing it to rotate whilst being held in the optical trap. This effect was theoretically modelled and experimentally observed. Using the optical spanners, the spin angular momentum of the laser was used to directly cancel the orbital angular momentum in the beam, which was observed as a cessation in rotation of the trapped particle. This demonstrated the mechanical equivalence of the spin and orbital components of angular momentum in a light beam, and gave experimental evidence for the well defined nature of the orbital angular momentum present in Laguerre-Gaussian laser modes.
1998-03-01T00:00:00ZSimpson, Neil B.This thesis describes the experimental and theoretical work that investigated the transfer of orbital angular momentum from light to matter. This was achieved by combining two established areas of laser physics which were "optical tweezers" and Laguerre-Gaussian laser modes. The optical tweezers are essentially a tightly focussed laser beam from a high numerical aperture microscope objective lens, which traps particles in three dimensions just below the beam focus. By incorporating a Laguerre- Gaussian laser mode into the tweezers system, the trapping efficiency was doubled. These improved optical tweezers have been successfully demonstrated both theoretically and experimentally. In addition to the spin angular momentum which is associated with the polarisation state, the Laguerre-Gaussian laser modes also possess orbital angular momentum. The "optical spanners" utilised this property by transferring orbital angular momentum from the laser beam to the trapped particle, causing it to rotate whilst being held in the optical trap. This effect was theoretically modelled and experimentally observed. Using the optical spanners, the spin angular momentum of the laser was used to directly cancel the orbital angular momentum in the beam, which was observed as a cessation in rotation of the trapped particle. This demonstrated the mechanical equivalence of the spin and orbital components of angular momentum in a light beam, and gave experimental evidence for the well defined nature of the orbital angular momentum present in Laguerre-Gaussian laser modes.Optimisation of a colliding-pulse modelocked dye laserWilliams, Edmond J. O.https://hdl.handle.net/10023/147962019-04-01T10:08:48Z1998-05-01T00:00:00ZThe work presented in this thesis describes the operation, characterisation and optimisation of a colliding-pulse modelocked (CPM) dye laser. A method of pulse analysis has been developed which is capable of determining the shape and chirp of the output pulses to a first approximation. It involves an iterative pulse-fitting to intensity autocorrelation, interferometric autocorrelation and spectral measurements. The use of a four-prism sequence for intracavity dispersion compensation in a CPM dye laser resulted in pulse durations of 40-50fs. However, operating the laser close to the instability regime so as to obtain strong focusing in the absorber dye jet enabled pulse durations as short as 19fs to be obtained. A detailed empirical study of the dispersion- compensated laser, together with a theoretical and experimental chirp analysis, indicated the presence of strong phase shaping arising from a net positive self-phase modulation, which was attributed to the optical Kerr effect occurring in the absorber dye solvent. Various modes of operation were observed, including unidirectional lasing and a higher- order solitonlike regime. The results of pulse-fitting were found to yield strong evidence for pulse asymmetry, the pulse profiles corresponding closely to an asymmetric sech2 pulse function with a longer leading edge. A computer simulation of the CPM dye laser provided a comprehensive understanding of the underlying pulse shaping dynamics of this system, elucidating fully the experimental behaviours observed, as well as providing a clear strategy for further optimisation of the laser. In particular, optimal performance was found to depend on strong amplitude and strong phase shaping, minimal spectral filtering, the control of higher-order dispersion and the provision of extracavity dispersion compensation. An experimental study of Gires-Tournois interferometers (GTI's) for intracavity cubic phase compensation identified the key requirements for cubic phase control in the CPM dye laser, while highlighting the limitations of utilising conventional GTI structures. A subsequent theoretical analysis enabled a more suitable strategy to be devised. It involved optimising the cavity optics and using a prism system with variable prism spacing, alone or in tandem with specially tailored GTI structures. Implementation of these findings resulted in pulse durations of around 30-40fs and the elimination of pulse asymmetry, which was attributed to a residual positive cubic phase. However, the appearance of a distinctive modulation in the wings of the pulse provided strong evidence that the pulse durations from the CPM dye laser had become limited by the next higher-order dispersion term; quartic phase. To demonstrate the direct relevance of this work to the more recently developed solid- state laser systems, an alternative all-solid-state femtosecond laser has been described. Based around a Ti:sapphire gain medium, the design of this laser incorporates the essential optimising principles and techniques developed for the CPM dye laser. The proposed system utilises a low-loss, broadband semiconductor saturable absorber mirror to initiate self-modelocking and a hybrid prism-chirped-mirror scheme for broadband intracavity and extracavity quintic-phase-limited dispersion compensation. When fully optimised, it is predicted that this laser should yield pulse durations as short as 5fs.
1998-05-01T00:00:00ZWilliams, Edmond J. O.The work presented in this thesis describes the operation, characterisation and optimisation of a colliding-pulse modelocked (CPM) dye laser. A method of pulse analysis has been developed which is capable of determining the shape and chirp of the output pulses to a first approximation. It involves an iterative pulse-fitting to intensity autocorrelation, interferometric autocorrelation and spectral measurements. The use of a four-prism sequence for intracavity dispersion compensation in a CPM dye laser resulted in pulse durations of 40-50fs. However, operating the laser close to the instability regime so as to obtain strong focusing in the absorber dye jet enabled pulse durations as short as 19fs to be obtained. A detailed empirical study of the dispersion- compensated laser, together with a theoretical and experimental chirp analysis, indicated the presence of strong phase shaping arising from a net positive self-phase modulation, which was attributed to the optical Kerr effect occurring in the absorber dye solvent. Various modes of operation were observed, including unidirectional lasing and a higher- order solitonlike regime. The results of pulse-fitting were found to yield strong evidence for pulse asymmetry, the pulse profiles corresponding closely to an asymmetric sech2 pulse function with a longer leading edge. A computer simulation of the CPM dye laser provided a comprehensive understanding of the underlying pulse shaping dynamics of this system, elucidating fully the experimental behaviours observed, as well as providing a clear strategy for further optimisation of the laser. In particular, optimal performance was found to depend on strong amplitude and strong phase shaping, minimal spectral filtering, the control of higher-order dispersion and the provision of extracavity dispersion compensation. An experimental study of Gires-Tournois interferometers (GTI's) for intracavity cubic phase compensation identified the key requirements for cubic phase control in the CPM dye laser, while highlighting the limitations of utilising conventional GTI structures. A subsequent theoretical analysis enabled a more suitable strategy to be devised. It involved optimising the cavity optics and using a prism system with variable prism spacing, alone or in tandem with specially tailored GTI structures. Implementation of these findings resulted in pulse durations of around 30-40fs and the elimination of pulse asymmetry, which was attributed to a residual positive cubic phase. However, the appearance of a distinctive modulation in the wings of the pulse provided strong evidence that the pulse durations from the CPM dye laser had become limited by the next higher-order dispersion term; quartic phase. To demonstrate the direct relevance of this work to the more recently developed solid- state laser systems, an alternative all-solid-state femtosecond laser has been described. Based around a Ti:sapphire gain medium, the design of this laser incorporates the essential optimising principles and techniques developed for the CPM dye laser. The proposed system utilises a low-loss, broadband semiconductor saturable absorber mirror to initiate self-modelocking and a hybrid prism-chirped-mirror scheme for broadband intracavity and extracavity quintic-phase-limited dispersion compensation. When fully optimised, it is predicted that this laser should yield pulse durations as short as 5fs.Investigations on two-level maser oscillationsFirth, Ian Masonhttps://hdl.handle.net/10023/147952019-04-01T10:09:50Z1962-01-01T00:00:00ZAbstract Not Provided
1962-01-01T00:00:00ZFirth, Ian MasonAbstract Not ProvidedThe kinetics of copper HyBrID lasersWhyte, Colinhttps://hdl.handle.net/10023/147942019-04-01T10:09:41Z1996-07-01T00:00:00ZSpatially and temporally resolved measurements of many of the parameters of a 25mm bore, 800mm active length copper HyBrID laser have been performed. The laser was operated under excitation conditions for maximum efficiency of light generation at all times. The hook method has been used to investigate the population densities in the copper ground state, the upper laser levels, the lower laser levels and one of the quartet levels in copper. The population density in one of the neon metastable levels has also been measured. All of these measurements have been performed with 5 ns time resolution and 2 mm spatial resolution. A two wavelength laser interferometric technique has been used to measure the electron density with nanosecond time resolution and 2mm spatial resolution. The waveforms of the voltage across the laser head and current through the laser tube have been measured. The temporally resolved laser pulse radial profile has been investigated. The results from these investigations have been compared to those for a 42 mm bore CVL previously studied with the same experimental apparatus. Comparison has also been made with a detailed computer model of a 19mm bore CVL. Deductions as to the effect of the remanent electron density prior to the excitation pulse on the current and voltage waveforms have been made. The effect of added HBr has been discussed, relative to the remanent electron density. The laser kinetics are discussed in terms of the electron temperature and the relative rate coefficients of the mechanisms affecting the level populations.
1996-07-01T00:00:00ZWhyte, ColinSpatially and temporally resolved measurements of many of the parameters of a 25mm bore, 800mm active length copper HyBrID laser have been performed. The laser was operated under excitation conditions for maximum efficiency of light generation at all times. The hook method has been used to investigate the population densities in the copper ground state, the upper laser levels, the lower laser levels and one of the quartet levels in copper. The population density in one of the neon metastable levels has also been measured. All of these measurements have been performed with 5 ns time resolution and 2 mm spatial resolution. A two wavelength laser interferometric technique has been used to measure the electron density with nanosecond time resolution and 2mm spatial resolution. The waveforms of the voltage across the laser head and current through the laser tube have been measured. The temporally resolved laser pulse radial profile has been investigated. The results from these investigations have been compared to those for a 42 mm bore CVL previously studied with the same experimental apparatus. Comparison has also been made with a detailed computer model of a 19mm bore CVL. Deductions as to the effect of the remanent electron density prior to the excitation pulse on the current and voltage waveforms have been made. The effect of added HBr has been discussed, relative to the remanent electron density. The laser kinetics are discussed in terms of the electron temperature and the relative rate coefficients of the mechanisms affecting the level populations.Ion processes and effects in CO₂ laser dischargesShields, Henryhttps://hdl.handle.net/10023/147932019-04-01T10:10:31Z1987-01-01T00:00:00ZPresently, a major factor restricting the achievement of higher power outputs from the CO2 laser is the occurrence of plasma instability. This instability may manifest itself in two forms causing either discharge striations (ionisation instability) or, more importantly, discharge arcing (thermal instability). Recent experimental and theoretical work, has identified these instabilities with the electron and ion kinetics of the CO2 laser discharge. The CO2 laser discharge, its development for high power output and the conditions leading to the two types of instability are reviewed. The requirement for full data on electron and ion processes for discharge modelling is clearly established. The techniques involved in mass spectrometric analysis of ions in gas discharge plasmas are reviewed and subsequently applied to the CO2 laser discharge. The major processes affecting the positive ion species in CO2 laser plasmas are determined in this way. The important negative ion species in the CO2 laser plasma are most easily and comprehensively determined by computational methods. These methods have allowed considerable insight into the dominant negative ion processes of the CO2 laser discharge. By application to specific laser situations correlation has been shown to exist between ion densities and the onset of experimentally observed discharge instability. The elucidation of the main ion processes, both by mass spectrometer and computer, has contributed detailed information for modelling of the CO2 laser discharge, and its instabilities, in a variety of configurations.
1987-01-01T00:00:00ZShields, HenryPresently, a major factor restricting the achievement of higher power outputs from the CO2 laser is the occurrence of plasma instability. This instability may manifest itself in two forms causing either discharge striations (ionisation instability) or, more importantly, discharge arcing (thermal instability). Recent experimental and theoretical work, has identified these instabilities with the electron and ion kinetics of the CO2 laser discharge. The CO2 laser discharge, its development for high power output and the conditions leading to the two types of instability are reviewed. The requirement for full data on electron and ion processes for discharge modelling is clearly established. The techniques involved in mass spectrometric analysis of ions in gas discharge plasmas are reviewed and subsequently applied to the CO2 laser discharge. The major processes affecting the positive ion species in CO2 laser plasmas are determined in this way. The important negative ion species in the CO2 laser plasma are most easily and comprehensively determined by computational methods. These methods have allowed considerable insight into the dominant negative ion processes of the CO2 laser discharge. By application to specific laser situations correlation has been shown to exist between ion densities and the onset of experimentally observed discharge instability. The elucidation of the main ion processes, both by mass spectrometer and computer, has contributed detailed information for modelling of the CO2 laser discharge, and its instabilities, in a variety of configurations.Heat transfer studies of strontium recombination lasersPugsley, Trevor R.https://hdl.handle.net/10023/147922019-04-01T10:06:19Z1995-07-01T00:00:00ZThermal loading models have been developed in order to predict the optimum input powers of free-convection cooled strontium recombination lasers (SRLs) and forced-convection cooled SRLs. Gas temperature models have also been developed for circular bore and rectangular bore SRLs, to complement the thermal loading models, so that the performances of circular and rectangular bore SRLs can be compared. It has been shown, in theory, that rectangular bore tubes offer operation at lower gas temperatures than do circular bore tubes of the same cross-sectional area, with the same input power per unit length. Laser performance is, therefore, expected to be improved through the use of rectangular bores in the construction of SRLs. The results of the thermal loading models are validated by experiments. The need to oven process alumina tubes in forming gas, prior to their use in the SRL, is demonstrated by an increase in output power from tubes processed using this technique. Enhanced radiative heat extraction, accompanied by an increase in average laser output power, is obtained from a free-convection cooled SRL by increasing the emissivity of the outer surface of the ceramic discharge tube. Three rectangular bore water-cooled SRLs, optimising at different input powers, have been constructed, operated and compared in performance to a free-convection cooled SRL, constructed using the same ceramic discharge tube. An average output power of 2.3 W has been obtained from a 40 cm long rectangular bore beryllia discharge tube through the use of active water cooling. This represents an improvement of over 200% when compared to the performance of a free-convection cooled SRL utilizing the same ceramic discharge tube.
1995-07-01T00:00:00ZPugsley, Trevor R.Thermal loading models have been developed in order to predict the optimum input powers of free-convection cooled strontium recombination lasers (SRLs) and forced-convection cooled SRLs. Gas temperature models have also been developed for circular bore and rectangular bore SRLs, to complement the thermal loading models, so that the performances of circular and rectangular bore SRLs can be compared. It has been shown, in theory, that rectangular bore tubes offer operation at lower gas temperatures than do circular bore tubes of the same cross-sectional area, with the same input power per unit length. Laser performance is, therefore, expected to be improved through the use of rectangular bores in the construction of SRLs. The results of the thermal loading models are validated by experiments. The need to oven process alumina tubes in forming gas, prior to their use in the SRL, is demonstrated by an increase in output power from tubes processed using this technique. Enhanced radiative heat extraction, accompanied by an increase in average laser output power, is obtained from a free-convection cooled SRL by increasing the emissivity of the outer surface of the ceramic discharge tube. Three rectangular bore water-cooled SRLs, optimising at different input powers, have been constructed, operated and compared in performance to a free-convection cooled SRL, constructed using the same ceramic discharge tube. An average output power of 2.3 W has been obtained from a 40 cm long rectangular bore beryllia discharge tube through the use of active water cooling. This represents an improvement of over 200% when compared to the performance of a free-convection cooled SRL utilizing the same ceramic discharge tube.Tunable femtosecond lasers with low pump thresholdsOppo, Karenhttps://hdl.handle.net/10023/147912019-04-01T10:05:03Z1996-07-01T00:00:00ZThe work in this thesis is concerned with the development of tunable, femtosecond laser systems, exhibiting low pump threshold powers. The main motive for this work was the development of a low threshold, self-modelocked Ti:Al2O3 laser in order to replace the conventional large-frame argon-ion pump laser with a more compact and efficient all-solid-state alternative. Results are also presented for an all-solid-state, self-modelocked Cr:LiSAF laser, however most of this work is concerned with self-modelocked Ti:Al2O3 laser systems. In chapter 2, the operation of a regeneratively-initiated, and a hard-aperture self- modelocked Ti:Al2O3 laser, pumped by an argon-ion laser, is discussed. Continuous- wave oscillation thresholds as low as 160mW have been demonstrated, along with self-modelocked threshold powers as low as 500mW. The measurement and suppression of phase noise on modelocked lasers is discussed in chapter 3. This is followed by a comparison of the phase noise characteristics of the regeneratively-initiated, and hard-aperture self-modelocked Ti:Al2O3 lasers. The use of a synchronously-operating, high resolution electron-optical streak camera in the evaluation of timing jitter is also presented. In chapter 4, the construction and self-modelocked operation of an all-solid-state Ti:Al2O3 laser is described. The all-solid-state alternative to the conventional argon-ion pump laser was a continuous-wave, intracavity-frequency doubled, diode-laser pumped Nd:YLF ring laser. At a total diode-laser pump power of 10W, this minilaser was capable of producing a single frequency output of ~1W, at 523.5nm in a TEM00 beam. The remainder of this thesis looks at the operation of a self-modelocked Ti:Al2O3 laser generating ultrashort pulses at wavelengths as long as 1053nm. The motive for this work was the development of an all-solid-state, self- modelocked Ti:Al2O3 laser operating at 1053nm, for use as a master oscillator in a Nd:glass power chain.
1996-07-01T00:00:00ZOppo, KarenThe work in this thesis is concerned with the development of tunable, femtosecond laser systems, exhibiting low pump threshold powers. The main motive for this work was the development of a low threshold, self-modelocked Ti:Al2O3 laser in order to replace the conventional large-frame argon-ion pump laser with a more compact and efficient all-solid-state alternative. Results are also presented for an all-solid-state, self-modelocked Cr:LiSAF laser, however most of this work is concerned with self-modelocked Ti:Al2O3 laser systems. In chapter 2, the operation of a regeneratively-initiated, and a hard-aperture self- modelocked Ti:Al2O3 laser, pumped by an argon-ion laser, is discussed. Continuous- wave oscillation thresholds as low as 160mW have been demonstrated, along with self-modelocked threshold powers as low as 500mW. The measurement and suppression of phase noise on modelocked lasers is discussed in chapter 3. This is followed by a comparison of the phase noise characteristics of the regeneratively-initiated, and hard-aperture self-modelocked Ti:Al2O3 lasers. The use of a synchronously-operating, high resolution electron-optical streak camera in the evaluation of timing jitter is also presented. In chapter 4, the construction and self-modelocked operation of an all-solid-state Ti:Al2O3 laser is described. The all-solid-state alternative to the conventional argon-ion pump laser was a continuous-wave, intracavity-frequency doubled, diode-laser pumped Nd:YLF ring laser. At a total diode-laser pump power of 10W, this minilaser was capable of producing a single frequency output of ~1W, at 523.5nm in a TEM00 beam. The remainder of this thesis looks at the operation of a self-modelocked Ti:Al2O3 laser generating ultrashort pulses at wavelengths as long as 1053nm. The motive for this work was the development of an all-solid-state, self- modelocked Ti:Al2O3 laser operating at 1053nm, for use as a master oscillator in a Nd:glass power chain.Development of all-solid-state modelocked laser sources at 1.55 μMValentine, Gareth J.https://hdl.handle.net/10023/147902019-04-01T10:02:57Z1999-03-01T00:00:00ZThis thesis concerns the generation of tunable ultrashort pulses near the 1.55 mum telecommunications window. Two principal laser systems are considered: i) the NaCl:OH colour-centre laser, which employs the technique of synchronously-pumped modelocking to generate tunable picosecond pulses and ii) the self-modelocked Cr4:YAG laser to generate femtosecond pulses tunable from 1.5-1.56 mum. Details are given for an all-solid-state cw and cw-modelocked pump source for Cr4:YAG and colour-centre lasers based on Nd:YAG. Fibre-coupled AlGaAs laser diodes are employed as the solid-state pump source to this laser. When operated cw, up to 8.5 W of linearly polarised output power in a TEM00 beam is obtained. A compact cw actively-modelocked Nd:YAG laser is described having a pulse repetition rate of 194 MHz. Pulse durations down to 34 ps and output powers up to 6.0 W are obtained from this system. An 82 MHz Nd:YVO4 laser is also detailed producing pulsewidths down to 75 ps and average output powers up to 3.5 W. The intrinsic noise source associated with the synchronous modelocking technique is discussed and a simple passive stabilisation scheme, coherent-photon-seeding (CPS), is described and applied to the synchronously-modelocked NaCl:OH laser. Results of a simulation of this laser are reported and a comparison is made with the practical observations of the stabilised laser. For the first time, theoretical and experimental evidence for the presence of high frequency pulse jitter in synchronously-pumped- modelocked (SPML) lasers is presented and the coherent photon seeding technique is shown to eliminate this noise. Details are also given for the construction of a compact, all-solid-state, femtosecond Cr4+:YAG laser. A design prescription for laser resonators having a high propensity for self-modelocking is presented and an unconventional 3-mirror resonator is adopted for optimised self-modelocked operation. Using this design, modelocked output powers up to 300 mW with 120 fs pulses from a compact, regeneratively initiated laser having a pulse repetition rate of 320 MHz is reported for 4.7W incident pump power. Self- modelocking is demonstrated for pump powers down to ~1W with this cavity design. A compact cavity design for self-modelocking is also assessed, with a footprint of just 20 X 25 cm, which places a prism in each cavity arm. 470 fs pulses at 220 mW average output power are reported.
1999-03-01T00:00:00ZValentine, Gareth J.This thesis concerns the generation of tunable ultrashort pulses near the 1.55 mum telecommunications window. Two principal laser systems are considered: i) the NaCl:OH colour-centre laser, which employs the technique of synchronously-pumped modelocking to generate tunable picosecond pulses and ii) the self-modelocked Cr4:YAG laser to generate femtosecond pulses tunable from 1.5-1.56 mum. Details are given for an all-solid-state cw and cw-modelocked pump source for Cr4:YAG and colour-centre lasers based on Nd:YAG. Fibre-coupled AlGaAs laser diodes are employed as the solid-state pump source to this laser. When operated cw, up to 8.5 W of linearly polarised output power in a TEM00 beam is obtained. A compact cw actively-modelocked Nd:YAG laser is described having a pulse repetition rate of 194 MHz. Pulse durations down to 34 ps and output powers up to 6.0 W are obtained from this system. An 82 MHz Nd:YVO4 laser is also detailed producing pulsewidths down to 75 ps and average output powers up to 3.5 W. The intrinsic noise source associated with the synchronous modelocking technique is discussed and a simple passive stabilisation scheme, coherent-photon-seeding (CPS), is described and applied to the synchronously-modelocked NaCl:OH laser. Results of a simulation of this laser are reported and a comparison is made with the practical observations of the stabilised laser. For the first time, theoretical and experimental evidence for the presence of high frequency pulse jitter in synchronously-pumped- modelocked (SPML) lasers is presented and the coherent photon seeding technique is shown to eliminate this noise. Details are also given for the construction of a compact, all-solid-state, femtosecond Cr4+:YAG laser. A design prescription for laser resonators having a high propensity for self-modelocking is presented and an unconventional 3-mirror resonator is adopted for optimised self-modelocked operation. Using this design, modelocked output powers up to 300 mW with 120 fs pulses from a compact, regeneratively initiated laser having a pulse repetition rate of 320 MHz is reported for 4.7W incident pump power. Self- modelocking is demonstrated for pump powers down to ~1W with this cavity design. A compact cavity design for self-modelocking is also assessed, with a footprint of just 20 X 25 cm, which places a prism in each cavity arm. 470 fs pulses at 220 mW average output power are reported.The design of a two-level solid-state maserCampbell, Colin Kyddhttps://hdl.handle.net/10023/147892019-04-01T10:07:28Z1960-07-01T00:00:00ZAbstract not Provided
1960-07-01T00:00:00ZCampbell, Colin KyddAbstract not ProvidedDiode-pumped IμM neodymium lasers and their internal frequency doublingYelland, Carlhttps://hdl.handle.net/10023/147882019-04-01T10:02:49Z1997-06-01T00:00:00ZIn this thesis the design, construction and performance of several diode-laser pumped continuous-wave neodymium lasers are described. These lasers were operated both around 1 mum and, by internal frequency-doubling, at 0.5 mum. The main emphasis has been on the assessment of the various laser designs with regard to their potential for efficient, high-power visible operation. A variety of pumping geometries, resonator configurations, gain media and internal frequency-doubling schemes were investigated, and their relative merits explored. Both side-pumping and end-pumping arrangements were employed, with Nd:YAG, Nd:YLF and Nd:YVO4 being used as gain media. Travelling-wave and standing-wave resonator designs were used. The polarisation-rotation effect in non- planar ring resonators was investigated and used to obtain single-frequency output. Single-frequency 0.5 mum powers up to 1.2 W were generated, and the highest 0.5 mum output power achieved was 4 W on two-longitudinal modes spaced by 450 MHz. The highest 1 mum output power achieved was 10 W, with a slope efficiency of 43%. Maximum pump powers for the lasers were in the region 15 - 35 W. A review of diode-laser pumped devices is included, with particular emphasis on the role of the spatial distributions of the pump and signal fields, because this is an important limiting factor in the performance of diode-pumped bulk laser systems. The criteria governing the harmonic output power when internally frequency-doubling are discussed. Issues relating to noise in the harmonic output, and techniques for its avoidance, are also discussed.
1997-06-01T00:00:00ZYelland, CarlIn this thesis the design, construction and performance of several diode-laser pumped continuous-wave neodymium lasers are described. These lasers were operated both around 1 mum and, by internal frequency-doubling, at 0.5 mum. The main emphasis has been on the assessment of the various laser designs with regard to their potential for efficient, high-power visible operation. A variety of pumping geometries, resonator configurations, gain media and internal frequency-doubling schemes were investigated, and their relative merits explored. Both side-pumping and end-pumping arrangements were employed, with Nd:YAG, Nd:YLF and Nd:YVO4 being used as gain media. Travelling-wave and standing-wave resonator designs were used. The polarisation-rotation effect in non- planar ring resonators was investigated and used to obtain single-frequency output. Single-frequency 0.5 mum powers up to 1.2 W were generated, and the highest 0.5 mum output power achieved was 4 W on two-longitudinal modes spaced by 450 MHz. The highest 1 mum output power achieved was 10 W, with a slope efficiency of 43%. Maximum pump powers for the lasers were in the region 15 - 35 W. A review of diode-laser pumped devices is included, with particular emphasis on the role of the spatial distributions of the pump and signal fields, because this is an important limiting factor in the performance of diode-pumped bulk laser systems. The criteria governing the harmonic output power when internally frequency-doubling are discussed. Issues relating to noise in the harmonic output, and techniques for its avoidance, are also discussed.Stable isotope dynamic labeling of secretomes (SIDLS) identifies authentic secretory proteins released by cancer and stromal cellsHammond, Dean EKumar, J. DineshRaymond, LornaSimpson, Deborah M.Beynon, Robert J.Dockray, Graham J.Varro, Andreahttps://hdl.handle.net/10023/147262023-04-25T23:53:40Z2018-06-18T00:00:00ZAnalysis of secretomes critically underpins the capacity to understand the mechanisms determining interactions between cells and between cells and their environment. In the context of cancer cell micro-environments, the relevant interactions are recognised to be an important determinant of tumor progression. Global proteomic analyses of secretomes are often performed at a single time point and frequently identify both classical secreted proteins (possessing an N-terminal signal sequence), as well as many intracellular proteins, the release of which is of uncertain biological significance. Here, we describe a mass spectrometry-based method for stable isotope dynamic labeling of secretomes (SIDLS) that, by dynamic SILAC, discriminates the secretion kinetics of classical secretory proteins and intracellular proteins released from cancer and stromal cells in culture. SIDLS is a robust classifier of the different cellular origins of proteins within the secretome and should be broadly applicable to non-proliferating cells and cells grown in short term culture.
Supported by a grant from North West Cancer Research.
2018-06-18T00:00:00ZHammond, Dean EKumar, J. DineshRaymond, LornaSimpson, Deborah M.Beynon, Robert J.Dockray, Graham J.Varro, AndreaAnalysis of secretomes critically underpins the capacity to understand the mechanisms determining interactions between cells and between cells and their environment. In the context of cancer cell micro-environments, the relevant interactions are recognised to be an important determinant of tumor progression. Global proteomic analyses of secretomes are often performed at a single time point and frequently identify both classical secreted proteins (possessing an N-terminal signal sequence), as well as many intracellular proteins, the release of which is of uncertain biological significance. Here, we describe a mass spectrometry-based method for stable isotope dynamic labeling of secretomes (SIDLS) that, by dynamic SILAC, discriminates the secretion kinetics of classical secretory proteins and intracellular proteins released from cancer and stromal cells in culture. SIDLS is a robust classifier of the different cellular origins of proteins within the secretome and should be broadly applicable to non-proliferating cells and cells grown in short term culture.Nuclear magnetic relaxation in ionic single crystalsSwanson, Kenneth M.https://hdl.handle.net/10023/147002019-04-01T10:04:31Z1958-01-01T00:00:00ZIt has been known since 1946 that energy can be absorbed from a radiofrequency field by the nuclear spins in bulk material placed in a uniform magnetic field. For this absorption of energy to be a continuous process it is necessary for the spins to have some thermal contact with the surrounding lattice, so that they can pass on the absorbed energy and then take part in further absorption. In general a nucleus can exchange energy with the lattice by means of interaction between its magnetic dipole moment and fluctuating magnetic fields supplied by the lattice, or by interaction between the electric quadrupole moment of the nucleus and fluctuating electric field supplied by the lattice. Either or both of these interactions can provide the relaxation mechanism which allows the nucleus to lose to the surrounding lattice the excess energy gained from the applied radiofrequency field in nuclear magnetic resonance absorption. This thesis describes a method of showing experimentally which of these interactions is dominant in providing the relaxation mechanism in cases where either mechanism can operate.
1958-01-01T00:00:00ZSwanson, Kenneth M.It has been known since 1946 that energy can be absorbed from a radiofrequency field by the nuclear spins in bulk material placed in a uniform magnetic field. For this absorption of energy to be a continuous process it is necessary for the spins to have some thermal contact with the surrounding lattice, so that they can pass on the absorbed energy and then take part in further absorption. In general a nucleus can exchange energy with the lattice by means of interaction between its magnetic dipole moment and fluctuating magnetic fields supplied by the lattice, or by interaction between the electric quadrupole moment of the nucleus and fluctuating electric field supplied by the lattice. Either or both of these interactions can provide the relaxation mechanism which allows the nucleus to lose to the surrounding lattice the excess energy gained from the applied radiofrequency field in nuclear magnetic resonance absorption. This thesis describes a method of showing experimentally which of these interactions is dominant in providing the relaxation mechanism in cases where either mechanism can operate.Nuclear magnetic resonance applied to the study of single crystalsHyndman, Danielhttps://hdl.handle.net/10023/146972019-04-01T10:09:02Z1955-01-01T00:00:00ZSince its discovery in 1946 over 400 papers have been published on nuclear magnetic resonance of which more than 50% are related to problems in the structure of matter. This fact illustrates the scope of this comparatively new technique in the field of structural investigations. In addition nuclear magnetic resonance can give information concerning molecular motion in the solid state. The work reported in this thesis is concerned with this field of research. An account is given of the application of nuclear magnetic resonance to the study of single crystals of Urea and Rochelle Salt.
1955-01-01T00:00:00ZHyndman, DanielSince its discovery in 1946 over 400 papers have been published on nuclear magnetic resonance of which more than 50% are related to problems in the structure of matter. This fact illustrates the scope of this comparatively new technique in the field of structural investigations. In addition nuclear magnetic resonance can give information concerning molecular motion in the solid state. The work reported in this thesis is concerned with this field of research. An account is given of the application of nuclear magnetic resonance to the study of single crystals of Urea and Rochelle Salt.Nuclear magnetic resonance in some solid hydrocarbonsEades, Robert G.https://hdl.handle.net/10023/146952019-04-01T10:03:25Z1952-01-01T00:00:00ZThe phenomenon of nuclear magnetic resonance is closely related to the molecular beam experiments and to microwave spectroscopy. Its significant feature is that the magnetic resonance principle, first applied to the molecular beam technique, has been extended to solids, liquids and gases in their normal physical states. In addition to providing yet another important method of measuring nuclear magnetic properties, this newer technique gives a means of investigating the establishment of the thermal equilibrium which is essential to the methods of obtaining very low temperatures; further, the resonance absorption spectrum yields information of crystal structures, phase transitions in solids and information about hindered rotation of molecules in solids. Thus the phenomenon can be used to study certain problems of the solid state. This thesis gives an account of such an application.
1952-01-01T00:00:00ZEades, Robert G.The phenomenon of nuclear magnetic resonance is closely related to the molecular beam experiments and to microwave spectroscopy. Its significant feature is that the magnetic resonance principle, first applied to the molecular beam technique, has been extended to solids, liquids and gases in their normal physical states. In addition to providing yet another important method of measuring nuclear magnetic properties, this newer technique gives a means of investigating the establishment of the thermal equilibrium which is essential to the methods of obtaining very low temperatures; further, the resonance absorption spectrum yields information of crystal structures, phase transitions in solids and information about hindered rotation of molecules in solids. Thus the phenomenon can be used to study certain problems of the solid state. This thesis gives an account of such an application.Nuclear magnetic resonance applied to problems of molecular motion in solidsRushworth, F. A.https://hdl.handle.net/10023/146942019-04-01T10:05:46Z1953-01-01T00:00:00ZThe discovery of nuclear magnetic resonance in bulk matter in 1945 has led to advances in several branches of physics. One of the most interesting applications of these new techniques has been to problems of the solid state, where information about molecular motion and thermal relaxation effects can be obtained. It is with this field of research that the nuclear magnetic resonance absorption experiments reported in this thesis are concerned.
The general theory of nuclear magnetic absorption is developed first and the relevant apparatus and experiment methods are then described. The design and performance of a large permanent magnet made especially for nuclear resonance work is considered and experiments on suitable solids are reported and discussed in the final section.
1953-01-01T00:00:00ZRushworth, F. A.The discovery of nuclear magnetic resonance in bulk matter in 1945 has led to advances in several branches of physics. One of the most interesting applications of these new techniques has been to problems of the solid state, where information about molecular motion and thermal relaxation effects can be obtained. It is with this field of research that the nuclear magnetic resonance absorption experiments reported in this thesis are concerned.
The general theory of nuclear magnetic absorption is developed first and the relevant apparatus and experiment methods are then described. The design and performance of a large permanent magnet made especially for nuclear resonance work is considered and experiments on suitable solids are reported and discussed in the final section.The effect of volume changes on the order-disorder transition in substitutional alloysRoss, Archie Walterhttps://hdl.handle.net/10023/146912019-04-01T10:02:27Z1957-01-01T00:00:00Z1957-01-01T00:00:00ZRoss, Archie WalterThe nuclear polarization of gold 198 in dilute solution in gadolinium metalLeitch, Norman Mathiesonhttps://hdl.handle.net/10023/146872019-04-01T10:08:31Z1964-01-01T00:00:00ZIn 1960, Samoilov, Sklyarevskii and Stepanovil reported a series of experiments on the nucles polarization of the nuclei of diamagnetic elements in dilute solution in magnetically saturated iron. These experiments were successful attempts to obtain nuclear polarization of the gamma-ray emitting nuclei of indium, antimony, and gold in dilute solution of iron. The specimens were cooled to about 0.03°K bt adiabatic demagnetization of a pill of a paramagnetic salt in thermal contact with them, and were magnetized to saturation. Nuclear orientation in the specimens was detected by anisotropy in the gamma-ray emission which was measured by two scintillation counters arranged parallel and perpendicular to the direction of magnetization of the specimens. They concluded that nuclear orientation was due to interaction of the nuclei with a strong internal magnetic field within the alloy, and from the anisotropy of the gamma-ray emission they determined the order of magnitudes of the magnetic fields at these nuclei. Effective magnetic fields of the order of 7x10⁵ gauss were estimated to act on the AU¹⁹⁸ nuclei in iron where the magnetic 3d shell is close to the surface of the atom.
It was our intention to determine whether the deeply imbedded 4f electron shell of gadolinium could produce such large hyperfine field as would polarize the gold nuclei in a dilute alloy of gold in gadolinium. A series of experiments on the nuclear polarization of gold in gadolinium by the method of Samoilov are reported in this thesis. A review of the current state of the theory of internal magnetic fields in ferromagnetic materials is given in Chapter III. Much of the theoretical work reviewed here was published after the commencement of the work described in this thesis, and would certainly have suggested other experiments to be performed in this field, had it been published earlier. We intended also, as part of this series of experiments, to investigate the nuclear polarization of the nuclei of suitable gadolinium isotopes in pure gadolinium metal in order to have complementary estimates of the internal magnetic field in gadolinium.
It was essential to use specimens in the form of thin discs for two reasons. It was desirable to decrease the demagnetization factor of the specimens, and it was necessary to have them circular in section so that the angular distribution of gamma radiation from a warm sample should be isotropic. The specimens were therefore punched from foils of the metal. Unfortunately, the bulk metal proved to be too brittle to roll them into thin sheets at room temperature. We did not have facilities for heat treatment of the metal, and delivery dates for samples in laminar form were too long to allow the work to be completed in time.
We decided also to carry out an experiment on the nuclear orientation of Co⁶⁰ in a 50% Co-Ni alloy to test the apparatus and the results are given in chapter IX for interest and are compared with results obtained by other investigators.
1964-01-01T00:00:00ZLeitch, Norman MathiesonIn 1960, Samoilov, Sklyarevskii and Stepanovil reported a series of experiments on the nucles polarization of the nuclei of diamagnetic elements in dilute solution in magnetically saturated iron. These experiments were successful attempts to obtain nuclear polarization of the gamma-ray emitting nuclei of indium, antimony, and gold in dilute solution of iron. The specimens were cooled to about 0.03°K bt adiabatic demagnetization of a pill of a paramagnetic salt in thermal contact with them, and were magnetized to saturation. Nuclear orientation in the specimens was detected by anisotropy in the gamma-ray emission which was measured by two scintillation counters arranged parallel and perpendicular to the direction of magnetization of the specimens. They concluded that nuclear orientation was due to interaction of the nuclei with a strong internal magnetic field within the alloy, and from the anisotropy of the gamma-ray emission they determined the order of magnitudes of the magnetic fields at these nuclei. Effective magnetic fields of the order of 7x10⁵ gauss were estimated to act on the AU¹⁹⁸ nuclei in iron where the magnetic 3d shell is close to the surface of the atom.
It was our intention to determine whether the deeply imbedded 4f electron shell of gadolinium could produce such large hyperfine field as would polarize the gold nuclei in a dilute alloy of gold in gadolinium. A series of experiments on the nuclear polarization of gold in gadolinium by the method of Samoilov are reported in this thesis. A review of the current state of the theory of internal magnetic fields in ferromagnetic materials is given in Chapter III. Much of the theoretical work reviewed here was published after the commencement of the work described in this thesis, and would certainly have suggested other experiments to be performed in this field, had it been published earlier. We intended also, as part of this series of experiments, to investigate the nuclear polarization of the nuclei of suitable gadolinium isotopes in pure gadolinium metal in order to have complementary estimates of the internal magnetic field in gadolinium.
It was essential to use specimens in the form of thin discs for two reasons. It was desirable to decrease the demagnetization factor of the specimens, and it was necessary to have them circular in section so that the angular distribution of gamma radiation from a warm sample should be isotropic. The specimens were therefore punched from foils of the metal. Unfortunately, the bulk metal proved to be too brittle to roll them into thin sheets at room temperature. We did not have facilities for heat treatment of the metal, and delivery dates for samples in laminar form were too long to allow the work to be completed in time.
We decided also to carry out an experiment on the nuclear orientation of Co⁶⁰ in a 50% Co-Ni alloy to test the apparatus and the results are given in chapter IX for interest and are compared with results obtained by other investigators.Nuclear spin-lattice relaxation times in metalsBrown, Davidhttps://hdl.handle.net/10023/146812019-04-01T10:10:34Z1970-01-01T00:00:00ZThis thesis is a report of an experimental and theoretical investigation of dipolar spin-lattice relaxation times in pure metals. Both the usual Zeeman as well as the dipolar relaxation times were measured as a function of temperature in Al⁸⁰ Cu, V, Cd⁸⁰ and Pt. The metals Al, Cu and V all have nuclear spin > ½; so they show strong quadrupole effects which complicate the analysis. Non-exponential spin-lattice decays are observed in these metals. A model explaining this and leading to the elucidation of the true dipolar relaxation time is presented. These complications are not present for Cd and Pt since they both have nuclear spin ½; and hence no quadrupole moment. In these metals however the dipolar relaxation is strongly influenced by the presence of indirect nuclear-nuclear couplings. These measurements require the use of a phase-coherent pulse spectrometer capable of measuring spin-lattice relaxation times over a wide temperature range. A suitable apparatus and the experimental techniques are described. The parameter discussed in the relevant theories of dipolar relaxation is δ, the ratio of Zeeman to dipolar relaxation times. The following values were found.
Al δ = 2.15 ± .07; Cu δ= 2.08 ± .15; V δ = 2.15 ± .20; Pt δ = 1.28 ± .07; Cd δ = 1.43 ± .15 The overlap with previous investigations concerned the metals A1 and Cu. The results reported here are in considerably better agreement with theory. The general characteristic of the results is the need to invoke electron-electron interactions in an explanation of the values of 5. The measurements in Pt and V are difficult to interpret because their dominant relaxation mechanisms are not discussed in existing theories. A theory of nuclear relaxation which considers the effects of a δ-function Interaction between electrons partially explains the remaining results but a residual discrepancy exists in all cases, This may be due to the restrictive assumptions of the theory which make It relevant only to simple metals" Of the metals investigated here Cu approaches the requirements most closely. However application of the theory to a simple metal such as Na still leaves a discrepancy between predicted and measured values of B. The effects of introducing a finite range to the electron-electron interaction are discussed. It appears that failure to fully explain the results is due in part to the inherent inadequacies of existing theories and in part to the complicated electronic structures of the metals Investigated which make the formulation of a more general theory very difficult.
1970-01-01T00:00:00ZBrown, DavidThis thesis is a report of an experimental and theoretical investigation of dipolar spin-lattice relaxation times in pure metals. Both the usual Zeeman as well as the dipolar relaxation times were measured as a function of temperature in Al⁸⁰ Cu, V, Cd⁸⁰ and Pt. The metals Al, Cu and V all have nuclear spin > ½; so they show strong quadrupole effects which complicate the analysis. Non-exponential spin-lattice decays are observed in these metals. A model explaining this and leading to the elucidation of the true dipolar relaxation time is presented. These complications are not present for Cd and Pt since they both have nuclear spin ½; and hence no quadrupole moment. In these metals however the dipolar relaxation is strongly influenced by the presence of indirect nuclear-nuclear couplings. These measurements require the use of a phase-coherent pulse spectrometer capable of measuring spin-lattice relaxation times over a wide temperature range. A suitable apparatus and the experimental techniques are described. The parameter discussed in the relevant theories of dipolar relaxation is δ, the ratio of Zeeman to dipolar relaxation times. The following values were found.
Al δ = 2.15 ± .07; Cu δ= 2.08 ± .15; V δ = 2.15 ± .20; Pt δ = 1.28 ± .07; Cd δ = 1.43 ± .15 The overlap with previous investigations concerned the metals A1 and Cu. The results reported here are in considerably better agreement with theory. The general characteristic of the results is the need to invoke electron-electron interactions in an explanation of the values of 5. The measurements in Pt and V are difficult to interpret because their dominant relaxation mechanisms are not discussed in existing theories. A theory of nuclear relaxation which considers the effects of a δ-function Interaction between electrons partially explains the remaining results but a residual discrepancy exists in all cases, This may be due to the restrictive assumptions of the theory which make It relevant only to simple metals" Of the metals investigated here Cu approaches the requirements most closely. However application of the theory to a simple metal such as Na still leaves a discrepancy between predicted and measured values of B. The effects of introducing a finite range to the electron-electron interaction are discussed. It appears that failure to fully explain the results is due in part to the inherent inadequacies of existing theories and in part to the complicated electronic structures of the metals Investigated which make the formulation of a more general theory very difficult.The behaviour of ions in the presence of the liquid vapour interface in heliumKennedy, Stuart G.https://hdl.handle.net/10023/146772019-04-01T10:08:39Z1972-01-01T00:00:00ZThe thesis is concerned with some of the properties of positive and negative ions in liquid Helium, and in particular with the passage of ions, which are generated inside the liquid, through the free liquid surface into vapour. It was found, for negative ions, that this process was inhibited by an energy barrier, in agreement with other workers who have examined this problem, although there is considerable disagreement as to the magnitude of the barrier. It has been observed in the present work that the energy barrier depends upon the field, the position of the liquid surface in an ion cell, and the nature of the ion cell itself. The dependence of the barrier on these three parameters probably explains why diverse values for it have been reported. It was originally intended to use the ions as probes to examine dissipation mechanisms in the mobile superfluid Helium film. It had been reliably reported that ions preferentially travelled in the film. This was observed not to be the case however. The negative ion currents sin the film were found to be extremely small, when they existed at all, and no real positive ion currents in the film could be detected. This has been interpreted as being due to the combination of the large image potential binding the ion to the substrate, and the intrinsic roughness of the substrate. The present work has revealed that what appears at first sight to be a current of positive ions crossing the liquid surface can be attributed to a current of photoelectrons due to photoelectric emission from the surface of the collector. Such photoemission can arise from the uv radiation produced largely by ionic recombination in the region close to the a-emitting source. Screening the collector from the direct view of the a-source greatly reduced the photoelectron current. In order to extend the temperature range available, for the study of ion currents, a vortex refrigerator was designed and constructed. This enabled measurements to be extended down to 0.8K. Since there has been only one publication on the vortex refrigerator, which appeared during the building of the present one, its properties and operation with various design parameters were studied in detail.
1972-01-01T00:00:00ZKennedy, Stuart G.The thesis is concerned with some of the properties of positive and negative ions in liquid Helium, and in particular with the passage of ions, which are generated inside the liquid, through the free liquid surface into vapour. It was found, for negative ions, that this process was inhibited by an energy barrier, in agreement with other workers who have examined this problem, although there is considerable disagreement as to the magnitude of the barrier. It has been observed in the present work that the energy barrier depends upon the field, the position of the liquid surface in an ion cell, and the nature of the ion cell itself. The dependence of the barrier on these three parameters probably explains why diverse values for it have been reported. It was originally intended to use the ions as probes to examine dissipation mechanisms in the mobile superfluid Helium film. It had been reliably reported that ions preferentially travelled in the film. This was observed not to be the case however. The negative ion currents sin the film were found to be extremely small, when they existed at all, and no real positive ion currents in the film could be detected. This has been interpreted as being due to the combination of the large image potential binding the ion to the substrate, and the intrinsic roughness of the substrate. The present work has revealed that what appears at first sight to be a current of positive ions crossing the liquid surface can be attributed to a current of photoelectrons due to photoelectric emission from the surface of the collector. Such photoemission can arise from the uv radiation produced largely by ionic recombination in the region close to the a-emitting source. Screening the collector from the direct view of the a-source greatly reduced the photoelectron current. In order to extend the temperature range available, for the study of ion currents, a vortex refrigerator was designed and constructed. This enabled measurements to be extended down to 0.8K. Since there has been only one publication on the vortex refrigerator, which appeared during the building of the present one, its properties and operation with various design parameters were studied in detail.On the thermo-electric power and thermal conductivity of semi-conductivity of semi-conductors and metalsNeaves, Angushttps://hdl.handle.net/10023/146732019-04-01T10:08:32Z1955-01-01T00:00:00ZThe first successful attempt to explain the electrical and thermal properties of metals was made by the Drude-Lorentz theory at the beginning of this century. According to Drude, certain electrons were free to move from atom to atom throughout the metal and it was those electrons which undertook the conduction of electricity and heat. The electrons were then treated as a “gas” and in order to apply the statistical theory of gases to such an electron cloud Lorentz postulated his well-known assumptions. The theory was immediately successful in the derivation of the Wiedemann-Franz law, relating the electrical and thermal conductivities. The major drawback to the theory lay in the evaluation of the electron specific heat. Lorentz had ascribed to the electrons a Maxwell distribution of velocities, the only reasonable choice at that time. On such a picture the electron specific heat was large and such an addition to the specific heat completely destroyed the agreement of Debye’s theory with the experimentally observed specific heats.
The theory remained in this state until the discovery by Pauli, in 1925, of the Pauli Exclusion Principle. In its simplest form the principle states that in an atom not more than two electrons can have the same three quantum numbers. This allowed Dirac and Fermi, working independently, to develop the statistics of particles obeying such a principle and gave birth to the Fermi-Dirac statistics. The use of new statistics enabled the discrepancy in the specific heats to be explained. Pauli was able to account for the paramagnetism of the alkali metals and it was left to Sommerfeld to consider the problems of transport phenomena in the light of the Fermi-Dirac statistics.
Such were the foundations of the modern electron theory of metals. The modern development of the theory was begun by Block, Sommerfeld, Bether, Peierls and Wilson. It is based on the quantum-mechanical analysis of the motion of an electron in the periodic field of a crystal lattice. Considered from this point of view, the electrons in a metal are distributed over a number of allowed energy bands, forbidden bands occurring in the regions between the allowed energies. Most of these allowed bands are filled completely by the electrons, and it is only the electrons which are contained in incompletely filled bands which contribute to the resultant current. It is these electrons which are regarded as “free” in conduction theory. This picture of a metal also enables us to obtain a better understanding of the “mean free path” of an electron, a quantity which is treated as an arbitrary parameter in the Sommerfeld theory.
1955-01-01T00:00:00ZNeaves, AngusThe first successful attempt to explain the electrical and thermal properties of metals was made by the Drude-Lorentz theory at the beginning of this century. According to Drude, certain electrons were free to move from atom to atom throughout the metal and it was those electrons which undertook the conduction of electricity and heat. The electrons were then treated as a “gas” and in order to apply the statistical theory of gases to such an electron cloud Lorentz postulated his well-known assumptions. The theory was immediately successful in the derivation of the Wiedemann-Franz law, relating the electrical and thermal conductivities. The major drawback to the theory lay in the evaluation of the electron specific heat. Lorentz had ascribed to the electrons a Maxwell distribution of velocities, the only reasonable choice at that time. On such a picture the electron specific heat was large and such an addition to the specific heat completely destroyed the agreement of Debye’s theory with the experimentally observed specific heats.
The theory remained in this state until the discovery by Pauli, in 1925, of the Pauli Exclusion Principle. In its simplest form the principle states that in an atom not more than two electrons can have the same three quantum numbers. This allowed Dirac and Fermi, working independently, to develop the statistics of particles obeying such a principle and gave birth to the Fermi-Dirac statistics. The use of new statistics enabled the discrepancy in the specific heats to be explained. Pauli was able to account for the paramagnetism of the alkali metals and it was left to Sommerfeld to consider the problems of transport phenomena in the light of the Fermi-Dirac statistics.
Such were the foundations of the modern electron theory of metals. The modern development of the theory was begun by Block, Sommerfeld, Bether, Peierls and Wilson. It is based on the quantum-mechanical analysis of the motion of an electron in the periodic field of a crystal lattice. Considered from this point of view, the electrons in a metal are distributed over a number of allowed energy bands, forbidden bands occurring in the regions between the allowed energies. Most of these allowed bands are filled completely by the electrons, and it is only the electrons which are contained in incompletely filled bands which contribute to the resultant current. It is these electrons which are regarded as “free” in conduction theory. This picture of a metal also enables us to obtain a better understanding of the “mean free path” of an electron, a quantity which is treated as an arbitrary parameter in the Sommerfeld theory.Some magneto-dynamic properties of type II superconductorsChou, Chan Shinhttps://hdl.handle.net/10023/146712019-04-01T10:09:17Z1978-01-01T00:00:00ZThe possibility of making a refrigerator by mechanically induced flux flow to transfer heat in a type II superconductor has been examined. Heat generation due to flux flow was sufficient to destroy the cooling effect over most of our experimental temperature range, although some cooling was obtained near T[sub]c. The dissipation due to flux pinning was determined by the pinning strength of the specimen. The pinning forces in Nb and Pb-In were determined by mechanically sweeping a magnetic field over them and measuring the resulting force. Three different forces due to the interaction of the magnetic field, with (a) the trapped flux lines, (b) the Meissner screening current, and (c) the pinning sites of the specimen, were identified. The magnitude of the force due to the screening current was proportional to the field strength, and was reduced by the trapped, flux lines inside the specimen. The magnitude of the bulk pinning force was in agreement with that calculated from the magnetization via the Irie-Yamafujii model; F[sub]p = 𝛼B<p>[super] 𝛾. The dependence of the pinning force and the magnetic properties on the temperature and on the surface treatment of the specimen was also studied. The temperature dependence of the pinning force was found to be given by F[sub]p(T) = F[sub]p(o)(1 -T/T[sub]c ) it is suggested that this can be understood in terms of Anderson's flux creep model. The critical current of various specimens was obtained from the pinning force and the magnetization. In applying the Irie-Yamafuji model, the demagnetization and the surface effects of a specimen had to be taken into account, and methods of doing these have been suggested and experimentally checked. The role of the magneto-caloric effect on the mixed state specific heat was discussed. A 'two-fluid' type equation was proposed and used to calculate the mixed state specific heat. The results were compared with the experimental measurements. With the above experiments, the dissipation associated with flux flow was studied in terms of the pinning and viscous forces. It was shown that dissipation due to pinning was dominant in low fields and that viscous forces gradually became important as the field increased. It was found that the motion of the flux lattice inside a type II superconductor could not be induced by moving the source of a uniform magnetic field. This showed that the concept of 'lines of flux' has no meaning for the case of a uniform field.
1978-01-01T00:00:00ZChou, Chan ShinThe possibility of making a refrigerator by mechanically induced flux flow to transfer heat in a type II superconductor has been examined. Heat generation due to flux flow was sufficient to destroy the cooling effect over most of our experimental temperature range, although some cooling was obtained near T[sub]c. The dissipation due to flux pinning was determined by the pinning strength of the specimen. The pinning forces in Nb and Pb-In were determined by mechanically sweeping a magnetic field over them and measuring the resulting force. Three different forces due to the interaction of the magnetic field, with (a) the trapped flux lines, (b) the Meissner screening current, and (c) the pinning sites of the specimen, were identified. The magnitude of the force due to the screening current was proportional to the field strength, and was reduced by the trapped, flux lines inside the specimen. The magnitude of the bulk pinning force was in agreement with that calculated from the magnetization via the Irie-Yamafujii model; F[sub]p = 𝛼B<p>[super] 𝛾. The dependence of the pinning force and the magnetic properties on the temperature and on the surface treatment of the specimen was also studied. The temperature dependence of the pinning force was found to be given by F[sub]p(T) = F[sub]p(o)(1 -T/T[sub]c ) it is suggested that this can be understood in terms of Anderson's flux creep model. The critical current of various specimens was obtained from the pinning force and the magnetization. In applying the Irie-Yamafuji model, the demagnetization and the surface effects of a specimen had to be taken into account, and methods of doing these have been suggested and experimentally checked. The role of the magneto-caloric effect on the mixed state specific heat was discussed. A 'two-fluid' type equation was proposed and used to calculate the mixed state specific heat. The results were compared with the experimental measurements. With the above experiments, the dissipation associated with flux flow was studied in terms of the pinning and viscous forces. It was shown that dissipation due to pinning was dominant in low fields and that viscous forces gradually became important as the field increased. It was found that the motion of the flux lattice inside a type II superconductor could not be induced by moving the source of a uniform magnetic field. This showed that the concept of 'lines of flux' has no meaning for the case of a uniform field.Destruction of superconductivity by a currentMukherjee, Benoy Kumarhttps://hdl.handle.net/10023/146702019-04-01T10:09:55Z1971-01-01T00:00:00ZWe have carried out an experimental investigation of the resistance transition in Indium and Thallium wires when superconductivity is destroyed by a current. Our results, as well as those obtained previously by other workers, do not agree with the theories put forward by London (1937) and Gorter (1957) and the consideration of secondary effects does not satisfactorily account for the discrepancies. We present a new model of the intermediate state in Type-I current-carrying superconductors. In addition to predicting a resistance transition in reasonable agreement with experimental observations, the model gives good agreement with experimental values of the radius of the intermediate state core as obtained by Rinderer (1956). A treatment of secondary effects is also given and together with the basic resistance transition predicted by the model, they provide a better understanding of the destruction of superconductivity in Type-I wires by a current.
1971-01-01T00:00:00ZMukherjee, Benoy KumarWe have carried out an experimental investigation of the resistance transition in Indium and Thallium wires when superconductivity is destroyed by a current. Our results, as well as those obtained previously by other workers, do not agree with the theories put forward by London (1937) and Gorter (1957) and the consideration of secondary effects does not satisfactorily account for the discrepancies. We present a new model of the intermediate state in Type-I current-carrying superconductors. In addition to predicting a resistance transition in reasonable agreement with experimental observations, the model gives good agreement with experimental values of the radius of the intermediate state core as obtained by Rinderer (1956). A treatment of secondary effects is also given and together with the basic resistance transition predicted by the model, they provide a better understanding of the destruction of superconductivity in Type-I wires by a current.Some properties of the dynamic intermediate state in type 1 superconductorsLerski, Richard A.https://hdl.handle.net/10023/146692019-04-01T10:03:21Z1974-01-01T00:00:00ZThe high resolution magneto-optic method using the Faraday effect in thin films of EuS:EuF₂has been used to observe the dynamic intermediate state induced by the passage of an electric current or a heat current through thin slabs of the superconductors Pb, In and Sn. The ease with which the various intermediate state topologies could be made to move has been studied and several features of the interaction of moving flux with pinning sites have been noted. In the case of the current induced motion the measured characteristics of flux flow velocity versus current have been found to exhibit two distinct regions. Firstly, a linear region where the observed velocity was found to agree reasonably well with the predictions of the recent general theory of Andreev and Dzhikaev when allowance was made for the effects of pinning by the introduction of a velocity independent pinning force. Secondly, a curved region was found for currents close to the critical current J₀ in agreement with earlier work using other methods of observation. Possible reasons for the existence of this curvature were examined in detail, and. it was found, that a phenomenological model based, on the presence of a Gaussian distribution of critical current values throughout the sample could account satisfactorily for the observations. The presence of such a Gaussian distribution was confirmed by observing the variations in distance travelled by a domain subjected, to a pulsed, driving current. The curvature was found in disagreement with the theory of thermal activation and. no evidence could be found, for the presence of a velocity dependent pinning force. In the case of the thermally induced, motion which was investigated only in Pb, it was found, in agreement with the very recent work of Laeng and Rinderer, that there exist two competing mechanisms driving the flux. The first of these, that treated, by Andreev and Dzhikaev in their general theory, which acts in a direction parallel to the heat flow, was found to be effective at low temperatures (T ≪ 4.2K) but to be negligible at high temperatures. The magnitude of the velocity produced by this mechanism agreed reasonably well with the theory at low temperatures but was in complete disagreement at high temperatures when the theory predicts that it should still be observable. The second mechanism which acts perpendicularly to the heat flow dominated the motion at high temperatures but its magnitude did not agree with the predictions of the recent theory of Rothen, which ascribes the effect to the thermoelectric power of the normal state. It should be noted that to overcome the pinning all of the low temperature observations (T ≪ 4.2k) were performed in the presence of an electric current. Clearly, much more work requires to be done to clarify these observations.
1974-01-01T00:00:00ZLerski, Richard A.The high resolution magneto-optic method using the Faraday effect in thin films of EuS:EuF₂has been used to observe the dynamic intermediate state induced by the passage of an electric current or a heat current through thin slabs of the superconductors Pb, In and Sn. The ease with which the various intermediate state topologies could be made to move has been studied and several features of the interaction of moving flux with pinning sites have been noted. In the case of the current induced motion the measured characteristics of flux flow velocity versus current have been found to exhibit two distinct regions. Firstly, a linear region where the observed velocity was found to agree reasonably well with the predictions of the recent general theory of Andreev and Dzhikaev when allowance was made for the effects of pinning by the introduction of a velocity independent pinning force. Secondly, a curved region was found for currents close to the critical current J₀ in agreement with earlier work using other methods of observation. Possible reasons for the existence of this curvature were examined in detail, and. it was found, that a phenomenological model based, on the presence of a Gaussian distribution of critical current values throughout the sample could account satisfactorily for the observations. The presence of such a Gaussian distribution was confirmed by observing the variations in distance travelled by a domain subjected, to a pulsed, driving current. The curvature was found in disagreement with the theory of thermal activation and. no evidence could be found, for the presence of a velocity dependent pinning force. In the case of the thermally induced, motion which was investigated only in Pb, it was found, in agreement with the very recent work of Laeng and Rinderer, that there exist two competing mechanisms driving the flux. The first of these, that treated, by Andreev and Dzhikaev in their general theory, which acts in a direction parallel to the heat flow, was found to be effective at low temperatures (T ≪ 4.2K) but to be negligible at high temperatures. The magnitude of the velocity produced by this mechanism agreed reasonably well with the theory at low temperatures but was in complete disagreement at high temperatures when the theory predicts that it should still be observable. The second mechanism which acts perpendicularly to the heat flow dominated the motion at high temperatures but its magnitude did not agree with the predictions of the recent theory of Rothen, which ascribes the effect to the thermoelectric power of the normal state. It should be noted that to overcome the pinning all of the low temperature observations (T ≪ 4.2k) were performed in the presence of an electric current. Clearly, much more work requires to be done to clarify these observations.AC losses in superconducting niobiumAl-Huseini, Fahad A.https://hdl.handle.net/10023/146662019-04-01T10:09:52Z1973-01-01T00:00:00ZThis thesis is concerned with measurements of alternating field losses in cylindrical rods of niobium in its superconducting state. Six samples have been investigated each with a different surface condition, Magnetization measurements of the samples have been studied which show the effect of the surface condition on the first critical magnetic field H[sub]c₁. The effect of the surface on flux pinning are discussed. Power losses have been studied experimentally for single crystal niobium and for polycrystalline niobium each with three different surface conditions in order to study the effect of the surface properties on these ac losses. A wattmeter technique was developed for measurements of power losses in the samples. Experiments were conducted over a range of applied fields at 50 Hz and at 4.2 K. This technique will be described together with the experimental results. The results will be compared with predictions from a modified critical state theory and their significance discussed. The critical current density has been measured for each sample using a method based on the measurements of the power dissipation in a small modulating field, in the presence of a steady bias field. The technique will be described together with experimental results. The shielding field △H in the mixed state has been studied for the samples. The technique and the results will be discussed for each sample.
1973-01-01T00:00:00ZAl-Huseini, Fahad A.This thesis is concerned with measurements of alternating field losses in cylindrical rods of niobium in its superconducting state. Six samples have been investigated each with a different surface condition, Magnetization measurements of the samples have been studied which show the effect of the surface condition on the first critical magnetic field H[sub]c₁. The effect of the surface on flux pinning are discussed. Power losses have been studied experimentally for single crystal niobium and for polycrystalline niobium each with three different surface conditions in order to study the effect of the surface properties on these ac losses. A wattmeter technique was developed for measurements of power losses in the samples. Experiments were conducted over a range of applied fields at 50 Hz and at 4.2 K. This technique will be described together with the experimental results. The results will be compared with predictions from a modified critical state theory and their significance discussed. The critical current density has been measured for each sample using a method based on the measurements of the power dissipation in a small modulating field, in the presence of a steady bias field. The technique will be described together with experimental results. The shielding field △H in the mixed state has been studied for the samples. The technique and the results will be discussed for each sample.Vibrational and electronic properties of impurities in semiconductorsNoras, James M.https://hdl.handle.net/10023/146652019-04-01T10:07:39Z1978-01-01T00:00:00ZProperties of impurities in semiconductors have been investigated by means of absorption spectroscopy. The studies have concentrated not on one topic but on a range of types of impurity behaviour: they may be summarized under three main headings. (1) Fine Structure in Absorption Bands. The shapes and temperature-dependent widths of absorption bands of impurities may be understood qualitatively in terms of vibrational coupling with the lattice. The spectra studied show evidence of Jahn-Teller coupling, one particular form of such an interaction, and it has been found possible in most cases to relate the fine structure observed to features in the phonon spectra of the host lattices. (2) Antiresonances. Since the materials studied are such that in some cases a clear and complete ascription of spectral fine structure can be made in terms of Jahn-Teller coupling, it is possible unambiguously to identify certain other features not previously observed in any systems. These are vibronic anti-resonances, resulting from interference between vibrational levels of the lattice and impurity states of mixed electronic-vibrational nature. In the absence of adequate models, these features are discussed phenomeno- logically. An electronic analogue of these effects, involving spontaneous ionization of excited impurities, has been looked for, but no convincing instances have been found, (3) Photoionization. The charge state and electronic configuration of an impurity depend not only on bonding requirements at the lattice site, but on other factors such as the position of the Fermi level in the bulk medium, and may be changed by photo-excitation. Electrons or holes being removed from ar. impurity and released into crystal bancs results in intense absorption. The form of the absorption cross-section for these processes follows a simple power law near the threshold of ionization, so the value of the ionization energy may be determined quite readily. In this way the positions of a deep level of nickel in zinc selenide and in zinc sulphide have been determined with respect to the conduction and valence bands of these materials. This important information has not been obtained directly for any other transition metal impurity. In addition, fine structure has been seen near the ionization thresholds in these materials. This is due to the coulomb potential of the ionized centre, charged with respect to the lattice, being able to hind holes in hydrogenic orbits. The phonon coupling to this shallow level is found to be much stronger than is observed in absorption bands due to photoionization.
1978-01-01T00:00:00ZNoras, James M.Properties of impurities in semiconductors have been investigated by means of absorption spectroscopy. The studies have concentrated not on one topic but on a range of types of impurity behaviour: they may be summarized under three main headings. (1) Fine Structure in Absorption Bands. The shapes and temperature-dependent widths of absorption bands of impurities may be understood qualitatively in terms of vibrational coupling with the lattice. The spectra studied show evidence of Jahn-Teller coupling, one particular form of such an interaction, and it has been found possible in most cases to relate the fine structure observed to features in the phonon spectra of the host lattices. (2) Antiresonances. Since the materials studied are such that in some cases a clear and complete ascription of spectral fine structure can be made in terms of Jahn-Teller coupling, it is possible unambiguously to identify certain other features not previously observed in any systems. These are vibronic anti-resonances, resulting from interference between vibrational levels of the lattice and impurity states of mixed electronic-vibrational nature. In the absence of adequate models, these features are discussed phenomeno- logically. An electronic analogue of these effects, involving spontaneous ionization of excited impurities, has been looked for, but no convincing instances have been found, (3) Photoionization. The charge state and electronic configuration of an impurity depend not only on bonding requirements at the lattice site, but on other factors such as the position of the Fermi level in the bulk medium, and may be changed by photo-excitation. Electrons or holes being removed from ar. impurity and released into crystal bancs results in intense absorption. The form of the absorption cross-section for these processes follows a simple power law near the threshold of ionization, so the value of the ionization energy may be determined quite readily. In this way the positions of a deep level of nickel in zinc selenide and in zinc sulphide have been determined with respect to the conduction and valence bands of these materials. This important information has not been obtained directly for any other transition metal impurity. In addition, fine structure has been seen near the ionization thresholds in these materials. This is due to the coulomb potential of the ionized centre, charged with respect to the lattice, being able to hind holes in hydrogenic orbits. The phonon coupling to this shallow level is found to be much stronger than is observed in absorption bands due to photoionization.Studies relating to cyclotron emission from semiconductorsCowan, David A.https://hdl.handle.net/10023/146642019-04-01T10:05:00Z1985-01-01T00:00:00ZIn this thesis studies relating to cyclotron emission in GaAs, InSb, InP and CdₓHg₁- ₓTe (C.M.T.) are described. The effect of the electric field bias on the emission intensity and linewidth in GaAs and InP is studied and a theoretical model is proposed. The study indicates the importance of nonparabolicity and polar mode scattering in determining the intensity and linewidth in these materials. Inter-excited state transitions are observed for the first time in emission- Nonparabolicity splitting is observed in InP and that in GaAs, originally observed in by Gornik (1983),is confirmed. The application of hydrostatic pressure to InSb is observed to narrow the emission line by a factor of three. In experiments on the magnetoimpurity effect in GaAs transitions between the 1S and 2P_, 2P₀, 3D-₂ and 3D-₁ impurity states are observed and central cell structure is observed for the first time. In bulk C.M.T. cyclotron resonance is studied as a function of hydrostatic pressure and reasonable agreement is achieved with accepted values for the variation of band gap with pressure. In the surface layer cyclotron resonance is observed as are Shubnikov de Haas oscillations and electric subband energies are calculated for each of these experiments. Finally the use of cyclotron emission as a spectroscopic source is demonstrated and future developments discussed.
1985-01-01T00:00:00ZCowan, David A.In this thesis studies relating to cyclotron emission in GaAs, InSb, InP and CdₓHg₁- ₓTe (C.M.T.) are described. The effect of the electric field bias on the emission intensity and linewidth in GaAs and InP is studied and a theoretical model is proposed. The study indicates the importance of nonparabolicity and polar mode scattering in determining the intensity and linewidth in these materials. Inter-excited state transitions are observed for the first time in emission- Nonparabolicity splitting is observed in InP and that in GaAs, originally observed in by Gornik (1983),is confirmed. The application of hydrostatic pressure to InSb is observed to narrow the emission line by a factor of three. In experiments on the magnetoimpurity effect in GaAs transitions between the 1S and 2P_, 2P₀, 3D-₂ and 3D-₁ impurity states are observed and central cell structure is observed for the first time. In bulk C.M.T. cyclotron resonance is studied as a function of hydrostatic pressure and reasonable agreement is achieved with accepted values for the variation of band gap with pressure. In the surface layer cyclotron resonance is observed as are Shubnikov de Haas oscillations and electric subband energies are calculated for each of these experiments. Finally the use of cyclotron emission as a spectroscopic source is demonstrated and future developments discussed.Exciton saturation dynamics and spin gratings in multiple quantum well semiconductorsCameron, Alasdair R.https://hdl.handle.net/10023/146612019-04-01T10:04:41Z1997-01-01T00:00:00ZThe dynamics of exciton saturation in GaAs/AlGaAs multiple quantum wells are investigated using picosecond optical pulses. The main contributions to exciton saturation have been shown to be phase space filling. Coulomb screening and exciton lineshape broadening. The spin dependent nature of the phase space filling nonlinearity has allowed its separation from the effects due to Coulomb screening and broadening. The effect of lineshape broadening on exciton saturation has been investigated through its density dependence for a number of quantum well widths. The results show that the effects due to broadening are important in narrow wells of high quality. An investigation into electron spin relaxation at room temperature has been carried out as a function of the well width. The observed decrease in the electron spin relaxation time with decreasing well width is in good agreement with previously reported results and suggests the D'Yakonov-Perel mechanism is the dominant spin-flip mechanism for electrons in quantum wells at room temperature. The first demonstration of an electron spin grating in a quantum well semiconductor is reported by utilising the optical selection rules for quantum wells and optical polarisation gratings. Time resolved studies of the grating decay have allowed the first optical measurement of the in-plane electron diffusion coefficient in a quantum well semiconductor.
1997-01-01T00:00:00ZCameron, Alasdair R.The dynamics of exciton saturation in GaAs/AlGaAs multiple quantum wells are investigated using picosecond optical pulses. The main contributions to exciton saturation have been shown to be phase space filling. Coulomb screening and exciton lineshape broadening. The spin dependent nature of the phase space filling nonlinearity has allowed its separation from the effects due to Coulomb screening and broadening. The effect of lineshape broadening on exciton saturation has been investigated through its density dependence for a number of quantum well widths. The results show that the effects due to broadening are important in narrow wells of high quality. An investigation into electron spin relaxation at room temperature has been carried out as a function of the well width. The observed decrease in the electron spin relaxation time with decreasing well width is in good agreement with previously reported results and suggests the D'Yakonov-Perel mechanism is the dominant spin-flip mechanism for electrons in quantum wells at room temperature. The first demonstration of an electron spin grating in a quantum well semiconductor is reported by utilising the optical selection rules for quantum wells and optical polarisation gratings. Time resolved studies of the grating decay have allowed the first optical measurement of the in-plane electron diffusion coefficient in a quantum well semiconductor.A study of the high temperature superconductor YBa₂Cu₃O₇-𝛿 and its cobalt-doped derivativeWebster, Wendy J.https://hdl.handle.net/10023/146592019-04-01T10:03:26Z1994-01-01T00:00:00ZStatic nuclear magnetic resonance (NMR) techniques have been used to study the magnetic hyperfine interactions of ⁶³Cu and ⁸⁹Y nuclei in the superconducting (T=1.5K) and normal (T=160-300K) states of the pure and cobalt doped Y-Ba-Cu-O layered cuprate systems. Sample quality and physical characteristics are well supported by magnetic susceptibility. Hall effect and X-ray diffraction measurements. We report ⁶³Cu NMR spin echo experiments, performed at 1.5K and 141MHz, on a uniaxially aligned sample of undoped YBa₂Cu₃O₇. By working at such low temperatures T, we provide clear confirmation that the spin components of the ⁶³Cu Knight shift, at both crystallographic sites and in all directions, vanish as T→0, leaving the orbital shift as the residual shift. This is indicative of singlet spin pairing in the superconducting state. The size and anisotropy of the ⁶³Cu(1,2) orbital shifts are consistent with a localised moment model in which there is a single hole of dₓ₂-[sub]y₂ (d[sub]y₂-[sub]z₂) symmetry in the d-shell shell of the Cu(2) (Cu(1)) ion. By working at high fields, where the demagnetisation corrections are much smaller, we are able to conclude that the disparity in the Kᶜorb(1,2) results from previous studies can be traced directly to discrepancies in the demagnetisation corrections, while inconsistencies in the Kᵃ, ᵇorb(1,2) components result from difficulties in analysing the restricted powder pattern.
The effects of the atomic substitution of Co into the YBa₂(Cu₁-ₓMₓ)₃O₆.₉ (0≤x≤0.04) system have been investigated via an integrated ⁸⁹Y and ⁶³Cu NMR study on aligned powders. Combined susceptibility, X-ray and NMR measurements indicate that the Co ion substitutes preferentially at the Cu(1) site and bears a magnetic moment of around 3.7μB, consistent with a Co³⁺ localised moment of intermediate spin state (s=1). Theoretical modelling of the dipolar fields induced by such a moment suggest that the ⁸⁹Y and ⁶³Cu(2) NMR shifts and linewidths cannot be explained by a dipolar mechanism alone. The T[sub]c of the system is found to plateau at ~92K for x%<2.8, but then falls rapidly. This change in T[sub]c is found to coincide with a macroscopic orthorhombic (O) to tetragonal (T) structural transition at x=2.8%. The presence of a true O→T transition is also supported by changes in the ⁸⁹Y chemical shift. Falling hole concentration with increasing x indicates that T[sub]c is not determined by the planes hole concentration alone. Correlations between the ⁸⁹Y Knight shift and T[sub]c, and between ⁸⁹Y magnetic shifts for Co doped and oxygen depleted samples with the same T[sub]c, suggest that decreases in T[sub]c result from a reduction of the DOS at the Fermi level and that the mechanism responsible for T-dependent behaviour in both systems may be the same and directly related to T[sub]c. The complete devastation observed in the low temperature c//B ⁶³Cu(2) spectra for x%>0.5 is consistent with strong inhomogeneity in the Van Vleck component of the planes susceptibility. Finally, correlations between the ⁸⁹Y and ⁶³Cu(2) Knight shifts provide strong evidence in favour of a single quantum spin fluid.
1994-01-01T00:00:00ZWebster, Wendy J.Static nuclear magnetic resonance (NMR) techniques have been used to study the magnetic hyperfine interactions of ⁶³Cu and ⁸⁹Y nuclei in the superconducting (T=1.5K) and normal (T=160-300K) states of the pure and cobalt doped Y-Ba-Cu-O layered cuprate systems. Sample quality and physical characteristics are well supported by magnetic susceptibility. Hall effect and X-ray diffraction measurements. We report ⁶³Cu NMR spin echo experiments, performed at 1.5K and 141MHz, on a uniaxially aligned sample of undoped YBa₂Cu₃O₇. By working at such low temperatures T, we provide clear confirmation that the spin components of the ⁶³Cu Knight shift, at both crystallographic sites and in all directions, vanish as T→0, leaving the orbital shift as the residual shift. This is indicative of singlet spin pairing in the superconducting state. The size and anisotropy of the ⁶³Cu(1,2) orbital shifts are consistent with a localised moment model in which there is a single hole of dₓ₂-[sub]y₂ (d[sub]y₂-[sub]z₂) symmetry in the d-shell shell of the Cu(2) (Cu(1)) ion. By working at high fields, where the demagnetisation corrections are much smaller, we are able to conclude that the disparity in the Kᶜorb(1,2) results from previous studies can be traced directly to discrepancies in the demagnetisation corrections, while inconsistencies in the Kᵃ, ᵇorb(1,2) components result from difficulties in analysing the restricted powder pattern.
The effects of the atomic substitution of Co into the YBa₂(Cu₁-ₓMₓ)₃O₆.₉ (0≤x≤0.04) system have been investigated via an integrated ⁸⁹Y and ⁶³Cu NMR study on aligned powders. Combined susceptibility, X-ray and NMR measurements indicate that the Co ion substitutes preferentially at the Cu(1) site and bears a magnetic moment of around 3.7μB, consistent with a Co³⁺ localised moment of intermediate spin state (s=1). Theoretical modelling of the dipolar fields induced by such a moment suggest that the ⁸⁹Y and ⁶³Cu(2) NMR shifts and linewidths cannot be explained by a dipolar mechanism alone. The T[sub]c of the system is found to plateau at ~92K for x%<2.8, but then falls rapidly. This change in T[sub]c is found to coincide with a macroscopic orthorhombic (O) to tetragonal (T) structural transition at x=2.8%. The presence of a true O→T transition is also supported by changes in the ⁸⁹Y chemical shift. Falling hole concentration with increasing x indicates that T[sub]c is not determined by the planes hole concentration alone. Correlations between the ⁸⁹Y Knight shift and T[sub]c, and between ⁸⁹Y magnetic shifts for Co doped and oxygen depleted samples with the same T[sub]c, suggest that decreases in T[sub]c result from a reduction of the DOS at the Fermi level and that the mechanism responsible for T-dependent behaviour in both systems may be the same and directly related to T[sub]c. The complete devastation observed in the low temperature c//B ⁶³Cu(2) spectra for x%>0.5 is consistent with strong inhomogeneity in the Van Vleck component of the planes susceptibility. Finally, correlations between the ⁸⁹Y and ⁶³Cu(2) Knight shifts provide strong evidence in favour of a single quantum spin fluid.The pyrolysis of ethylbenzeneRhind, John L.https://hdl.handle.net/10023/146402018-07-19T23:17:41Z1963-01-01T00:00:00Z1963-01-01T00:00:00ZRhind, John L.Ferroelectricity in potassium dihydrogen phosphateFinlayson, David M.https://hdl.handle.net/10023/146352019-04-01T10:05:24Z1951-01-01T00:00:00ZA brief survey of ferroelectrics and a statement of the purpose of the present research are given. The necessary piezoelectric equations are developed and, using Mueller’s theory of Rochelle salt as a basis, a phenomenological theory for potassium dihydrogen phosphate is worked out. Measurements of the electro-mechanical constants in the Curie region are reported. The experimental results are discussed in terms of the phenomenological theory and compared with a theory proposed by W.P.Mason. Since neither offers on adequate explanation of the results, the possibility of domain wall movement is considered. Crystal growth and experimental techniques are also discussed.
1951-01-01T00:00:00ZFinlayson, David M.A brief survey of ferroelectrics and a statement of the purpose of the present research are given. The necessary piezoelectric equations are developed and, using Mueller’s theory of Rochelle salt as a basis, a phenomenological theory for potassium dihydrogen phosphate is worked out. Measurements of the electro-mechanical constants in the Curie region are reported. The experimental results are discussed in terms of the phenomenological theory and compared with a theory proposed by W.P.Mason. Since neither offers on adequate explanation of the results, the possibility of domain wall movement is considered. Crystal growth and experimental techniques are also discussed.The determination of structure factors from kinematic and dynamic effects in x-ray diffractionSharma, V. C.https://hdl.handle.net/10023/146322019-04-01T10:07:56Z1973-01-01T00:00:00Z1973-01-01T00:00:00ZSharma, V. C.The problem of the solar red shiftsForbes, Eric Grayhttps://hdl.handle.net/10023/146282019-04-01T10:08:30Z1961-01-01T00:00:00ZThe problem of interpreting the small systematic displacements of solar absorption lines towards longer wavelengths relative to the corresponding laboratory wave-lengths – the so-called solar red shifts – has ranked as one of the most controversial problems in solar physics ever since its discovery by Jewell in 1896. The observational and theoretical difficulties which confronted the pioneer workers in this field are reviewed in Chapter I of this thesis. The year 1920 marks the beginning of a new phase in the development of the problem, since this was when Saha introduced his Ionization Theory which formed a completely new conception of the physical conditions prevailing in the solar atmosphere. At the same time, the announcement that an eclipse experiment made in 1919 appeared to confirm Einstein’s prediction regarding the value of the light deflection, encouraged the belief that the gravitational red shift was implicit in the observed values of the solar red shifts; consequently, the latter were taken as resulting from a superposition of this predicted displacement upon the Doppler effects of radial currents in the solar atmosphere.
The validity of this relativity-radial current interpretation is examined in Chapter II on the basis of observational data presently at our disposal. Our survey serves to show that, although many features of the solar red shifts can be explained by assuming that the steady state of the solar atmosphere is being maintained by a microscopic circulation associated with the solar granulation, the well-established observational fact that the absolute (Sun-arc) displacements are generally in excess of the relativity value at the edge of the disk (where the Doppler effects should vanish) is in contradiction to the conventional interpretation.
This difficulty was fully appreciated by Professor Freundlich, who held the opinion that it might arise as a result of attempting to fit the observational data into a framework to which they did not rightly belong. It was this attitude which stimulated Freundlich (1954) to propose his revolutionary hypothesis that the red shifts observed in stellar spectra were produced as a result of some unknown interaction mechanism whereby light loses energy as it travels through space. Since this view did not appear to be compatible with the existence of the gravitational red shift, it was considered to be of the utmost importance to analyse the solar red shift data – which were much more reliable than those based on stellar observation – without taking it for granted that this effect existed, and determine whether the observed value supported Freundlich’s interpretation. This was the original object of the present research, begun in September 1955.
The major results of the initial two years’ work carried out by the writer in collaboration with Professor Freundlich at the Department of Astronomy of St Andrews University are contained in the three reprints from the Annales d’Astrophysique which are submitted along with this thesis: they appear to confirm the validity of Freundlich’s hypothesis, at the same time revealing no indication of the predicted gravitational red shift. Nevertheless, it was recognised that these conclusions were necessarily based upon a relatively small amount of observational material, and efforts were made by the writer to collect additional data with a view to investigating in more detail the observational properties of the solar red shifts, with particular reference to their dependency upon wavelength and excitation potential. Chapters III and IV are concerned with a description of the observational programme carried out with this intent at the Osservatorio Astrofisico di Arcetri, Italy, and the Universitäts-Sternwarte, Göttingen during the three-year period between October 1957 and September 1960. Much has happened during this time, however, which has cause us to reconsider the problem of the solar red shifts in an entirely different light. The present viewpoint is expressed in Chapter V.
1961-01-01T00:00:00ZForbes, Eric GrayThe problem of interpreting the small systematic displacements of solar absorption lines towards longer wavelengths relative to the corresponding laboratory wave-lengths – the so-called solar red shifts – has ranked as one of the most controversial problems in solar physics ever since its discovery by Jewell in 1896. The observational and theoretical difficulties which confronted the pioneer workers in this field are reviewed in Chapter I of this thesis. The year 1920 marks the beginning of a new phase in the development of the problem, since this was when Saha introduced his Ionization Theory which formed a completely new conception of the physical conditions prevailing in the solar atmosphere. At the same time, the announcement that an eclipse experiment made in 1919 appeared to confirm Einstein’s prediction regarding the value of the light deflection, encouraged the belief that the gravitational red shift was implicit in the observed values of the solar red shifts; consequently, the latter were taken as resulting from a superposition of this predicted displacement upon the Doppler effects of radial currents in the solar atmosphere.
The validity of this relativity-radial current interpretation is examined in Chapter II on the basis of observational data presently at our disposal. Our survey serves to show that, although many features of the solar red shifts can be explained by assuming that the steady state of the solar atmosphere is being maintained by a microscopic circulation associated with the solar granulation, the well-established observational fact that the absolute (Sun-arc) displacements are generally in excess of the relativity value at the edge of the disk (where the Doppler effects should vanish) is in contradiction to the conventional interpretation.
This difficulty was fully appreciated by Professor Freundlich, who held the opinion that it might arise as a result of attempting to fit the observational data into a framework to which they did not rightly belong. It was this attitude which stimulated Freundlich (1954) to propose his revolutionary hypothesis that the red shifts observed in stellar spectra were produced as a result of some unknown interaction mechanism whereby light loses energy as it travels through space. Since this view did not appear to be compatible with the existence of the gravitational red shift, it was considered to be of the utmost importance to analyse the solar red shift data – which were much more reliable than those based on stellar observation – without taking it for granted that this effect existed, and determine whether the observed value supported Freundlich’s interpretation. This was the original object of the present research, begun in September 1955.
The major results of the initial two years’ work carried out by the writer in collaboration with Professor Freundlich at the Department of Astronomy of St Andrews University are contained in the three reprints from the Annales d’Astrophysique which are submitted along with this thesis: they appear to confirm the validity of Freundlich’s hypothesis, at the same time revealing no indication of the predicted gravitational red shift. Nevertheless, it was recognised that these conclusions were necessarily based upon a relatively small amount of observational material, and efforts were made by the writer to collect additional data with a view to investigating in more detail the observational properties of the solar red shifts, with particular reference to their dependency upon wavelength and excitation potential. Chapters III and IV are concerned with a description of the observational programme carried out with this intent at the Osservatorio Astrofisico di Arcetri, Italy, and the Universitäts-Sternwarte, Göttingen during the three-year period between October 1957 and September 1960. Much has happened during this time, however, which has cause us to reconsider the problem of the solar red shifts in an entirely different light. The present viewpoint is expressed in Chapter V.The stimulated Raman effectPaul, Donald M.https://hdl.handle.net/10023/146242019-04-01T10:04:46Z1972-01-01T00:00:00ZThe present work describes a theoretical and experimental investigation of the stimulated Raman effect excited by the focused output of a slow, Q-switched ruby laser in the self-focusing liquids benzene, nitrobenzene, chlorobenzene, and carbon disulphide. Time relationships between the exciting pulse, the transmitted laser pulse, and the time resolved stimulated Raman spectra have been studied using a high speed streak camera in conjunction with fast photodiodes. The spontaneous Raman spectra of these liquids have been investigated using a photoelectric Raman spectrometer and a method is outlined for measuring the relative values of the stimulated Raman gain coefficients. It is experimentally shown that, on a nanosecond time scale, stimulated Raman lines are not generated simultaneously but in the sequence Stokes, Anti-Stokes, and second harmonic Stokes. Further, the time delay between the start of first and second harmonic Stokes is shown to be dependent on the rate of rise of the exciting pulse. It was found that during stimulated Raman generation the laser pulse transmitted through the liquid was heavily distorted and that each liquid produced its own characteristic pulse distortion. No distortion was found in the absence of stimulated Raman generation. Time correlations were found between the duration of features of the pulse distortion and the duration of first Stokes and second harmonic Stokes. These correlations show that forward stimulated Raman generation is controlled by the transmitted laser pulse not the exciting pulse. The threshold for the onset of pulse distortion is shown to be dependent on the rate of rise of the exciting pulse whilst, in all the liquids, apart from carbon disulphide, the 'cut-off' threshold is dependent on the peak power of the incident pulse. Investigation of the pulse distortion thresholds for the range of liquids produced relative values which did not agree with those predicted from either the optical Kerr coefficient or the calculated stimulated Raman gain coefficients. A theoretical model of the self-focusing of a focused beam in a medium for which both electrostriction and the optical Kerr effect are significant is presented and has been used to explain the anomalous threshold results. Within the experimental scatter of the results obtained, this model appears to explain the observed threshold effects. Since the forward stimulated Raman generation was weak in comparison to the transmitted laser pulse and followed this distorted pulse rather than the exciting pulse, it is concluded that stimulated Raman is not the dominant mechanism in the interaction. A brief review is presented of the theory and predictions of steady and non-steady state Brillouin scattering. Although the phonon lifetimes for the other liquids appear to be too short to be of significance, the results are similar in form to those of carbon disulphide. An explanation based on multiphoton absorption is suggested to explain the anomalous behaviour of these liquids. It is concluded that the observed effects in the forward stimulated Raman process depend upon the development in time of the non-linear field dependent self-focusing of the exciting beam and the strong backward scattering process.
1972-01-01T00:00:00ZPaul, Donald M.The present work describes a theoretical and experimental investigation of the stimulated Raman effect excited by the focused output of a slow, Q-switched ruby laser in the self-focusing liquids benzene, nitrobenzene, chlorobenzene, and carbon disulphide. Time relationships between the exciting pulse, the transmitted laser pulse, and the time resolved stimulated Raman spectra have been studied using a high speed streak camera in conjunction with fast photodiodes. The spontaneous Raman spectra of these liquids have been investigated using a photoelectric Raman spectrometer and a method is outlined for measuring the relative values of the stimulated Raman gain coefficients. It is experimentally shown that, on a nanosecond time scale, stimulated Raman lines are not generated simultaneously but in the sequence Stokes, Anti-Stokes, and second harmonic Stokes. Further, the time delay between the start of first and second harmonic Stokes is shown to be dependent on the rate of rise of the exciting pulse. It was found that during stimulated Raman generation the laser pulse transmitted through the liquid was heavily distorted and that each liquid produced its own characteristic pulse distortion. No distortion was found in the absence of stimulated Raman generation. Time correlations were found between the duration of features of the pulse distortion and the duration of first Stokes and second harmonic Stokes. These correlations show that forward stimulated Raman generation is controlled by the transmitted laser pulse not the exciting pulse. The threshold for the onset of pulse distortion is shown to be dependent on the rate of rise of the exciting pulse whilst, in all the liquids, apart from carbon disulphide, the 'cut-off' threshold is dependent on the peak power of the incident pulse. Investigation of the pulse distortion thresholds for the range of liquids produced relative values which did not agree with those predicted from either the optical Kerr coefficient or the calculated stimulated Raman gain coefficients. A theoretical model of the self-focusing of a focused beam in a medium for which both electrostriction and the optical Kerr effect are significant is presented and has been used to explain the anomalous threshold results. Within the experimental scatter of the results obtained, this model appears to explain the observed threshold effects. Since the forward stimulated Raman generation was weak in comparison to the transmitted laser pulse and followed this distorted pulse rather than the exciting pulse, it is concluded that stimulated Raman is not the dominant mechanism in the interaction. A brief review is presented of the theory and predictions of steady and non-steady state Brillouin scattering. Although the phonon lifetimes for the other liquids appear to be too short to be of significance, the results are similar in form to those of carbon disulphide. An explanation based on multiphoton absorption is suggested to explain the anomalous behaviour of these liquids. It is concluded that the observed effects in the forward stimulated Raman process depend upon the development in time of the non-linear field dependent self-focusing of the exciting beam and the strong backward scattering process.Optical properties of metals at low temperaturesBaird, David C.https://hdl.handle.net/10023/146212019-04-01T10:10:23Z1953-01-01T00:00:00ZThe work to be described deals with the reflectivity of metals at temperatures down to 20K. A historical introduction shows how the development of electronic optical theory led to an understanding of many of the important optical properties of metals while leaving unexplained an anomalously high absorbing power. The Maxwellian phenomenological and the electronic theories are given and their limitations noted. The method chosen for the measurement of reflectivity uses a double beam system with continuous balancing and the main features of the apparatus are described. The mirror surfaces have been prepared by electrolytic polishing, by casting and by annealing. The experimental procedure is described in detail and a discussion of the sources of error is given. The variation of reflectivity with both temperature and wavelength has been measured but no absolute measurements of reflectivity were made. The d.c. resistivity of comparable samples was measured over most of the above temperature range. The reflectivity results show features which are identified with the behaviour expected as a result of a theory due to Reuter and Sondheimer. On this basis it is concluded that present theories are still inadequate to account for all the optical absolution found in metals.
1953-01-01T00:00:00ZBaird, David C.The work to be described deals with the reflectivity of metals at temperatures down to 20K. A historical introduction shows how the development of electronic optical theory led to an understanding of many of the important optical properties of metals while leaving unexplained an anomalously high absorbing power. The Maxwellian phenomenological and the electronic theories are given and their limitations noted. The method chosen for the measurement of reflectivity uses a double beam system with continuous balancing and the main features of the apparatus are described. The mirror surfaces have been prepared by electrolytic polishing, by casting and by annealing. The experimental procedure is described in detail and a discussion of the sources of error is given. The variation of reflectivity with both temperature and wavelength has been measured but no absolute measurements of reflectivity were made. The d.c. resistivity of comparable samples was measured over most of the above temperature range. The reflectivity results show features which are identified with the behaviour expected as a result of a theory due to Reuter and Sondheimer. On this basis it is concluded that present theories are still inadequate to account for all the optical absolution found in metals.The radiative heating of plane-parallel and spherical atmosphereMorgan, David H.https://hdl.handle.net/10023/146172019-04-01T10:06:34Z1973-01-01T00:00:00Z1973-01-01T00:00:00ZMorgan, David H.Nuclear magnetic resonance in some hydrocarbons at low temperaturesLawrenson, Ian J.https://hdl.handle.net/10023/146152019-04-01T10:03:08Z1958-01-01T00:00:00ZThe nuclear resonance absorption spectrum of several hydrocarbons has been studied at temperatures down to 20°K, using the bridge method of detection. A special cryostat was constructed for use with liquid hydrogen. In the case of cyclopentene, experimental evidence has been found in favour of a non-planar molecule, rotating about an axis perpendicular to the plane defined by four of the carbon atoms. This motion starts near 87°k, the temperature of the reported specific heat transition. At 124°k, the molecules appear to diffuse through the lattice, so that at 128°k, 10° below the melting point, the line width is less than the field inhomogeneity. Measurements of the second moments of n-pentane, n-hexane and n-octane indicate reorientation of the methyl groups about the end C-C axes, even at 20°K. an approximate value for the height of the barrier restricting the rotation in the n-octane molecule has been determined from the spin-lattice relaxation time measurements. Second moment measurements on cyclooctatetraeene have provided strong evidence in favour of a "tub" (D2a) structure of the molecule, with conjugated double and single bonds.
1958-01-01T00:00:00ZLawrenson, Ian J.The nuclear resonance absorption spectrum of several hydrocarbons has been studied at temperatures down to 20°K, using the bridge method of detection. A special cryostat was constructed for use with liquid hydrogen. In the case of cyclopentene, experimental evidence has been found in favour of a non-planar molecule, rotating about an axis perpendicular to the plane defined by four of the carbon atoms. This motion starts near 87°k, the temperature of the reported specific heat transition. At 124°k, the molecules appear to diffuse through the lattice, so that at 128°k, 10° below the melting point, the line width is less than the field inhomogeneity. Measurements of the second moments of n-pentane, n-hexane and n-octane indicate reorientation of the methyl groups about the end C-C axes, even at 20°K. an approximate value for the height of the barrier restricting the rotation in the n-octane molecule has been determined from the spin-lattice relaxation time measurements. Second moment measurements on cyclooctatetraeene have provided strong evidence in favour of a "tub" (D2a) structure of the molecule, with conjugated double and single bonds.The Bauschinger effectWoolley, Robert Lewishttps://hdl.handle.net/10023/146062019-04-01T10:04:54Z1953-01-01T00:00:00ZIf a work-hardenable metal is deformed plastically by a tensile stress +σ₀ and unloaded, its mechanical properties become anisotropic; in particular, though its tensile yield stress is now +σ₀, it will deform plastically if compression stresses numerically smaller than σ₀ are applied. This is known as the Bauschinger effect (Bauschinger 1886).
When this research was commenced, very few experiments had previously been carried out to elucidate the effect as a function of the several possible variables and there was practically no satisfactory theory, except perhaps that of Masing (1923), which in any case could be severely criticised on purely theoretical grounds. It was clear that various classes of metals would probably show different types of effect. In particular the metals with face-centred and body-centred cubic lattices which deform by slip only, should show a very different effect from that exhibited by the metals of hexagonal lattice, where twinning plays an important part in the deformation. Single crystals might well show a different effect from polycrystals. In nominally pure metals the effect might well depend on grain size, elastic anisotropy, amount of previous work-hardening, temperature, purity, magnetic properties, degree of preferred orientation of a polycrystal, and orientation of a single crystal. In alloys the situation might be even more complex, especially if more than one phase were present.
For the purposes of this present research it was decided to examine the effect thoroughly and systematically in polycrystalline metals deforming entirely by slip, and to carry out exploratory experiments on other metals and in other conditions, including in particular, polycrystalline hexagonal metals and single crystals. Studies of macroscopic mechanical properties alone are not usually very conclusive in establishing the mechanism of the physical processes occurring, and it seemed very desirable to obtain additional information by examining the accompanying changes of other physical properties. Some experiments on these lines are discussed in sections III and V. The exploratory experiments of sections III, V, VI and VIII were deliberately restricted usually to one metal under well-defined conditions, in view of limitations of time. It is hoped to extend these experiments in the near future.
1953-01-01T00:00:00ZWoolley, Robert LewisIf a work-hardenable metal is deformed plastically by a tensile stress +σ₀ and unloaded, its mechanical properties become anisotropic; in particular, though its tensile yield stress is now +σ₀, it will deform plastically if compression stresses numerically smaller than σ₀ are applied. This is known as the Bauschinger effect (Bauschinger 1886).
When this research was commenced, very few experiments had previously been carried out to elucidate the effect as a function of the several possible variables and there was practically no satisfactory theory, except perhaps that of Masing (1923), which in any case could be severely criticised on purely theoretical grounds. It was clear that various classes of metals would probably show different types of effect. In particular the metals with face-centred and body-centred cubic lattices which deform by slip only, should show a very different effect from that exhibited by the metals of hexagonal lattice, where twinning plays an important part in the deformation. Single crystals might well show a different effect from polycrystals. In nominally pure metals the effect might well depend on grain size, elastic anisotropy, amount of previous work-hardening, temperature, purity, magnetic properties, degree of preferred orientation of a polycrystal, and orientation of a single crystal. In alloys the situation might be even more complex, especially if more than one phase were present.
For the purposes of this present research it was decided to examine the effect thoroughly and systematically in polycrystalline metals deforming entirely by slip, and to carry out exploratory experiments on other metals and in other conditions, including in particular, polycrystalline hexagonal metals and single crystals. Studies of macroscopic mechanical properties alone are not usually very conclusive in establishing the mechanism of the physical processes occurring, and it seemed very desirable to obtain additional information by examining the accompanying changes of other physical properties. Some experiments on these lines are discussed in sections III and V. The exploratory experiments of sections III, V, VI and VIII were deliberately restricted usually to one metal under well-defined conditions, in view of limitations of time. It is hoped to extend these experiments in the near future.Dissipation in the superfluid helium filmAdie, Christopher J.https://hdl.handle.net/10023/146022019-04-01T10:03:05Z1984-01-01T00:00:00ZExperimental apparatus to study dissipation in the saturated superfluid helium film has been developed. The low temperature parts comprise a sealed cell containing liquid helium, to which are affixed two parallel plate capacitors, functioning both as liquid reservoirs and as a way of measuring the liquid level. A small hole in a thin plastic film located in the flow path between the two capacitors forms the flow-limiting constriction. This arrangement introduces large velocity gradients in the vicinity of the hole. Film flow is initiated and sustained by an electric field in one capacitor, generated by a purpose-built Film Drive Unit (FDU) and a high-voltage amplifier. Detailed study of the helium film under steady flow conditions was not possible, but those results which were obtained indicate that the transfer rate is about 30% higher than was anticipated. By applying positive feedback to the film through the FDU, the inertial oscillations can be studied over many cycles. This new method has revealed some unexpected results, and a variety of types of oscillation behaviour have been observed. A theoretical model of dissipation has been developed, based on the premise that vortices in the film are oriented perpendicular to the film plane and are free to move and cross streamlines. According to this model, the large steady film transfer rates are due to the separation of the region of dissipation and the region of maximum velocity, an effect caused by the radial-flow geometry. Numerical simulation of the inertial oscillations using the model reproduces some of the behaviour observed experimentally, provided that the rate of vortex creation is taken to be a step function of the velocity. The shape of the liquid helium surface tension meniscus has been calculated numerically. The calculation is valid for the moving and static film in the absence of dissipation.
1984-01-01T00:00:00ZAdie, Christopher J.Experimental apparatus to study dissipation in the saturated superfluid helium film has been developed. The low temperature parts comprise a sealed cell containing liquid helium, to which are affixed two parallel plate capacitors, functioning both as liquid reservoirs and as a way of measuring the liquid level. A small hole in a thin plastic film located in the flow path between the two capacitors forms the flow-limiting constriction. This arrangement introduces large velocity gradients in the vicinity of the hole. Film flow is initiated and sustained by an electric field in one capacitor, generated by a purpose-built Film Drive Unit (FDU) and a high-voltage amplifier. Detailed study of the helium film under steady flow conditions was not possible, but those results which were obtained indicate that the transfer rate is about 30% higher than was anticipated. By applying positive feedback to the film through the FDU, the inertial oscillations can be studied over many cycles. This new method has revealed some unexpected results, and a variety of types of oscillation behaviour have been observed. A theoretical model of dissipation has been developed, based on the premise that vortices in the film are oriented perpendicular to the film plane and are free to move and cross streamlines. According to this model, the large steady film transfer rates are due to the separation of the region of dissipation and the region of maximum velocity, an effect caused by the radial-flow geometry. Numerical simulation of the inertial oscillations using the model reproduces some of the behaviour observed experimentally, provided that the rate of vortex creation is taken to be a step function of the velocity. The shape of the liquid helium surface tension meniscus has been calculated numerically. The calculation is valid for the moving and static film in the absence of dissipation.A theory of gravitation incorporating the quadratic action principle of relativityWynne, Valerie Annehttps://hdl.handle.net/10023/145992019-04-01T10:02:40Z1971-01-01T00:00:00ZThe hypothesis adopted in this work is that any permissible metric field whatsoever must satisfy the field equations deduced from an action principle in which the Lagrangian is quadratic in the components of the Riemann curvature tensor. The adoption of such a hypothesis is motivated by the precariousness of the general relativistic interpretation of Mach's principle, which is often used to justify a phenomenological approach to the theory. The quadratic action principle is chosen to provide the fundamental equations of the gravitational field because it is logically and aesthetically appealing, and causes us to lose nothing of the standard relativity theory based on Einstein's vacuum equations. The set of relationships, Rp𝜎 - ½9p𝜎R = -k Tp𝜎 (Equation) retained as a definition of the matter tensor Tp𝜎. Attention is concentrated on the solutions of the (generally fourth order) fundamental field equations in the static, spherically symmetric case. Sets of exact, series and numerical solutions are obtained corresponding to certain boundary conditions, or with certain properties in common. Study of the geometrical, topological and physical properties of several of the universes obtained as a result of our hypothesis leads us to believe that our theory is not implausible. We conclude by considering the further possibilities of the theory.
1971-01-01T00:00:00ZWynne, Valerie AnneThe hypothesis adopted in this work is that any permissible metric field whatsoever must satisfy the field equations deduced from an action principle in which the Lagrangian is quadratic in the components of the Riemann curvature tensor. The adoption of such a hypothesis is motivated by the precariousness of the general relativistic interpretation of Mach's principle, which is often used to justify a phenomenological approach to the theory. The quadratic action principle is chosen to provide the fundamental equations of the gravitational field because it is logically and aesthetically appealing, and causes us to lose nothing of the standard relativity theory based on Einstein's vacuum equations. The set of relationships, Rp𝜎 - ½9p𝜎R = -k Tp𝜎 (Equation) retained as a definition of the matter tensor Tp𝜎. Attention is concentrated on the solutions of the (generally fourth order) fundamental field equations in the static, spherically symmetric case. Sets of exact, series and numerical solutions are obtained corresponding to certain boundary conditions, or with certain properties in common. Study of the geometrical, topological and physical properties of several of the universes obtained as a result of our hypothesis leads us to believe that our theory is not implausible. We conclude by considering the further possibilities of the theory.Non-radiative transitionsRobertson, Neilhttps://hdl.handle.net/10023/145932019-04-01T10:02:48Z1977-01-01T00:00:00ZThis thesis describes theoretical research into multiphonon non-radiative transitions in solids. These processes occur when an impurity centre electron which has been raised to an excited state returns to its ground state by emitting phonons into the lattice rather than by emitting light. The original contribution of this thesis is concerned with certain theoretical aspects of non-radiative transitions. For the first time the Molecular Crystal Model originally developed by Holstein to study small-polaron motion is used to investigate this problem. In particular, the effect that weak dispersion of the vibrational modes has on the transition rates is studied. The inclusion of weak dispersion is essential for a rigorous treatment of non-radiative transitions, and without this feature the transition rates are not well defined. We also use the well-known single configuration coordinate model to derive recombination rates, giving particular attention to the unequal frequency case. Finally an adaptation of the Molecular Crystal Model is considered which more closely resembles real physical systems in that the electron is coupled to both crystalline modes and a local mode.
1977-01-01T00:00:00ZRobertson, NeilThis thesis describes theoretical research into multiphonon non-radiative transitions in solids. These processes occur when an impurity centre electron which has been raised to an excited state returns to its ground state by emitting phonons into the lattice rather than by emitting light. The original contribution of this thesis is concerned with certain theoretical aspects of non-radiative transitions. For the first time the Molecular Crystal Model originally developed by Holstein to study small-polaron motion is used to investigate this problem. In particular, the effect that weak dispersion of the vibrational modes has on the transition rates is studied. The inclusion of weak dispersion is essential for a rigorous treatment of non-radiative transitions, and without this feature the transition rates are not well defined. We also use the well-known single configuration coordinate model to derive recombination rates, giving particular attention to the unequal frequency case. Finally an adaptation of the Molecular Crystal Model is considered which more closely resembles real physical systems in that the electron is coupled to both crystalline modes and a local mode.The effect of local field corrections on the transport properties of solidsSaglam, Mesudehttps://hdl.handle.net/10023/145912019-04-01T10:10:28Z1977-01-01T00:00:00ZThe purpose of this work is to investigate local field type corrections to electrical transport properties and in particular small polaron hopping conductivity. The question of local field corrections to transport properties has been considered for some time. Recently this question has become of particular interest for small polaron hopping conduction. Such corrections were considered as a possible explanation for the large jump parameters inferred in fitting the electric field dependence of the electrical conductivity of certain transition metal oxide glasses in which conduction is believed to occur via thermally activated small polaron hopping.
Local field corrections are well established in determining the dielectric constant and optical properties at a single atomic site. Tessman et al have shown that the assumption of a normal Lorentz term gives excellent agreement between computed and measured polarizabilities in alkali halide crystals at optical frequencies. Adler has proved, using a self-consistent field theory that the standard Lorentz term arises in the tight binding limit. Also a full quantum mechanical treatment of the frequency and wave-number dependent dielectric constant including local field effect has been given by Wiser. It is shown that the dielectric constant splits into an atomic term that describes the motion of the electron around each atom and an acceleration term which describes the motion from atom to atom and that acceleration term contains no local field correction.
Lidiard has discussed Lorentz type corrections to the diffusion coefficient in ionic conductivity. He argued that no Lorentz correction should be applied, since the Lorentz cavity must always be centred on the hopping carrier; thus, the Lorentz internal field can do no work as the carrier moves from one site to another. This view is strongly supported by the close agreement between ionic diffusion coefficients obtained from conductivity data and NMR relaxation and isotopic diffusion.
It has been argued by Munn in several papers without detailed justification that the microscopic mobility does give the polaron velocity in terms of a local field. This argument is in conflict with that of Lidiard. However this question had not been considered explicitly for the case of electrical transport in the hopping limit.
It is the intent of the present work to study the local field problem. In the first chapter we give a general description of the local electric field concept which is straight review. Chapters II and III are concerned with the question of local field corrections to the small polaron hopping conduction and the Hall mobility. In Chapter IV the results of Chapter II are applied to ac impurity hopping conduction. Finally in Chapter V we study the band motion of electrons using a theory of transport based on quantum mechanical principles.
1977-01-01T00:00:00ZSaglam, MesudeThe purpose of this work is to investigate local field type corrections to electrical transport properties and in particular small polaron hopping conductivity. The question of local field corrections to transport properties has been considered for some time. Recently this question has become of particular interest for small polaron hopping conduction. Such corrections were considered as a possible explanation for the large jump parameters inferred in fitting the electric field dependence of the electrical conductivity of certain transition metal oxide glasses in which conduction is believed to occur via thermally activated small polaron hopping.
Local field corrections are well established in determining the dielectric constant and optical properties at a single atomic site. Tessman et al have shown that the assumption of a normal Lorentz term gives excellent agreement between computed and measured polarizabilities in alkali halide crystals at optical frequencies. Adler has proved, using a self-consistent field theory that the standard Lorentz term arises in the tight binding limit. Also a full quantum mechanical treatment of the frequency and wave-number dependent dielectric constant including local field effect has been given by Wiser. It is shown that the dielectric constant splits into an atomic term that describes the motion of the electron around each atom and an acceleration term which describes the motion from atom to atom and that acceleration term contains no local field correction.
Lidiard has discussed Lorentz type corrections to the diffusion coefficient in ionic conductivity. He argued that no Lorentz correction should be applied, since the Lorentz cavity must always be centred on the hopping carrier; thus, the Lorentz internal field can do no work as the carrier moves from one site to another. This view is strongly supported by the close agreement between ionic diffusion coefficients obtained from conductivity data and NMR relaxation and isotopic diffusion.
It has been argued by Munn in several papers without detailed justification that the microscopic mobility does give the polaron velocity in terms of a local field. This argument is in conflict with that of Lidiard. However this question had not been considered explicitly for the case of electrical transport in the hopping limit.
It is the intent of the present work to study the local field problem. In the first chapter we give a general description of the local electric field concept which is straight review. Chapters II and III are concerned with the question of local field corrections to the small polaron hopping conduction and the Hall mobility. In Chapter IV the results of Chapter II are applied to ac impurity hopping conduction. Finally in Chapter V we study the band motion of electrons using a theory of transport based on quantum mechanical principles.Transport problems in the theory of metalsMcGill, Neil Charleshttps://hdl.handle.net/10023/145882019-04-01T10:08:35Z1972-01-01T00:00:00ZOf all common substances, metals conduct heat and electricity to the greatest extent. They also exhibit very readily a range of more complicated phenomena – the galvanomagnetic, thermomagnetic and thermoelectric effects – which results when an electric field, a thermal gradient and a magnetic field are combined in various ways. Any theory of the metallic state, therefore, must explain why metals should demonstrate these properties as well as they do, and account for the variation of the physical quantities in question with temperature, electric field, atomic structure and so on. It is now well established that the simplest metals are characterised by an energy band structure in which the topmost occupied band is half full, so making it easy to impart extra energy to those electrons lying at or just below the Fermi level. The electrons in this band are loosely bound to the atomic cores, and can readily move through the material. When an external electric field is applied, the average motion of such electrons in the direction of the field constitutes an electric current, while a thermal current results (in the presence of a thermal gradient) from the diffusion of electrons from hot to cold areas. Similar qualitative explanations may be furnished for the more complicated thermal, electric and magnetic effects. Quantitatively, the key to the calculation of the transport coefficients – and the electrical and thermal conductivities, the Hall coefficient, and so on – lies in the evaluation of the distribution function 𝛽 (I, k, t) which describes how to the electrons are distributed in (I, k) space at any time t. This function is found by solving the Boltzmann transport equation, the mathematical expression of the statement that any change in 𝛽 with time is the sum of three contributions, one for each possible cause: diffusion, collisions, and the action of external fields. Even after simplifying assumptions have been made about the lattice through which the electrons move, the Boltzmann equation is, in its most general form, a complicated integral equation, the complezity stemming partly from the different types of electron collision which must all be represented: collisions with thermal vibrations of the lattive, with impurities, with other displaced atoms, with boundary surfaces, and so on. In practice relief is usually gained by considering only one or perhaps two of these types of collision at a time, assuming conditions which make these dominant. In the first problem to be considered here, attention is restricted mainly to the case where electrons are scattered by thermal vibrations of the lattice; in the second, the emphasis is on collisions which electrons make with the metal surface.
1972-01-01T00:00:00ZMcGill, Neil CharlesOf all common substances, metals conduct heat and electricity to the greatest extent. They also exhibit very readily a range of more complicated phenomena – the galvanomagnetic, thermomagnetic and thermoelectric effects – which results when an electric field, a thermal gradient and a magnetic field are combined in various ways. Any theory of the metallic state, therefore, must explain why metals should demonstrate these properties as well as they do, and account for the variation of the physical quantities in question with temperature, electric field, atomic structure and so on. It is now well established that the simplest metals are characterised by an energy band structure in which the topmost occupied band is half full, so making it easy to impart extra energy to those electrons lying at or just below the Fermi level. The electrons in this band are loosely bound to the atomic cores, and can readily move through the material. When an external electric field is applied, the average motion of such electrons in the direction of the field constitutes an electric current, while a thermal current results (in the presence of a thermal gradient) from the diffusion of electrons from hot to cold areas. Similar qualitative explanations may be furnished for the more complicated thermal, electric and magnetic effects. Quantitatively, the key to the calculation of the transport coefficients – and the electrical and thermal conductivities, the Hall coefficient, and so on – lies in the evaluation of the distribution function 𝛽 (I, k, t) which describes how to the electrons are distributed in (I, k) space at any time t. This function is found by solving the Boltzmann transport equation, the mathematical expression of the statement that any change in 𝛽 with time is the sum of three contributions, one for each possible cause: diffusion, collisions, and the action of external fields. Even after simplifying assumptions have been made about the lattice through which the electrons move, the Boltzmann equation is, in its most general form, a complicated integral equation, the complezity stemming partly from the different types of electron collision which must all be represented: collisions with thermal vibrations of the lattive, with impurities, with other displaced atoms, with boundary surfaces, and so on. In practice relief is usually gained by considering only one or perhaps two of these types of collision at a time, assuming conditions which make these dominant. In the first problem to be considered here, attention is restricted mainly to the case where electrons are scattered by thermal vibrations of the lattice; in the second, the emphasis is on collisions which electrons make with the metal surface.Some studies on excited states by E.S.R.Reekie, David D.https://hdl.handle.net/10023/145772019-04-01T10:04:16Z1966-01-01T00:00:00Z1966-01-01T00:00:00ZReekie, David D.The crystal structure of methly stubstituted 1:2-BenzanthraquinonesFerrier, Robert Pattonhttps://hdl.handle.net/10023/145732019-04-01T10:04:48Z1959-01-01T00:00:00Z1959-01-01T00:00:00ZFerrier, Robert PattonA nuclear magnetic resonance study of molecular disorder in the solid state of some medium ring hydrocarbonsBrookeman, James R.https://hdl.handle.net/10023/145652019-04-01T10:08:22Z1968-01-01T00:00:00ZSince the discovery of nuclear magnetic resonance in bulk matter in 1945, n.m.r studies of molecular motion in solids have provided a powerful insight into the details of molecular reorientation processes that occur in solids. By studying the variation of the n.m.r. parameters as a function of temperature it is usually possible to obtain information regarding such phenomena as solid-solid phase transitions, including the anomalous behaviour of heat capacity associated with order-disorder and co-operative processes, rotational disorder and self-diffusion. In suitable cases it is possible to estimate the origin and magnitude of the potential barriers of the crystalline field which hinder the various reorientation processes. The cycloalkane ring series (CH₂)[sub]n from cyclopropane n =3 to cyclohexane n=6 have all previously been studied in the solid state by n.m.r. The results indicate that considerable molecular mobility occurs in the solid state o these substances and that in general the solid-solid phase transitions detected by hear capacity measurements are associated with the onset of molecular reorientation and in some cases self-diffusion. The current interest in the molecular configuration of the medium ring hydrocarbons, and the extensive study by Finke et al. (1956) of the low temperature thermal properties of cycloheptane n=7, and cyclooctane n=8, showing three and two solid-solid phase transitions respectively, prompted an n.m.r. study of cycloheptane and cyclooctane. The results of this investigation together with those of a related seven membered ring hydrocarbon 1, 3, 5 cycloheptatriene C₇H₈ are presented in this thesis, and show that extensive molecular motion exists well below the melting points of all three substances, and information is given regarding the form of the motion in the various crystalline phases.
1968-01-01T00:00:00ZBrookeman, James R.Since the discovery of nuclear magnetic resonance in bulk matter in 1945, n.m.r studies of molecular motion in solids have provided a powerful insight into the details of molecular reorientation processes that occur in solids. By studying the variation of the n.m.r. parameters as a function of temperature it is usually possible to obtain information regarding such phenomena as solid-solid phase transitions, including the anomalous behaviour of heat capacity associated with order-disorder and co-operative processes, rotational disorder and self-diffusion. In suitable cases it is possible to estimate the origin and magnitude of the potential barriers of the crystalline field which hinder the various reorientation processes. The cycloalkane ring series (CH₂)[sub]n from cyclopropane n =3 to cyclohexane n=6 have all previously been studied in the solid state by n.m.r. The results indicate that considerable molecular mobility occurs in the solid state o these substances and that in general the solid-solid phase transitions detected by hear capacity measurements are associated with the onset of molecular reorientation and in some cases self-diffusion. The current interest in the molecular configuration of the medium ring hydrocarbons, and the extensive study by Finke et al. (1956) of the low temperature thermal properties of cycloheptane n=7, and cyclooctane n=8, showing three and two solid-solid phase transitions respectively, prompted an n.m.r. study of cycloheptane and cyclooctane. The results of this investigation together with those of a related seven membered ring hydrocarbon 1, 3, 5 cycloheptatriene C₇H₈ are presented in this thesis, and show that extensive molecular motion exists well below the melting points of all three substances, and information is given regarding the form of the motion in the various crystalline phases.The statistical aspects of Boltzmann's H-TheoremGreen, C. D.https://hdl.handle.net/10023/145582019-04-01T10:02:41Z1954-01-01T00:00:00ZThis thesis is concerned with the consideration of the H-theorem in a statistical manner and the information that may be derived from it as to the variation with time of an isolated mechanical system, and especially the approach to equilibrium. A historical introduction is given in which it shown how the need for such a statistical approach arose, and hoy the question of the behaviour of the fluctuations about the values of H predicted by the unrestricted H-theorem became important. The type of behaviour suggested by the Ehrenfests is quoted and to verify this it is found to be necessary to consider in detail actual models. Two classical models, the urn model and the wind-wood model, are considered, and then the procedure is generalized so as to include the whole class of models of the type consisting of two groups of particles, the one group moving and interacting with the members of the second group which are fixed. The transition probabilities and the rate of change of H, and the mean time of recurrence of a fluctuation are found for these models by considering the influence of fluctuations upon the Stosszahlansatz values for the numbers of collisions. The results confirm the postulates of the Ehrenfests. In assumptions common to the statistical treatment of collision processes.
1954-01-01T00:00:00ZGreen, C. D.This thesis is concerned with the consideration of the H-theorem in a statistical manner and the information that may be derived from it as to the variation with time of an isolated mechanical system, and especially the approach to equilibrium. A historical introduction is given in which it shown how the need for such a statistical approach arose, and hoy the question of the behaviour of the fluctuations about the values of H predicted by the unrestricted H-theorem became important. The type of behaviour suggested by the Ehrenfests is quoted and to verify this it is found to be necessary to consider in detail actual models. Two classical models, the urn model and the wind-wood model, are considered, and then the procedure is generalized so as to include the whole class of models of the type consisting of two groups of particles, the one group moving and interacting with the members of the second group which are fixed. The transition probabilities and the rate of change of H, and the mean time of recurrence of a fluctuation are found for these models by considering the influence of fluctuations upon the Stosszahlansatz values for the numbers of collisions. The results confirm the postulates of the Ehrenfests. In assumptions common to the statistical treatment of collision processes.Quantization of some generally covariant model field theoriesWan, Kong K.https://hdl.handle.net/10023/145542019-04-01T10:09:16Z1971-01-01T00:00:00ZThis thesis reports a study of the quantization of generally covariant and nonlinear field theories. It begins by reviewing some existing general theories in Chapter 2 and Chapter 3, Chapter 2 deals with general classical theories while Chapter 3 examines various quantization schemes. The model field derived from the Lagrangian density
𝓵 = 1/4 ℇ[super]lkμλ (A [sub] k,t – A [sub] k,t)(A, {sub{ A[sub]μ,λ – A[sub]λ,μ).
is proposed in Chapter 4 especially for the study of general covariance. It is demonstrated that for this field general covariance alone does not appear to bring in anything physically new, a discussion is given on the differences between general covariance and Lorentz covariance. In subsequent chapters a generally covariant and nonlinear model field, a 4-surface of stationary 4-volume embedded in a 5-dimensional Pseudo-Euclidean space, is investigated. Firstly a manifestly covariant quantization programme is carried out. The model field is then examined in a special coordinate frame for the study of its nonlinearity. Various treatments of the intrinsic nonlinearity are examined starting with conventional perturbation theory in Chapter 6. The usual divergence problem in quantum field theory appears, in particular in the self-energy calculation of a one-particle state. A new variational method is proposed in Chapter 8 which is able to lead to finite results for one-particle states. The thesis is concluded with a chapter discussing some general problems involved and a chapter containing suggestions for further work.
1971-01-01T00:00:00ZWan, Kong K.This thesis reports a study of the quantization of generally covariant and nonlinear field theories. It begins by reviewing some existing general theories in Chapter 2 and Chapter 3, Chapter 2 deals with general classical theories while Chapter 3 examines various quantization schemes. The model field derived from the Lagrangian density
𝓵 = 1/4 ℇ[super]lkμλ (A [sub] k,t – A [sub] k,t)(A, {sub{ A[sub]μ,λ – A[sub]λ,μ).
is proposed in Chapter 4 especially for the study of general covariance. It is demonstrated that for this field general covariance alone does not appear to bring in anything physically new, a discussion is given on the differences between general covariance and Lorentz covariance. In subsequent chapters a generally covariant and nonlinear model field, a 4-surface of stationary 4-volume embedded in a 5-dimensional Pseudo-Euclidean space, is investigated. Firstly a manifestly covariant quantization programme is carried out. The model field is then examined in a special coordinate frame for the study of its nonlinearity. Various treatments of the intrinsic nonlinearity are examined starting with conventional perturbation theory in Chapter 6. The usual divergence problem in quantum field theory appears, in particular in the self-energy calculation of a one-particle state. A new variational method is proposed in Chapter 8 which is able to lead to finite results for one-particle states. The thesis is concluded with a chapter discussing some general problems involved and a chapter containing suggestions for further work.Quantization in spaces of constant curvatureViazminsky, Cesar Peterhttps://hdl.handle.net/10023/145492019-04-01T10:05:16Z1978-01-01T00:00:00ZQuantization in generalized coordinate systems and in non-Euclidean spaces has attracted much recent attention. The aim of this thesis is to discuss the problem for spaces of constant curvature. Chapter I is a brief review of tensor analysis on manifolds and the Hamiltonian formulation of classical mechanics. Chapter II deals with the canonical quantization scheme. It is proved that this scheme is unsatisfactory since it often fails to produce an essentially self-adjoin operator corresponding to a generalized momentum. It is shown that the only case for which we can obtain a unique well-defined pair of canonical quantum observables is that when the range of the generalized coordinate is the entire real line. Chapter III is devoted for discussing the quantization in spaces of constant curvature. An approach by Mackey circumvents the difficulties facing the canonical quantization scheme and is adopted instead. It is shown that a more physical meaning is gained if we impose on Mackey's momenta the condition that they are metric- preserving. Such a condition makes the momenta constants of the free motion. §4 in essence demonstrates that the dynamics in a space of constant curvature are rooted in its geometry. Utilizing the metric-preserving momenta, the quantum Hamiltonian is built up from symmetry considerations, and is uniquely determined as the Laplacian up to a multiplicative constant and an additive constant. In §5-§8 the quantization in spaces of constant curvature is studied in detail. The quantum and classical momenta are found explicitly. Eigenvalues and Eigenfunctions of the momentum observables are evaluated. Also it is shown that spaces of different curvature are physically distinguishable. An interesting relation between the momenta, the curvature of the space and the Hamiltonian holds classically and quantum-mechanically. The well-known relation giving the Hamiltonian as proportional to the sum of squares of the momenta (in Euclidean spaces with Cartesian coordinates) is just a special case of a more general relation in which the momenta, the angular momenta and the curvature of the space take part in forming the Hamiltonian classically or quantum-mechanically. §9 of chapter III discusses the Lie algebraic treatment of quantization. It is shown that such a treatment cannot be taken as a general quantization scheme. The sets of all classical and quantal momenta (metric-preserving and non- metric-preserving) do not form Lie algebras. However when we confine ourselves to those momenta which are generated by motions of the space, then these sets form isomorphic Lie algebras. The striking parallelism between quantum and classical mechanics observed throughout §4-§8 is pushed further in §9. It is shown that the quantum and classical Hamiltonians are, respectively, related to the Casimir operators of the Lie algebras of quantum and classical momenta in the same way. Chapter IV envisages the problem of quantization from an intrinsic point of view. A space of constant curvature is looked on as a hypersurface in a flat space. In §4 we verify that Dirac's scheme for quantizing a constrained classical system works when the constraints are geometrical.
1978-01-01T00:00:00ZViazminsky, Cesar PeterQuantization in generalized coordinate systems and in non-Euclidean spaces has attracted much recent attention. The aim of this thesis is to discuss the problem for spaces of constant curvature. Chapter I is a brief review of tensor analysis on manifolds and the Hamiltonian formulation of classical mechanics. Chapter II deals with the canonical quantization scheme. It is proved that this scheme is unsatisfactory since it often fails to produce an essentially self-adjoin operator corresponding to a generalized momentum. It is shown that the only case for which we can obtain a unique well-defined pair of canonical quantum observables is that when the range of the generalized coordinate is the entire real line. Chapter III is devoted for discussing the quantization in spaces of constant curvature. An approach by Mackey circumvents the difficulties facing the canonical quantization scheme and is adopted instead. It is shown that a more physical meaning is gained if we impose on Mackey's momenta the condition that they are metric- preserving. Such a condition makes the momenta constants of the free motion. §4 in essence demonstrates that the dynamics in a space of constant curvature are rooted in its geometry. Utilizing the metric-preserving momenta, the quantum Hamiltonian is built up from symmetry considerations, and is uniquely determined as the Laplacian up to a multiplicative constant and an additive constant. In §5-§8 the quantization in spaces of constant curvature is studied in detail. The quantum and classical momenta are found explicitly. Eigenvalues and Eigenfunctions of the momentum observables are evaluated. Also it is shown that spaces of different curvature are physically distinguishable. An interesting relation between the momenta, the curvature of the space and the Hamiltonian holds classically and quantum-mechanically. The well-known relation giving the Hamiltonian as proportional to the sum of squares of the momenta (in Euclidean spaces with Cartesian coordinates) is just a special case of a more general relation in which the momenta, the angular momenta and the curvature of the space take part in forming the Hamiltonian classically or quantum-mechanically. §9 of chapter III discusses the Lie algebraic treatment of quantization. It is shown that such a treatment cannot be taken as a general quantization scheme. The sets of all classical and quantal momenta (metric-preserving and non- metric-preserving) do not form Lie algebras. However when we confine ourselves to those momenta which are generated by motions of the space, then these sets form isomorphic Lie algebras. The striking parallelism between quantum and classical mechanics observed throughout §4-§8 is pushed further in §9. It is shown that the quantum and classical Hamiltonians are, respectively, related to the Casimir operators of the Lie algebras of quantum and classical momenta in the same way. Chapter IV envisages the problem of quantization from an intrinsic point of view. A space of constant curvature is looked on as a hypersurface in a flat space. In §4 we verify that Dirac's scheme for quantizing a constrained classical system works when the constraints are geometrical.Higher spin relativistic wave-equationsKoutroulos, Christos G.https://hdl.handle.net/10023/145452019-04-01T10:08:59Z1980-01-01T00:00:00ZIn this work we have undertaken the study of some aspects of higher spin relativistic wave equations of the first order. These aspects are their definiteness of charge, their second quantization and their causal propagation. The work is divided into three parts. In part one we have studied the Bhabha equations based on the sixteen and twenty dimensional representations and showed that the charge associated with them is indefinite. In part two we have studied in detail the Gel'fand Yaglom equations and showed that certain equations can have definite charge. Finally in part three we have studied the propagation in the external electromagnetic field of certain higher spin relativistic wave equations.
1980-01-01T00:00:00ZKoutroulos, Christos G.In this work we have undertaken the study of some aspects of higher spin relativistic wave equations of the first order. These aspects are their definiteness of charge, their second quantization and their causal propagation. The work is divided into three parts. In part one we have studied the Bhabha equations based on the sixteen and twenty dimensional representations and showed that the charge associated with them is indefinite. In part two we have studied in detail the Gel'fand Yaglom equations and showed that certain equations can have definite charge. Finally in part three we have studied the propagation in the external electromagnetic field of certain higher spin relativistic wave equations.On the quantization and measurement of momentum observablesMcFarlane, Keithhttps://hdl.handle.net/10023/145432019-04-01T10:03:51Z1980-01-01T00:00:00ZThis thesis is the outcome of research conducted into the quantization of observables defined over a Riemannian configuration manifold, and is naturally divided into three chapters:
The first chapter is essentially concerned with the development from physical bases, of the concepts of classical and quantum “global measurability”, which, when combined with the requirement that all quantizable momenta shall be either classically or quantum mechanically globally measurable, result in the exclusion form the class of quantizable momenta of all those observables which are not quantizable in accordance with G. W. Mackey’s natural procedure. The refinement of exact global measurability is then introduced and the physically important class of “killing” momenta are found to be exactly classically and quantum mechanically globally measurable and moreover quantizable by means of Mackey’s scheme.
The second chapter seeks to analyse the algebraic structure of the Mackey-quantizable momenta, so as to compare and contrast his geometric scheme with the various algebraic schemes which have frequently been proposed. As an extension of this work, a natural geometric quantization is proposed for the more general class of observables “linear in momentum”, the set of quantizable such entities determines, and its algebraic properties discussed. It is concluded that, if Mackey’s procedure is, as we argue, correct and exhaustive of the quantizable momenta, then algebraic rules of quantization do not, in general, obtain among the momenta defined over a Riemannian space, ‘though in the case of momenta, such as the Killing momenta, reflecting the symmetry of the configuration manifold, such laws not only exist but are moreover of great practical importance.
The third chapter is concerned with the quantization of the observables “multilinear in momentum” and in particular with the delimitation of the possible operators of formal quantization as circumscribed by the requirement of operator symmetry, and the question of the essential self-adjointness of local observables associated with the multilinear momenta. Finally some possible means of determining and exact quantization are discussed and contrasted and a tentative selection made of a particular scheme which is then illustrated by concrete example on the real line with the usual metric.
We conclude this preface with a brief indication of the layout of the thesis: Each chapter or associated group of appendices has been treated for the purpose of all textual numbering and referencing systems, as a separate unit, and is prefixed by a detailed list of contents, and concluded by the appropriate group of reference, so that all pointers of a chapter or group of appendices refer, unless otherwise indicated, to the text of that chapter or group of appendices. Finally note that a bibliography has been included which will serve as a collected list of references to papers and books employed.
1980-01-01T00:00:00ZMcFarlane, KeithThis thesis is the outcome of research conducted into the quantization of observables defined over a Riemannian configuration manifold, and is naturally divided into three chapters:
The first chapter is essentially concerned with the development from physical bases, of the concepts of classical and quantum “global measurability”, which, when combined with the requirement that all quantizable momenta shall be either classically or quantum mechanically globally measurable, result in the exclusion form the class of quantizable momenta of all those observables which are not quantizable in accordance with G. W. Mackey’s natural procedure. The refinement of exact global measurability is then introduced and the physically important class of “killing” momenta are found to be exactly classically and quantum mechanically globally measurable and moreover quantizable by means of Mackey’s scheme.
The second chapter seeks to analyse the algebraic structure of the Mackey-quantizable momenta, so as to compare and contrast his geometric scheme with the various algebraic schemes which have frequently been proposed. As an extension of this work, a natural geometric quantization is proposed for the more general class of observables “linear in momentum”, the set of quantizable such entities determines, and its algebraic properties discussed. It is concluded that, if Mackey’s procedure is, as we argue, correct and exhaustive of the quantizable momenta, then algebraic rules of quantization do not, in general, obtain among the momenta defined over a Riemannian space, ‘though in the case of momenta, such as the Killing momenta, reflecting the symmetry of the configuration manifold, such laws not only exist but are moreover of great practical importance.
The third chapter is concerned with the quantization of the observables “multilinear in momentum” and in particular with the delimitation of the possible operators of formal quantization as circumscribed by the requirement of operator symmetry, and the question of the essential self-adjointness of local observables associated with the multilinear momenta. Finally some possible means of determining and exact quantization are discussed and contrasted and a tentative selection made of a particular scheme which is then illustrated by concrete example on the real line with the usual metric.
We conclude this preface with a brief indication of the layout of the thesis: Each chapter or associated group of appendices has been treated for the purpose of all textual numbering and referencing systems, as a separate unit, and is prefixed by a detailed list of contents, and concluded by the appropriate group of reference, so that all pointers of a chapter or group of appendices refer, unless otherwise indicated, to the text of that chapter or group of appendices. Finally note that a bibliography has been included which will serve as a collected list of references to papers and books employed.Problems of the gauge theory of weak, electromagnetic and strong interactionsPapantonopoulos, Eleftherios G.https://hdl.handle.net/10023/145342019-04-01T10:06:41Z1980-01-01T00:00:00ZThe aim of this thesis is to present and discuss some mathematical and physical problems in the theory of weak, electromagnetic and strong interactions. Our main concern is a parallel development of mathematical and physical concepts and when it is possible, an attempt to bridge the abstract mathematical formulations with physical ideas. A central role in this thesis is played by a general construction scheme, which enables us to calculate explicitly all the mathematical quantities like matrix elements, Clebsch-Gordan series, Clebsch-Gordan coefficients which are necessary for a Grand Unification model construction. In this content, we have followed two basic principles: simplicity and applicability. To meet the first principle, all the construction methods developed are based on first principles and basic concepts of the Lie algebras and its representation theory, like roots and weights. Moreover, the requirement of applicability is met with the implementation of all the algorithms into computer programs. In the physical area, we have concentrated on the problem of mass. The lepton mass spectrum us studied in a theory of weak and electromagnetic interactions, while the mass problem of the SO(10) Grand Unified theory is analysed as a direct application of our Lie group construction scheme.
1980-01-01T00:00:00ZPapantonopoulos, Eleftherios G.The aim of this thesis is to present and discuss some mathematical and physical problems in the theory of weak, electromagnetic and strong interactions. Our main concern is a parallel development of mathematical and physical concepts and when it is possible, an attempt to bridge the abstract mathematical formulations with physical ideas. A central role in this thesis is played by a general construction scheme, which enables us to calculate explicitly all the mathematical quantities like matrix elements, Clebsch-Gordan series, Clebsch-Gordan coefficients which are necessary for a Grand Unification model construction. In this content, we have followed two basic principles: simplicity and applicability. To meet the first principle, all the construction methods developed are based on first principles and basic concepts of the Lie algebras and its representation theory, like roots and weights. Moreover, the requirement of applicability is met with the implementation of all the algorithms into computer programs. In the physical area, we have concentrated on the problem of mass. The lepton mass spectrum us studied in a theory of weak and electromagnetic interactions, while the mass problem of the SO(10) Grand Unified theory is analysed as a direct application of our Lie group construction scheme.Geometric quantisation and quantum mechanics in Dirac's front formPowis, J. J.https://hdl.handle.net/10023/145332019-04-01T10:02:55Z1994-01-01T00:00:00ZWe give a brief review of geometric quantisation up to and including the Blattner-Kostant-Sternberg kernal. In general this leads to symmetric operators that are not essentially self-adjoint so motivating a study of Hermitian operators as observables in a generalised quantum mechanics. We show that a generalised squaring axiom can reproduce the results of Blattner-Kostant-Sternberg quantisation. We also show that quantisation with respect to polarisations with compact leaves gives results that conflict with the nonlocal nature of quantum mechanics. We develop a front form quantum mechanics of a free scalar particle using geometric quantisation. The front and instant forms are related via unitary maps derived from the pairing which intertwines quantisations with respect to the forms. The front form position operator has a maximally symmetric component so we are compelled to work within the framework of a generalised quantum mechanics; the result in there being no Hegerfeldt type instantaneous spreading of initially localised wavefunctions in the front form. Finally we show that this model can be lifted to a many particle free field theory.
1994-01-01T00:00:00ZPowis, J. J.We give a brief review of geometric quantisation up to and including the Blattner-Kostant-Sternberg kernal. In general this leads to symmetric operators that are not essentially self-adjoint so motivating a study of Hermitian operators as observables in a generalised quantum mechanics. We show that a generalised squaring axiom can reproduce the results of Blattner-Kostant-Sternberg quantisation. We also show that quantisation with respect to polarisations with compact leaves gives results that conflict with the nonlocal nature of quantum mechanics. We develop a front form quantum mechanics of a free scalar particle using geometric quantisation. The front and instant forms are related via unitary maps derived from the pairing which intertwines quantisations with respect to the forms. The front form position operator has a maximally symmetric component so we are compelled to work within the framework of a generalised quantum mechanics; the result in there being no Hegerfeldt type instantaneous spreading of initially localised wavefunctions in the front form. Finally we show that this model can be lifted to a many particle free field theory.On the interatomic potential of neonNicol, William Menzieshttps://hdl.handle.net/10023/145322019-04-01T10:05:31Z1956-01-01T00:00:00ZIt is not an exaggeration to say that the knowledge of the interatomic potential energy is the stepping stone to the physical properties of matter. In the case of gases the most important commutation on the path of development is between the interatomic potential energy on the one hand and the equation of state and the transport properties – viz. conductivity, viscosity and diffusion – on the other hand. However, for the solid state, the role of interatomic forces in crystal structures should not be overlooked. It is clear that the development is reversible in that the potential energy could be determined from one or more of the physical properties mentioned above and vice versa. A great many of the existing interatomic potential energies in the literature are of this phenomenological kind obtained by fitting the experimental results to an analytic function. Theoretically it should be possible to determine this energy from first principles and thereby arrive at values for the physical properties for comparison with the experimental results. The latter is the objective of this investigation.
1956-01-01T00:00:00ZNicol, William MenziesIt is not an exaggeration to say that the knowledge of the interatomic potential energy is the stepping stone to the physical properties of matter. In the case of gases the most important commutation on the path of development is between the interatomic potential energy on the one hand and the equation of state and the transport properties – viz. conductivity, viscosity and diffusion – on the other hand. However, for the solid state, the role of interatomic forces in crystal structures should not be overlooked. It is clear that the development is reversible in that the potential energy could be determined from one or more of the physical properties mentioned above and vice versa. A great many of the existing interatomic potential energies in the literature are of this phenomenological kind obtained by fitting the experimental results to an analytic function. Theoretically it should be possible to determine this energy from first principles and thereby arrive at values for the physical properties for comparison with the experimental results. The latter is the objective of this investigation.The stability of particle-like solutions of some non-linear field equationsAnderson, David Lessells Thomsonhttps://hdl.handle.net/10023/145302019-04-01T10:03:39Z1969-01-01T00:00:00ZThe object of this thesis is to examine the stability of particle-like solutions of the nonlinear field equation
▽²Ψ - 1/c² δ²Ψ/δt² = K²Ψ –μ² ΨΨ*Ψ+λ(ΨΨ*)²Ψ
with the particular form of time-dependence
Ψ = φ (r) e ⁻ ˡʷᵗ
Initially our interest is concentrated on the case λ = 0. We begin the analysis by finding spherically symmetric particle-like solutions, and then examining the stability of the lowest-order solution by first- order perturbation theory. Direct perturbation methods are then considered. This solution is found to be highly unstable whether it is time-independent (ω = 0) or not (ω ≠ 0).
The more general case λ ≠ 0 is next discussed. Particle-like solutions are found to exist in this case for
-∞ < λ (K² - w²/c²) μ⁴ < 3/16
On examining the stability of the lowest-order solutions of this generalised field equation, it is found that for correct choice of the field parameters stable time-dependent solutions can exist, some of which can also have the attractive feature that their energy density is positive definite. We conclude by considering some methods of extending the theory.
1969-01-01T00:00:00ZAnderson, David Lessells ThomsonThe object of this thesis is to examine the stability of particle-like solutions of the nonlinear field equation
▽²Ψ - 1/c² δ²Ψ/δt² = K²Ψ –μ² ΨΨ*Ψ+λ(ΨΨ*)²Ψ
with the particular form of time-dependence
Ψ = φ (r) e ⁻ ˡʷᵗ
Initially our interest is concentrated on the case λ = 0. We begin the analysis by finding spherically symmetric particle-like solutions, and then examining the stability of the lowest-order solution by first- order perturbation theory. Direct perturbation methods are then considered. This solution is found to be highly unstable whether it is time-independent (ω = 0) or not (ω ≠ 0).
The more general case λ ≠ 0 is next discussed. Particle-like solutions are found to exist in this case for
-∞ < λ (K² - w²/c²) μ⁴ < 3/16
On examining the stability of the lowest-order solutions of this generalised field equation, it is found that for correct choice of the field parameters stable time-dependent solutions can exist, some of which can also have the attractive feature that their energy density is positive definite. We conclude by considering some methods of extending the theory.Drag forces in liquid helium IIMartin, Colin N. B.https://hdl.handle.net/10023/145262019-04-01T10:03:40Z1969-01-01T00:00:00ZMeasurements of drag forces an spheres end a cylinder in open rectangular channels in liquid helium II heat flow and superflow wore mode at temperatures between 1.3°K and the λ-point. The drag forces were measured by the deflection of a torsion system suspended above the free surface of the liquid from a quarts fibre. In the heat flow experiments, the drag vas found to be similar to that which would be exerted by a classical fluid with the same velocity, viscosity and density as the norma1 component. Correlations of the drag coefficient D/( ½ ρ[sub]n²A) with the Reynolds numbers ρnᵛηᵈ/ηn and ρᵛnᵈ/ηn show that the former is much more suitable both in tor of eliminating temperature dependence and agreement with the classical value. Above 1.6°K, a small decrease in drag with increasing velocity was usually noticed; this was attributed to the onset of turbulence in the superfluid, giving rise to a component of drag in the direction of superfluid flow. Describing the turbulent superfluid so a laminarly flowing fluid with an effective viscosity ηs makes possible an ardor of magnitude estimate of ηs the decrease in drag; it is found to lie between 10 and 100 micropoise. At temperatures nearer the λ -point, the simple two fluid description appears to become lies adequate. In the superflow experiments, sphere an the cylinder arc dragged in the direction of the superfluid flow. Correlations of drag coefficient with Reynolds number suggest value for the effective viscosity of between 20 and 100 micropoise. In the cylinder superflow experiments, below a velocity of 2±1 am sec⁻¹, no drag was observable. This is attributed to an ideal flow regime and is bellowed to be the first direct demonstration of D'Alembert's wades, rely that an inviscid fluid can exert no drag on a body.
1969-01-01T00:00:00ZMartin, Colin N. B.Measurements of drag forces an spheres end a cylinder in open rectangular channels in liquid helium II heat flow and superflow wore mode at temperatures between 1.3°K and the λ-point. The drag forces were measured by the deflection of a torsion system suspended above the free surface of the liquid from a quarts fibre. In the heat flow experiments, the drag vas found to be similar to that which would be exerted by a classical fluid with the same velocity, viscosity and density as the norma1 component. Correlations of the drag coefficient D/( ½ ρ[sub]n²A) with the Reynolds numbers ρnᵛηᵈ/ηn and ρᵛnᵈ/ηn show that the former is much more suitable both in tor of eliminating temperature dependence and agreement with the classical value. Above 1.6°K, a small decrease in drag with increasing velocity was usually noticed; this was attributed to the onset of turbulence in the superfluid, giving rise to a component of drag in the direction of superfluid flow. Describing the turbulent superfluid so a laminarly flowing fluid with an effective viscosity ηs makes possible an ardor of magnitude estimate of ηs the decrease in drag; it is found to lie between 10 and 100 micropoise. At temperatures nearer the λ -point, the simple two fluid description appears to become lies adequate. In the superflow experiments, sphere an the cylinder arc dragged in the direction of the superfluid flow. Correlations of drag coefficient with Reynolds number suggest value for the effective viscosity of between 20 and 100 micropoise. In the cylinder superflow experiments, below a velocity of 2±1 am sec⁻¹, no drag was observable. This is attributed to an ideal flow regime and is bellowed to be the first direct demonstration of D'Alembert's wades, rely that an inviscid fluid can exert no drag on a body.Dissipative flow in the superfluid helium film in the temperature region 1.63K to 0.01KToft, Michael W.https://hdl.handle.net/10023/145222019-04-01T10:07:53Z1978-01-01T00:00:00ZExperiments on helium film flow over a stainless steel beaker rim were carried out in the temperature region 1.63K to 11mK. No previous measurements have been made below 35mK. The results of other studies of film flow below 1K are mutually conflicting; this is thought to originate from poor temperature stability, inadequate vibration isolation and contamination of the substrate. Careful attention was paid to these points in the design and construction of a demagnetisation cryostat, full details of which are given. Flow experiments from initial level differences of ~8mm confirmed the existence of a range of metastable transfer rates at all temperatures. The variation of the mean rate with rim height suggests that the film profile should be calculated using a van der Waals' exponent, n, of 2.85 ± .25. The low temperature increase in transfer rate reported by others was observed, but did not extend below 1K. Instead, the mean transfer rate was approximately constant from 1K to 400mK, in direct conflict with the predictions of thermal fluctuation theories of superfluid dissipation. Below 250mK the results corroborated closely those of Crum et al in that the transfer rate fell sharply with decreasing temperature. Discrepancies arose between runs below 60mK, some showing the transfer rate to decrease monotonically with temperature down to 20mK whilst others indicated a levelling out of the transfer rate, followed by a slight increase at the lowest temperatures. These effects were thought to originate from changes in the concentration of the ³He impurity in the film. Steady state driven flow experiments at level differences of <500μm provided the first evidence for the existence of dissipation at subcritical transfer rates at temperatures below 1K. That is, superfluid flow was shown to be never strictly frictionless. Above 1K, the form of the subcritical dissipation curve conformed to the predictions of thermal fluctuation theories, but the values of the parameters 3 and f₀ for flow over stainless steel were considerably smaller than those reported for flow over glass. The parameter f₀ was also found to be strongly temperature dependent. Below 1K, the onset of dissipation was seen to be much more gradual, and the form of the subcritical dissipation curve was independent of temperature down to 20mK. These findings were corroborated by measurements of the damping of the inertial oscillations occurring at the end of each flow experiment. At the lowest temperature attained, 11mK, a change in subcritical dissipative behaviour was observed; exponential damping of the oscillations indicated that the frictional force on unit mass of superfluid was now directly proportional to the superfluid velocity, and was of magnitude (4.56 ± .28) x 10⁻³ dynes g⁻¹ cm ⁻¹ s. Exponential damping was also observed at temperatures above 1K, where Robinson type thermal dissipation was the dominant mechanism at small transfer rates.
1978-01-01T00:00:00ZToft, Michael W.Experiments on helium film flow over a stainless steel beaker rim were carried out in the temperature region 1.63K to 11mK. No previous measurements have been made below 35mK. The results of other studies of film flow below 1K are mutually conflicting; this is thought to originate from poor temperature stability, inadequate vibration isolation and contamination of the substrate. Careful attention was paid to these points in the design and construction of a demagnetisation cryostat, full details of which are given. Flow experiments from initial level differences of ~8mm confirmed the existence of a range of metastable transfer rates at all temperatures. The variation of the mean rate with rim height suggests that the film profile should be calculated using a van der Waals' exponent, n, of 2.85 ± .25. The low temperature increase in transfer rate reported by others was observed, but did not extend below 1K. Instead, the mean transfer rate was approximately constant from 1K to 400mK, in direct conflict with the predictions of thermal fluctuation theories of superfluid dissipation. Below 250mK the results corroborated closely those of Crum et al in that the transfer rate fell sharply with decreasing temperature. Discrepancies arose between runs below 60mK, some showing the transfer rate to decrease monotonically with temperature down to 20mK whilst others indicated a levelling out of the transfer rate, followed by a slight increase at the lowest temperatures. These effects were thought to originate from changes in the concentration of the ³He impurity in the film. Steady state driven flow experiments at level differences of <500μm provided the first evidence for the existence of dissipation at subcritical transfer rates at temperatures below 1K. That is, superfluid flow was shown to be never strictly frictionless. Above 1K, the form of the subcritical dissipation curve conformed to the predictions of thermal fluctuation theories, but the values of the parameters 3 and f₀ for flow over stainless steel were considerably smaller than those reported for flow over glass. The parameter f₀ was also found to be strongly temperature dependent. Below 1K, the onset of dissipation was seen to be much more gradual, and the form of the subcritical dissipation curve was independent of temperature down to 20mK. These findings were corroborated by measurements of the damping of the inertial oscillations occurring at the end of each flow experiment. At the lowest temperature attained, 11mK, a change in subcritical dissipative behaviour was observed; exponential damping of the oscillations indicated that the frictional force on unit mass of superfluid was now directly proportional to the superfluid velocity, and was of magnitude (4.56 ± .28) x 10⁻³ dynes g⁻¹ cm ⁻¹ s. Exponential damping was also observed at temperatures above 1K, where Robinson type thermal dissipation was the dominant mechanism at small transfer rates.Measuring the Hubble constant from reverberating accretion discs in active galaxiesCollier, Stefan Jhttps://hdl.handle.net/10023/145172019-04-01T10:07:38Z1999-01-01T00:00:00ZThe standard paradigm of active galactic nuclei (AGN) postulates that their luminosity, L ~ 10³⁹⁻⁴⁸erg s⁻¹, derives from the accretion of gas onto a supermassive black hole, mass M ~ 10⁶⁻⁹M☉, at the centre of a host galaxy. Echo or reverberation mapping affords a method of relating flux variations at different wavelengths to determine the nature of the flux emitting regions, with μ-arcsecond resolution. The results of an intensive two-month campaign of ground based spectrophotometric monitoring of the Seyfert 1 galaxy NGC 7469, with a temporal resolution of ≤1 day, are presented. Application of echo mapping techniques reveal the virial mass of the central source to be MNGC 7469 ~10⁶⁻⁷ M☉, and a compact broad Balmer line emitting region ~ 5 light days from the central source. Together, this evidence suggests the existence of a supermassive black hole in NGC 7469. Further, evidence for significant wavelength- dependent continuum time delays is presented, with optical continuum variations lagging those at UV wavelengths by about 1-2 days. The wavelength-dependent time delays, (λ), are consistent with the predicted T ∝ λ ⁴/ ³ relationship for an irradiated blackbody accretion disc with temperature structure T(R) ∝ R⁻³/⁴ and hence may represent the indirect detection of an accretion disc structure in NGC 7469. It is shown that wavelength-dependent time delays test the standard black-hole accretion disc paradigm of AGN, by measuring T(R) of the gaseous material surrounding the purported black hole. Moreover, a new direct method is presented that combines observed time delays and the spectral energy distribution of an AGN to derive a redshift-independent luminosity distance; assuming the observed time delays are indeed due to a classical accretion disc structure. The luminosity distance permits an estimate of the Hubble constant, H₀-the expansion rate of the Universe. The first application of the method yields H₀(cos i/0.7)¹/² = 38 ± 7km s⁻¹ Mp ⁻¹. A more accurate determination of H₀ requires either an independent accurate determination of the disc inclination i or statistical average of a moderate sample of active galaxies. This method permits determination of redshift-independent luminosity distances to AGNs, thereby, giving a new route to H₀, and by extension to fainter objects at z ~ 1, q₀.
1999-01-01T00:00:00ZCollier, Stefan JThe standard paradigm of active galactic nuclei (AGN) postulates that their luminosity, L ~ 10³⁹⁻⁴⁸erg s⁻¹, derives from the accretion of gas onto a supermassive black hole, mass M ~ 10⁶⁻⁹M☉, at the centre of a host galaxy. Echo or reverberation mapping affords a method of relating flux variations at different wavelengths to determine the nature of the flux emitting regions, with μ-arcsecond resolution. The results of an intensive two-month campaign of ground based spectrophotometric monitoring of the Seyfert 1 galaxy NGC 7469, with a temporal resolution of ≤1 day, are presented. Application of echo mapping techniques reveal the virial mass of the central source to be MNGC 7469 ~10⁶⁻⁷ M☉, and a compact broad Balmer line emitting region ~ 5 light days from the central source. Together, this evidence suggests the existence of a supermassive black hole in NGC 7469. Further, evidence for significant wavelength- dependent continuum time delays is presented, with optical continuum variations lagging those at UV wavelengths by about 1-2 days. The wavelength-dependent time delays, (λ), are consistent with the predicted T ∝ λ ⁴/ ³ relationship for an irradiated blackbody accretion disc with temperature structure T(R) ∝ R⁻³/⁴ and hence may represent the indirect detection of an accretion disc structure in NGC 7469. It is shown that wavelength-dependent time delays test the standard black-hole accretion disc paradigm of AGN, by measuring T(R) of the gaseous material surrounding the purported black hole. Moreover, a new direct method is presented that combines observed time delays and the spectral energy distribution of an AGN to derive a redshift-independent luminosity distance; assuming the observed time delays are indeed due to a classical accretion disc structure. The luminosity distance permits an estimate of the Hubble constant, H₀-the expansion rate of the Universe. The first application of the method yields H₀(cos i/0.7)¹/² = 38 ± 7km s⁻¹ Mp ⁻¹. A more accurate determination of H₀ requires either an independent accurate determination of the disc inclination i or statistical average of a moderate sample of active galaxies. This method permits determination of redshift-independent luminosity distances to AGNs, thereby, giving a new route to H₀, and by extension to fainter objects at z ~ 1, q₀.A spectroscopic study of high mass X-ray binariesReynolds, Alastair P.https://hdl.handle.net/10023/145092019-04-01T10:03:46Z1992-01-01T00:00:00ZObservations of four massive x-ray binary stars are presented, based on data accumulated between February 1989 and August 1991. Using modern techniques of spectroscopic data analysis, velocity curves are derived for three of these systems. Two of these curves (SMC X-1, QV Nor) yield very precise mass estimates for the component stars, while the third (Cen X-3) offers a constraint on the possible masses. The fourth system (X Per) is not shown to exhibit periodic variations, despite an extensive study conducted over more than two years. For the two systems that yielded precise masses, the component neutron stars are shown to lie within the theoretical mass range based on theories of their formation via the supernova explosion of a helium star in a close binary system. This is a marked improvement on previous studies where both stars had estimated masses which lay well outside of the expected range. The derivation of these masses incorporates the use of non-Keplerian velocity corrections, arising from the non-spherical, asymmetrically illuminated primary stars. A study of the line profiles showed that the temperatures around both primary stars were consistent with the parameters in these calculations. For the third system, the inaccuracy of the published ephemeris resulted in a lack of observations at the times of maximum and minimum velocity. The semi-amplitude is thus not well constrained, but it is shown that the observations are consistent with the assumption of a normal mass neutron star secondary. The system is shown to have undergone a gradual decrease in its orbital period which follows a parabolic trend, suggesting substantial mass-transfer. For the fourth system, a periodicity analysis of 130 spectroscopic velocity measurements of a Be star, via Fouriergram and string-length techniques, failed to highlight any strong periodicity. The scatter in the data appears larger than would be expected for a non-variable B star. The absence of periodic velocity variations at the expected period is discussed in terms of the binarity (or otherwise) of the Be star. A transition from Be to shell-star or ordinary B star phase occurred during the study, which is not evident from the spectral variations observed in the blue.
1992-01-01T00:00:00ZReynolds, Alastair P.Observations of four massive x-ray binary stars are presented, based on data accumulated between February 1989 and August 1991. Using modern techniques of spectroscopic data analysis, velocity curves are derived for three of these systems. Two of these curves (SMC X-1, QV Nor) yield very precise mass estimates for the component stars, while the third (Cen X-3) offers a constraint on the possible masses. The fourth system (X Per) is not shown to exhibit periodic variations, despite an extensive study conducted over more than two years. For the two systems that yielded precise masses, the component neutron stars are shown to lie within the theoretical mass range based on theories of their formation via the supernova explosion of a helium star in a close binary system. This is a marked improvement on previous studies where both stars had estimated masses which lay well outside of the expected range. The derivation of these masses incorporates the use of non-Keplerian velocity corrections, arising from the non-spherical, asymmetrically illuminated primary stars. A study of the line profiles showed that the temperatures around both primary stars were consistent with the parameters in these calculations. For the third system, the inaccuracy of the published ephemeris resulted in a lack of observations at the times of maximum and minimum velocity. The semi-amplitude is thus not well constrained, but it is shown that the observations are consistent with the assumption of a normal mass neutron star secondary. The system is shown to have undergone a gradual decrease in its orbital period which follows a parabolic trend, suggesting substantial mass-transfer. For the fourth system, a periodicity analysis of 130 spectroscopic velocity measurements of a Be star, via Fouriergram and string-length techniques, failed to highlight any strong periodicity. The scatter in the data appears larger than would be expected for a non-variable B star. The absence of periodic velocity variations at the expected period is discussed in terms of the binarity (or otherwise) of the Be star. A transition from Be to shell-star or ordinary B star phase occurred during the study, which is not evident from the spectral variations observed in the blue.A spectroscopic study of binary systemsMcLean, Brian Johnhttps://hdl.handle.net/10023/145032019-04-01T10:09:53Z1981-01-01T00:00:00ZSpectroscopic observations of some of the brighter W Ursae Majoris type contact binaries in the northern hemisphere have been obtained at a medium dispersion of 20-30 Å/mm. A computer program has been developed to process and analyse these spectra using modem digital reduction techniques in order to obtain more accurate spectroscopic data for such systems. Radial velocity measurements have provided spectroscopic mass ratios which have been compared with the values derived from light curve synthesis. The improved mass ratios have removed the discrepancy which existed between the spectroscopic and photometric values in most of the systems observed. The application of spectrum deconvolution techniques has revealed an asymmetry in the structure of two W subclass systems shown by the variation in the strength of the primary components' spectral lines. Measurement, of the relative line strengths of the two components has provided further evidence for the presence of extensive magnetic starspots on the surface of the primary component of these systems.
1981-01-01T00:00:00ZMcLean, Brian JohnSpectroscopic observations of some of the brighter W Ursae Majoris type contact binaries in the northern hemisphere have been obtained at a medium dispersion of 20-30 Å/mm. A computer program has been developed to process and analyse these spectra using modem digital reduction techniques in order to obtain more accurate spectroscopic data for such systems. Radial velocity measurements have provided spectroscopic mass ratios which have been compared with the values derived from light curve synthesis. The improved mass ratios have removed the discrepancy which existed between the spectroscopic and photometric values in most of the systems observed. The application of spectrum deconvolution techniques has revealed an asymmetry in the structure of two W subclass systems shown by the variation in the strength of the primary components' spectral lines. Measurement, of the relative line strengths of the two components has provided further evidence for the presence of extensive magnetic starspots on the surface of the primary component of these systems.A spectroscopic investigation of the hydro-deficient binary Upsilon Sgr, and related objectsDudley, Richard E.https://hdl.handle.net/10023/144262019-04-01T10:06:15Z1992-01-01T00:00:00ZThe bright hydrogen-deficient binary Upsilon Sgr has been studied in detail in the areas of radial velocities, flux distribution, temperature, surface gravity, composition and mass loss using the analysis of spectroscopic data. From IUE high resolution spectra the primary radial velocity orbit has been confirmed (K₁ = 47.3±0.8kms⁻¹) and the secondary radial velocity curve determined for the first time (K₂ = 29.7±1.7kms⁻¹). The orbit has been constrained to an inclination of 65 - 78°. The masses of the primary and secondary are then 3.0 ± 0.3 and 4.8 ± 0.5M☉ respectively, with an orbital separation of 230±20R☉.The primary is expected to fill its Roche Lobe at a radius of ~60R☉.Using all the available photometry and line-blanketed hydrogen-deficient model atmospheres the primary effective temperature has been determined to be 11,800±500K. IUE spectra were used to determine an extinction of E[sub]B-v = 0.20±0.05 leading to a distance of ~1400pc and a luminosity of 61,000(+11,000)/(-5,000) L☉.. The long standing problem of the flux distribution has been mostly solved and little of the UV flux shortward of 1400 Å comes from a hot bright secondary. The same model atmospheres and a high resolution CCD/echelle spectrum was used to confirm the temperature (Teff= 11,750±750K) and to obtain a surface gravity of log g = 1.5±0.5. 𝜐 Sgr is ~ 99.6% helium, 0.016% H with CNO abundances indicative of advanced evolution and high main sequence mass.
From profile fitting of the UV resonance lines of C, N and Si the mass-loss rate from 𝜐 Sgr is at least 2.5 x10⁻¹°M☉yr⁻¹. The upper limit to the mass-loss rate is 1.0 x10 ⁻⁵M☉yr⁻¹ from the lack of observed changes in the orbital parameters. The mass of the primary component and the low likely mass-loss rate indicate that the primary component will not be able to shed enough material before core collapse, an event that will be classed as a type Ib supernova. This is the first solid evidence that hydrogen-deficient binaries are progenitors of these objects although other candidates are not ruled out. The other known hydrogen-deficient binaries (KS Per, LSS 1922 and LSS 4300) have had their temperatures (12,500±500K, 12,000±500K, 12,000±1,000K) and interstellar extinctions (0.55± 0.05, 0.80±0.05, 0.90±0.10) re-determined from a flux distribution analysis. Mass-loss rates from 6 O stars, 3 Extreme Helium stars and 2 sdO stars have also been determined. The O and EHe results broadly agree with the previous determinations. Results for the 2 sdO stars, BD H-37° 1977 and BD +37 ° 442, are presented from profile fitting for the first time and indicate a mass-loss rate slightly higher than for the EHe stars.
1992-01-01T00:00:00ZDudley, Richard E.The bright hydrogen-deficient binary Upsilon Sgr has been studied in detail in the areas of radial velocities, flux distribution, temperature, surface gravity, composition and mass loss using the analysis of spectroscopic data. From IUE high resolution spectra the primary radial velocity orbit has been confirmed (K₁ = 47.3±0.8kms⁻¹) and the secondary radial velocity curve determined for the first time (K₂ = 29.7±1.7kms⁻¹). The orbit has been constrained to an inclination of 65 - 78°. The masses of the primary and secondary are then 3.0 ± 0.3 and 4.8 ± 0.5M☉ respectively, with an orbital separation of 230±20R☉.The primary is expected to fill its Roche Lobe at a radius of ~60R☉.Using all the available photometry and line-blanketed hydrogen-deficient model atmospheres the primary effective temperature has been determined to be 11,800±500K. IUE spectra were used to determine an extinction of E[sub]B-v = 0.20±0.05 leading to a distance of ~1400pc and a luminosity of 61,000(+11,000)/(-5,000) L☉.. The long standing problem of the flux distribution has been mostly solved and little of the UV flux shortward of 1400 Å comes from a hot bright secondary. The same model atmospheres and a high resolution CCD/echelle spectrum was used to confirm the temperature (Teff= 11,750±750K) and to obtain a surface gravity of log g = 1.5±0.5. 𝜐 Sgr is ~ 99.6% helium, 0.016% H with CNO abundances indicative of advanced evolution and high main sequence mass.
From profile fitting of the UV resonance lines of C, N and Si the mass-loss rate from 𝜐 Sgr is at least 2.5 x10⁻¹°M☉yr⁻¹. The upper limit to the mass-loss rate is 1.0 x10 ⁻⁵M☉yr⁻¹ from the lack of observed changes in the orbital parameters. The mass of the primary component and the low likely mass-loss rate indicate that the primary component will not be able to shed enough material before core collapse, an event that will be classed as a type Ib supernova. This is the first solid evidence that hydrogen-deficient binaries are progenitors of these objects although other candidates are not ruled out. The other known hydrogen-deficient binaries (KS Per, LSS 1922 and LSS 4300) have had their temperatures (12,500±500K, 12,000±500K, 12,000±1,000K) and interstellar extinctions (0.55± 0.05, 0.80±0.05, 0.90±0.10) re-determined from a flux distribution analysis. Mass-loss rates from 6 O stars, 3 Extreme Helium stars and 2 sdO stars have also been determined. The O and EHe results broadly agree with the previous determinations. Results for the 2 sdO stars, BD H-37° 1977 and BD +37 ° 442, are presented from profile fitting for the first time and indicate a mass-loss rate slightly higher than for the EHe stars.Photospheric parameters of early-type starsStewart, Gordon C.https://hdl.handle.net/10023/144222019-04-01T10:07:18Z1984-01-01T00:00:00ZThe development of an interactive data reduction and analysis system for astronomical spectra written in the FORTH computing language and operated on the Nova 820 minicomputer at the University Observatory, St. Andrews is discussed. Spectra of a number of B stars, many of which were obtained by the author at the South African Astronomical Observatory, were measured and reduced using this system. Equivalent widths of metal absorption lines are used to deduce the stars' fundamental photospheric parameters and abundances by comparison with the predictions of both non-LTE and line-blanketed LTE models. It is shown that effective temperatures derived with the line-blanketed LTE models are in good agreement with those found by other authors for near main sequence B stars using measurements of the integrated flux of the stars. Unblanketed non-LTE models give effective temperatures which are systematically high by approximately 7%, similar to the difference between blanketed and unblanketed LTE models. The major differences between the predictions for absorption lines using LTE and non-LTE models are due to the different level populations predicted and not to the different photospheric structures. The use of non-LTE models is found to reduce the microturbulence required to bring element abundances derived from strong lines into agreement with those found for weak lines but not to remove the need for the inclusion of microturbulence in the solutions for most B stars. An investigation of the suitability of the analysis of high resolution Michelson interferometry using Fourier decomposition techniques as a means of determining the broadening mechanisms and velocity fields in the photospheres of early A stars is made.
1984-01-01T00:00:00ZStewart, Gordon C.The development of an interactive data reduction and analysis system for astronomical spectra written in the FORTH computing language and operated on the Nova 820 minicomputer at the University Observatory, St. Andrews is discussed. Spectra of a number of B stars, many of which were obtained by the author at the South African Astronomical Observatory, were measured and reduced using this system. Equivalent widths of metal absorption lines are used to deduce the stars' fundamental photospheric parameters and abundances by comparison with the predictions of both non-LTE and line-blanketed LTE models. It is shown that effective temperatures derived with the line-blanketed LTE models are in good agreement with those found by other authors for near main sequence B stars using measurements of the integrated flux of the stars. Unblanketed non-LTE models give effective temperatures which are systematically high by approximately 7%, similar to the difference between blanketed and unblanketed LTE models. The major differences between the predictions for absorption lines using LTE and non-LTE models are due to the different level populations predicted and not to the different photospheric structures. The use of non-LTE models is found to reduce the microturbulence required to bring element abundances derived from strong lines into agreement with those found for weak lines but not to remove the need for the inclusion of microturbulence in the solutions for most B stars. An investigation of the suitability of the analysis of high resolution Michelson interferometry using Fourier decomposition techniques as a means of determining the broadening mechanisms and velocity fields in the photospheres of early A stars is made.A spectroscopic study of some early-type emission-line starsJarad, Majeed M.https://hdl.handle.net/10023/144182019-04-01T10:06:27Z1986-01-01T00:00:00ZSpectroscopic observations of 18 of the brighter Be and 0e stars in the Northern Hemisphere have been presented in this project. About 900 spectrograms were secured and measured using the more objective numerical technique (REDUCE and VCROSS) to obtain as many accurate radial velocities as possible for these stars. The analysis of these measurements has demonstrated clearly, that most of these early-type stars are indeed variable in radial velocity, while a real periodicity in the radial velocities of some of the programme stars is found. The long-term periodicities were attributed to the binary nature, while the short ones could be ascribed to the pulsation phenomenon. Orbital elements are determined for four newly discovered binary systems, and improved orbits are determined for four known binaries. Five other stars display short time-scale periodicity (less than 1.5 days) attributable to radial or non-radial pulsation, whilst three variables show little evidence for periodicity. Only two stars are found to have constant radial velocities. A brief summary of the properties of Be stars and all the available models to explain the Be phenomenon is presented in chapter one. Chapter two contains a brief description of the observing equipment and the techniques used. The reasons for selecting the programme stars are given in the same chapter. A complete explanation of the measuring technique used in this investigation is given in chapter three. In chapter four, all the analyses and the results for each star are given individually, while a discussion of previous work on these stars appears in the relevant sections. The final chapter contains a discussion of the results from this study together with an analysis of the energy distributions of the programme stars, compiled by the author from published fluxes and photometry from the ultraviolet to the infra-red parts of the electromagnetic spectrum. Some consideration of future work to be done on these types of stars is also given.
1986-01-01T00:00:00ZJarad, Majeed M.Spectroscopic observations of 18 of the brighter Be and 0e stars in the Northern Hemisphere have been presented in this project. About 900 spectrograms were secured and measured using the more objective numerical technique (REDUCE and VCROSS) to obtain as many accurate radial velocities as possible for these stars. The analysis of these measurements has demonstrated clearly, that most of these early-type stars are indeed variable in radial velocity, while a real periodicity in the radial velocities of some of the programme stars is found. The long-term periodicities were attributed to the binary nature, while the short ones could be ascribed to the pulsation phenomenon. Orbital elements are determined for four newly discovered binary systems, and improved orbits are determined for four known binaries. Five other stars display short time-scale periodicity (less than 1.5 days) attributable to radial or non-radial pulsation, whilst three variables show little evidence for periodicity. Only two stars are found to have constant radial velocities. A brief summary of the properties of Be stars and all the available models to explain the Be phenomenon is presented in chapter one. Chapter two contains a brief description of the observing equipment and the techniques used. The reasons for selecting the programme stars are given in the same chapter. A complete explanation of the measuring technique used in this investigation is given in chapter three. In chapter four, all the analyses and the results for each star are given individually, while a discussion of previous work on these stars appears in the relevant sections. The final chapter contains a discussion of the results from this study together with an analysis of the energy distributions of the programme stars, compiled by the author from published fluxes and photometry from the ultraviolet to the infra-red parts of the electromagnetic spectrum. Some consideration of future work to be done on these types of stars is also given.The calculation of stellar opacityHollingsworth, Helen M.https://hdl.handle.net/10023/144142019-04-01T10:03:22Z1966-01-01T00:00:00Z1966-01-01T00:00:00ZHollingsworth, Helen M.An investigation of a supercluster of galaxies in Piscis AustrinisBunclark, Peter Shttps://hdl.handle.net/10023/144112019-04-01T10:04:12Z1986-01-01T00:00:00ZAn examination of a IIIaJ plate taken on the United Kingdom Schmidt Telescope of Survey field 405 revealed a possible supercluster of clusters of galaxies. Three rich clusters could be seen grouped within one degree, and which appeared to be of similar distance. The project described in this thesis has investigated the supercluster hypothesis by determining relative (and less precisely, absolute) distances to the three component clusters. It is described how the photographic material was painstakingly reduced to relative magnitudes and colours, and how these values were calibrated using a stellar sequence photo-electrically observed using the 1m telescope at South African Astronomical Observatory. Radial velocities were determined by a process which maximises the amount of information derived from intrinsically low-precision objective prism material. It is found that the Supercluster has, within the errors, a line-of-sight dimension of twice its projected dimension; this suggests that in fact the clusters of galaxies are in as close proximity spatially as they are apparently. The distance derived to the supercluster is 550 Mpc, giving a projected diameter of 10Mpc, with a recession velocity of 47700 kms⁻¹ which Leads to a determination of Hubble's constant: H₀ = 87 ±20kms⁻¹ Mpc⁻¹ .
1986-01-01T00:00:00ZBunclark, Peter SAn examination of a IIIaJ plate taken on the United Kingdom Schmidt Telescope of Survey field 405 revealed a possible supercluster of clusters of galaxies. Three rich clusters could be seen grouped within one degree, and which appeared to be of similar distance. The project described in this thesis has investigated the supercluster hypothesis by determining relative (and less precisely, absolute) distances to the three component clusters. It is described how the photographic material was painstakingly reduced to relative magnitudes and colours, and how these values were calibrated using a stellar sequence photo-electrically observed using the 1m telescope at South African Astronomical Observatory. Radial velocities were determined by a process which maximises the amount of information derived from intrinsically low-precision objective prism material. It is found that the Supercluster has, within the errors, a line-of-sight dimension of twice its projected dimension; this suggests that in fact the clusters of galaxies are in as close proximity spatially as they are apparently. The distance derived to the supercluster is 550 Mpc, giving a projected diameter of 10Mpc, with a recession velocity of 47700 kms⁻¹ which Leads to a determination of Hubble's constant: H₀ = 87 ±20kms⁻¹ Mpc⁻¹ .Multicolour photometry of globular cluster starsPike, Christopher Davidhttps://hdl.handle.net/10023/144082019-04-01T10:04:44Z1977-01-01T00:00:00ZThe underlying theme of this thesis is the application of electronography to the study of stars in globular clusters through the use of both broad and intermediate-band photometric systems. Introductions to both the study of globular clusters and to the use of electronography are given in Chapter 1. In Chapters 2 and 3 observations based upon electronographic exposures are presented for two little-studied clusters, NGC 5053 and NGC 6366. Both are loose, sparsely populated clusters but despite their similarity in appearance, the photometry shows that NGC 5053 is an unreddened, metal-poor globular, while NGC 6366 is found to be a highly reddened metal-rich cluster. One variable star in NGC 6366 is shown to be an a-type RR Lyrae - an unusual occurrence in a cluster of high metallicity. Chapter 4 describes developments in the reduction of stellar electronographs made by the author. An automatic method of fitting Gaussian profiles to the density volumes is shown to produce photometry as accurate and linear as previous manual techniques, but with a much increased efficiency. This work is extended to investigate the use of more flexible profiles which are then shown to be useful for allowing photometry of crowded images. Finally, an application of the Gaussian profile fitting routines to photographic stellar photometry is discussed. Chapter 5 reports the use of this reduction scheme on photographic plates of the globular cluster M 15. Using the David Dunlap Observatory (DDO) intermediate-band photometric system, cluster members are easily distinguished from foreground dwarfs. Chapter 6 describes the results of a programme of DDO electronography of the globular clusters M 5 and M 13. An accuracy of 0.03 is obtained for the DDO colours which, although comparable with some earlier photoelectric work, shows that, with the observational and reduction techniques employed, electronographic stellar photometry has not yet attained its potential as two-dimensional photoelectric photometry.
1977-01-01T00:00:00ZPike, Christopher DavidThe underlying theme of this thesis is the application of electronography to the study of stars in globular clusters through the use of both broad and intermediate-band photometric systems. Introductions to both the study of globular clusters and to the use of electronography are given in Chapter 1. In Chapters 2 and 3 observations based upon electronographic exposures are presented for two little-studied clusters, NGC 5053 and NGC 6366. Both are loose, sparsely populated clusters but despite their similarity in appearance, the photometry shows that NGC 5053 is an unreddened, metal-poor globular, while NGC 6366 is found to be a highly reddened metal-rich cluster. One variable star in NGC 6366 is shown to be an a-type RR Lyrae - an unusual occurrence in a cluster of high metallicity. Chapter 4 describes developments in the reduction of stellar electronographs made by the author. An automatic method of fitting Gaussian profiles to the density volumes is shown to produce photometry as accurate and linear as previous manual techniques, but with a much increased efficiency. This work is extended to investigate the use of more flexible profiles which are then shown to be useful for allowing photometry of crowded images. Finally, an application of the Gaussian profile fitting routines to photographic stellar photometry is discussed. Chapter 5 reports the use of this reduction scheme on photographic plates of the globular cluster M 15. Using the David Dunlap Observatory (DDO) intermediate-band photometric system, cluster members are easily distinguished from foreground dwarfs. Chapter 6 describes the results of a programme of DDO electronography of the globular clusters M 5 and M 13. An accuracy of 0.03 is obtained for the DDO colours which, although comparable with some earlier photoelectric work, shows that, with the observational and reduction techniques employed, electronographic stellar photometry has not yet attained its potential as two-dimensional photoelectric photometry.Magnetic activity in late-type starsHussain, Gaiteehttps://hdl.handle.net/10023/144042019-04-22T08:50:41Z1999-01-01T00:00:00ZHigh resolution spectroscopic techniques are used to investigate the magnetic topology of the young, rapidly rotating star, AB Dor. Doppler images of AB Dor are produced using the Li I 6708 Å line and compared to images produced using Ca I and Fe i lines. A Li abundance of 2.9±0.3 dex is measured using LTE profile synthesis. By evaluating the effects of enhanced spot Li abundances we find that starspots are unlikely to lead to an overestimation of the photospheric Li abundance. While the Li I 6708 Å line is strengthened in the presence of starspots, the extent to which the Li I line equivalent width displays rotational modulation is diluted. A Zeeman Doppler imaging code, which maps the surface magnetic flux distributions on rapid rotators, has been developed and tested in this thesis. Reliability tests indicate that this technique is robust with respect to small errors in line and stellar parameters. Spectropolarimetric observations of AB Dor from three years are presented here. The technique of least squares deconvolution, combines the signal from over 1500 lines, enhancing the S/N of the observed dataset. Brightness and magnetic maps of AB Dor are reconstructed using the deconvolved profiles and show excellent agreement with maps produced using independently developed codes by J.-F. Donati. This points to the consistency of Doppler imaging techniques. Magnetic field maps show the presence of strong radial and azimuthal fields at almost all latitudes. This non-solar like distribution of magnetic flux may indicate the presence of a distributed dynamo component operating in the convection zones of rapid rotators. By tracing the positions of absorption transients in Ha, we find prominence complexes at almost all longitudes. Footpoint locations remain difficult to ascertain. Prominences are ejected over much shorter timescales than the timescale over which differential rotation should act further observations with closer time sampling are necessary to investigate the coronal topology further.
1999-01-01T00:00:00ZHussain, GaiteeHigh resolution spectroscopic techniques are used to investigate the magnetic topology of the young, rapidly rotating star, AB Dor. Doppler images of AB Dor are produced using the Li I 6708 Å line and compared to images produced using Ca I and Fe i lines. A Li abundance of 2.9±0.3 dex is measured using LTE profile synthesis. By evaluating the effects of enhanced spot Li abundances we find that starspots are unlikely to lead to an overestimation of the photospheric Li abundance. While the Li I 6708 Å line is strengthened in the presence of starspots, the extent to which the Li I line equivalent width displays rotational modulation is diluted. A Zeeman Doppler imaging code, which maps the surface magnetic flux distributions on rapid rotators, has been developed and tested in this thesis. Reliability tests indicate that this technique is robust with respect to small errors in line and stellar parameters. Spectropolarimetric observations of AB Dor from three years are presented here. The technique of least squares deconvolution, combines the signal from over 1500 lines, enhancing the S/N of the observed dataset. Brightness and magnetic maps of AB Dor are reconstructed using the deconvolved profiles and show excellent agreement with maps produced using independently developed codes by J.-F. Donati. This points to the consistency of Doppler imaging techniques. Magnetic field maps show the presence of strong radial and azimuthal fields at almost all latitudes. This non-solar like distribution of magnetic flux may indicate the presence of a distributed dynamo component operating in the convection zones of rapid rotators. By tracing the positions of absorption transients in Ha, we find prominence complexes at almost all longitudes. Footpoint locations remain difficult to ascertain. Prominences are ejected over much shorter timescales than the timescale over which differential rotation should act further observations with closer time sampling are necessary to investigate the coronal topology further.Southern hemisphere early-type stars at intermediate and high galactic latitudesKilkenny, Davidhttps://hdl.handle.net/10023/143932019-04-01T10:02:28Z1973-01-01T00:00:00ZA survey of early-type stars at intermediate galactic latitudes was carried out in the southern hemisphere winters of 1970 and 1971. The observing programme was limited to negative declinations and covered a range in right ascension of approximately 12ʰ to 20ʰ. At the Royal Observatory, Cape Town, in 1970, UBV photoelectric measurements were made of 56 stars for which no UBV data existed and 20 stars which had been observed on one or two previous occasions, the intention being to obtain four separate measures of each star. In 19711 the Bochum University telescope at the E.S.O. site in Chile was used for H/3 photoelectric photometry of over 200 intermediate and high latitude stars. Shortly afterwards, spectra for radial velocity determination and MK classification were obtained with the two-prism spectrograph and 74" reflector of Radcliffe Observatory, Pretoria. Work was concentrated upon some 60 stars not previously observed with spectroscopic equipment and selected on the basis of blue colour or possible high luminosity from photometric considerations. A few southern standard stars and stars from earlier Radcliffe programmes were re-observed as control or overlap stars. Chapters II - IV describe the observational procedures and reduction methods. Tables in chapter V contain results from the 1970-71 programmes plus UBV and spectroscopic data for intermediate and high latitude stars from various other sources. The remaining chapters are concerned with analysis and discussion of the observations. Chapter VI summarises some optical and radio determinations of the spiral structure of the Galaxy and compares the spatial distribution of the programme stars with these results. The possibility that early-type stars may be formed well away from the galactic plane is considered by comparison of kinematic and evolutionary lifetimes of some stars at appreciable distances from the plane. In chapter VII, intermediate and high latitude stars are shown to participate in the differential rotation of the Galaxy and detailed analysis of the space motions of a number of high velocity stars leads to the conclusion that some may .have sufficient energy to escape from the galactic system. The radial velocities of interstellar Ca II lines are shown in chapter VIII to be as expected for material in the solar neighbourhood involved in differential galactic rotation. An apparent deviation from circular motion reported by observers investigating H II regions is also present in the Ca II gas. Constants in the cosecant equation of interstellar reddening are re-determined and show an apparently significant difference between northern and southern galactic hemispheres. Appendix I describes attempts to simulate the effect of filters in order to explain the curvature in the transformations from instrumental to standard photometric systems. Appendix II gives details of the method used to compute stellar space velocities from proper motions and radial velocities and includes a short Fortran IV programme which implements the operations described.
1973-01-01T00:00:00ZKilkenny, DavidA survey of early-type stars at intermediate galactic latitudes was carried out in the southern hemisphere winters of 1970 and 1971. The observing programme was limited to negative declinations and covered a range in right ascension of approximately 12ʰ to 20ʰ. At the Royal Observatory, Cape Town, in 1970, UBV photoelectric measurements were made of 56 stars for which no UBV data existed and 20 stars which had been observed on one or two previous occasions, the intention being to obtain four separate measures of each star. In 19711 the Bochum University telescope at the E.S.O. site in Chile was used for H/3 photoelectric photometry of over 200 intermediate and high latitude stars. Shortly afterwards, spectra for radial velocity determination and MK classification were obtained with the two-prism spectrograph and 74" reflector of Radcliffe Observatory, Pretoria. Work was concentrated upon some 60 stars not previously observed with spectroscopic equipment and selected on the basis of blue colour or possible high luminosity from photometric considerations. A few southern standard stars and stars from earlier Radcliffe programmes were re-observed as control or overlap stars. Chapters II - IV describe the observational procedures and reduction methods. Tables in chapter V contain results from the 1970-71 programmes plus UBV and spectroscopic data for intermediate and high latitude stars from various other sources. The remaining chapters are concerned with analysis and discussion of the observations. Chapter VI summarises some optical and radio determinations of the spiral structure of the Galaxy and compares the spatial distribution of the programme stars with these results. The possibility that early-type stars may be formed well away from the galactic plane is considered by comparison of kinematic and evolutionary lifetimes of some stars at appreciable distances from the plane. In chapter VII, intermediate and high latitude stars are shown to participate in the differential rotation of the Galaxy and detailed analysis of the space motions of a number of high velocity stars leads to the conclusion that some may .have sufficient energy to escape from the galactic system. The radial velocities of interstellar Ca II lines are shown in chapter VIII to be as expected for material in the solar neighbourhood involved in differential galactic rotation. An apparent deviation from circular motion reported by observers investigating H II regions is also present in the Ca II gas. Constants in the cosecant equation of interstellar reddening are re-determined and show an apparently significant difference between northern and southern galactic hemispheres. Appendix I describes attempts to simulate the effect of filters in order to explain the curvature in the transformations from instrumental to standard photometric systems. Appendix II gives details of the method used to compute stellar space velocities from proper motions and radial velocities and includes a short Fortran IV programme which implements the operations described.An investigation of certain problems related to the classification and physical properties of faint blue starsBrown, Alexanderhttps://hdl.handle.net/10023/143902019-04-01T10:03:35Z1978-01-01T00:00:00ZFaint blue stars situated out of the galactic plane have been studied using a variety of techniques. Stromgren photometry has been obtained for a number of these stars and classifications derived from the photometry. An interference filter centred at 3775 A near the Balmer discontinuity has been used in conjunction with the Stromgren filters. Observation of Stromgren standard stars, bright B stars and faint blue stars have been used to study the behaviour of the colours (u-38) and (38-b). Reddening-free parameters [u-38] and [38-b] have been evaluated and [38-b] is found to be linearly related to the Crawford H𝛽 index for B-type dwarfs, giants and supergiants. This relationship has been used to derive absolute magnitudes and distances for faint apparently normal B stars. [38-b] allows this work to be done at fainter magnitudes than possible with H𝛽 photometry. A filter pair centred on the HeII 𝜆4686 line has been used to study 0-type stars including 0 subdwarfs. The resulting index is strongly correlated with the equivalent width of the Hell line and has a range of 0ᵐ.13. It is possible to separate sd0, 0 and 0f stars using this index. Although the index is correlated with absolute magnitude for 0 and 0f stars, the detailed behaviour precludes the accurate determination of absolute magnitude for dwarfs and giants. Theoretical indices have been computed from the models of Kudritzki and agree well with the results for stars with high helium abundance. These computations suggest that it is possible to separate sdO and DO stars with this system. The results have also been compared with the work of Auer and Mihalas. 30 A/mm image tube spectra have been used for Ealmer line width measurement and radial velocity determination. The line widths were used to estimate surface gravities for faint blue stars in conjunction with temperatures determined from the Stromgren [u-b] index. The results indicate that 30 A/mm image tube spectra can be used successfully for surface gravity determination. 75 A/mm image tube spectra have been used for spectral classification as a check on the classification provided by Stromgren photometry. The kinematics of certain subdwarfs and main sequence stars at high galactic latitude have been considered. Proper motions have been employed as a check for subluminosity using appropriate absolute magnitudes. Galactic orbits have been computed using the Schmidt model of the galactic force field. Several B-type dwarfs and subdwarfs were found to have been ejected from the galactic plane at high velocity. Thirty percent of the stars previously classified as normal dwarfs were found to be subluminous. The evolutionary implications of these results are discussed.
1978-01-01T00:00:00ZBrown, AlexanderFaint blue stars situated out of the galactic plane have been studied using a variety of techniques. Stromgren photometry has been obtained for a number of these stars and classifications derived from the photometry. An interference filter centred at 3775 A near the Balmer discontinuity has been used in conjunction with the Stromgren filters. Observation of Stromgren standard stars, bright B stars and faint blue stars have been used to study the behaviour of the colours (u-38) and (38-b). Reddening-free parameters [u-38] and [38-b] have been evaluated and [38-b] is found to be linearly related to the Crawford H𝛽 index for B-type dwarfs, giants and supergiants. This relationship has been used to derive absolute magnitudes and distances for faint apparently normal B stars. [38-b] allows this work to be done at fainter magnitudes than possible with H𝛽 photometry. A filter pair centred on the HeII 𝜆4686 line has been used to study 0-type stars including 0 subdwarfs. The resulting index is strongly correlated with the equivalent width of the Hell line and has a range of 0ᵐ.13. It is possible to separate sd0, 0 and 0f stars using this index. Although the index is correlated with absolute magnitude for 0 and 0f stars, the detailed behaviour precludes the accurate determination of absolute magnitude for dwarfs and giants. Theoretical indices have been computed from the models of Kudritzki and agree well with the results for stars with high helium abundance. These computations suggest that it is possible to separate sdO and DO stars with this system. The results have also been compared with the work of Auer and Mihalas. 30 A/mm image tube spectra have been used for Ealmer line width measurement and radial velocity determination. The line widths were used to estimate surface gravities for faint blue stars in conjunction with temperatures determined from the Stromgren [u-b] index. The results indicate that 30 A/mm image tube spectra can be used successfully for surface gravity determination. 75 A/mm image tube spectra have been used for spectral classification as a check on the classification provided by Stromgren photometry. The kinematics of certain subdwarfs and main sequence stars at high galactic latitude have been considered. Proper motions have been employed as a check for subluminosity using appropriate absolute magnitudes. Galactic orbits have been computed using the Schmidt model of the galactic force field. Several B-type dwarfs and subdwarfs were found to have been ejected from the galactic plane at high velocity. Thirty percent of the stars previously classified as normal dwarfs were found to be subluminous. The evolutionary implications of these results are discussed.Stellar pulsation: a study of the influence of opacity on numerical modelsAl-Mostafa, Zaki A.https://hdl.handle.net/10023/143862019-04-01T10:04:46Z1995-01-01T00:00:00ZA theoretical study of Population II variables has been carried out using the nonlinear approach and using the more recent molecular opacities of Carson and Sharp (1991) with the atomic opacities of Iglesias and Rogers (1991). The calculations have been done using a computer code created by Dr. T. R. Carson. More than fifty models have been constructed for different compositions for Hydrogen (X), 0.745, 0.749 and 0.750 and for Helium (Y) fixed at 0.250. These models belong to three types of stars, RR Lyrae, BL Herculis and W Virginis. The aim of this study was to test the new opacities and compare them with the old ones (especially those of Carson). Generally the periods of the old and new models are in very good agreement. However, the amplitudes of the new models tend to be fairly consistently smaller than those of the old models, tending towards greater agreement with observation. The new blue edges are shifted redward (toward lower temperature) with respect to the old, particularly for the larger values of the metal content Z, and less for smaller Z. However, for the same composition all results are in excellent agreement with those of Carson, Stothers and Vemury (1981) using the opacities of Carson The non-linear results obtained here show that the mass of RR Lyrae cannot be less than 0.6 Msun, while on the other hand, the BL Her variables have mass greater than 0.7 Msun.
1995-01-01T00:00:00ZAl-Mostafa, Zaki A.A theoretical study of Population II variables has been carried out using the nonlinear approach and using the more recent molecular opacities of Carson and Sharp (1991) with the atomic opacities of Iglesias and Rogers (1991). The calculations have been done using a computer code created by Dr. T. R. Carson. More than fifty models have been constructed for different compositions for Hydrogen (X), 0.745, 0.749 and 0.750 and for Helium (Y) fixed at 0.250. These models belong to three types of stars, RR Lyrae, BL Herculis and W Virginis. The aim of this study was to test the new opacities and compare them with the old ones (especially those of Carson). Generally the periods of the old and new models are in very good agreement. However, the amplitudes of the new models tend to be fairly consistently smaller than those of the old models, tending towards greater agreement with observation. The new blue edges are shifted redward (toward lower temperature) with respect to the old, particularly for the larger values of the metal content Z, and less for smaller Z. However, for the same composition all results are in excellent agreement with those of Carson, Stothers and Vemury (1981) using the opacities of Carson The non-linear results obtained here show that the mass of RR Lyrae cannot be less than 0.6 Msun, while on the other hand, the BL Her variables have mass greater than 0.7 Msun.Accurate astrophysical parameters for algol-type binary starsMaxted, P. F. L.https://hdl.handle.net/10023/143832019-04-01T10:05:56Z1995-01-01T00:00:00ZWe present new, accurate astrophysical parameters for both components of the six short-period Algol-type binary stars RZ Cas, AT Peg, TX UMa, AF Gem, RU UMi and HU Tau. The accuracy of the astrophysical parameters is ensured by the determination of a mass ratio for each of the systems from the spectroscopic orbits of both components via the cross correlation technique. For RZ Cas and AT Peg we have developed a simple technique to correct the observed photometry for the presence of the secondary component. Spectral classification of the primary component of AT Peg was aided by the use of an improved Doppler tomography technique. The systems RZ Cas, AT Peg, TX UMa, AF Gem and HU Tau are found to be apparently normal semi-detached Algol-type systems and accurate masses and radii have been determined with a typical precision of a few percent. For RU UMi we have been able to confirm the suspected semi-detached configuration. We have compiled a list of nine Algol-type binary stars, five from this study, for which the observed astrophysical parameters have been determined accurately using a combination of both spectroscopic and photometric data in a self-consistent solution. The general model of non-conservative case B evolution adequately explains the properties of these systems as a group. Angular momentum loss via a magnetic stellar wind is shown to be an important factor in the evolution of these systems. However, there appear to be no published evolutionary models which can be used to estimate the properties of the progenitors of these systems. For the more massive systems the best available models predict periods that are too large by ~50-250% and luminosities for the faint components that are too high by factors of 10-20.
1995-01-01T00:00:00ZMaxted, P. F. L.We present new, accurate astrophysical parameters for both components of the six short-period Algol-type binary stars RZ Cas, AT Peg, TX UMa, AF Gem, RU UMi and HU Tau. The accuracy of the astrophysical parameters is ensured by the determination of a mass ratio for each of the systems from the spectroscopic orbits of both components via the cross correlation technique. For RZ Cas and AT Peg we have developed a simple technique to correct the observed photometry for the presence of the secondary component. Spectral classification of the primary component of AT Peg was aided by the use of an improved Doppler tomography technique. The systems RZ Cas, AT Peg, TX UMa, AF Gem and HU Tau are found to be apparently normal semi-detached Algol-type systems and accurate masses and radii have been determined with a typical precision of a few percent. For RU UMi we have been able to confirm the suspected semi-detached configuration. We have compiled a list of nine Algol-type binary stars, five from this study, for which the observed astrophysical parameters have been determined accurately using a combination of both spectroscopic and photometric data in a self-consistent solution. The general model of non-conservative case B evolution adequately explains the properties of these systems as a group. Angular momentum loss via a magnetic stellar wind is shown to be an important factor in the evolution of these systems. However, there appear to be no published evolutionary models which can be used to estimate the properties of the progenitors of these systems. For the more massive systems the best available models predict periods that are too large by ~50-250% and luminosities for the faint components that are too high by factors of 10-20.Multicolour photometry of Mira variablesKelly, B. D.https://hdl.handle.net/10023/143782019-04-01T10:08:54Z1977-01-01T00:00:00ZIn Part 1, a background to the project is provided by summarizing the general properties of Miras. A section is devoted to describing pulsating variables from a theoretical viewpoint, and this is followed by a discussion of past photometric work. Finally, a comparison is made between the theory and observations of Miras, which points out the well- known incompatibilities between the two approaches. Part 2 begins by underlining the desirability of observing Miras in UBVRI, and then goes on to describe the mechanical design of the St. Andrews ten colour, two photo multiplier, automated photometer. This photometer was designed as a general user instrument for the South African Astronomical Observatory, where the observing programme was carried out between May 1973 and October 1974. The photometric characteristics of Miras are then reconsidered, and the adopted ten colour photometric system, which adds five 200 Å wide filters to UBVRI, is described. The procedures used for obtaining the observations, and the reduction methods are summarized, along with the likely sources of error. This is followed by the analysis of the results, which consists mainly of a description of the properties of the various two-colour diagrams, particular emphasis being placed on those such as (V-R, R-l) and (V, R-I) which show unexpectedly narrow sequences. In addition, considerable attention is directed at the scatter in the (U-B, R-I) diagram, and also at the interesting R Aqr system. The data are then considered from the period-luminosity- colour viewpoint, enabling a discussion of absolute magnitudes and the value of the pulsation constant, as well as an investigation, of the possible harmonic relationships between Miras of differing periods. Finally, an attempt is made to place the Me Miras in the (M[sub]bol, log Te) diagram, and the conclusion is drawn that the discrepancies between theory and observation can probably be mostly attributed to the methods of interpretation of the latter.
1977-01-01T00:00:00ZKelly, B. D.In Part 1, a background to the project is provided by summarizing the general properties of Miras. A section is devoted to describing pulsating variables from a theoretical viewpoint, and this is followed by a discussion of past photometric work. Finally, a comparison is made between the theory and observations of Miras, which points out the well- known incompatibilities between the two approaches. Part 2 begins by underlining the desirability of observing Miras in UBVRI, and then goes on to describe the mechanical design of the St. Andrews ten colour, two photo multiplier, automated photometer. This photometer was designed as a general user instrument for the South African Astronomical Observatory, where the observing programme was carried out between May 1973 and October 1974. The photometric characteristics of Miras are then reconsidered, and the adopted ten colour photometric system, which adds five 200 Å wide filters to UBVRI, is described. The procedures used for obtaining the observations, and the reduction methods are summarized, along with the likely sources of error. This is followed by the analysis of the results, which consists mainly of a description of the properties of the various two-colour diagrams, particular emphasis being placed on those such as (V-R, R-l) and (V, R-I) which show unexpectedly narrow sequences. In addition, considerable attention is directed at the scatter in the (U-B, R-I) diagram, and also at the interesting R Aqr system. The data are then considered from the period-luminosity- colour viewpoint, enabling a discussion of absolute magnitudes and the value of the pulsation constant, as well as an investigation, of the possible harmonic relationships between Miras of differing periods. Finally, an attempt is made to place the Me Miras in the (M[sub]bol, log Te) diagram, and the conclusion is drawn that the discrepancies between theory and observation can probably be mostly attributed to the methods of interpretation of the latter.A spectroscopic and photometric investigation of the extreme helium-rich star HD 168476Walker, H. J.https://hdl.handle.net/10023/143732019-04-01T10:05:03Z1979-01-01T00:00:00ZSpectra were obtained from several sources for a fine abundance analysis of the extreme helium-rich star HD 168476. The atmospheric parameters are found using theoretical models and abundances determined for the ions identified in the spectra. The star is found to have an effective temperature of 14000°K, log(surface gravity) of 1.5 and a microturbulent velocity of 10 km/s. Over 1400 lines are identified on the spectra ranging in wavelength from 3100A to 4925A and 5490A to 6585A, and about 530 lines are suitable for the abundance analysis. It is confirmed that helium, carbon, nitrogen and neon are overabundant, with hydrogen and oxygen underabundant. An ultraviolet spectrum of the star is also obtained and lines identified. In addition to the ions found in the visible region of the spectrum, neutral ions are also present, indicating the possibility of a cool outer shell to the star. Photometric and spectroscopic observations are made to study the light and radial velocity of the star, to determine if it is variable. Statistical tests showed that the star is variable in its V magnitude, and most probably in its colours, as well as being variable in its radial velocity. No period is found and it is suspected that the variability may be complex. Several theories for the causes of the variability are discussed and a possible origin for the extreme helium-rich nature of the star mentioned.
1979-01-01T00:00:00ZWalker, H. J.Spectra were obtained from several sources for a fine abundance analysis of the extreme helium-rich star HD 168476. The atmospheric parameters are found using theoretical models and abundances determined for the ions identified in the spectra. The star is found to have an effective temperature of 14000°K, log(surface gravity) of 1.5 and a microturbulent velocity of 10 km/s. Over 1400 lines are identified on the spectra ranging in wavelength from 3100A to 4925A and 5490A to 6585A, and about 530 lines are suitable for the abundance analysis. It is confirmed that helium, carbon, nitrogen and neon are overabundant, with hydrogen and oxygen underabundant. An ultraviolet spectrum of the star is also obtained and lines identified. In addition to the ions found in the visible region of the spectrum, neutral ions are also present, indicating the possibility of a cool outer shell to the star. Photometric and spectroscopic observations are made to study the light and radial velocity of the star, to determine if it is variable. Statistical tests showed that the star is variable in its V magnitude, and most probably in its colours, as well as being variable in its radial velocity. No period is found and it is suspected that the variability may be complex. Several theories for the causes of the variability are discussed and a possible origin for the extreme helium-rich nature of the star mentioned.Studies of low-mass interacting binary starsRainger, Paul P.https://hdl.handle.net/10023/143692019-04-01T10:07:59Z1990-01-01T00:00:00ZSpectroscopic and photometric observations of eight contact/near-contact binaries are presented and analysed. Spectroscopic observations were obtained at 4200 Å (radial velocity spectra) and 6563 Å (hydrogen-alpha line profiles). New photometric observations were obtained at visual and infrared wavelengths, and other previously published light curves are also re-analysed. Absolute dimensions have been obtained for five systems; TY Boo, VW Boo, BX And, SS Ari and AG Vir, and their evolutionary positions discussed. Four of the systems are found to be in marginal but poor thermal contact, exhibiting regions of apparent "excess luminosity" in their light curves. A qualitative analysis of these "hot spot" regions has been attempted for the first time using spot models now incorporated into a light curve synthesis programme. Substantial time for this project was awarded on telescopes funded by the United Kingdom Science and Engineering Research Council (SERC), comprising 14 nights at the Issac Newton Telescope (INT) on La Palma, and 4 nights at the United Kingdom Infrared Telescope (UKIRT) on Mauna Kea. Additional observations were made during an 8 night commissioning run on the Jacobus Kapteyn Telescope (JKT) on La Palma, and extensive observations were made with the Twin Photometric Telescope (TPT) at St Andrews University Observatory between 1985 and 1989. These resulted in over 100 spectra at 4200 Å and over 50 spectra at 6563 Å (INT and JKT observations), over 300 infrared photometric observations (UKIRT), and over 3500 visual photometric observations (TPT). Of the five systems analysed in detail in this work, TY Boo appears to be a normal shallow-contact W-type system. Both VW Boo and BX And exhibit regions of "excess luminosity" around the ingress and egress of secondary minimum which are well modelled by a warm spot on the cooler component sitting symmetrically around the neck joining the pair. Such a phenomenon may be expected to arise naturally in systems which have come into contact but are not yet/currently in thermal contact, exhibiting a temperature difference between the components. BXAnd like other B-type systems seems to be reaching this contact state for the first time, but the position of VW Boo is uncertain, and whilst evidence that it could be in the "broken contact" state predicted by the TRO Theory is far from conclusive, its lower orbital angular momentum clearly marks the system as worthy of further study. SS Ari and AG Vir exhibit light curves with unequal quadrature heights. Attempts to treat the higher quadrature as a region of "excess luminosity" due to an energy transfer "warm spot" does not however provide a good model of this phenomenon. Since invoking a dark starspot model also does not provide a good explanation for such systems, it may be that this form of light curve distortion is due to an entirely different form of distorting surface phenomenon. Like BX And, AG Vir appears to be just reaching contact for the first time, but like VW Boo, the slightly lower angular momentum of SS Ari warrants further study.
1990-01-01T00:00:00ZRainger, Paul P.Spectroscopic and photometric observations of eight contact/near-contact binaries are presented and analysed. Spectroscopic observations were obtained at 4200 Å (radial velocity spectra) and 6563 Å (hydrogen-alpha line profiles). New photometric observations were obtained at visual and infrared wavelengths, and other previously published light curves are also re-analysed. Absolute dimensions have been obtained for five systems; TY Boo, VW Boo, BX And, SS Ari and AG Vir, and their evolutionary positions discussed. Four of the systems are found to be in marginal but poor thermal contact, exhibiting regions of apparent "excess luminosity" in their light curves. A qualitative analysis of these "hot spot" regions has been attempted for the first time using spot models now incorporated into a light curve synthesis programme. Substantial time for this project was awarded on telescopes funded by the United Kingdom Science and Engineering Research Council (SERC), comprising 14 nights at the Issac Newton Telescope (INT) on La Palma, and 4 nights at the United Kingdom Infrared Telescope (UKIRT) on Mauna Kea. Additional observations were made during an 8 night commissioning run on the Jacobus Kapteyn Telescope (JKT) on La Palma, and extensive observations were made with the Twin Photometric Telescope (TPT) at St Andrews University Observatory between 1985 and 1989. These resulted in over 100 spectra at 4200 Å and over 50 spectra at 6563 Å (INT and JKT observations), over 300 infrared photometric observations (UKIRT), and over 3500 visual photometric observations (TPT). Of the five systems analysed in detail in this work, TY Boo appears to be a normal shallow-contact W-type system. Both VW Boo and BX And exhibit regions of "excess luminosity" around the ingress and egress of secondary minimum which are well modelled by a warm spot on the cooler component sitting symmetrically around the neck joining the pair. Such a phenomenon may be expected to arise naturally in systems which have come into contact but are not yet/currently in thermal contact, exhibiting a temperature difference between the components. BXAnd like other B-type systems seems to be reaching this contact state for the first time, but the position of VW Boo is uncertain, and whilst evidence that it could be in the "broken contact" state predicted by the TRO Theory is far from conclusive, its lower orbital angular momentum clearly marks the system as worthy of further study. SS Ari and AG Vir exhibit light curves with unequal quadrature heights. Attempts to treat the higher quadrature as a region of "excess luminosity" due to an energy transfer "warm spot" does not however provide a good model of this phenomenon. Since invoking a dark starspot model also does not provide a good explanation for such systems, it may be that this form of light curve distortion is due to an entirely different form of distorting surface phenomenon. Like BX And, AG Vir appears to be just reaching contact for the first time, but like VW Boo, the slightly lower angular momentum of SS Ari warrants further study.The law of interstellar extinction in the southern Milky WayWhittet, D. C. B. (Doug C. B.)https://hdl.handle.net/10023/143632019-04-01T10:10:41Z1975-01-01T00:00:00ZPhotoelectric scanner observations are combined with infrared photometry in the JHKL bands to give interstellar extinction curves by the colour difference method for 25 reddened stars in the Southern Milky Way.
The observed stars are distributed from galactic longitude 𝓁 ᴵᴵ= 260°; through the Galactic Centre to 𝓁 ᴵᴵ= 30°. The scanner data ranges in wavelength from 3A80 to 5240 A with a resolution of 40 A in the second order of diffraction, and from 5080 to 8040 A with a resolution of 80 A in the first order. The infrared photometry extends the wavelength coverage to 3.5 µ. The normalized extinction curves show appreciable variations from star to star which are not apparently related to galactic structure. In several cases stars quite close together in the sky show appreciably different curves, suggesting that fluctuations in the grain size distribution may be occurring on a fairly localized scale.
Using a graphite-iron-silicate grain model, extinction curves computed from the Mie theory are fitted to the observational data. This provides a theoretical basis for extrapolation of the curves to 𝜆 ⁻¹=0, allowing the ratio of total to selective extinction, R, to be deduced. R varies between 2.6 and 3.5 for 22 stars lying within 10° of the galactic plane, and between 3.8 and 4.3 for 3 stars in Upper Scorpius.
One of the Upper Scorpius stars studied, HD 147889, lies in an HII region near the centre of the p Oph complex of dust and nebulosity, A variable extinction analysis applied to members of the Sco OB-2 association indicates an increase in the value of R for stars lying towards nebulosity in the complex; for these stars alone a result of R = 4.2 ± 0.5 is derived, whilst for the remainder, R = 3.3 ± 0.3. The distance modulus of the association is deduced to be 6.1 ± 0.1, which is in good agreement with the kinematical value.
The scanner extinction curves contain considerable broadband structure. In particular, a depressed region between wavenumbers 1.6 and 1.97 µ⁻¹ is a common feature of the curves. A similar depression was noted by Whiteoak in the extinction curves of Northern Milky Way stars. Its depth correlates well with reddening, suggesting that the feature originates in the grains. The extinction curve of HD 147889 shows a -1 cusp-like depression at 1.97 µ⁻¹ which divides the curve into two distinct sections. The ratio of the 4430 A diffuse interstellar band height to colour excess shows signs of systematic variation with longitude for stars whose reddening occurs mainly in the local spiral arm.
1975-01-01T00:00:00ZWhittet, D. C. B. (Doug C. B.)Photoelectric scanner observations are combined with infrared photometry in the JHKL bands to give interstellar extinction curves by the colour difference method for 25 reddened stars in the Southern Milky Way.
The observed stars are distributed from galactic longitude 𝓁 ᴵᴵ= 260°; through the Galactic Centre to 𝓁 ᴵᴵ= 30°. The scanner data ranges in wavelength from 3A80 to 5240 A with a resolution of 40 A in the second order of diffraction, and from 5080 to 8040 A with a resolution of 80 A in the first order. The infrared photometry extends the wavelength coverage to 3.5 µ. The normalized extinction curves show appreciable variations from star to star which are not apparently related to galactic structure. In several cases stars quite close together in the sky show appreciably different curves, suggesting that fluctuations in the grain size distribution may be occurring on a fairly localized scale.
Using a graphite-iron-silicate grain model, extinction curves computed from the Mie theory are fitted to the observational data. This provides a theoretical basis for extrapolation of the curves to 𝜆 ⁻¹=0, allowing the ratio of total to selective extinction, R, to be deduced. R varies between 2.6 and 3.5 for 22 stars lying within 10° of the galactic plane, and between 3.8 and 4.3 for 3 stars in Upper Scorpius.
One of the Upper Scorpius stars studied, HD 147889, lies in an HII region near the centre of the p Oph complex of dust and nebulosity, A variable extinction analysis applied to members of the Sco OB-2 association indicates an increase in the value of R for stars lying towards nebulosity in the complex; for these stars alone a result of R = 4.2 ± 0.5 is derived, whilst for the remainder, R = 3.3 ± 0.3. The distance modulus of the association is deduced to be 6.1 ± 0.1, which is in good agreement with the kinematical value.
The scanner extinction curves contain considerable broadband structure. In particular, a depressed region between wavenumbers 1.6 and 1.97 µ⁻¹ is a common feature of the curves. A similar depression was noted by Whiteoak in the extinction curves of Northern Milky Way stars. Its depth correlates well with reddening, suggesting that the feature originates in the grains. The extinction curve of HD 147889 shows a -1 cusp-like depression at 1.97 µ⁻¹ which divides the curve into two distinct sections. The ratio of the 4430 A diffuse interstellar band height to colour excess shows signs of systematic variation with longitude for stars whose reddening occurs mainly in the local spiral arm.A three-colour photometric survey of Virgo 'core' lenticular galaxiesMalcolm, Gordon Jhttps://hdl.handle.net/10023/143562019-04-01T10:10:02Z1987-01-01T00:00:00ZPhotographic Surface photometry in the Johnson B, R and I passbands of eighteen galaxies within 3° of the Virgo cluster 'core' is presented in this work. U.K. National facilities and STARLIKK common-user software were used to produce major and minor-axis luminosity profiles, colour-difference profiles, isophotal maps, equivalent profiles, and standard luminosity parameters for each galaxy in each passband. Standard techniques have been applied to the equivalent profiles to produce decomposed bulge and disk components for each galaxy. Solutions which are demonstrated to be realistic representations of the programme galaxies are amalgamated with previously-published independent data to construct homogeneous data sets. Nearly all the lenticular systems surveyed display vestigial disk structure or unusual features in the colour-difference profiles. Statistical analysis of the constructed data sets reveals no significant structural differences in the bulge components of disk systems as a function of morphological type, in contradiction with the earlier results of Dressier (1980). Significant differences in the disk components as a function of morphological type are found in the B-band data, but not the I-band data, in agreement with the results of Hamabe (1982) and Boroson et al. (1983b) respectively. The primary objections to the production of lenticular galaxies from spiral progenitors appear to have been discredited, together with several intrinsic-formation theories, and the results presented in this survey are considered strongly supportive of the hypothesis of a common origin of disk systems of all morphological types. The conclusion that at least some fraction of lenticular systems must have looked like spiral galaxies at some time in their history seems almost inescapable.
1987-01-01T00:00:00ZMalcolm, Gordon JPhotographic Surface photometry in the Johnson B, R and I passbands of eighteen galaxies within 3° of the Virgo cluster 'core' is presented in this work. U.K. National facilities and STARLIKK common-user software were used to produce major and minor-axis luminosity profiles, colour-difference profiles, isophotal maps, equivalent profiles, and standard luminosity parameters for each galaxy in each passband. Standard techniques have been applied to the equivalent profiles to produce decomposed bulge and disk components for each galaxy. Solutions which are demonstrated to be realistic representations of the programme galaxies are amalgamated with previously-published independent data to construct homogeneous data sets. Nearly all the lenticular systems surveyed display vestigial disk structure or unusual features in the colour-difference profiles. Statistical analysis of the constructed data sets reveals no significant structural differences in the bulge components of disk systems as a function of morphological type, in contradiction with the earlier results of Dressier (1980). Significant differences in the disk components as a function of morphological type are found in the B-band data, but not the I-band data, in agreement with the results of Hamabe (1982) and Boroson et al. (1983b) respectively. The primary objections to the production of lenticular galaxies from spiral progenitors appear to have been discredited, together with several intrinsic-formation theories, and the results presented in this survey are considered strongly supportive of the hypothesis of a common origin of disk systems of all morphological types. The conclusion that at least some fraction of lenticular systems must have looked like spiral galaxies at some time in their history seems almost inescapable.On the mass and luminosity within isolated and binary galaxiesBlackman, Clinton Paulhttps://hdl.handle.net/10023/143502020-02-20T12:01:02Z1977-01-01T00:00:00ZThe aim of this project has been to study the detailed mass and luminosity distributions within spiral galaxies, with particular emphasis on the comparison of isolated and binary systems. A large programme of photographic U, B, Y and E surface photometry has been completed using the St. Andrews 1m Schmidt-Cassegrain telescope, and a series of computer programmes have been written to evaluate the detailed luminosity distribution of galaxies, using data from a computer -controlled, plate-scanning. Spectroscopic observations of a number of binary galaxy systems have also been made, using the Isaac Newton Telescope and the 195cm telescope at mute Provence, and well-defined rotation curves have been obtained for the galaxies in three systems. The photometry of these galaxies and of ten isolated galaxies with known rotation curves has revealed an extra outer component in the integrated luminosity distributions which has not been widely noted before. For these galaxies, the variation in mass to luminosity ratio (M/L) has been studied at large radii by extrapolating the observed rotation curves, using an empirical formula. This has revealed a general tendency for M/L to decrease with increasing radius. For most galaxies, a small, but well-defined outer peak is also seen, coinciding with the boundary of the outer component in the luminosity distribution. It is postulated that the outer component corresponds to those parts of the galaxies lying outside the outer Lindblad resonance, and this has been confirmed comparing the spiral pattern obtained from density wave theory with the 034ervod structure of the galaxies. This implies that the rotation curves are not at large radii, as they would be if a massive halo were present. This is by the fact that the luminosities of the outer components are too large for much a nalo, according to recent estimates of halo properties. In their gross properties, the binary galaxies do not differ from the isolated Two of the galaxies are, however, very luminous for their mass, and this is explained qualitatively by the tidal forces due to the neighbouring galaxy, with increase the strength of the shocks associated with the spiral arms, extrapolating to the density wave model, in turn giving rise to enhanced star formation.
1977-01-01T00:00:00ZBlackman, Clinton PaulThe aim of this project has been to study the detailed mass and luminosity distributions within spiral galaxies, with particular emphasis on the comparison of isolated and binary systems. A large programme of photographic U, B, Y and E surface photometry has been completed using the St. Andrews 1m Schmidt-Cassegrain telescope, and a series of computer programmes have been written to evaluate the detailed luminosity distribution of galaxies, using data from a computer -controlled, plate-scanning. Spectroscopic observations of a number of binary galaxy systems have also been made, using the Isaac Newton Telescope and the 195cm telescope at mute Provence, and well-defined rotation curves have been obtained for the galaxies in three systems. The photometry of these galaxies and of ten isolated galaxies with known rotation curves has revealed an extra outer component in the integrated luminosity distributions which has not been widely noted before. For these galaxies, the variation in mass to luminosity ratio (M/L) has been studied at large radii by extrapolating the observed rotation curves, using an empirical formula. This has revealed a general tendency for M/L to decrease with increasing radius. For most galaxies, a small, but well-defined outer peak is also seen, coinciding with the boundary of the outer component in the luminosity distribution. It is postulated that the outer component corresponds to those parts of the galaxies lying outside the outer Lindblad resonance, and this has been confirmed comparing the spiral pattern obtained from density wave theory with the 034ervod structure of the galaxies. This implies that the rotation curves are not at large radii, as they would be if a massive halo were present. This is by the fact that the luminosities of the outer components are too large for much a nalo, according to recent estimates of halo properties. In their gross properties, the binary galaxies do not differ from the isolated Two of the galaxies are, however, very luminous for their mass, and this is explained qualitatively by the tidal forces due to the neighbouring galaxy, with increase the strength of the shocks associated with the spiral arms, extrapolating to the density wave model, in turn giving rise to enhanced star formation.Dynamics and kinematics of systems consisting of spherical and spheroidal bodiesPapadakos, Dimitrios N.https://hdl.handle.net/10023/143462019-04-01T10:03:08Z1981-01-01T00:00:00ZThere are three principal aims of this work; firstly to derive the analytical expressions for the potential energy and the mutual gravitational attraction between two homogeneous or non-homogeneous oblate spheroids with coplanar equatorial planes; secondly, to construct and study the equations of motion of dynamical systems consisting of particles and rigid homogeneous spheroidal bodies whose equatorial planes are coplanar; thirdly, to investigate by numerical integration and compare the evolution of dynamical models of interacting galaxies. Two different types of dynamical models of galaxies were used in this work:(a) galaxies consisting of gravitating particles, (b) galaxies comprising of gravitationally interacting particles and heavy central rigid homogeneous oblate spheroids. Chapter (2) and appendices (4), (5), (6) and (7) are an account of the method used to derive the expressions for the potential energy and the mutual gravitational attraction between two rigid bodies bounded by spheroidal surfaces with coplanar equatorial planes, when the densities of the bodj.es are either constant or inversely proportional to the square of the radial distance from the centres of the bodies. These expressions were at first obtained in the form of non-elementary integrals over the complete elliptic integrals of the first and second kind, a result due to the fact that the common volume between two ellipsoids cannot be expressed in finite terms. The evaluation of these integrals was achieved with the aid of MacLaurin' s theorem by collapsing one of the spheroids to its confocal disc; their final form is that of rapidly convergent series in terms of the parameters which determine the shape and orientation of the spheroids. In the course of obtaining the expressions mentioned above we derived some other useful formulae, for example, the formulae giving the gravitational attraction between two homoeoids and the potential and force law of a non-homogeneous oblate spheroid. Having found the expressions for the gravitational potential and attraction between two spheroids we proceeded by constructing and solving the equations of motion of dynamical systems consisting of either particles or particles and rigid homogeneous oblate spheroids with coplanar equatorial planes [chapters (1) and (3), appendices (2) and (3)]. The solutions of the equations of motion - being non-algebraic - were obtained in the form of power series. In particular, for the first type of dynamical systems (particles only) they were found in the form of three different types of series. The first type is based on the development and implementation of generalized f and g series for the N-body problem, the second type is based on recurrent formulae used for the evaluation of the terms of the series and the last type of series is a power series in terms of ln ɼijτ/ɼij and polynomial of ɼijτ/ɼij. All the series mentioned above lend themselves easily, to numerical calculations since their convergence, which was analytically proved, is a rapid one. We concluded the present work by numerically integrating the equations of motion of dynamical models of pairs of interacting galaxies. The galaxies were of the types (a) and (b) mentioned previously. The numerical integration was performed with variable time steps in order to reduce computing time. A general method was developed for the precise evaluation of the length of the time step determined by the desired integration accuracy. The calculations were carried out to an extremely high degree of accuracy. The most significant of the results of the numerical experiments on the evolution of dynamical models of interacting galaxies were:(i) the development of long-lived spiral structure when galaxies of type (b) were lased even for intrinsic velocities and impact parameters so high that galaxies of type (a) were unable to produce such structure,(ii) the roughly periodic appearance and disappearance of the spiral structure,(iii) the fact that in the early stages of their evolution the dynamical models take up rather long-lived triaxial ellipsoidal shapes.
1981-01-01T00:00:00ZPapadakos, Dimitrios N.There are three principal aims of this work; firstly to derive the analytical expressions for the potential energy and the mutual gravitational attraction between two homogeneous or non-homogeneous oblate spheroids with coplanar equatorial planes; secondly, to construct and study the equations of motion of dynamical systems consisting of particles and rigid homogeneous spheroidal bodies whose equatorial planes are coplanar; thirdly, to investigate by numerical integration and compare the evolution of dynamical models of interacting galaxies. Two different types of dynamical models of galaxies were used in this work:(a) galaxies consisting of gravitating particles, (b) galaxies comprising of gravitationally interacting particles and heavy central rigid homogeneous oblate spheroids. Chapter (2) and appendices (4), (5), (6) and (7) are an account of the method used to derive the expressions for the potential energy and the mutual gravitational attraction between two rigid bodies bounded by spheroidal surfaces with coplanar equatorial planes, when the densities of the bodj.es are either constant or inversely proportional to the square of the radial distance from the centres of the bodies. These expressions were at first obtained in the form of non-elementary integrals over the complete elliptic integrals of the first and second kind, a result due to the fact that the common volume between two ellipsoids cannot be expressed in finite terms. The evaluation of these integrals was achieved with the aid of MacLaurin' s theorem by collapsing one of the spheroids to its confocal disc; their final form is that of rapidly convergent series in terms of the parameters which determine the shape and orientation of the spheroids. In the course of obtaining the expressions mentioned above we derived some other useful formulae, for example, the formulae giving the gravitational attraction between two homoeoids and the potential and force law of a non-homogeneous oblate spheroid. Having found the expressions for the gravitational potential and attraction between two spheroids we proceeded by constructing and solving the equations of motion of dynamical systems consisting of either particles or particles and rigid homogeneous oblate spheroids with coplanar equatorial planes [chapters (1) and (3), appendices (2) and (3)]. The solutions of the equations of motion - being non-algebraic - were obtained in the form of power series. In particular, for the first type of dynamical systems (particles only) they were found in the form of three different types of series. The first type is based on the development and implementation of generalized f and g series for the N-body problem, the second type is based on recurrent formulae used for the evaluation of the terms of the series and the last type of series is a power series in terms of ln ɼijτ/ɼij and polynomial of ɼijτ/ɼij. All the series mentioned above lend themselves easily, to numerical calculations since their convergence, which was analytically proved, is a rapid one. We concluded the present work by numerically integrating the equations of motion of dynamical models of pairs of interacting galaxies. The galaxies were of the types (a) and (b) mentioned previously. The numerical integration was performed with variable time steps in order to reduce computing time. A general method was developed for the precise evaluation of the length of the time step determined by the desired integration accuracy. The calculations were carried out to an extremely high degree of accuracy. The most significant of the results of the numerical experiments on the evolution of dynamical models of interacting galaxies were:(i) the development of long-lived spiral structure when galaxies of type (b) were lased even for intrinsic velocities and impact parameters so high that galaxies of type (a) were unable to produce such structure,(ii) the roughly periodic appearance and disappearance of the spiral structure,(iii) the fact that in the early stages of their evolution the dynamical models take up rather long-lived triaxial ellipsoidal shapes.A spectroscopic and photometric investigation of some extremely hydrogen-deficient starsMorrison, Keithhttps://hdl.handle.net/10023/143432019-04-01T10:09:00Z1988-01-01T00:00:00ZA photometric study of the extreme helium stars has detected three new variables
(BD+1°4381, BD-1°3438, LSIV-1°2) and confirmed the suspected variability of two others (LSII+33°5, BD-9°4395). The timescale of the variations in BD+1°4381 (~"21 days), BD-1°3438 (5-8 days), LSIV-1°2 (~11 days) and LSII+33°5 (3-4 days), and the presence of colour changes concomitant with the luminosity variations, indicates that they are radial pulsators. In contrast, BD-9°4395 is believed to be pulsating non-radially. Photometric investigations of the extremely hydrogen-deficient binaries Upsilon Sagittarii, CPD-58°2721 and KS Persei are presented. KS Persei is suspected of having a ~5-day periodicity in addition to the ~30 day previously reported. A frequency analysis of the complex light curve of CPD-58°2721 shows it may be reconstructed from two sine-waves with periods of 9.3 and 14.1 days. For Upsilon Sagittarii, the results indicate that the variability previously ascribed to eclipses can instead be understood in terms of radial pulsation of the visible component, with a period of ~20 days. Additional rapid, ~0.02 mag luminosity variations with a period of 239 seconds are reported. Their origin is uncertain, and they are the shortest form of variability reported for this type of object. Theoretical modelling of the light curve indicates an early B-type main-sequence secondary with a mass of ~11M[sub] ⊙ , much higher than previously thought. A possible future merging of the binary components would result in a Type I supernova. There is no evidence of eclipses in any of the stars. A fine abundance analysis of CPD-58°2721 yielded T[sub]eff=14,000K, log g=1.25, n[sub]H:n[sub]He=0.005, n[sub]N:n[sub]c=40 and suggests an overabundance of heavy metals when compared to related objects. Its spectrum shows marked changes in the strengths of low excitation metallic lines which are evidence of temperature changes during pulsation. Spectroscopy of the surrounding nebulosity shows it to have an emission spectrum typical of an HII region. Radial-velocity measurements indicate it is probably part of the surrounding Eta Carina nebula, whilst the star is more distant.
1988-01-01T00:00:00ZMorrison, KeithA photometric study of the extreme helium stars has detected three new variables
(BD+1°4381, BD-1°3438, LSIV-1°2) and confirmed the suspected variability of two others (LSII+33°5, BD-9°4395). The timescale of the variations in BD+1°4381 (~"21 days), BD-1°3438 (5-8 days), LSIV-1°2 (~11 days) and LSII+33°5 (3-4 days), and the presence of colour changes concomitant with the luminosity variations, indicates that they are radial pulsators. In contrast, BD-9°4395 is believed to be pulsating non-radially. Photometric investigations of the extremely hydrogen-deficient binaries Upsilon Sagittarii, CPD-58°2721 and KS Persei are presented. KS Persei is suspected of having a ~5-day periodicity in addition to the ~30 day previously reported. A frequency analysis of the complex light curve of CPD-58°2721 shows it may be reconstructed from two sine-waves with periods of 9.3 and 14.1 days. For Upsilon Sagittarii, the results indicate that the variability previously ascribed to eclipses can instead be understood in terms of radial pulsation of the visible component, with a period of ~20 days. Additional rapid, ~0.02 mag luminosity variations with a period of 239 seconds are reported. Their origin is uncertain, and they are the shortest form of variability reported for this type of object. Theoretical modelling of the light curve indicates an early B-type main-sequence secondary with a mass of ~11M[sub] ⊙ , much higher than previously thought. A possible future merging of the binary components would result in a Type I supernova. There is no evidence of eclipses in any of the stars. A fine abundance analysis of CPD-58°2721 yielded T[sub]eff=14,000K, log g=1.25, n[sub]H:n[sub]He=0.005, n[sub]N:n[sub]c=40 and suggests an overabundance of heavy metals when compared to related objects. Its spectrum shows marked changes in the strengths of low excitation metallic lines which are evidence of temperature changes during pulsation. Spectroscopy of the surrounding nebulosity shows it to have an emission spectrum typical of an HII region. Radial-velocity measurements indicate it is probably part of the surrounding Eta Carina nebula, whilst the star is more distant.A study of early and intermediate type stars at the galactic polesMcFadzean, A. D.https://hdl.handle.net/10023/143312019-04-01T10:04:36Z1985-01-01T00:00:00ZA catalogue of faint blue stars at the North Galactic Pole, compiled from the literature, is presented. Spectral classifications for catalogue stars within 3° of the pole have been obtained from U.K.S.T. objective prism and St. Andrews grism plates. Photometric data on the uvby𝛽 system is presented for 572 U-F8 stars at the South Galactic Pole, with radial velocities being given for 161 of these stars. From this South Galactic Pole data the interstellar reddening towards the Pole is shown to be negligible, in agreement with the findings of other authors. A number of photometrically odd stars are isolated, including several intermediate Population II, Population II and Am stars. From available data at both Poles the relative proportions of various population groups as a function of height are discussed. There is an apparent excess of PI A over iPII stars out to Ikpc., relative to the numbers expected on the basis of the 'thick disk' of iPII stars reported by Gilmore and Reid (1983). The w-velocity distributions of Pop.I A and F stars within 200pc. of both Poles are shown to be well fitted by gaussians and these gaussians are shown to be the same for both Poles. The Pop.I A stars are shown to have a mean w-velocity of 0,6 kms⁻¹ (rms 11.1 kms.⁻¹) and the corresponding F stars to have a mean w-velocity of -2.9 kms⁻¹ (rms 10.9 kms⁻¹), implying negligible net streaming through the galactic plane.<p>
1985-01-01T00:00:00ZMcFadzean, A. D.A catalogue of faint blue stars at the North Galactic Pole, compiled from the literature, is presented. Spectral classifications for catalogue stars within 3° of the pole have been obtained from U.K.S.T. objective prism and St. Andrews grism plates. Photometric data on the uvby𝛽 system is presented for 572 U-F8 stars at the South Galactic Pole, with radial velocities being given for 161 of these stars. From this South Galactic Pole data the interstellar reddening towards the Pole is shown to be negligible, in agreement with the findings of other authors. A number of photometrically odd stars are isolated, including several intermediate Population II, Population II and Am stars. From available data at both Poles the relative proportions of various population groups as a function of height are discussed. There is an apparent excess of PI A over iPII stars out to Ikpc., relative to the numbers expected on the basis of the 'thick disk' of iPII stars reported by Gilmore and Reid (1983). The w-velocity distributions of Pop.I A and F stars within 200pc. of both Poles are shown to be well fitted by gaussians and these gaussians are shown to be the same for both Poles. The Pop.I A stars are shown to have a mean w-velocity of 0,6 kms⁻¹ (rms 11.1 kms.⁻¹) and the corresponding F stars to have a mean w-velocity of -2.9 kms⁻¹ (rms 10.9 kms⁻¹), implying negligible net streaming through the galactic plane.<p>An investigation of galactic structure made from the study of the northern-hemisphere early type stars at intermediate galactic latitudesLynas-Gray, A. E.https://hdl.handle.net/10023/143292019-04-01T10:04:33Z1976-01-01T00:00:00ZPhotometric and spectroscopic observations have been made of northern hemisphere early type stars at intermediate galactic latitudes. Stellar distances, the corresponding distances from the galactic plane and the interstellar reddening along the lines of sight have been derived. The available published data was used to support the observational results. The HI spiral features seem to have corresponding optical counterparts, and the agreement between the two patterns is remarkably good. The neutral hydrogen kinematic distances depart from the stellar distances. This can be explained in terms of the density wave theory, with a small modification which may be the result of a non-zero distance from the galactic plane. The Local Arm extends to about 500pc above the galactic plane and the Perseus Arm to 1 kpc. Thus it seems that the interpretation of intermediate latitude high and intermediate velocity features, as vertical extensions to the spiral arms in the galactic plane, may be correct. Stellar and interstellar calcium radial velocities suggest that these intermediate latitude spiral features adhere closely to differential galactic rotation, and that the small departures from this motion exhibit a significant correlation with the predictions of the density wave theory. Published proper motions are used together with the stellar radial velocities to derive the components of the stellar space motions with respect to their local standards of rest. In cases where the component normal to the galactic plane was significantly different from zero, the dynamical lifetime was calculated on the assumption that the star was formed in the galactic plane. These were found to be compatible with the evolutionary lifetimes for only half of the stars considered, suggesting that about 50% of intermediate latitude OB stars were formed in or near the galactic plane and subsequently ejected from it. The remainder seem to have been formed at considerable distances from the galactic plane, and a scheme for explaining this is proposed. This scheme also explains the apparent asymmetry between the northern and southern galactic hemispheres. A few interesting high velocity stars and OB star associations are also considered.
1976-01-01T00:00:00ZLynas-Gray, A. E.Photometric and spectroscopic observations have been made of northern hemisphere early type stars at intermediate galactic latitudes. Stellar distances, the corresponding distances from the galactic plane and the interstellar reddening along the lines of sight have been derived. The available published data was used to support the observational results. The HI spiral features seem to have corresponding optical counterparts, and the agreement between the two patterns is remarkably good. The neutral hydrogen kinematic distances depart from the stellar distances. This can be explained in terms of the density wave theory, with a small modification which may be the result of a non-zero distance from the galactic plane. The Local Arm extends to about 500pc above the galactic plane and the Perseus Arm to 1 kpc. Thus it seems that the interpretation of intermediate latitude high and intermediate velocity features, as vertical extensions to the spiral arms in the galactic plane, may be correct. Stellar and interstellar calcium radial velocities suggest that these intermediate latitude spiral features adhere closely to differential galactic rotation, and that the small departures from this motion exhibit a significant correlation with the predictions of the density wave theory. Published proper motions are used together with the stellar radial velocities to derive the components of the stellar space motions with respect to their local standards of rest. In cases where the component normal to the galactic plane was significantly different from zero, the dynamical lifetime was calculated on the assumption that the star was formed in the galactic plane. These were found to be compatible with the evolutionary lifetimes for only half of the stars considered, suggesting that about 50% of intermediate latitude OB stars were formed in or near the galactic plane and subsequently ejected from it. The remainder seem to have been formed at considerable distances from the galactic plane, and a scheme for explaining this is proposed. This scheme also explains the apparent asymmetry between the northern and southern galactic hemispheres. A few interesting high velocity stars and OB star associations are also considered.Stellar astrophysics: a study of stellar physics with particular reference to low mass starsLuo, Guo Quanhttps://hdl.handle.net/10023/143262019-04-01T10:06:20Z1991-01-01T00:00:00ZIn this thesis we investigate the importance of various elements of the input physics and other parameters which affect the structure and evolution of models of low mass stars. Of the elements of input physics, the nuclear generation rates, the electron screening effect on thermo-nuclear reactions, and the conductive opacities are adopted from the formulation or data tables by other contributors. The low temperature opacities are taken from the data calculated by Carson and Sharp, as well as those by Alexander. In our study, we establish a sophisticated model for the determination of the equation of state. It is formulated by a new method based on the theory of the grand canonical ensemble. The interatomic interactions, which are responsible for the nonideal effects and pressure ionization, are treated carefully in the equation of state. In addition, and consistent with the equation of state, we also evaluate the radiative opacity according to the average atom model for heavy elements and the hydrogen-like model for hydrogen and helium. Negative hydrogen absorption and free electron scattering are also included. A computing code is constructed to calculate the radiative opacity data which are required in the study of low mass stars. To investigate the importance of each element of the input physics, we exclude it or replace it with an alternative for the model calculations. The parameters of the stellar code, such as the element abundances, the surface condition and the mixing length ratio are as well investigated by alternative values or formulation. In the computation, all elements of input physics, except the energy generation rates, are incorporated into the main code by means of data tables. A bicubic interpolation is used for their input. Our numerical calculations cover the zero age main sequence of the stars ranging from a solar mass down to the hydrogen burning minimum mass. The calculated results of the zero age models for the Standard Population I and Population II indicate good agreement with the observed data for the objects with effective temperatures above 3000 K, or with masses greater than 0,15 M[sub]sun. Below this limit, in agreement with other theoretical work, there is still a small discrepancy between the theoretical models and the observed data. The investigations of the effects of the input physics and model parameters show that some of them affect considerably the minimum mass limit for hydrogen burning on the lower main sequence. The low temperature opacities, the nonideal effects in the EOS and the conductive opacities lead to a limiting mass ranging from 0.08 M[sub]sun to 0.15 M[sub]sun although they do not affect the models with masses above 0.15 M[sub]sun obviously. The investigations also show that the massive models of the lower main sequence (i.e. those with mass near the sun) are dependent very much on the element abundances and the mixing length ratio while the lower mass models are not. The lower main sequence models are found to be insensitive to the surface condition (photospheric model or atmospheric model) used. We also perform the calculation of the evolutionary models of the lower main sequence from the zero age up to an age of 10¹⁰ years. A perfect theoretical model of the solar evolution is obtained when the atmospheric model is used as surface condition, the element abundances are chosen to be 0.70/0.28/0.02 and the mixing length ratio to be 1.5. The investigations show that the element abundances are the most important parameters in the determination of the solar models. The mixing length ratio is the second most important one being much more important than the physical model used as surface condition. The evolutionary models of the masses below 0.8 M[sub]sun are found to have only small changes in their properties within 10¹⁰ years. None of them can deplete their central hydrogen in that time. We find one problem in that the models with masses around 0.1 M[sub]sun have oscillating solutions for both the zero age models and evolutionary models. According to our investigation, the nonideal effects in the EOS can be responsible. These oscillating solutions imply the existence of more than one stable configuration for stellar masses.
1991-01-01T00:00:00ZLuo, Guo QuanIn this thesis we investigate the importance of various elements of the input physics and other parameters which affect the structure and evolution of models of low mass stars. Of the elements of input physics, the nuclear generation rates, the electron screening effect on thermo-nuclear reactions, and the conductive opacities are adopted from the formulation or data tables by other contributors. The low temperature opacities are taken from the data calculated by Carson and Sharp, as well as those by Alexander. In our study, we establish a sophisticated model for the determination of the equation of state. It is formulated by a new method based on the theory of the grand canonical ensemble. The interatomic interactions, which are responsible for the nonideal effects and pressure ionization, are treated carefully in the equation of state. In addition, and consistent with the equation of state, we also evaluate the radiative opacity according to the average atom model for heavy elements and the hydrogen-like model for hydrogen and helium. Negative hydrogen absorption and free electron scattering are also included. A computing code is constructed to calculate the radiative opacity data which are required in the study of low mass stars. To investigate the importance of each element of the input physics, we exclude it or replace it with an alternative for the model calculations. The parameters of the stellar code, such as the element abundances, the surface condition and the mixing length ratio are as well investigated by alternative values or formulation. In the computation, all elements of input physics, except the energy generation rates, are incorporated into the main code by means of data tables. A bicubic interpolation is used for their input. Our numerical calculations cover the zero age main sequence of the stars ranging from a solar mass down to the hydrogen burning minimum mass. The calculated results of the zero age models for the Standard Population I and Population II indicate good agreement with the observed data for the objects with effective temperatures above 3000 K, or with masses greater than 0,15 M[sub]sun. Below this limit, in agreement with other theoretical work, there is still a small discrepancy between the theoretical models and the observed data. The investigations of the effects of the input physics and model parameters show that some of them affect considerably the minimum mass limit for hydrogen burning on the lower main sequence. The low temperature opacities, the nonideal effects in the EOS and the conductive opacities lead to a limiting mass ranging from 0.08 M[sub]sun to 0.15 M[sub]sun although they do not affect the models with masses above 0.15 M[sub]sun obviously. The investigations also show that the massive models of the lower main sequence (i.e. those with mass near the sun) are dependent very much on the element abundances and the mixing length ratio while the lower mass models are not. The lower main sequence models are found to be insensitive to the surface condition (photospheric model or atmospheric model) used. We also perform the calculation of the evolutionary models of the lower main sequence from the zero age up to an age of 10¹⁰ years. A perfect theoretical model of the solar evolution is obtained when the atmospheric model is used as surface condition, the element abundances are chosen to be 0.70/0.28/0.02 and the mixing length ratio to be 1.5. The investigations show that the element abundances are the most important parameters in the determination of the solar models. The mixing length ratio is the second most important one being much more important than the physical model used as surface condition. The evolutionary models of the masses below 0.8 M[sub]sun are found to have only small changes in their properties within 10¹⁰ years. None of them can deplete their central hydrogen in that time. We find one problem in that the models with masses around 0.1 M[sub]sun have oscillating solutions for both the zero age models and evolutionary models. According to our investigation, the nonideal effects in the EOS can be responsible. These oscillating solutions imply the existence of more than one stable configuration for stellar masses.Stellar photometry from above the earth's atmosphereCampbell, J. W.https://hdl.handle.net/10023/143232019-04-01T10:05:33Z1980-01-01T00:00:00ZAlthough relative ground based stellar photometry has been an established investigative technique for many years, there have been very few programmes which have provided absolute stellar fluxes and indeed where such results have been published, they have been derived mainly from spectro-photometric data for a few bright stars. Such measurements are essential for the confirmation of model atmosphere calculations and for those models relating to early type stars, it is essential to extend such observations well into what is known as the vacuum ultraviolet region, i.e. below 3000A. The measurements are technically very difficult because of the need to use balloon or rocket platforms and when this work was started in 1962, only a very few observations had been conducted by the Naval Research Laboratory in Washington using very simple photometers with no absolute calibration. Under the auspices of a Senior Research Fellowship (the first in Scotland), held at the Royal Observatory, Edinburgh, the author proposed to the newly formed European Space Research Organisation, a number of stellar investigations aimed at obtaining absolute stellar fluxes in the region 1200Å - 2800Å. This thesis gives an account of almost a decade spent in developing the techniques of rocket astronomy and absolute calibration in the vacuum ultraviolet. Since no other European groups were engaged in similar studies, it was necessary for the author to essentially begin at the very beginning and a substantial period was spent in designing and testing optical systems which would withstand a typical Skylark Rocket launch. Systems of this type had never been flown before and many hours of vibration testing were necessary to ensure a satisfactory optical and mechanical configuration. The choice of a suitable ultraviolet detector was not easy and a substantial part of the early laboratory research was aimed at evaluating commercially available detectors and in the end an entirely new range of photo-multipliers was developed as a co-operative venture between the author and E.M.I. Hayes, Middlesex, resulting in a commercially saleable product. Wavelength isolation techniques were developed and the use of reflective mirror surfaces was successfully pioneered. A complete range of rocket-borne electronic systems was developed including solid state electrometers and high voltage power supplies. The techniques for encapsulating photomultipliers to withstand the rocket environment and the mounting of large optical mirrors were successfully developed and over fifty telescopes constructed and launched. The problems of calibrating on an absolute basis these rocket borne telescopes of large aperture were investigated and a network of traceable sub-standards was developed which ensured comparison with other (American) data. Highly stable microwave sources were developed and detectors calibrated at several national laboratories. These techniques were eventually used in the calibration of the T.D.I. S2/68 satellite experiment. The problem of integration of the payloads at centres throughout Europe was successfully overcome and the development of range facilities undertaken. Seven fully instrumented Skylarks were proposed and launched with a total of twenty photometers out of fifty-five photometers actually observing the Sky. The remainder having failed to produce any data due to rocket failure. During the period of the investigation, i.e. (1962-1970) the author obtained the first absolute stellar fluxes above the Earth's atmosphere and the total observations obtained equalled that obtained by all the other U.S. Groups including the satellite results of A. Smith. The results were compared with current model atmosphere calculations and showed that if blanketed models were used, the results would be to within ±0.25 magnitude in the region 1400Å-2800Å. The results also showed that it was possible to establish an absolute calibration network which was capable of not only permitting inter-comparison between the author's photometers, but also with other rocket groups, a situation that had hitherto been impossible. These results were the only stellar data to be obtained by any Group within the ESRO rocket programme. No other European results were obtained during that period.
1980-01-01T00:00:00ZCampbell, J. W.Although relative ground based stellar photometry has been an established investigative technique for many years, there have been very few programmes which have provided absolute stellar fluxes and indeed where such results have been published, they have been derived mainly from spectro-photometric data for a few bright stars. Such measurements are essential for the confirmation of model atmosphere calculations and for those models relating to early type stars, it is essential to extend such observations well into what is known as the vacuum ultraviolet region, i.e. below 3000A. The measurements are technically very difficult because of the need to use balloon or rocket platforms and when this work was started in 1962, only a very few observations had been conducted by the Naval Research Laboratory in Washington using very simple photometers with no absolute calibration. Under the auspices of a Senior Research Fellowship (the first in Scotland), held at the Royal Observatory, Edinburgh, the author proposed to the newly formed European Space Research Organisation, a number of stellar investigations aimed at obtaining absolute stellar fluxes in the region 1200Å - 2800Å. This thesis gives an account of almost a decade spent in developing the techniques of rocket astronomy and absolute calibration in the vacuum ultraviolet. Since no other European groups were engaged in similar studies, it was necessary for the author to essentially begin at the very beginning and a substantial period was spent in designing and testing optical systems which would withstand a typical Skylark Rocket launch. Systems of this type had never been flown before and many hours of vibration testing were necessary to ensure a satisfactory optical and mechanical configuration. The choice of a suitable ultraviolet detector was not easy and a substantial part of the early laboratory research was aimed at evaluating commercially available detectors and in the end an entirely new range of photo-multipliers was developed as a co-operative venture between the author and E.M.I. Hayes, Middlesex, resulting in a commercially saleable product. Wavelength isolation techniques were developed and the use of reflective mirror surfaces was successfully pioneered. A complete range of rocket-borne electronic systems was developed including solid state electrometers and high voltage power supplies. The techniques for encapsulating photomultipliers to withstand the rocket environment and the mounting of large optical mirrors were successfully developed and over fifty telescopes constructed and launched. The problems of calibrating on an absolute basis these rocket borne telescopes of large aperture were investigated and a network of traceable sub-standards was developed which ensured comparison with other (American) data. Highly stable microwave sources were developed and detectors calibrated at several national laboratories. These techniques were eventually used in the calibration of the T.D.I. S2/68 satellite experiment. The problem of integration of the payloads at centres throughout Europe was successfully overcome and the development of range facilities undertaken. Seven fully instrumented Skylarks were proposed and launched with a total of twenty photometers out of fifty-five photometers actually observing the Sky. The remainder having failed to produce any data due to rocket failure. During the period of the investigation, i.e. (1962-1970) the author obtained the first absolute stellar fluxes above the Earth's atmosphere and the total observations obtained equalled that obtained by all the other U.S. Groups including the satellite results of A. Smith. The results were compared with current model atmosphere calculations and showed that if blanketed models were used, the results would be to within ±0.25 magnitude in the region 1400Å-2800Å. The results also showed that it was possible to establish an absolute calibration network which was capable of not only permitting inter-comparison between the author's photometers, but also with other rocket groups, a situation that had hitherto been impossible. These results were the only stellar data to be obtained by any Group within the ESRO rocket programme. No other European results were obtained during that period.Atomic laser-spectroscopy in the UV and visibleKane, D.M.https://hdl.handle.net/10023/143212019-04-01T10:03:06Z1984-07-01T00:00:00ZThis thesis describes the development of an intracavity frequency doubled dye laser from a commercial Spectra-Physics 380D single frequency ring dye laser, and the application of this system to a number of spectroscopic studies in discharges. Evaluation of the stability of the laser cavity is carried out, by computer calculation, in order to optimise the optical component set used in the frequency doubled dye laser. Using an ADA (ammonium dihydrogen arsenate) crystal, 17 mW of continuous wave ultra-violet, tunable over the range 292-302 nm, has been produced. The linewidth, when the laser is stabilised, is about 500 kHz in the UV. The CW UV output has been used to measure helium triplet (23S) metastable densities in a positive column discharge as a function of pressure (1-8 Torr) and current (2.5-80 mA). A maximum density of 10x10<super> 12</super> cm-3 is measured for a pressure of 2 Torr and current of 60mA. The measured metastable densities are used in a rate equation analysis to extract values of the rate coefficient for distraction (by collisions with electrons) of the 23S metastables (1.6x10<super>-8</super> cm3s-1 for p = 2 Torr). A study of the optogalvanic signal generated when irradiating a neon positive column discharge with laser light of wavelength 588.2 nm, corresponding to the ls5-2p2 transition, is carried out. The signal is found to change sign as the dye laser power is increased, for some discharge conditions. Spatial studies of the optogalvanic signal in conjunction with absorption and emission studies are carried out to investigate the evolution of the population densities of levels connected to the 1s5 and 2p2 levels. A rate equation model is developed which demonstrates the importance of the 1s3 population density in explaining the observed sign reversal. A high resolution study of two transitions in the UV, in the tungsten spectrum, is presented. The even isotope splittings and the hyperfine splitting between the two strongest hyperfine components on transitions at 294.4 nm and 294.7 nm have been resolved by saturated absorption spectroscopy. From centre of gravity considerations the hyperfine splittings of the three levels involved in the two transitions have been predicted as 1630 MHz for the7s3 level, -506 MHz for the 3692 level and 225 MHz for the 3683 level. A simple wavemeter using a corner-cube Michelson interferometer is described. Digital counting circuits to provide a direct reading of either the fundamental or second harmonic wavelength have been designed and constructed. Wavelength readings accurate to 1 part in 106 are achieved.
1984-07-01T00:00:00ZKane, D.M.This thesis describes the development of an intracavity frequency doubled dye laser from a commercial Spectra-Physics 380D single frequency ring dye laser, and the application of this system to a number of spectroscopic studies in discharges. Evaluation of the stability of the laser cavity is carried out, by computer calculation, in order to optimise the optical component set used in the frequency doubled dye laser. Using an ADA (ammonium dihydrogen arsenate) crystal, 17 mW of continuous wave ultra-violet, tunable over the range 292-302 nm, has been produced. The linewidth, when the laser is stabilised, is about 500 kHz in the UV. The CW UV output has been used to measure helium triplet (23S) metastable densities in a positive column discharge as a function of pressure (1-8 Torr) and current (2.5-80 mA). A maximum density of 10x10<super> 12</super> cm-3 is measured for a pressure of 2 Torr and current of 60mA. The measured metastable densities are used in a rate equation analysis to extract values of the rate coefficient for distraction (by collisions with electrons) of the 23S metastables (1.6x10<super>-8</super> cm3s-1 for p = 2 Torr). A study of the optogalvanic signal generated when irradiating a neon positive column discharge with laser light of wavelength 588.2 nm, corresponding to the ls5-2p2 transition, is carried out. The signal is found to change sign as the dye laser power is increased, for some discharge conditions. Spatial studies of the optogalvanic signal in conjunction with absorption and emission studies are carried out to investigate the evolution of the population densities of levels connected to the 1s5 and 2p2 levels. A rate equation model is developed which demonstrates the importance of the 1s3 population density in explaining the observed sign reversal. A high resolution study of two transitions in the UV, in the tungsten spectrum, is presented. The even isotope splittings and the hyperfine splitting between the two strongest hyperfine components on transitions at 294.4 nm and 294.7 nm have been resolved by saturated absorption spectroscopy. From centre of gravity considerations the hyperfine splittings of the three levels involved in the two transitions have been predicted as 1630 MHz for the7s3 level, -506 MHz for the 3692 level and 225 MHz for the 3683 level. A simple wavemeter using a corner-cube Michelson interferometer is described. Digital counting circuits to provide a direct reading of either the fundamental or second harmonic wavelength have been designed and constructed. Wavelength readings accurate to 1 part in 106 are achieved.A unified state variable analysis of repetitively pulsed recombination lasersKidd, Andrew Keithhttps://hdl.handle.net/10023/143202019-04-01T10:08:45Z1992-07-01T00:00:00ZA unified state-space model of a high repetition rate electrical discharge in a helium-strontium mixture is presented. The atomic (number density of particles in a particular state), optical (number density of photons), thermodynamic (local particle temperature and pressure) and electrical (voltage and current) internal states of the system are the state variables. A generalised circuit analysis program (GCAP) provides a description of the excitation circuit used to power the strontium laser. Rate equations describing the time evolution of the state variables are simultaneously numerically integrated to provide a description of the laser system during the discharge of a capacitor through the laser load and in the immediate afterglow. The effects of parametric variation of the circuit on strontium laser performance are examined. The model predicts that Sr++ ions are formed during the discharge current pulse by step-wise excitation from SrII states lying lower in energy. A population inversion is achieved on the 62S1/2-52P3/2 transition in SrII (lambda=430.5nm) in the current pulse afterglow by rapid three-body recombination of Sr++ ions. A strong recombination flux is established on rapid termination of the discharge current pulse. Impedance matching of excitation circuit to load is essential for stimulated emission under recombination conditions. The results of the state variable analysis are validated by experiment. The operating characteristics of a discharge-heated strontium vapour laser are presented. An average power of 0.3W is obtained in a high heat-loss configuration. The fall-time of the discharge current pulse is reduced by means of a saturable inductor placed in parallel with the laser load. The circuit generates current pulses with peak amplitudes up to 300A and fall-times of less than 70ns. GCAP is used to estimate the theoretical limits on the fall-time of the laser current by state variable analysis of a flux-controlled model of the saturable inductor.
1992-07-01T00:00:00ZKidd, Andrew KeithA unified state-space model of a high repetition rate electrical discharge in a helium-strontium mixture is presented. The atomic (number density of particles in a particular state), optical (number density of photons), thermodynamic (local particle temperature and pressure) and electrical (voltage and current) internal states of the system are the state variables. A generalised circuit analysis program (GCAP) provides a description of the excitation circuit used to power the strontium laser. Rate equations describing the time evolution of the state variables are simultaneously numerically integrated to provide a description of the laser system during the discharge of a capacitor through the laser load and in the immediate afterglow. The effects of parametric variation of the circuit on strontium laser performance are examined. The model predicts that Sr++ ions are formed during the discharge current pulse by step-wise excitation from SrII states lying lower in energy. A population inversion is achieved on the 62S1/2-52P3/2 transition in SrII (lambda=430.5nm) in the current pulse afterglow by rapid three-body recombination of Sr++ ions. A strong recombination flux is established on rapid termination of the discharge current pulse. Impedance matching of excitation circuit to load is essential for stimulated emission under recombination conditions. The results of the state variable analysis are validated by experiment. The operating characteristics of a discharge-heated strontium vapour laser are presented. An average power of 0.3W is obtained in a high heat-loss configuration. The fall-time of the discharge current pulse is reduced by means of a saturable inductor placed in parallel with the laser load. The circuit generates current pulses with peak amplitudes up to 300A and fall-times of less than 70ns. GCAP is used to estimate the theoretical limits on the fall-time of the laser current by state variable analysis of a flux-controlled model of the saturable inductor.Kinetic modelling of fundamental (00̊1) and sequence (00̊2) band CO₂ lasersMellis, Johnhttps://hdl.handle.net/10023/143192019-04-01T10:06:25Z1984-07-01T00:00:00ZThe vibrational kinetics of the CO2 laser system are studied experimentally and theoretically. A sequence (00°2) band/ fundamental (00°1) band gain ratioing technique is used to measure the CO2 asymmetric stretch mode temperature (T3) in low-pressure cw laser discharges; the relationship of discharge current to electron density is determined by X-band microwave cavity resonance. The experimental measurements are compared to theory using a comprehensive computer model of CO2 laser kinetics, based on the vibrational temperature approximation. It is demonstrated that the observed saturation of vibrational temperature with increasing discharge current is caused by the de-activation of excited molecules by electron superelastic collisions, at a rate predicted by the principle of detailed balance. Superelastic collisions crucially determine the attainable vibrational temperatures, and limit T3 to values below the optimum for 00°1 or 00°2 band laser action. The associated laser gain limitations are investigated, and it is shown that superelastic collisions inflict efficiency losses on pulsed TE CO2 lasers even at moderate input energies. The operating characteristics of CO2 sequence band lasers are also examined. A comparison of oscillator performance with corresponding small-signal gain measurements indicates a sequence band saturation intensity which, is higher than that of the fundamental band. This observation is supported by model computations, which predict that the. extractable 00°2 band laser power (alphao Is) is typically 60% of that available on the 00°1 band.
1984-07-01T00:00:00ZMellis, JohnThe vibrational kinetics of the CO2 laser system are studied experimentally and theoretically. A sequence (00°2) band/ fundamental (00°1) band gain ratioing technique is used to measure the CO2 asymmetric stretch mode temperature (T3) in low-pressure cw laser discharges; the relationship of discharge current to electron density is determined by X-band microwave cavity resonance. The experimental measurements are compared to theory using a comprehensive computer model of CO2 laser kinetics, based on the vibrational temperature approximation. It is demonstrated that the observed saturation of vibrational temperature with increasing discharge current is caused by the de-activation of excited molecules by electron superelastic collisions, at a rate predicted by the principle of detailed balance. Superelastic collisions crucially determine the attainable vibrational temperatures, and limit T3 to values below the optimum for 00°1 or 00°2 band laser action. The associated laser gain limitations are investigated, and it is shown that superelastic collisions inflict efficiency losses on pulsed TE CO2 lasers even at moderate input energies. The operating characteristics of CO2 sequence band lasers are also examined. A comparison of oscillator performance with corresponding small-signal gain measurements indicates a sequence band saturation intensity which, is higher than that of the fundamental band. This observation is supported by model computations, which predict that the. extractable 00°2 band laser power (alphao Is) is typically 60% of that available on the 00°1 band.Some aspects of nonlinear laser plasma interactionsJohnson, David A.https://hdl.handle.net/10023/143182019-04-01T10:03:34Z1995-07-01T00:00:00ZRecent advances in the development of high power short pulse laser systems has opened a new regime of laser plasma interactions for study. The thesis is presented in two parts. In Part I, we consider the implications of these high power laser pulses for the interaction with a uniform underdense plasma, with particular regard to plasma-based accelerators. We present a scheme for the resonant excitation of large electrostatic Wakefields in these plasmas using a train of ultra-intense laser pulses. We also present an analysis of the resonant mechanism of this excitation based on consideration of phase space trajectories. In Part II, we consider the transition from linear Resonance Absorption to nonlinear absorption processes in a linear electron density profile as the intensity of the incident radiation increases and the scale length of the density profile decreases. We find that the electron motion excited by an electrostatic field exhibits some extremely complicated dynamics with bifurcations to period doubling and chaotic motion as the strength of the driving field is increased or the density scale length is decreased. We also present some results obtained from particle simulations of these interactions.
1995-07-01T00:00:00ZJohnson, David A.Recent advances in the development of high power short pulse laser systems has opened a new regime of laser plasma interactions for study. The thesis is presented in two parts. In Part I, we consider the implications of these high power laser pulses for the interaction with a uniform underdense plasma, with particular regard to plasma-based accelerators. We present a scheme for the resonant excitation of large electrostatic Wakefields in these plasmas using a train of ultra-intense laser pulses. We also present an analysis of the resonant mechanism of this excitation based on consideration of phase space trajectories. In Part II, we consider the transition from linear Resonance Absorption to nonlinear absorption processes in a linear electron density profile as the intensity of the incident radiation increases and the scale length of the density profile decreases. We find that the electron motion excited by an electrostatic field exhibits some extremely complicated dynamics with bifurcations to period doubling and chaotic motion as the strength of the driving field is increased or the density scale length is decreased. We also present some results obtained from particle simulations of these interactions.Generation and nonlinear propagation of ultrashort near infrared laser pulsesKean, Peter N.https://hdl.handle.net/10023/143172019-04-01T10:07:58Z1990-07-01T00:00:00ZBy utilising a CW mode-locked Nd:YAG pump laser an experimental study of self-phase modulation (SPM) and stimulated Raman scattering (SRS) in single mode optical fibres has been conducted. The dependence of the spectral broadening due to SPM upon the launched optical power was observed to obey a linear relationship in agreement with a simple theory. A deviation from this occurred for high input powers due to the onset of stimulated Raman scattering which caused a preferential depletion of the leading edge of the pump pulse and an increased spectral broadening to the long wavelength side of the spectrum. The pulses exiting the fibre were then compressed using a pair of holographic diffraction gratings, which were able to compensate for the linear part of the frequency chirp imposed on the pulse by SPM and the 1.06 ?m pulses were reduced in duration from ~ 100 ps to approximately 4 ps by this method. By making use of Raman generation in the fibre, a synchronously pumped fibre Raman oscillator was constructed. This enabled the generation of frequency tunable (1.07 - 1.12 ?m) near infrared pulses by the method of time dispersion tuning. By incorporating two fibre grating reflectors onto the ends of the optical fibre, an all-fibre device was constructed having the potential advantages of compactness and stability. The generation of mode-locked pulses around the 1.5 jim wavelength region was accomplished with the use of a colour centre laser based upon a stabilised F2+ centre in NaC1 or a thallium centre in KCl. Both of these lasers were examined, although to date the poor quality of our NaC1 laser crystals has meant that most of the work reported here was performed with KC1:T1. This laser produced pulses of ? 20 ps duration, tunable over 1.45 - 1.55 ?m with average powers ? 200 mW. A simple experiment to observe soliton propagation of these pulses in an optical fibre was conducted and this compressed the pulses to ? 0.8 ps, although this does not represent the optimum compression that could be achieved. Using nonlinear pulse propagation in an optical fibre, the mode-locked characteristics of the colour centre laser were dramatically improved with the duration of the pulses from the laser being reduced to ? 200 fs. This enhancement was achieved by the use of a nonlinear external cavity containing the optical fibre, which reinjected the pulses back into the main laser cavity, with an increased spectral bandwidth due to SPM. It was initially thought that the explanation to this effect was due to soliton formation within the control cavity, however experimental evidence is presented here which shows that the mode-locking enhancement phenomena is in fact quite general and does not rely on dispersion in the control cavity.
1990-07-01T00:00:00ZKean, Peter N.By utilising a CW mode-locked Nd:YAG pump laser an experimental study of self-phase modulation (SPM) and stimulated Raman scattering (SRS) in single mode optical fibres has been conducted. The dependence of the spectral broadening due to SPM upon the launched optical power was observed to obey a linear relationship in agreement with a simple theory. A deviation from this occurred for high input powers due to the onset of stimulated Raman scattering which caused a preferential depletion of the leading edge of the pump pulse and an increased spectral broadening to the long wavelength side of the spectrum. The pulses exiting the fibre were then compressed using a pair of holographic diffraction gratings, which were able to compensate for the linear part of the frequency chirp imposed on the pulse by SPM and the 1.06 ?m pulses were reduced in duration from ~ 100 ps to approximately 4 ps by this method. By making use of Raman generation in the fibre, a synchronously pumped fibre Raman oscillator was constructed. This enabled the generation of frequency tunable (1.07 - 1.12 ?m) near infrared pulses by the method of time dispersion tuning. By incorporating two fibre grating reflectors onto the ends of the optical fibre, an all-fibre device was constructed having the potential advantages of compactness and stability. The generation of mode-locked pulses around the 1.5 jim wavelength region was accomplished with the use of a colour centre laser based upon a stabilised F2+ centre in NaC1 or a thallium centre in KCl. Both of these lasers were examined, although to date the poor quality of our NaC1 laser crystals has meant that most of the work reported here was performed with KC1:T1. This laser produced pulses of ? 20 ps duration, tunable over 1.45 - 1.55 ?m with average powers ? 200 mW. A simple experiment to observe soliton propagation of these pulses in an optical fibre was conducted and this compressed the pulses to ? 0.8 ps, although this does not represent the optimum compression that could be achieved. Using nonlinear pulse propagation in an optical fibre, the mode-locked characteristics of the colour centre laser were dramatically improved with the duration of the pulses from the laser being reduced to ? 200 fs. This enhancement was achieved by the use of a nonlinear external cavity containing the optical fibre, which reinjected the pulses back into the main laser cavity, with an increased spectral bandwidth due to SPM. It was initially thought that the explanation to this effect was due to soliton formation within the control cavity, however experimental evidence is presented here which shows that the mode-locking enhancement phenomena is in fact quite general and does not rely on dispersion in the control cavity.Plasma kinetics of sealed off high power carbon monoxide lasersMurray, Gordon Alexanderhttps://hdl.handle.net/10023/143162018-06-21T08:38:40Z1979-10-01T00:00:00ZThe plasma kinetics of sealed off room temperature CO lasers have been investigated by measuring the gain, electron density, and discharge voltage in various gas mixtures which are used for 'such lasers. In certain cases the positive ion spectrum has also been studied. This has led to an understanding of the effects of Xe, O2 N2, H2 and CO2 on the plasma kinetics of CO lasers, and these effects can be broadly divided into three classes, (a) The additives which primarily alter the electron kinetics and change the electron impact excitation rates to CO, these are also usually found to alter the positive ion spectrum, eg Xe, O2. (b) Those additives which are important in altering the vibrational kinetics by changing the V-V-T near resonant exchange rate, eg N2. (c) Those additives which alter the V-T relaxation rate and hence affect the vibrational kinetics of CO, eg H2. In general in a working laser all three of the above processes must be considered. By understanding how each of the gases mentioned earlier can affect the CO laser plasma, high power, high efficiency single line and multiline CO lasers have been produced.
1979-10-01T00:00:00ZMurray, Gordon AlexanderThe plasma kinetics of sealed off room temperature CO lasers have been investigated by measuring the gain, electron density, and discharge voltage in various gas mixtures which are used for 'such lasers. In certain cases the positive ion spectrum has also been studied. This has led to an understanding of the effects of Xe, O2 N2, H2 and CO2 on the plasma kinetics of CO lasers, and these effects can be broadly divided into three classes, (a) The additives which primarily alter the electron kinetics and change the electron impact excitation rates to CO, these are also usually found to alter the positive ion spectrum, eg Xe, O2. (b) Those additives which are important in altering the vibrational kinetics by changing the V-V-T near resonant exchange rate, eg N2. (c) Those additives which alter the V-T relaxation rate and hence affect the vibrational kinetics of CO, eg H2. In general in a working laser all three of the above processes must be considered. By understanding how each of the gases mentioned earlier can affect the CO laser plasma, high power, high efficiency single line and multiline CO lasers have been produced.Frequency doubled continuous wave dye lasersFerguson, Allister Ianhttps://hdl.handle.net/10023/143152019-04-01T10:03:24Z1977-09-01T00:00:00ZThis thesis describes the design and development of a frequency doubled, continuous wave dye laser and its application to a study of the high Rydberg states of Rubidium. The laser uses the dye rhodamine 6G as the active medium and is optically pumped with an argon ion laser. Frequency doubling is by an ADA (ammonium dihydrogen arsenate) or ADP (ammonium dihydrogen phosphate) crystal located within the laser cavity. Continuous output powers in the ultra-violet in excess of 30 mW and tunable over the wavelength range 285-315 nm have been produced. The linewidth can be chosen to be 0.02 nm broadband system) or 0.002 nm (narrowband system) depending on the frequency selecting elements used. In order to keep insertion losses small the crystals have optical faces cut at Brewster's angle, and in order to increase generation efficiency the intracavity radiation field is focused into the crystal. Such an arrangement introduces the aberrations of coma and astigmatism which must be compensated by suitable cavity design. A variety of cavity and crystal configurations have been analysed for aberrations, and a novel arrangement for the simultaneous elimination of coma and astigmatism developed. Several practical frequency doubled dye lasers have been investigated. In particular the performances of ADA and ADP as the frequency doubling crystals are compared and contrasted. ADA has the advantage that it can be non-critically phase matched at these wavelengths and this results in a higher generation efficiency and a better UV beam quality than encountered with ADP. However, since it can only be temperature tuned, the tuning range (292-302 nm for temperature range 20-80°C) is more limited than that for ADP (285-315 nm) which can also be angled tuned. For both types of crystal, thermal phase mismatching is identified as the process limiting generation efficiency. Evidence is also presented that thermal focusing ultimately limits the UV output power by upsetting cavity stability. A computer model of intracavity frequency doubling has been developed. Thermal phase mismatching effects in the crystal as well as excited state absorption in the dye are included. This model is used to investigate the influence of cavity losses and crystal absorption on generation efficiency. Optimization of conversion efficiency by correct choice of crystal parameters is considered. Two systems have been developed to allow continuous scanning over an extended frequency range. One allows the broadband laser (0.02 nm) to be continuously tuned over 3 nm, the other allows the narrowband laser (0.002 nm) to be continuously tuned over 2 nm. The operation of a single frequency version of the laser and its stabilization on an external reference cavity is also described. The excitation of high Rydberg states in Rubidium using the frequency doubled laser is described. The states are detected by a space-charge limited ionization detector. The principal series of Rb up to a principal quantum number of n = 74 has been detected. A novel triode arrangement of electrodes in the space charge detector has enabled a small electric field to be applied to the rubidium vapour. The consequent Stark mixing of n2S, n2P and n2S states has allowed the n2S and n2D states to be excited from the 52S ground state. New term values of the n2S and n2D series are reported.
1977-09-01T00:00:00ZFerguson, Allister IanThis thesis describes the design and development of a frequency doubled, continuous wave dye laser and its application to a study of the high Rydberg states of Rubidium. The laser uses the dye rhodamine 6G as the active medium and is optically pumped with an argon ion laser. Frequency doubling is by an ADA (ammonium dihydrogen arsenate) or ADP (ammonium dihydrogen phosphate) crystal located within the laser cavity. Continuous output powers in the ultra-violet in excess of 30 mW and tunable over the wavelength range 285-315 nm have been produced. The linewidth can be chosen to be 0.02 nm broadband system) or 0.002 nm (narrowband system) depending on the frequency selecting elements used. In order to keep insertion losses small the crystals have optical faces cut at Brewster's angle, and in order to increase generation efficiency the intracavity radiation field is focused into the crystal. Such an arrangement introduces the aberrations of coma and astigmatism which must be compensated by suitable cavity design. A variety of cavity and crystal configurations have been analysed for aberrations, and a novel arrangement for the simultaneous elimination of coma and astigmatism developed. Several practical frequency doubled dye lasers have been investigated. In particular the performances of ADA and ADP as the frequency doubling crystals are compared and contrasted. ADA has the advantage that it can be non-critically phase matched at these wavelengths and this results in a higher generation efficiency and a better UV beam quality than encountered with ADP. However, since it can only be temperature tuned, the tuning range (292-302 nm for temperature range 20-80°C) is more limited than that for ADP (285-315 nm) which can also be angled tuned. For both types of crystal, thermal phase mismatching is identified as the process limiting generation efficiency. Evidence is also presented that thermal focusing ultimately limits the UV output power by upsetting cavity stability. A computer model of intracavity frequency doubling has been developed. Thermal phase mismatching effects in the crystal as well as excited state absorption in the dye are included. This model is used to investigate the influence of cavity losses and crystal absorption on generation efficiency. Optimization of conversion efficiency by correct choice of crystal parameters is considered. Two systems have been developed to allow continuous scanning over an extended frequency range. One allows the broadband laser (0.02 nm) to be continuously tuned over 3 nm, the other allows the narrowband laser (0.002 nm) to be continuously tuned over 2 nm. The operation of a single frequency version of the laser and its stabilization on an external reference cavity is also described. The excitation of high Rydberg states in Rubidium using the frequency doubled laser is described. The states are detected by a space-charge limited ionization detector. The principal series of Rb up to a principal quantum number of n = 74 has been detected. A novel triode arrangement of electrodes in the space charge detector has enabled a small electric field to be applied to the rubidium vapour. The consequent Stark mixing of n2S, n2P and n2S states has allowed the n2S and n2D states to be excited from the 52S ground state. New term values of the n2S and n2D series are reported.Compact, low-threshold femtosecond lasersHopkins, John-Markhttps://hdl.handle.net/10023/143142019-04-01T10:06:45Z1999-05-01T00:00:00ZThis thesis is concerned with the design and development of compact, all-solid-state femtosecond pulse lasers with low pump power requirements. A number of directly-diode- pumped laser systems based on the gain materials Cr3:LiSrGaF6 (chromium-doped lithium strontium aluminium fluoride) and Cr3:LiSrGaF6 (chromium-doped lithium strontium gallium fluoride) pumped with AlGaInP laser diodes are described. The motivation behind this work was the development of portable, low-noise and lower cost ultrashort pulse lasers for a number of low-power applications such as the characterisation of electron-optical streak camera systems. The investigation into the modelocking of lasers with modest intracavity powers was also an important challenge. The achievement of a battery-powered, compact and efficient laser system represents an excellent outcome for this research programme. Major consideration is given to the key factors that determine both the cw and modelocking thresholds of an ultrashort-pulse laser. In particular, the reduction of intracavity optical losses by designing the laser to operate with fewer cavity elements, the optimisation of second-order and higher-order dispersion for efficient modelocked operation and the inclusion of a semiconductor saturable absorber mirror for increased stability are discussed. This has enabled pump thresholds to be reduced to a level permitting, for the first time, the use of diffraction-limited, narrow-stripe laser diodes for efficient, low-power optical pumping. A number of laser oscillators with novel cavity designs and progressively lower pump thresholds are described. Pulses as short as 57 fs and average output powers as high as 9 mW for only 80 mW of incident pump power are reported for a battery powered femtosecond Cr:LiSAF laser. This represents an overall electrical-to-optical conversion efficiency of approximately 1% which is excellent for a femtosecond pulse laser system. In addition, the amplitude and phase noise performance is shown to be exceptionally good and is believed to be the best yet reported for this type of ultrashort pulse laser. The design and demonstration of highly compact, ultrashort-pulse lasers incorporating novel resonator configurations and simplified dispersion compensation schemes are then described. These lasers produced sub-ps pulses at cavity frequencies as high as 450 MHz.
1999-05-01T00:00:00ZHopkins, John-MarkThis thesis is concerned with the design and development of compact, all-solid-state femtosecond pulse lasers with low pump power requirements. A number of directly-diode- pumped laser systems based on the gain materials Cr3:LiSrGaF6 (chromium-doped lithium strontium aluminium fluoride) and Cr3:LiSrGaF6 (chromium-doped lithium strontium gallium fluoride) pumped with AlGaInP laser diodes are described. The motivation behind this work was the development of portable, low-noise and lower cost ultrashort pulse lasers for a number of low-power applications such as the characterisation of electron-optical streak camera systems. The investigation into the modelocking of lasers with modest intracavity powers was also an important challenge. The achievement of a battery-powered, compact and efficient laser system represents an excellent outcome for this research programme. Major consideration is given to the key factors that determine both the cw and modelocking thresholds of an ultrashort-pulse laser. In particular, the reduction of intracavity optical losses by designing the laser to operate with fewer cavity elements, the optimisation of second-order and higher-order dispersion for efficient modelocked operation and the inclusion of a semiconductor saturable absorber mirror for increased stability are discussed. This has enabled pump thresholds to be reduced to a level permitting, for the first time, the use of diffraction-limited, narrow-stripe laser diodes for efficient, low-power optical pumping. A number of laser oscillators with novel cavity designs and progressively lower pump thresholds are described. Pulses as short as 57 fs and average output powers as high as 9 mW for only 80 mW of incident pump power are reported for a battery powered femtosecond Cr:LiSAF laser. This represents an overall electrical-to-optical conversion efficiency of approximately 1% which is excellent for a femtosecond pulse laser system. In addition, the amplitude and phase noise performance is shown to be exceptionally good and is believed to be the best yet reported for this type of ultrashort pulse laser. The design and demonstration of highly compact, ultrashort-pulse lasers incorporating novel resonator configurations and simplified dispersion compensation schemes are then described. These lasers produced sub-ps pulses at cavity frequencies as high as 450 MHz.Radial profile studies in metal vapour laser dischargesMcKenzie, Alan L.https://hdl.handle.net/10023/143132019-04-01T10:09:58Z1976-01-01T00:00:00ZPositive-column metal-vapour lasers are important because of the many visible and ultra-violet c.w. wavelengths attainable. In many such systems population inversion among the metal ion states is produced in charge-exchange reactions with helium ions. Although the power of such lasers should apparently increase linearly with the helium ion density and, hence, with the discharge current, several observers have noted a power saturation. This work describes a phenomenon which will account for this. The spontaneous emission from laser states in a helium-selenium discharge is observed across the tube, and the radial profiles of intensity show a central dip which deepens with discharge current and helium pressure. This axial depletion of laser states sets a limit to the achievable power. The dip effect is shown to be due to the removal of metal atoms from the centre of the tube by ionisation followed by a build-up of the vapour at the walls where the; metal is neutralised. States excited from the axially reduced neutral population will reflect the depletion, to some extent in their own distribution. The helium-cadmium discharge is selected as representative of metal-vapour systems for further study using the dip effect. By comparing the radial profiles of different excited states under the same discharge conditions, it is shown that electron collisions probably contribute to the excitation of levels which are normally assumed to result from charge-exchange and Penning collisions. Radial profiles of helium and cadmium metastables are examined. The behaviour of the helium profiles at high cadmium partial pressures is consistent with an electron temperature which depends upon the local cadmium concentration. The shape of the cadmium metastable profile indicates that this species is being destroyed in the discharge - either by electron collision or collisional mixing with the cadmium neutral resonance state. Observations of cadmium ion profiles at various cadmium partial pressures indicate that electron collisions are the principal source of cadmium ionisation. From the magnitude of the radial dip, the ionisation rate constant is calculated (1 - 5 x 10<super>-8</super> cm3 s-1). This varies with E/N, the reduced axial electric field. The ionisation rate is found another way, by modulating the discharge current and observing the phase lag of the profile dip. This experiment yields a value similar to the above.
1976-01-01T00:00:00ZMcKenzie, Alan L.Positive-column metal-vapour lasers are important because of the many visible and ultra-violet c.w. wavelengths attainable. In many such systems population inversion among the metal ion states is produced in charge-exchange reactions with helium ions. Although the power of such lasers should apparently increase linearly with the helium ion density and, hence, with the discharge current, several observers have noted a power saturation. This work describes a phenomenon which will account for this. The spontaneous emission from laser states in a helium-selenium discharge is observed across the tube, and the radial profiles of intensity show a central dip which deepens with discharge current and helium pressure. This axial depletion of laser states sets a limit to the achievable power. The dip effect is shown to be due to the removal of metal atoms from the centre of the tube by ionisation followed by a build-up of the vapour at the walls where the; metal is neutralised. States excited from the axially reduced neutral population will reflect the depletion, to some extent in their own distribution. The helium-cadmium discharge is selected as representative of metal-vapour systems for further study using the dip effect. By comparing the radial profiles of different excited states under the same discharge conditions, it is shown that electron collisions probably contribute to the excitation of levels which are normally assumed to result from charge-exchange and Penning collisions. Radial profiles of helium and cadmium metastables are examined. The behaviour of the helium profiles at high cadmium partial pressures is consistent with an electron temperature which depends upon the local cadmium concentration. The shape of the cadmium metastable profile indicates that this species is being destroyed in the discharge - either by electron collision or collisional mixing with the cadmium neutral resonance state. Observations of cadmium ion profiles at various cadmium partial pressures indicate that electron collisions are the principal source of cadmium ionisation. From the magnitude of the radial dip, the ionisation rate constant is calculated (1 - 5 x 10<super>-8</super> cm3 s-1). This varies with E/N, the reduced axial electric field. The ionisation rate is found another way, by modulating the discharge current and observing the phase lag of the profile dip. This experiment yields a value similar to the above.Temporal and frequency characteristics of distributed feedback dye lasersLusty, Michael E.https://hdl.handle.net/10023/143122019-04-01T10:03:59Z1989-07-01T00:00:00ZPrevious studies of distributed feedback dye lasers (DFDL's) have identified that the linewidth of the device scales, to a first approximation, with the level of pumping employed. A more recent development is that the DFDL can be used to produce single ultrashort pulses. To produce such pulses the main requirement is that the laser is operated close to its threshold. An apparent contradiction exists here since, by lowering the pump power to achieve narrow linewidth operation, the near threshold region must be avoided since pulsing operation acts to increase the linewidth (to at least the Fourier transform of the pulse duration). This thesis further investigates the mechanisms which contribute to the temporal and linewidth properties of the laser. It is identified that by judicious choice of operating conditions a regime exists where the DFDL may be operated with a linewidth approaching that of the transform limit for the nanosecond pulse durations involved. After introducing the different types of distributed feedback lasers the thesis first reviews previously understood DFDL behaviour. Different DFDL geometries are considered with a view to their particular temporal and linewidth properties. A strategy for the development of a narrow linewidth DFDL is presented. The experimental laser system is described detailing the operation and characteristics of the frequency doubled Q switched Nd:YAG pump laser and the two different DFDL geometries. A high resolution computer aided interferometry (CAIN) system is described which provided single shot linewidth measurements. This system was used extensively in the experiments reported. DFDL linewidth is seen to depend on the thermo-optical properties of the dye's host solvent and as such a full characterization of commonly used solvents is presented. The temporal behaviour of the laser is considered theoretically with the aid of a coupled rate equation model which describes the interplay between the population inversion and the cavity photon flux. The model is used to predict short (picosecond) and smooth (nanosecond) pulse operation of the laser. Finally, a description of and the results obtained from various experimental investigations into the DFDL are presented. Temporal analysis, using a streak camera, revealed that, as expected, under certain circumstances multiple pulsing of picosecond duration could occur. Different conditions however, lead to narrow linewidth (~100 MHz) operation. A description of the two operating regimes is presented and these are related to the particular parameters involved e.g. the grating length or the level of pumping employed.
1989-07-01T00:00:00ZLusty, Michael E.Previous studies of distributed feedback dye lasers (DFDL's) have identified that the linewidth of the device scales, to a first approximation, with the level of pumping employed. A more recent development is that the DFDL can be used to produce single ultrashort pulses. To produce such pulses the main requirement is that the laser is operated close to its threshold. An apparent contradiction exists here since, by lowering the pump power to achieve narrow linewidth operation, the near threshold region must be avoided since pulsing operation acts to increase the linewidth (to at least the Fourier transform of the pulse duration). This thesis further investigates the mechanisms which contribute to the temporal and linewidth properties of the laser. It is identified that by judicious choice of operating conditions a regime exists where the DFDL may be operated with a linewidth approaching that of the transform limit for the nanosecond pulse durations involved. After introducing the different types of distributed feedback lasers the thesis first reviews previously understood DFDL behaviour. Different DFDL geometries are considered with a view to their particular temporal and linewidth properties. A strategy for the development of a narrow linewidth DFDL is presented. The experimental laser system is described detailing the operation and characteristics of the frequency doubled Q switched Nd:YAG pump laser and the two different DFDL geometries. A high resolution computer aided interferometry (CAIN) system is described which provided single shot linewidth measurements. This system was used extensively in the experiments reported. DFDL linewidth is seen to depend on the thermo-optical properties of the dye's host solvent and as such a full characterization of commonly used solvents is presented. The temporal behaviour of the laser is considered theoretically with the aid of a coupled rate equation model which describes the interplay between the population inversion and the cavity photon flux. The model is used to predict short (picosecond) and smooth (nanosecond) pulse operation of the laser. Finally, a description of and the results obtained from various experimental investigations into the DFDL are presented. Temporal analysis, using a streak camera, revealed that, as expected, under certain circumstances multiple pulsing of picosecond duration could occur. Different conditions however, lead to narrow linewidth (~100 MHz) operation. A description of the two operating regimes is presented and these are related to the particular parameters involved e.g. the grating length or the level of pumping employed.The evolution of population III stars: a study of selected models with regard to the early enrichment of the interstellar mediumThacker, Peter D.https://hdl.handle.net/10023/143082019-04-01T10:10:50Z1996-01-01T00:00:00ZModelling of the evolution of 15M[sub]sun, 10M[sub]sun, 5M[sub]sun arid 2M[sub]sun stars has been carried out at three different initial compositions that could be thought of as 'population III' - Z=10⁻¹⁰, 10⁻¹⁰ and zero. The effects of mass loss due to a stellar wind have been included in the modelling, the mass loss rates taken from an empirical formula by Nieuwenhuijzen and de Jager (1990). An original and amended form of a FORTRAN 77 quasi-hydrostatic evolutionary code, written by Dr. T. R. Carson and modified by the author of this work, were used to create and evolve the models. The aims of the evolutionary modelling were to confirm that the sensitivity of stellar parameters to the initial metallicity of a star continues to a lower level of Z than has previously been thought, and to provide some evidence that nucleosynthesis and subsequent mass loss in a population III is a plausible mechanism for the prompt enrichment of the early interstellar medium. We find that there is sensitivity of stellar parameters to the initial metallicity Z in the range 0≤Z≤10⁻¹⁰ but this is less apparent in models of intermediate or low masses. During the evolution of the models, no significant loss of enriched material occurs, due to low rates of mass loss and the absence of any 'dredge-up' of enriched material. The results of the modelling were connected with bare-core studies of helium stars to determine the amount and composition of the material that would be returned to the interstellar medium during the endstates of the models. Functions for the rates of enrichment due to a population III and the distribution of mass in a population III were determined and their implications examined. From our results, we find that the enrichment of the interstellar medium due to population III stars must have come about entirely due to rapid mass loss during endstates, such as supernovae, as opposed to a slower rate of mass loss during the lifetime of the stars. We also find that our results indicate that the level of helium enrichment from a population III would be negligible compared to that due to cosmological mechanisms. Our distribution functions for a population III indicate that massive stars are very much fewer by number that would be expected for stars in our epoch. Overall, we can consider that a primordial population III can be considered a plausible mechanism for a prompt enrichment of metals in the early universe, but not for the prompt enrichment of helium.
1996-01-01T00:00:00ZThacker, Peter D.Modelling of the evolution of 15M[sub]sun, 10M[sub]sun, 5M[sub]sun arid 2M[sub]sun stars has been carried out at three different initial compositions that could be thought of as 'population III' - Z=10⁻¹⁰, 10⁻¹⁰ and zero. The effects of mass loss due to a stellar wind have been included in the modelling, the mass loss rates taken from an empirical formula by Nieuwenhuijzen and de Jager (1990). An original and amended form of a FORTRAN 77 quasi-hydrostatic evolutionary code, written by Dr. T. R. Carson and modified by the author of this work, were used to create and evolve the models. The aims of the evolutionary modelling were to confirm that the sensitivity of stellar parameters to the initial metallicity of a star continues to a lower level of Z than has previously been thought, and to provide some evidence that nucleosynthesis and subsequent mass loss in a population III is a plausible mechanism for the prompt enrichment of the early interstellar medium. We find that there is sensitivity of stellar parameters to the initial metallicity Z in the range 0≤Z≤10⁻¹⁰ but this is less apparent in models of intermediate or low masses. During the evolution of the models, no significant loss of enriched material occurs, due to low rates of mass loss and the absence of any 'dredge-up' of enriched material. The results of the modelling were connected with bare-core studies of helium stars to determine the amount and composition of the material that would be returned to the interstellar medium during the endstates of the models. Functions for the rates of enrichment due to a population III and the distribution of mass in a population III were determined and their implications examined. From our results, we find that the enrichment of the interstellar medium due to population III stars must have come about entirely due to rapid mass loss during endstates, such as supernovae, as opposed to a slower rate of mass loss during the lifetime of the stars. We also find that our results indicate that the level of helium enrichment from a population III would be negligible compared to that due to cosmological mechanisms. Our distribution functions for a population III indicate that massive stars are very much fewer by number that would be expected for stars in our epoch. Overall, we can consider that a primordial population III can be considered a plausible mechanism for a prompt enrichment of metals in the early universe, but not for the prompt enrichment of helium.Studies in stellar structure and evolutionJeffery, C. Simonhttps://hdl.handle.net/10023/143042019-04-01T10:03:18Z1983-01-01T00:00:00ZWe investigate stellar models for main-sequence and horizontal-branch stars constructed using the Carson opacities and make comparisons with models based on the Cox-Stewart opacities. A Henyey code based on the prescription of Kippenhahn et al (1967) is used for most of the calculations of stellar structure and evolution. In the equation of state we treat ionisation equilibrium and non-relativistic degeneracy for separate temperature-density regimes. The opacity is obtained by 4-dimensional linear interpolation in the Carson opacity tables. Nuclear energy generation rates are taken from Fowler et al (1975) and neutrino losses from the approximation due to Beaudet et al (1967). Electron-screening factors are from Reeves (1965). The standard local mixing-length theory of Bohm-Vitense (1958) is used to treat non-adiabatic convection, although some models are calculated with modifications due to Deupree et al (1979, 1980). We neglect semi convection. The Carson opacities have only a small effect on the position of ZAHB models, but this may be metallicity dependent. The drop in the hydrogen- shell luminosity due to the helium-core expansion during HB evolution is greater than that obtained with the Cox-Stewart opacities. Allowing for the inclusion of semi convection and convective overshooting, we find that adoption of the Carson opacities leads to a reduction of approximately 25% in the HB lifetimes. For a given range of values for the masses and envelope helium abundances of stars on a synthetic HB, the width in effective temperature is increased, and in luminosity the width is decreased. The dependence of the core luminosity on the falling core helium abundance is increased by approximately 16%. Studies of main-sequence stars lead to agreement with Stothers' (1974a, 1974b, 1976) results for homogeneous models constructed with the Carson opacities. The evolution of main-sequence stars of intermediate mass is unaffected by the change in the opacity. Two evolutionary sequences (for 1 Mo stars) suggest that the main-sequence lifetimes of low mass stars may be reduced by as much as 30%. Combined with a shift in the ZAMS position this will move isochrones for low mass stars towards lower effective temperatures and densities. If studies of red-giant evolution indicate little change in the luminosity level of the horizontal branch, globular cluster ages determined from the position of the main-sequence turnoff point may be substantially reduced (possibly by as much as 50%). This could save a conflict between observed values for globular cluster ages and a value for the Hubble constant of 90. Studies of the apsidal motion constant, k₂, for evolved MS stars shows that the discrepancy between observed values of k₁ for eclipsing binary systems and theoretical values obtained from homogeneous stellar models may be resolved by considering the evolution of the binary components. CO Lac is an exception to this result, but analysis of the observations suggests that a redetermination of the orbital semi-amplitudes may resolve the conflict.
1983-01-01T00:00:00ZJeffery, C. SimonWe investigate stellar models for main-sequence and horizontal-branch stars constructed using the Carson opacities and make comparisons with models based on the Cox-Stewart opacities. A Henyey code based on the prescription of Kippenhahn et al (1967) is used for most of the calculations of stellar structure and evolution. In the equation of state we treat ionisation equilibrium and non-relativistic degeneracy for separate temperature-density regimes. The opacity is obtained by 4-dimensional linear interpolation in the Carson opacity tables. Nuclear energy generation rates are taken from Fowler et al (1975) and neutrino losses from the approximation due to Beaudet et al (1967). Electron-screening factors are from Reeves (1965). The standard local mixing-length theory of Bohm-Vitense (1958) is used to treat non-adiabatic convection, although some models are calculated with modifications due to Deupree et al (1979, 1980). We neglect semi convection. The Carson opacities have only a small effect on the position of ZAHB models, but this may be metallicity dependent. The drop in the hydrogen- shell luminosity due to the helium-core expansion during HB evolution is greater than that obtained with the Cox-Stewart opacities. Allowing for the inclusion of semi convection and convective overshooting, we find that adoption of the Carson opacities leads to a reduction of approximately 25% in the HB lifetimes. For a given range of values for the masses and envelope helium abundances of stars on a synthetic HB, the width in effective temperature is increased, and in luminosity the width is decreased. The dependence of the core luminosity on the falling core helium abundance is increased by approximately 16%. Studies of main-sequence stars lead to agreement with Stothers' (1974a, 1974b, 1976) results for homogeneous models constructed with the Carson opacities. The evolution of main-sequence stars of intermediate mass is unaffected by the change in the opacity. Two evolutionary sequences (for 1 Mo stars) suggest that the main-sequence lifetimes of low mass stars may be reduced by as much as 30%. Combined with a shift in the ZAMS position this will move isochrones for low mass stars towards lower effective temperatures and densities. If studies of red-giant evolution indicate little change in the luminosity level of the horizontal branch, globular cluster ages determined from the position of the main-sequence turnoff point may be substantially reduced (possibly by as much as 50%). This could save a conflict between observed values for globular cluster ages and a value for the Hubble constant of 90. Studies of the apsidal motion constant, k₂, for evolved MS stars shows that the discrepancy between observed values of k₁ for eclipsing binary systems and theoretical values obtained from homogeneous stellar models may be resolved by considering the evolution of the binary components. CO Lac is an exception to this result, but analysis of the observations suggests that a redetermination of the orbital semi-amplitudes may resolve the conflict.Photographic surface photometry of galaxies in the Virgo clusterFraser, Christopher W.https://hdl.handle.net/10023/142992019-04-01T10:04:18Z1971-01-01T00:00:00ZSurface photometry is one area of extragalactic studies in which information is urgently needed by optical as well as radio astronomers. The aim of this thesis is to supply some of the photometric parameters for 48 galaxies in two international colour systems. The reader will notice that descriptions are included of many detailed aspects of the work and little attention is paid to the more basic problems. This is because the discussion of the basic problems in photo-graphic photometry of galaxies formed the contents of an M.Sc. thesis.
Volume one is devoted to a detailed description of the methods used for the reduction techniques together with a discussion of the results, while volumes two and three contain the photometric data. In the first volume, the first chapter is concerned with previous investigations and the following chapter contains detailed descriptions of the instruments and methods used in the preliminary plate reductions. The subject of isophotometry is dealt with in the third chapter, while in the fourth and fifth chapters respectively, mention is made of the reduction techniques, together with conclusions which may be deduced from the photometric data. This information is given in volume two for those galaxies with NGC catalogue numbers from 4189 to 4459, while the data for objects from NGC 4461 to NGC 4762 are listed in the third volume of the thesis.
1971-01-01T00:00:00ZFraser, Christopher W.Surface photometry is one area of extragalactic studies in which information is urgently needed by optical as well as radio astronomers. The aim of this thesis is to supply some of the photometric parameters for 48 galaxies in two international colour systems. The reader will notice that descriptions are included of many detailed aspects of the work and little attention is paid to the more basic problems. This is because the discussion of the basic problems in photo-graphic photometry of galaxies formed the contents of an M.Sc. thesis.
Volume one is devoted to a detailed description of the methods used for the reduction techniques together with a discussion of the results, while volumes two and three contain the photometric data. In the first volume, the first chapter is concerned with previous investigations and the following chapter contains detailed descriptions of the instruments and methods used in the preliminary plate reductions. The subject of isophotometry is dealt with in the third chapter, while in the fourth and fifth chapters respectively, mention is made of the reduction techniques, together with conclusions which may be deduced from the photometric data. This information is given in volume two for those galaxies with NGC catalogue numbers from 4189 to 4459, while the data for objects from NGC 4461 to NGC 4762 are listed in the third volume of the thesis.The long-period effects of perturbation by Jupiter on the orbits of minor planets with commensurable mean motionsVincent, Fionahttps://hdl.handle.net/10023/142912019-04-01T10:03:30Z1979-01-01T00:00:00ZSix minor planets with mean motions approximately commensurable with that of Jupiter were observed photographically: (87) Sylvia, (107) Camilla, (414) Liriope, (909) Ulla and (1574) Meyer (all with ratio 9/5), and (334) Chicago (ratio 3/2). The plates were measured on a two-coordinate measuring-machine, and reduced by the method of plate constants. Orbits were derived for all except (1574) Meyer; for (334) Chicago and for (909) Ulla, two orbits were obtained for successive years, though in the former case the orbits were not accurate. The orbits of all six planets were also integrated numerically by computer, together with that of (153) Hilda, using a technique applied to the latter planet by J. Schubart. Based on the three-body problem, this applies three mutually-perpendicular components of the disturbing force to the orbital parameters, which consequently vary with time. Short-period variables are eliminated by averaging; only the longitudes of the two planets are allowed to vary, around one cycle of the commensurability. The derivatives of the orbital elements are calculated at intervals around the cycle, and their average values used in the integration. Schubart's original method neglects the orbital inclinations of both planets; this investigation extends the equations to include the inclinations. His results for (153) Hilda are reproduced, and shown not to change greatly whether the inclinations are included or omitted. For (909) Ulla, however, considerable differences are found. The results of the integrations on all seven orbits are presented in the form of graphs, covering time-intervals of 3000 years and more. The orbit of (909) Ulla was integrated twice, once taking the ratio of mean motions as 9/5, and again as 7/4. The integrations of (153) Hilda and (334) Chicago took the ratio as 3/2. All the orbits considered appear stable over long time-intervals. The perihelion advance of (334) Chicago is interrupted by long retrogressions, as noted by Schubart. The orbit of (909) Ulla shows similar, though less marked, behaviour. Both planets are somewhat removed from exact commensurability. Comparison was made between the orbits of four of the planets, derived from observations, and the orbits predicted by the numerical integrations. In most cases, the orbital parameters changed in the direction predicted but .to a greater degree, indicating that more variables affect the real orbits of the minor planets than are taken into account in this theoretical investigation.
1979-01-01T00:00:00ZVincent, FionaSix minor planets with mean motions approximately commensurable with that of Jupiter were observed photographically: (87) Sylvia, (107) Camilla, (414) Liriope, (909) Ulla and (1574) Meyer (all with ratio 9/5), and (334) Chicago (ratio 3/2). The plates were measured on a two-coordinate measuring-machine, and reduced by the method of plate constants. Orbits were derived for all except (1574) Meyer; for (334) Chicago and for (909) Ulla, two orbits were obtained for successive years, though in the former case the orbits were not accurate. The orbits of all six planets were also integrated numerically by computer, together with that of (153) Hilda, using a technique applied to the latter planet by J. Schubart. Based on the three-body problem, this applies three mutually-perpendicular components of the disturbing force to the orbital parameters, which consequently vary with time. Short-period variables are eliminated by averaging; only the longitudes of the two planets are allowed to vary, around one cycle of the commensurability. The derivatives of the orbital elements are calculated at intervals around the cycle, and their average values used in the integration. Schubart's original method neglects the orbital inclinations of both planets; this investigation extends the equations to include the inclinations. His results for (153) Hilda are reproduced, and shown not to change greatly whether the inclinations are included or omitted. For (909) Ulla, however, considerable differences are found. The results of the integrations on all seven orbits are presented in the form of graphs, covering time-intervals of 3000 years and more. The orbit of (909) Ulla was integrated twice, once taking the ratio of mean motions as 9/5, and again as 7/4. The integrations of (153) Hilda and (334) Chicago took the ratio as 3/2. All the orbits considered appear stable over long time-intervals. The perihelion advance of (334) Chicago is interrupted by long retrogressions, as noted by Schubart. The orbit of (909) Ulla shows similar, though less marked, behaviour. Both planets are somewhat removed from exact commensurability. Comparison was made between the orbits of four of the planets, derived from observations, and the orbits predicted by the numerical integrations. In most cases, the orbital parameters changed in the direction predicted but .to a greater degree, indicating that more variables affect the real orbits of the minor planets than are taken into account in this theoretical investigation.Far-infrared laser spectroscopy of neutral and negatively charged shallow donors in GaAs and InPArmistead, C.J.https://hdl.handle.net/10023/141672022-01-07T09:14:49Z1987-07-01T00:00:00ZAn optically pumped far-infrared laser and superconducting magnet have been used to perform high resolution studies of the energy levels of neutral and negatively charged shallow donors in high purity n-GaAs and n-InP in magnetic fields where the dimensionless magnetic field gamma is approximately one (where gamma=hoc/(2R*), hoc is the cyclotron energy and R* is the Coulomb binding energy). The central cell structure caused by the presence of different shallow donor species has been studied on the 1s-2p+1,0 transitions of neutral shallow donors in undoped GaAs samples grown by molecular beam epitaxy, liquid phase epitaxy and vapour phase epitaxy (VPE). VPE material showed two new shallow donor species with negative central cell shifts. The ls-2p-1 transition at magnetic fields where gamma>1 shows exceptionally well resolved central cell structure. Detailed structure at magnetic fields below the 1s-2p+2 transition is due to transitions from the is to higher excited states. Samples of undoped high purity InP grown by the VPE, metal organic chemical vapour deposition and bulk growth techniques have been studied. VPE samples always show a strong component related to sulphur though some also show a strong silicon related component, and some show up to 7 components. A bulk, sample showed two strong components shallower than silicon which may have negative central cell shifts. Transitions between the excited states of neutral shallow donors in GaAs have been studied. Recent theoretical work by Makado (1982) describes the transition energies very well. Clearly resolved central cell structure is observed on inter-excited state transitions involving the 2s state. The first unambiguous observation of negatively charged shallow donors (D-states) in GaAs is reported. Simultaneous observations of transitions involving D-states, the cyclotron resonance and inter-excited state transitions of neutral donors over a wide magnetic field range, 0.03<gamma<3.5, highlight the differences between the transitions and the relative effects of optical excitation, temperature, magnetic field and electric field bias.
1987-07-01T00:00:00ZArmistead, C.J.An optically pumped far-infrared laser and superconducting magnet have been used to perform high resolution studies of the energy levels of neutral and negatively charged shallow donors in high purity n-GaAs and n-InP in magnetic fields where the dimensionless magnetic field gamma is approximately one (where gamma=hoc/(2R*), hoc is the cyclotron energy and R* is the Coulomb binding energy). The central cell structure caused by the presence of different shallow donor species has been studied on the 1s-2p+1,0 transitions of neutral shallow donors in undoped GaAs samples grown by molecular beam epitaxy, liquid phase epitaxy and vapour phase epitaxy (VPE). VPE material showed two new shallow donor species with negative central cell shifts. The ls-2p-1 transition at magnetic fields where gamma>1 shows exceptionally well resolved central cell structure. Detailed structure at magnetic fields below the 1s-2p+2 transition is due to transitions from the is to higher excited states. Samples of undoped high purity InP grown by the VPE, metal organic chemical vapour deposition and bulk growth techniques have been studied. VPE samples always show a strong component related to sulphur though some also show a strong silicon related component, and some show up to 7 components. A bulk, sample showed two strong components shallower than silicon which may have negative central cell shifts. Transitions between the excited states of neutral shallow donors in GaAs have been studied. Recent theoretical work by Makado (1982) describes the transition energies very well. Clearly resolved central cell structure is observed on inter-excited state transitions involving the 2s state. The first unambiguous observation of negatively charged shallow donors (D-states) in GaAs is reported. Simultaneous observations of transitions involving D-states, the cyclotron resonance and inter-excited state transitions of neutral donors over a wide magnetic field range, 0.03<gamma<3.5, highlight the differences between the transitions and the relative effects of optical excitation, temperature, magnetic field and electric field bias.Microwave excited copper halide and strontium-ion recombination lasersBethel, Jason W.https://hdl.handle.net/10023/141662018-06-19T11:29:50Z1995-07-01T00:00:00ZMicrowave excitation of pulsed metal vapour lasers (copper halide and strontium-ion recombination lasers) is investigated. Two waveguide coupling structures were designed and built, one based on a ridge waveguide; and the other based on a rectangular waveguide with a tapered narrow wall which was designed to produce a uniform, travelling microwave field. These coupling structures were designed to produce a transverse electric field at high pressure, with high electron densities, using pulsed microwaves with peak powers of up to 2.5 MW, compatible with the necessary requirements for copper and strontium based systems. The magnetron power supply was modified to produce double pulses of variable spacing (between 15 and 500 mus). Laser oscillation was observed on the cyclic transitions of neutral copper (=510.6 and 578.2 nm) and on the recombination transition of singly ionised strontium (=430.5 nm) for the first time in microwave excited systems. The performance of the tapered waveguide coupling structure was found to be superior to the ridge waveguide coupling structure. The efficiency of coupling of microwave power into the discharge was mainly dependent the buffer gas type and pressure and the electron density. At higher pressures, higher coupling efficiencies are observed (over 70 percent for pressures of over 500 mbar of helium). The performance of the lasers using both the coupling stractures was poor when compared to conventional copper based lasers. This was attributed to the interaction of the copper halide with the discharge and a nonuniform temperature distribution along the axis of the quartz tube. An average output power of 18 mW was achieved for both lines in a copper bromide laser. The strontium-ion recombination laser was operated at threshold average microwave input powers, no output powers were measured. The electric field and the electron density of the discharges in both laser systems were estimated and compared with those occurring in the respective conventional laser systems.
1995-07-01T00:00:00ZBethel, Jason W.Microwave excitation of pulsed metal vapour lasers (copper halide and strontium-ion recombination lasers) is investigated. Two waveguide coupling structures were designed and built, one based on a ridge waveguide; and the other based on a rectangular waveguide with a tapered narrow wall which was designed to produce a uniform, travelling microwave field. These coupling structures were designed to produce a transverse electric field at high pressure, with high electron densities, using pulsed microwaves with peak powers of up to 2.5 MW, compatible with the necessary requirements for copper and strontium based systems. The magnetron power supply was modified to produce double pulses of variable spacing (between 15 and 500 mus). Laser oscillation was observed on the cyclic transitions of neutral copper (=510.6 and 578.2 nm) and on the recombination transition of singly ionised strontium (=430.5 nm) for the first time in microwave excited systems. The performance of the tapered waveguide coupling structure was found to be superior to the ridge waveguide coupling structure. The efficiency of coupling of microwave power into the discharge was mainly dependent the buffer gas type and pressure and the electron density. At higher pressures, higher coupling efficiencies are observed (over 70 percent for pressures of over 500 mbar of helium). The performance of the lasers using both the coupling stractures was poor when compared to conventional copper based lasers. This was attributed to the interaction of the copper halide with the discharge and a nonuniform temperature distribution along the axis of the quartz tube. An average output power of 18 mW was achieved for both lines in a copper bromide laser. The strontium-ion recombination laser was operated at threshold average microwave input powers, no output powers were measured. The electric field and the electron density of the discharges in both laser systems were estimated and compared with those occurring in the respective conventional laser systems.DC glow discharge electron guns for the excitation of rare gases / R.J. Carman.Carman, R.J.https://hdl.handle.net/10023/141652019-04-01T10:04:40Z1986-07-01T00:00:00ZGlow discharge electron guns are used to generate continuous electron beams at 0.5keV-3.0keV in the intermediate range of gas pressures (0.1mb-10.0mb). Cathodes incorporating internal cavities are used to generate distinct electron beam filaments in both Helium and Argon. The formation of such beam filaments has been investigated using a number of different cathode types, and criteria for the production of stable electron beams are established. The production of an electron beam in a glow discharge is largely determined by the motion of electrons in the Cathode dark space sheath region next to the cathode, and other discharge processes in this region. A theoretical model has been developed to simulate electron motion in the sheath region, and in the Negative glow plasma region, of a Helium discharge with a Cathode fall of between 150V and 1000V. It is shown that the electron flux at the 'sheath/Negative glow boundary becomes increasingly monoenergetic as the Cathode fall rises to 1000V. The results are also compared with experimental spatial emission profiles of the glow in the Cathode dark space and Negative glow regions of a helium discharge. In particular, properties of the Cathode glow region in the sheath are discussed. Aspects of the theoretical model and results from the experimental measurements are also used to discuss discharge processes in the sheath region of cathodes incorporating internal cavities, and mechanisms leading to the formation of the electron beam filaments. The production of fast electrons in a glow discharge has a number of applications, including the excitation of gases leading to laser action. Aspects relating to the excitation of high lying energy states in gases, corresponding to known laser transitions, are discussed. It is shown that the production of helium ions, which are responsible for the excitation of metal atoms via asymmetric charge transfer in metal ion lasers, is theoretically more efficient in an electron beam discharge. The results are compared with the theoretical ion production rates in Hollow cathode discharges, and high-voltage Hollow cathode devices. Several electrode geometries using multiple arrays of electron gun cathodes have been developed. Investigations of an electron beam excited argon plasma suggest that Ar II excited states are pumped directly by single electron impacts, even at very low current densities (~10<super>-3</super> A cm<super>-2</super>). From previous calculations using the 'sudden perturbation' approximation, those ion states known to have large cross-sections for direct electron impact excitation (3p44p2P) appear to be favourably pumped in the electron beam plasma.
1986-07-01T00:00:00ZCarman, R.J.Glow discharge electron guns are used to generate continuous electron beams at 0.5keV-3.0keV in the intermediate range of gas pressures (0.1mb-10.0mb). Cathodes incorporating internal cavities are used to generate distinct electron beam filaments in both Helium and Argon. The formation of such beam filaments has been investigated using a number of different cathode types, and criteria for the production of stable electron beams are established. The production of an electron beam in a glow discharge is largely determined by the motion of electrons in the Cathode dark space sheath region next to the cathode, and other discharge processes in this region. A theoretical model has been developed to simulate electron motion in the sheath region, and in the Negative glow plasma region, of a Helium discharge with a Cathode fall of between 150V and 1000V. It is shown that the electron flux at the 'sheath/Negative glow boundary becomes increasingly monoenergetic as the Cathode fall rises to 1000V. The results are also compared with experimental spatial emission profiles of the glow in the Cathode dark space and Negative glow regions of a helium discharge. In particular, properties of the Cathode glow region in the sheath are discussed. Aspects of the theoretical model and results from the experimental measurements are also used to discuss discharge processes in the sheath region of cathodes incorporating internal cavities, and mechanisms leading to the formation of the electron beam filaments. The production of fast electrons in a glow discharge has a number of applications, including the excitation of gases leading to laser action. Aspects relating to the excitation of high lying energy states in gases, corresponding to known laser transitions, are discussed. It is shown that the production of helium ions, which are responsible for the excitation of metal atoms via asymmetric charge transfer in metal ion lasers, is theoretically more efficient in an electron beam discharge. The results are compared with the theoretical ion production rates in Hollow cathode discharges, and high-voltage Hollow cathode devices. Several electrode geometries using multiple arrays of electron gun cathodes have been developed. Investigations of an electron beam excited argon plasma suggest that Ar II excited states are pumped directly by single electron impacts, even at very low current densities (~10<super>-3</super> A cm<super>-2</super>). From previous calculations using the 'sudden perturbation' approximation, those ion states known to have large cross-sections for direct electron impact excitation (3p44p2P) appear to be favourably pumped in the electron beam plasma.Optical parametric oscillators pumped by excimer lasersEbrahimzadeh, M. (Majid)https://hdl.handle.net/10023/141642019-04-01T10:05:15Z1990-07-01T00:00:00ZThis thesis describes the development of a new generation of pulsed optical parametric oscillators (OPO's) based on two new non-linear materials, urea and beta-BaB2O4 (or BBO), and pumped by a new class of laser pump sources, namely, excimer lasers, to provide broadly tunable coherent radiation in new regions of the electromagnetic spectrum, particularly in the ultraviolet and the visible, which have previously been inaccessible. The laser pump source used during this work was a pulsed ultraviolet XeCl excimer laser operating at 308nm. Because of the stringent demands on the pump beam quality (with regard to both spatial and spectral coherence) for successful operation of OPO's, the pump laser was designed and constructed as an injection-seeded system, to provide a narrow-linewidth, near-diffraction-limited output beam, with sufficiently high peak powers to enable OPO operation. In this way, we were able to obtain an output beam with a linewidth ≤ 0.2cm-1, and a full-angle of divergence as low as 60muR (~3 times the diffraction limit). The maximum energy available from the pump laser was 30mJ, in pulses measuring typically 10ns in duration. The output beam was also linearly polarised to better than 95%, and the pulse repetition rate was 1 Hz. In the early part of this work, we used the constructed pump laser to investigate spontaneous parametric fluorescence in a home-grown urea sample, in order to characterise the crystal, and to compare the observed spectrum with the calculated OPO tuning curves. The results of these experiments were found to be in good agreement with the theoretical predictions. The main thrust of the project, however, was the development of an OPO based on urea as the non-linear medium and pumped at 308nm by the narrowband XeCl excimer laser. We were successful in constructing such an OPO, using an 8-mm-long, home-grown crystal, and were able to generate, continuously tunable output from 572 to 667nm, with a 2.5% energy conversion efficiency. The timing range of the device was later extended to 537-720nm, by utilising a 15-mm-long, home-grown urea sample, and its conversion efficiency was improved to as high as 37% at 90° phase-matching, with ≥ 10% efficiency over a 100-nm range in the visible (from 570 to 670nm). Finally, in an effort to achieve even higher efficiencies, we performed experiments in a 25-mm-long commercial urea crystal, and demonstrated exceptionally high external energy conversion efficiencies of up to 66%, with evidence of even higher levels of pump depletion (as high as 85%) at 90° phase-matching. The latter part of the project was concerned with the design and development of a similar device based on the new non-linear material, beta-BaB2O4, to provide continuously tunable radiation over a much broader tuning range, particularly in the blue and the near ultraviolet, not accessed by the urea OPO. We used a 12-mm-long commercially available beta-BaB2O4 crystal to construct this OPO, and successfully operated this device over the entire wavelength range from 354nm in the near ultraviolet, throughout the visible, to 2.37mum in the near infrared, with an energy conversion efficiency in excess of 10% over the range 450-960 nm. The constructed OPO's were also characterised with regard to several operating parameters, including oscillation threshold, spectral linewidth, as well as spatial and temporal variation and, where appropriate, the experimental results were compared with the predictions of theory.
1990-07-01T00:00:00ZEbrahimzadeh, M. (Majid)This thesis describes the development of a new generation of pulsed optical parametric oscillators (OPO's) based on two new non-linear materials, urea and beta-BaB2O4 (or BBO), and pumped by a new class of laser pump sources, namely, excimer lasers, to provide broadly tunable coherent radiation in new regions of the electromagnetic spectrum, particularly in the ultraviolet and the visible, which have previously been inaccessible. The laser pump source used during this work was a pulsed ultraviolet XeCl excimer laser operating at 308nm. Because of the stringent demands on the pump beam quality (with regard to both spatial and spectral coherence) for successful operation of OPO's, the pump laser was designed and constructed as an injection-seeded system, to provide a narrow-linewidth, near-diffraction-limited output beam, with sufficiently high peak powers to enable OPO operation. In this way, we were able to obtain an output beam with a linewidth ≤ 0.2cm-1, and a full-angle of divergence as low as 60muR (~3 times the diffraction limit). The maximum energy available from the pump laser was 30mJ, in pulses measuring typically 10ns in duration. The output beam was also linearly polarised to better than 95%, and the pulse repetition rate was 1 Hz. In the early part of this work, we used the constructed pump laser to investigate spontaneous parametric fluorescence in a home-grown urea sample, in order to characterise the crystal, and to compare the observed spectrum with the calculated OPO tuning curves. The results of these experiments were found to be in good agreement with the theoretical predictions. The main thrust of the project, however, was the development of an OPO based on urea as the non-linear medium and pumped at 308nm by the narrowband XeCl excimer laser. We were successful in constructing such an OPO, using an 8-mm-long, home-grown crystal, and were able to generate, continuously tunable output from 572 to 667nm, with a 2.5% energy conversion efficiency. The timing range of the device was later extended to 537-720nm, by utilising a 15-mm-long, home-grown urea sample, and its conversion efficiency was improved to as high as 37% at 90° phase-matching, with ≥ 10% efficiency over a 100-nm range in the visible (from 570 to 670nm). Finally, in an effort to achieve even higher efficiencies, we performed experiments in a 25-mm-long commercial urea crystal, and demonstrated exceptionally high external energy conversion efficiencies of up to 66%, with evidence of even higher levels of pump depletion (as high as 85%) at 90° phase-matching. The latter part of the project was concerned with the design and development of a similar device based on the new non-linear material, beta-BaB2O4, to provide continuously tunable radiation over a much broader tuning range, particularly in the blue and the near ultraviolet, not accessed by the urea OPO. We used a 12-mm-long commercially available beta-BaB2O4 crystal to construct this OPO, and successfully operated this device over the entire wavelength range from 354nm in the near ultraviolet, throughout the visible, to 2.37mum in the near infrared, with an energy conversion efficiency in excess of 10% over the range 450-960 nm. The constructed OPO's were also characterised with regard to several operating parameters, including oscillation threshold, spectral linewidth, as well as spatial and temporal variation and, where appropriate, the experimental results were compared with the predictions of theory.Microwave excitation of argon ion and helium-krypton ion lasersDobie, Paul J.https://hdl.handle.net/10023/141632019-04-01T10:03:53Z1989-07-01T00:00:00ZPulsed microwave excitation of noble gas ion lasers at frequencies between 3 and 17 GHz is investigated. The advantages of using microwaves instead of conventional DC sources to pump a laser are explained. These include the lower electrode and discharge tube wear due to the oscillating nature of a microwave electric field. The propagation of microwave radiation in an ionised gas is examined. At the frequencies used, the skin depth of an Argon ion laser discharge is shown to be approximately 1 mm, indicating good microwave power absorption. The dependence of the microwave power absorption on the frequency is shown to be weak. Microwave transmission at a dielectric/gas-discharge boundary, similar to those in the laser coupling structures used, is found to be around 1% of the incident power. It is suggested that for maximum laser efficiency, microwave power should be introduced directly into the gas discharge. Two microwave coupling structure designs for supplying microwave power to the laser discharges are described. The first of these, a waveguide coupler device based on the 3dB branch guide coupler, produces a transverse electric field across the laser tube. The procedure used to design a branch guide coupler using a Chebyshev impedance taper and T- junction discontinuity corrections is outlined, and a description of the entire laser coupling structure is given. The second design comprises a helix wrapped round the laser tube and produces an axial electric field. The electric field distribution around a helix is calculated as a function of helix parameters and the effects of surrounding objects are considered. The best helix dimensions are found for optimum laser operation. The characteristics of conventional Argon ion and Helium-Krypton ion lasers are given. No significant differences between conventional noble gas ion lasers and the microwave excited lasers reported here are observed. At the input powers used (~100 kW peak, 1 uS pulses, 1000 pps), 100 mW, 1 uS and 30 mW, 5 uS laser pulses are observed from Argon and Helium-Krypton gas mixtures, respectively. The transverse and axially excited lasers perform equally well.
1989-07-01T00:00:00ZDobie, Paul J.Pulsed microwave excitation of noble gas ion lasers at frequencies between 3 and 17 GHz is investigated. The advantages of using microwaves instead of conventional DC sources to pump a laser are explained. These include the lower electrode and discharge tube wear due to the oscillating nature of a microwave electric field. The propagation of microwave radiation in an ionised gas is examined. At the frequencies used, the skin depth of an Argon ion laser discharge is shown to be approximately 1 mm, indicating good microwave power absorption. The dependence of the microwave power absorption on the frequency is shown to be weak. Microwave transmission at a dielectric/gas-discharge boundary, similar to those in the laser coupling structures used, is found to be around 1% of the incident power. It is suggested that for maximum laser efficiency, microwave power should be introduced directly into the gas discharge. Two microwave coupling structure designs for supplying microwave power to the laser discharges are described. The first of these, a waveguide coupler device based on the 3dB branch guide coupler, produces a transverse electric field across the laser tube. The procedure used to design a branch guide coupler using a Chebyshev impedance taper and T- junction discontinuity corrections is outlined, and a description of the entire laser coupling structure is given. The second design comprises a helix wrapped round the laser tube and produces an axial electric field. The electric field distribution around a helix is calculated as a function of helix parameters and the effects of surrounding objects are considered. The best helix dimensions are found for optimum laser operation. The characteristics of conventional Argon ion and Helium-Krypton ion lasers are given. No significant differences between conventional noble gas ion lasers and the microwave excited lasers reported here are observed. At the input powers used (~100 kW peak, 1 uS pulses, 1000 pps), 100 mW, 1 uS and 30 mW, 5 uS laser pulses are observed from Argon and Helium-Krypton gas mixtures, respectively. The transverse and axially excited lasers perform equally well.A study of the hollow-cathode metal-vapour laser dischargeBelal, Ibrahim K.https://hdl.handle.net/10023/141622019-04-01T10:03:48Z1977-07-01T00:00:00ZThis work is a plasma diagnostic study of the hollow; cathode metal vapour laser. It can be divided into three sections. Section one concerns the study of the basic parameters (cathode fall potential electric field, cathode fall width; negative glow length and gas temperature) of the hollow cathode discharge. Section two concerns the use of absorption, linewidth and laser lieterodyne techniques to measure the number densities of He- (21S), (23S) states and He+ ions (electrons) since these species are involved in the pumping mechanisms (Penning and Duffendack) leading to population inversions. Section three concerns the emission, radial profiles of the excited states of He I and He II so the influence of the high energy electrons which enter the negative glow from, the cathode fall region can be studied as they cross the glow. Although the metastable number densities in the hollow cathode are comparable to those in the positive column, they behave differently. In the positive column both singlet and triplet metastables saturate with respect to the discharge current. In the hollow cathode the singlets saturate not only with respect to discharge current but also with respect to pressure. On the other hand the triplets increase linearly with the discharge current and follows' the cathode fall potential as a function of pressure. In the positive column the saturation of the metastable number densities with current combined with the cadmium ion drive-out to the walls lead to the laser power output limitation. In the hollow cathodo cadmium ion drive-out still occurs however this can be compensated by increasing the metal atom, number density since in this case the cathode fall potential, and hence electron energy, is not thereby decreased. Stark broadening and laser heterodyning were used to measure the ion number density and both techniques agree well. They show that the ion number density increases linearly with current, and follows the cathode fall potential as a function of pressure. Also they show that the electron (ion) number density in a hollow cathode is about two orders of magnitude larger than that in a positive column which may lead to an enhancement in the Duffendack reaction. The excitation radial profiles show that the negative glow receives a flux of electrons with energy derived directly from the cathode fall potential. These electrons play an important par in the collision processes in the negative glow. The correlation between these regions is shown clearly by studying the rate equations for different species. The behaviour of the species number densities in the negative glow with the discharge parameters can be related to the fundamental process occuring in the dark space.
1977-07-01T00:00:00ZBelal, Ibrahim K.This work is a plasma diagnostic study of the hollow; cathode metal vapour laser. It can be divided into three sections. Section one concerns the study of the basic parameters (cathode fall potential electric field, cathode fall width; negative glow length and gas temperature) of the hollow cathode discharge. Section two concerns the use of absorption, linewidth and laser lieterodyne techniques to measure the number densities of He- (21S), (23S) states and He+ ions (electrons) since these species are involved in the pumping mechanisms (Penning and Duffendack) leading to population inversions. Section three concerns the emission, radial profiles of the excited states of He I and He II so the influence of the high energy electrons which enter the negative glow from, the cathode fall region can be studied as they cross the glow. Although the metastable number densities in the hollow cathode are comparable to those in the positive column, they behave differently. In the positive column both singlet and triplet metastables saturate with respect to the discharge current. In the hollow cathode the singlets saturate not only with respect to discharge current but also with respect to pressure. On the other hand the triplets increase linearly with the discharge current and follows' the cathode fall potential as a function of pressure. In the positive column the saturation of the metastable number densities with current combined with the cadmium ion drive-out to the walls lead to the laser power output limitation. In the hollow cathodo cadmium ion drive-out still occurs however this can be compensated by increasing the metal atom, number density since in this case the cathode fall potential, and hence electron energy, is not thereby decreased. Stark broadening and laser heterodyning were used to measure the ion number density and both techniques agree well. They show that the ion number density increases linearly with current, and follows the cathode fall potential as a function of pressure. Also they show that the electron (ion) number density in a hollow cathode is about two orders of magnitude larger than that in a positive column which may lead to an enhancement in the Duffendack reaction. The excitation radial profiles show that the negative glow receives a flux of electrons with energy derived directly from the cathode fall potential. These electrons play an important par in the collision processes in the negative glow. The correlation between these regions is shown clearly by studying the rate equations for different species. The behaviour of the species number densities in the negative glow with the discharge parameters can be related to the fundamental process occuring in the dark space.Perturbation spectroscopy of the argon ion laserDunn, Malcolm H.https://hdl.handle.net/10023/141612019-04-01T10:06:33Z1974-06-01T00:00:00ZThe technique of perturbation spectroscopy has been applied to study the argon ion laser discharge. In this technique the laser radiation field present within the active medium is switched on and off, and this modulates the population of the upper laser level. By studying the magnitude of these modulations as discharge conditions within the active medium are changed, the copulation inversions on the 4p2D5/2 - 4s2P3/2 and 4p4D5/2 - 4s2P3/2 laser transitions have been explored. By studying, in the presence of an applied magnetic field, the polarization induced in the spontaneous emission sidelight through these modulations, the lifetime of the 4p2D5/2 upper laser level has been estimated. By studying the population changes induced on other 4p and 4d levels as a consequence of the population change on the upper laser level, the relative importance of various proposed excitation mechanisms for the 4p upper laser levels have been assessed. The population inversions (1 - g1N2/g2N1, where N1 is the population of the upper laser level and N2. the population of the lower laser level) on the 4p2D5/2 - 4s2P3/2 and 4p4D5/2 - 4s2P3/2 transitions of Ar II are destroyed by increasing the argon pressure above 0.2 - 0.3 Torr, and for higher pressures the medium exhibits absorption. Both population inversions increase slowly with discharge current. By using a flow graph technique to analyse the population inversions, this latter is shown to imply that the electron temperature in the discharge increases with current. The polarization experiments have been analysed by both a simple rate equation treatment, and by a more complete approach based on Lamb theory. This latter treatment demonstrates that the dependence of polarization ratio on magnetic field can be used to determine the lifetime of the upper laser level. Polarization ratios on the 4p2D5/2 - 4s2P3/2 and 4p4D5/2 - 4s2P3/2 transitions have been measured as a function of discharge parameters and applied axial magnetic field. From the variation of polarization ratio with magnetic field, the lifetime of the 4p2D5/2 level of Ar II under discharge conditions has been estimated. The perturbations in the populations of other 4p and 4d levels of Ar II resulting from the modulation of the upper laser level have been measured as a function of discharge parameters. Their magnitudes and dependence on discharge current provide direct evidence that an important excitation pathway for the 4p upper laser levels of Ar II is step wise excitation through the quartet 4s levels of Ar II.
1974-06-01T00:00:00ZDunn, Malcolm H.The technique of perturbation spectroscopy has been applied to study the argon ion laser discharge. In this technique the laser radiation field present within the active medium is switched on and off, and this modulates the population of the upper laser level. By studying the magnitude of these modulations as discharge conditions within the active medium are changed, the copulation inversions on the 4p2D5/2 - 4s2P3/2 and 4p4D5/2 - 4s2P3/2 laser transitions have been explored. By studying, in the presence of an applied magnetic field, the polarization induced in the spontaneous emission sidelight through these modulations, the lifetime of the 4p2D5/2 upper laser level has been estimated. By studying the population changes induced on other 4p and 4d levels as a consequence of the population change on the upper laser level, the relative importance of various proposed excitation mechanisms for the 4p upper laser levels have been assessed. The population inversions (1 - g1N2/g2N1, where N1 is the population of the upper laser level and N2. the population of the lower laser level) on the 4p2D5/2 - 4s2P3/2 and 4p4D5/2 - 4s2P3/2 transitions of Ar II are destroyed by increasing the argon pressure above 0.2 - 0.3 Torr, and for higher pressures the medium exhibits absorption. Both population inversions increase slowly with discharge current. By using a flow graph technique to analyse the population inversions, this latter is shown to imply that the electron temperature in the discharge increases with current. The polarization experiments have been analysed by both a simple rate equation treatment, and by a more complete approach based on Lamb theory. This latter treatment demonstrates that the dependence of polarization ratio on magnetic field can be used to determine the lifetime of the upper laser level. Polarization ratios on the 4p2D5/2 - 4s2P3/2 and 4p4D5/2 - 4s2P3/2 transitions have been measured as a function of discharge parameters and applied axial magnetic field. From the variation of polarization ratio with magnetic field, the lifetime of the 4p2D5/2 level of Ar II under discharge conditions has been estimated. The perturbations in the populations of other 4p and 4d levels of Ar II resulting from the modulation of the upper laser level have been measured as a function of discharge parameters. Their magnitudes and dependence on discharge current provide direct evidence that an important excitation pathway for the 4p upper laser levels of Ar II is step wise excitation through the quartet 4s levels of Ar II.Optical characterization and device application of the semiconductors ZnSe and ZnSZhang, Jiazhenhttps://hdl.handle.net/10023/141602019-04-01T10:06:04Z1991-07-01T00:00:00ZWe have presented evidence that the blue emission band, which is dominant at room temperature in ZnSe made under a wide variety of conditions, arises from a transition between a free hole and an electron bound to a donor, but not all donors contribute. There is a cutoff energy within the spread of donor levels above which there is negligible contribution to the emission. It is suggested that the cutoff corresponds to a localization edge of the same nature as the mobility edge. The line shape calculation based on a simple model agrees well with the experimental data. The origin of the blue emission seen at room temperature in the electroluminescence is examined to be the same. We have also discussed the injection mechanism of minority carriers in the ZnSe MIS diodes. It has been shown that annealing as-grown MOCVD ZnSe in the temperature range 300-400 °C can lead to large increases in resistivity. The effect is large for annealing in air or selenium and smaller for annealing in vacuum or zinc vapour. The process involved has an activation energy of only 0.26 eV and appears to be caused by a lattice defect acting as an acceptor. The photocapacitance spectra show that the acceptor is likely to be the so-called M-centre in ZnSe. We have shown that the attribution of the M-centre to copper-red centre is by no means conclusive. The possibility is still open that the M-centre is a lattice defect. We have made double light source steady-state photocapacitance measurements on ZnS single crystals. The Schottky diodes were made by evaporating a metal contact onto a chemically cleaned ZnS surface. Levels were found at 0.9 eV and 2.0 eV below the conduction band and 0.8 eV above the valence band in both melt-grown and iodine-transported material. These centres might be due to lattice defects. An additional level at 1.6 eV below the conduction band occurred in the iodine-transported material.
1991-07-01T00:00:00ZZhang, JiazhenWe have presented evidence that the blue emission band, which is dominant at room temperature in ZnSe made under a wide variety of conditions, arises from a transition between a free hole and an electron bound to a donor, but not all donors contribute. There is a cutoff energy within the spread of donor levels above which there is negligible contribution to the emission. It is suggested that the cutoff corresponds to a localization edge of the same nature as the mobility edge. The line shape calculation based on a simple model agrees well with the experimental data. The origin of the blue emission seen at room temperature in the electroluminescence is examined to be the same. We have also discussed the injection mechanism of minority carriers in the ZnSe MIS diodes. It has been shown that annealing as-grown MOCVD ZnSe in the temperature range 300-400 °C can lead to large increases in resistivity. The effect is large for annealing in air or selenium and smaller for annealing in vacuum or zinc vapour. The process involved has an activation energy of only 0.26 eV and appears to be caused by a lattice defect acting as an acceptor. The photocapacitance spectra show that the acceptor is likely to be the so-called M-centre in ZnSe. We have shown that the attribution of the M-centre to copper-red centre is by no means conclusive. The possibility is still open that the M-centre is a lattice defect. We have made double light source steady-state photocapacitance measurements on ZnS single crystals. The Schottky diodes were made by evaporating a metal contact onto a chemically cleaned ZnS surface. Levels were found at 0.9 eV and 2.0 eV below the conduction band and 0.8 eV above the valence band in both melt-grown and iodine-transported material. These centres might be due to lattice defects. An additional level at 1.6 eV below the conduction band occurred in the iodine-transported material.Two-contact semiconductor devices for ultra-fast switching and synchronisationBarnsley, Peter E.https://hdl.handle.net/10023/141592021-04-02T06:49:27Z1993-07-01T00:00:00ZThe information contained within this document is a study of the properties of two-contact semiconductor laser devices. The emphasis of this study was on understanding the behaviour and properties of semiconductor laser devices where regions of saturable absorption are introduced into the Fabry-Perot cavity, and to demonstrate that the inherent nonlinearity of such a device can be harnessed to provide all-optical switching and routing functions that could be incorporated within future multi-gigabit optical telecommunications networks. Important advances in the field are presented in the assessment and performance of such two-contact functional components when operated both below threshold, as nonlinear optical amplifiers, and above threshold, as self-pulsating lasers. Detailed measurements on the nonlinear optical amplifiers are presented defining the dependence of both the steady state and dynamic characteristics on optical input power and wavelength. These results are compared with theoretical predictions from a model incorporating material gain calculations based on strict momentum conservation. It is shown that such devices can be used to convert the wavelength of Gb/s data between the 1.3mum and 1.55mum telecommunications window of silica fibre, and that fast all-optical switching of gigabit/s data packets with sub-ns rise and fall times is achievable. Detailed system data Bit-Error-Ratio (BER) measurements are used to highlight the systems potential of such functions. Also presented are significant advances in the field of self-pulsating laser devices and their use in multi-gigabit all-optical clock recovery circuits. The pulsation behaviour is investigated in detail and a detailed study of the locking behaviour of these devices for Return-to-Zero-'soliton' data format made. Some of the properties investigated are: tuning and locking range, clock purity, data pattern dependence, clock lock-up time, pulse spectral variation (chirp). In addition BER measurements using the clock recovery circuit in a 20 Gb/s OTDM system has demonstrated their potential application. Operation with Non-Return-to-Zero format data is also demonstrated in combination with the nonlinear amplifier devices. In summary the results contained within this thesis show that two-contact devices have characteristics and functionality suitable for use in many application areas consistent with the development of future synchronous gigabit all-optical telecommunication networks.
1993-07-01T00:00:00ZBarnsley, Peter E.The information contained within this document is a study of the properties of two-contact semiconductor laser devices. The emphasis of this study was on understanding the behaviour and properties of semiconductor laser devices where regions of saturable absorption are introduced into the Fabry-Perot cavity, and to demonstrate that the inherent nonlinearity of such a device can be harnessed to provide all-optical switching and routing functions that could be incorporated within future multi-gigabit optical telecommunications networks. Important advances in the field are presented in the assessment and performance of such two-contact functional components when operated both below threshold, as nonlinear optical amplifiers, and above threshold, as self-pulsating lasers. Detailed measurements on the nonlinear optical amplifiers are presented defining the dependence of both the steady state and dynamic characteristics on optical input power and wavelength. These results are compared with theoretical predictions from a model incorporating material gain calculations based on strict momentum conservation. It is shown that such devices can be used to convert the wavelength of Gb/s data between the 1.3mum and 1.55mum telecommunications window of silica fibre, and that fast all-optical switching of gigabit/s data packets with sub-ns rise and fall times is achievable. Detailed system data Bit-Error-Ratio (BER) measurements are used to highlight the systems potential of such functions. Also presented are significant advances in the field of self-pulsating laser devices and their use in multi-gigabit all-optical clock recovery circuits. The pulsation behaviour is investigated in detail and a detailed study of the locking behaviour of these devices for Return-to-Zero-'soliton' data format made. Some of the properties investigated are: tuning and locking range, clock purity, data pattern dependence, clock lock-up time, pulse spectral variation (chirp). In addition BER measurements using the clock recovery circuit in a 20 Gb/s OTDM system has demonstrated their potential application. Operation with Non-Return-to-Zero format data is also demonstrated in combination with the nonlinear amplifier devices. In summary the results contained within this thesis show that two-contact devices have characteristics and functionality suitable for use in many application areas consistent with the development of future synchronous gigabit all-optical telecommunication networks.Fibre-compatible modelocked lasers at 1.5μmBurns, Davidhttps://hdl.handle.net/10023/141582019-04-01T10:05:50Z1991-07-01T00:00:00ZThis thesis describes techniques for the generation of ultrashort optical pulses using semiconductor lasers. Active modelocking through gain modulation is applied to 1.?m InGaAsP semiconductor lasers which utilise microlensed optical fibres as external-cavity components, and tunable optical pulses of ~5ps duration have been generated. Particular attention has been given to suppression of the noisy subpulse features associated with low-frequency modelocked semiconductor lasers. This has led to two suppression methods applicable to symmetric (or balanced) and asymmetric cavity configurations, allowing the generation of clean, single feature short pulses of 7-10ps duration with peak powers in excess of 300mW. The physical phenomena relating to such processes will be outlined. Further amplification of these pulses using an erbium- doped fibre amplifier has resulted in output peak powers in the 5W range. A study of the phase-noise characteristics of these modelocked laser systems was undertaken using a high-speed photodetector and wide-band spectrum analyser arrangement. This characterisation allowed improvements in the rms pulse timing jitter from >2ps to below 300fs in the frequency range 50-5000Hz. Novel modelocked laser configurations were also constructed where the cavity components included both a semiconductor amplifier and an erbium-doped fibre amplifier. Linear and ring hybrid lasers were investigated with resultant pulse durations as short as 3ps with high peak power and excellent stability. CW and mechanical Q-switched doped fibre lasers were configured in diffraction grating tuned cavities with high output coupling (R=4%). Unprecedented output powers of up to 700mW tunable over ~100nm around 1.55mum, and 800mW tunable around 1.08mum were obtained. Also the value of 800W peak power for the 80ns output pulses from the Q-switched erbium-doped fibre laser amounts to the most intense pulse created thus far from any fibre laser device. Optimisation of the total tuning range attainable from erbium-doped fibre lasers via length tuning resulted in a variety of nonlinear phenomena, namely self Q-switching and optical bistability at the long wavelength tail of the tunable range.
1991-07-01T00:00:00ZBurns, DavidThis thesis describes techniques for the generation of ultrashort optical pulses using semiconductor lasers. Active modelocking through gain modulation is applied to 1.?m InGaAsP semiconductor lasers which utilise microlensed optical fibres as external-cavity components, and tunable optical pulses of ~5ps duration have been generated. Particular attention has been given to suppression of the noisy subpulse features associated with low-frequency modelocked semiconductor lasers. This has led to two suppression methods applicable to symmetric (or balanced) and asymmetric cavity configurations, allowing the generation of clean, single feature short pulses of 7-10ps duration with peak powers in excess of 300mW. The physical phenomena relating to such processes will be outlined. Further amplification of these pulses using an erbium- doped fibre amplifier has resulted in output peak powers in the 5W range. A study of the phase-noise characteristics of these modelocked laser systems was undertaken using a high-speed photodetector and wide-band spectrum analyser arrangement. This characterisation allowed improvements in the rms pulse timing jitter from >2ps to below 300fs in the frequency range 50-5000Hz. Novel modelocked laser configurations were also constructed where the cavity components included both a semiconductor amplifier and an erbium-doped fibre amplifier. Linear and ring hybrid lasers were investigated with resultant pulse durations as short as 3ps with high peak power and excellent stability. CW and mechanical Q-switched doped fibre lasers were configured in diffraction grating tuned cavities with high output coupling (R=4%). Unprecedented output powers of up to 700mW tunable over ~100nm around 1.55mum, and 800mW tunable around 1.08mum were obtained. Also the value of 800W peak power for the 80ns output pulses from the Q-switched erbium-doped fibre laser amounts to the most intense pulse created thus far from any fibre laser device. Optimisation of the total tuning range attainable from erbium-doped fibre lasers via length tuning resulted in a variety of nonlinear phenomena, namely self Q-switching and optical bistability at the long wavelength tail of the tunable range.Spectroscopic studies of the helium-cadmium laser dischargeBrowne, Peter G.https://hdl.handle.net/10023/141572019-04-01T10:08:53Z1974-07-01T00:00:00ZThe techniques of absorption and perturbation spectroscopy have been applied to the He-Cd laser discharge to determine species densities, excitation rates and de-excitation rates of relevance to laser oscillation on transitions in the cadmium ion. The first section, of this work describes the use of a line absorption technique to measure helium singlet and triplet metastable densities in a 3 mm bore capillary tube for both pure helium discharges (current range 10 - 200 mA., pressure range 0.5 - 15 torr) and for He-Cd discharges. For the pure helium discharge, with constant discharge current, the singlet and triplet densities show pronounced maxima of 2 X 10<super>12</super> cm<super> -3</super> and 9 x 10<super>12</super> cm<super>-3</super> respectively around 2 torr. At constant pressure the metastable densities saturate for currents above about 20 mA. The measured triplet densities are in fair agreement with values calculated using known cross-sections for production and loss processes. The addition of cadmium vapour has two principal effects: the metastable populations are almost halved when the optimum cadmium pressure for lasing is present while the current saturation is displaced towards higher currents (60 - 120 mA). The observed optimum performance of the He-Cd laser with respect to discharge current, helium filling pressure and cadmium partial pressure is related directly to the behaviour of the helium metastable densities as these parameters are varied. The second section describes the application of the technique of perturbation spectroscopy to the helium-cadmium laser. Chopping the radiation field inside the cavity of the later operating at 4416A induces perturbations in the populations of the 5s2 2D3/2,5/2 and 5p 2P3/2 levels of the cadmium ion. Analysis of the perturbations of the 5p 2P3/2 and 5s2 2P3/2 populations shows that the ratio of non-radiative to radiative de-excitation rates for the latter level (the upper level of the 4416A laser transition) is 0.9 +/- 0.2, with little dependence on discharge conditions. From the 5s2 2D3/2 population changes the cross-section for excitation of this level from the 5s2 2D5/2 level by electron collisions is found to be ~ 7 x 10<super>-16</super> cm2.
1974-07-01T00:00:00ZBrowne, Peter G.The techniques of absorption and perturbation spectroscopy have been applied to the He-Cd laser discharge to determine species densities, excitation rates and de-excitation rates of relevance to laser oscillation on transitions in the cadmium ion. The first section, of this work describes the use of a line absorption technique to measure helium singlet and triplet metastable densities in a 3 mm bore capillary tube for both pure helium discharges (current range 10 - 200 mA., pressure range 0.5 - 15 torr) and for He-Cd discharges. For the pure helium discharge, with constant discharge current, the singlet and triplet densities show pronounced maxima of 2 X 10<super>12</super> cm<super> -3</super> and 9 x 10<super>12</super> cm<super>-3</super> respectively around 2 torr. At constant pressure the metastable densities saturate for currents above about 20 mA. The measured triplet densities are in fair agreement with values calculated using known cross-sections for production and loss processes. The addition of cadmium vapour has two principal effects: the metastable populations are almost halved when the optimum cadmium pressure for lasing is present while the current saturation is displaced towards higher currents (60 - 120 mA). The observed optimum performance of the He-Cd laser with respect to discharge current, helium filling pressure and cadmium partial pressure is related directly to the behaviour of the helium metastable densities as these parameters are varied. The second section describes the application of the technique of perturbation spectroscopy to the helium-cadmium laser. Chopping the radiation field inside the cavity of the later operating at 4416A induces perturbations in the populations of the 5s2 2D3/2,5/2 and 5p 2P3/2 levels of the cadmium ion. Analysis of the perturbations of the 5p 2P3/2 and 5s2 2P3/2 populations shows that the ratio of non-radiative to radiative de-excitation rates for the latter level (the upper level of the 4416A laser transition) is 0.9 +/- 0.2, with little dependence on discharge conditions. From the 5s2 2D3/2 population changes the cross-section for excitation of this level from the 5s2 2D5/2 level by electron collisions is found to be ~ 7 x 10<super>-16</super> cm2.An X-ray preionised mercury bromide discharge laserBrown, Andrew J.W.https://hdl.handle.net/10023/141562019-04-01T10:02:36Z1988-07-01T00:00:00ZAn X-ray preionised mercury bromide discharge laser (502-504 nm) has been designed, constructed and optimised. The double pulse forming line system built to drive the laser has proven to be both versatile and reliable. It utilises multiple-paralleled thyratrons and has demonstrated the characteristics required for long life operation of high average power gas discharge lasers. The supply produces a voltage pulse of 240 nsec duration and its output impedance is variable from 300 mO to 3.6O. With twelve thyratrons in parallel current risetimes of 83 kA musec-1 into a 300 mO matched load and 2 nsec jitter have been achieved. A voltage transient on the leading edge of the discharge pulse, to aid gas breakdown, up to 1½ times the line charge voltage is possible through a 'spike line' incorporated in the system. The X-ray preioniser, based on a cold cathode field emitter, produces an exposure of 5 mR in the discharge region which is adequate for effective preionisation of the laser. A full parametric study of the laser covering gas composition and pressure. X-ray source effects energy loading and other pulsed power supply effects has been carried out. The peak output energy recorded was 710 mJ with neon buffer gas at 4 atmospheres pressure. This represents an energy extraction of 0.5 JL-1. Pulse lengths of 92 nsec (FWHM) and efficiencies greater than 1.5 % have been achieved.
1988-07-01T00:00:00ZBrown, Andrew J.W.An X-ray preionised mercury bromide discharge laser (502-504 nm) has been designed, constructed and optimised. The double pulse forming line system built to drive the laser has proven to be both versatile and reliable. It utilises multiple-paralleled thyratrons and has demonstrated the characteristics required for long life operation of high average power gas discharge lasers. The supply produces a voltage pulse of 240 nsec duration and its output impedance is variable from 300 mO to 3.6O. With twelve thyratrons in parallel current risetimes of 83 kA musec-1 into a 300 mO matched load and 2 nsec jitter have been achieved. A voltage transient on the leading edge of the discharge pulse, to aid gas breakdown, up to 1½ times the line charge voltage is possible through a 'spike line' incorporated in the system. The X-ray preioniser, based on a cold cathode field emitter, produces an exposure of 5 mR in the discharge region which is adequate for effective preionisation of the laser. A full parametric study of the laser covering gas composition and pressure. X-ray source effects energy loading and other pulsed power supply effects has been carried out. The peak output energy recorded was 710 mJ with neon buffer gas at 4 atmospheres pressure. This represents an energy extraction of 0.5 JL-1. Pulse lengths of 92 nsec (FWHM) and efficiencies greater than 1.5 % have been achieved.A distributed control system for the St Andrews twin photometric telescopeGears, Richard T.https://hdl.handle.net/10023/140572019-04-01T10:09:04Z1996-01-01T00:00:00ZMany astronomers require large amounts of observational data to solve astrophysical problems and to validate theoretical hypotheses. It is therefore imperative that both the observer and telescope work efficiently, maximising data collection whilst minimising object selection and acquisition time. One method in which this can be achieved is through telescope automation. The advent of cheap integrated process controllers enables the system designer to realise novel control system architectures which were previously prohibitive to all but the largest of sites. This thesis reviews the development of processor based control systems in the astronomical and industrial environment and compares distributed and centralised control system architecture. It describes the design and construction of one such distributed control system for the St Andrews Twin Photometric Telescope.
1996-01-01T00:00:00ZGears, Richard T.Many astronomers require large amounts of observational data to solve astrophysical problems and to validate theoretical hypotheses. It is therefore imperative that both the observer and telescope work efficiently, maximising data collection whilst minimising object selection and acquisition time. One method in which this can be achieved is through telescope automation. The advent of cheap integrated process controllers enables the system designer to realise novel control system architectures which were previously prohibitive to all but the largest of sites. This thesis reviews the development of processor based control systems in the astronomical and industrial environment and compares distributed and centralised control system architecture. It describes the design and construction of one such distributed control system for the St Andrews Twin Photometric Telescope.New low pressure gas switchesWeatherup, Clifford Roberthttps://hdl.handle.net/10023/140402019-04-01T10:08:56Z1991-07-01T00:00:00ZThis thesis describes an investigation the aim of which was the development of low pressure gas switches with the advantages of zero standby power consumption and instant readiness. Hydrogen thyratrons use a hollow anode to give the switch a convenient reverse conduction capability. The hollow anode structure has been shown to pass a 4 kA pulse current at 500 Hz for 10<super>10 </super>shots. The use of the hollow anode structure as a cold cathode for a low pressure switch is proposed and triggering of the structure by ions is demonstrated. Under conditions of low gas pressure and high discharge voltage, electrons make few collisions in the cathode dark space of a glow discharge and form extensive beams which travel many centimetres in the gas. Current/voltage characteristics of this 'electron beam' type of discharge are presented for deuterium at pressures between 0.2 and 1.0 torr. The electron beam discharge was found to be space-charge limited with I V3/2 at pressures below about 0.25 torr and I V3/2 at pressures above about 0.25 torr. It is proposed that the current in the electron beam discharge is limited by the flow of positive ions in the cathode dark space. Control of the emission area of a discharge in a hollow metal cylinder is demonstrated and is used as a triggering method for a new type of low pressure gas switch. Tests in a pulse modulator at repetition rates up to 1 kHz show that the switch operates satisfactorily. The triggering mechanism is shown to depend on the properties of the cold cathode glow discharge which, in certain circumstances, leads to the unusual phenomenon of post trigger-pulse firing of the switch. The phenomenon is shown to result from the interaction of the trigger discharge cathode dark space and the geometry of the switch. The glow discharge electron beam is successfully applied as a triggering method in several new low pressure gas switches. In one arrangement, the electron beam is used to pre-ionise the switch and subsidiary grids are used to trigger main conduction. In another arrangement, the electron beam is directed into the high voltage region to trigger conduction directly. The designs of these switches are discussed and their operation is demonstrated.
1991-07-01T00:00:00ZWeatherup, Clifford RobertThis thesis describes an investigation the aim of which was the development of low pressure gas switches with the advantages of zero standby power consumption and instant readiness. Hydrogen thyratrons use a hollow anode to give the switch a convenient reverse conduction capability. The hollow anode structure has been shown to pass a 4 kA pulse current at 500 Hz for 10<super>10 </super>shots. The use of the hollow anode structure as a cold cathode for a low pressure switch is proposed and triggering of the structure by ions is demonstrated. Under conditions of low gas pressure and high discharge voltage, electrons make few collisions in the cathode dark space of a glow discharge and form extensive beams which travel many centimetres in the gas. Current/voltage characteristics of this 'electron beam' type of discharge are presented for deuterium at pressures between 0.2 and 1.0 torr. The electron beam discharge was found to be space-charge limited with I V3/2 at pressures below about 0.25 torr and I V3/2 at pressures above about 0.25 torr. It is proposed that the current in the electron beam discharge is limited by the flow of positive ions in the cathode dark space. Control of the emission area of a discharge in a hollow metal cylinder is demonstrated and is used as a triggering method for a new type of low pressure gas switch. Tests in a pulse modulator at repetition rates up to 1 kHz show that the switch operates satisfactorily. The triggering mechanism is shown to depend on the properties of the cold cathode glow discharge which, in certain circumstances, leads to the unusual phenomenon of post trigger-pulse firing of the switch. The phenomenon is shown to result from the interaction of the trigger discharge cathode dark space and the geometry of the switch. The glow discharge electron beam is successfully applied as a triggering method in several new low pressure gas switches. In one arrangement, the electron beam is used to pre-ionise the switch and subsidiary grids are used to trigger main conduction. In another arrangement, the electron beam is directed into the high voltage region to trigger conduction directly. The designs of these switches are discussed and their operation is demonstrated.Studies of hollow cathode He-Cd lasersPirrie, C.A.https://hdl.handle.net/10023/140392019-04-01T10:04:05Z1985-07-01T00:00:00ZNovel segmented-electrode discharge tubes which operate as sealed-off He-Cd hollow cathode lasers and which do not require a flowing gas system to distribute cadmium vapour are reported. The electrode geometry is arranged to produce a net axial electric field so that cadmium distribution is enhanced by cataphoresis,' a process which is not normally associated with He-Cd hollow cathode lasers. The conditions under which the segmented-electrode tubes exhibit the properties of hollow cathode discharges are determined from their current and voltage characteristics. One system studied consists of alternate anode and cathode segments with glass insulating sections arranged axially, and behaves as a series of hollow cathode discharges under appropriate conditions. Each anode/hollow cathode pair is separated from its neighbouring segments by insulating glass sections of length D, which provide a recombination site and prevent adjacent discharges from overlapping each other. A calculation shows that adjacent discharges will remain isolated provided that D> 2.85cm, in helium at 10 Torr and 700 K. Another segmented-electrode system studied consists of a series of metal segments equal in length. This system does not have discrete anode segments and exhibits the positive Volt-Ampere (VI) characteristics normally expected of, hollow cathode discharges only at low values of discharge current. A transition point in the characteristics exists, and, for discharge currents above this, the VI slope is negative. This behaviour is explained in terms of the plasma length, which is defined as the axial extent of the negative glow within a cylindrical segment, and which increases with increasing discharge current. At low values of discharge current (several mA), the plasma length is less than the length of the metal segments, and the discharge tube operates as a series of hollow cathodes. Under these conditions, each segment has a negative glow discharge partially penetrating the bore from one end, while the other end performs as an anode for the next segment, and so on. The last segment is at power supply earth potential and has no anodic function. The transition points in the VI characteristics occur when the discharge current is increased to a value such that the plasma length equals the length of the metal segments. For discharge currents above this value, only the last segment at power supply earth potential continues to operate as a hollow cathode. The results of these studies provide guidelines for the design of segmented-electrode He-Cd hollow cathode laser discharge tubes in terms of the axial order and optimum spacing of anode and cathode segments. It is shown that large axial temperature variations can exist in thin-walled (0.9mm) hollow cathode structures, implying that the current density distribution at the cathode surface is not uniform. A differential equation is derived to describe heat flow within a cylindrical conductor heated at one end and cooled by natural convection and radiation along its length. Approximate solutions are obtained for both thick-walled and thin-walled conductors. The results of the thick-walled approximation yield thermal design criteria for hollow cathodes, where moderate cathode temperatures (350°C) and minimal axial temperature variations are required. The thin-walled approximation yields results which provide design criteria for the protection from high temperature failure of components such as cylindrical glass/metal seals and 'O' rings. (Abstract shortened by ProQuest.)
1985-07-01T00:00:00ZPirrie, C.A.Novel segmented-electrode discharge tubes which operate as sealed-off He-Cd hollow cathode lasers and which do not require a flowing gas system to distribute cadmium vapour are reported. The electrode geometry is arranged to produce a net axial electric field so that cadmium distribution is enhanced by cataphoresis,' a process which is not normally associated with He-Cd hollow cathode lasers. The conditions under which the segmented-electrode tubes exhibit the properties of hollow cathode discharges are determined from their current and voltage characteristics. One system studied consists of alternate anode and cathode segments with glass insulating sections arranged axially, and behaves as a series of hollow cathode discharges under appropriate conditions. Each anode/hollow cathode pair is separated from its neighbouring segments by insulating glass sections of length D, which provide a recombination site and prevent adjacent discharges from overlapping each other. A calculation shows that adjacent discharges will remain isolated provided that D> 2.85cm, in helium at 10 Torr and 700 K. Another segmented-electrode system studied consists of a series of metal segments equal in length. This system does not have discrete anode segments and exhibits the positive Volt-Ampere (VI) characteristics normally expected of, hollow cathode discharges only at low values of discharge current. A transition point in the characteristics exists, and, for discharge currents above this, the VI slope is negative. This behaviour is explained in terms of the plasma length, which is defined as the axial extent of the negative glow within a cylindrical segment, and which increases with increasing discharge current. At low values of discharge current (several mA), the plasma length is less than the length of the metal segments, and the discharge tube operates as a series of hollow cathodes. Under these conditions, each segment has a negative glow discharge partially penetrating the bore from one end, while the other end performs as an anode for the next segment, and so on. The last segment is at power supply earth potential and has no anodic function. The transition points in the VI characteristics occur when the discharge current is increased to a value such that the plasma length equals the length of the metal segments. For discharge currents above this value, only the last segment at power supply earth potential continues to operate as a hollow cathode. The results of these studies provide guidelines for the design of segmented-electrode He-Cd hollow cathode laser discharge tubes in terms of the axial order and optimum spacing of anode and cathode segments. It is shown that large axial temperature variations can exist in thin-walled (0.9mm) hollow cathode structures, implying that the current density distribution at the cathode surface is not uniform. A differential equation is derived to describe heat flow within a cylindrical conductor heated at one end and cooled by natural convection and radiation along its length. Approximate solutions are obtained for both thick-walled and thin-walled conductors. The results of the thick-walled approximation yield thermal design criteria for hollow cathodes, where moderate cathode temperatures (350°C) and minimal axial temperature variations are required. The thin-walled approximation yields results which provide design criteria for the protection from high temperature failure of components such as cylindrical glass/metal seals and 'O' rings. (Abstract shortened by ProQuest.)Gas discharges confined by metallic wallsRidge, Natalie Annehttps://hdl.handle.net/10023/140382019-04-01T10:06:52Z1992-07-01T00:00:00ZThis thesis describes experimental and theoretical aspects of low pressure gas discharges confined by metallic walls. Simple considerations predict that the current between the anode and the cathode of a discharge tube which contains isolated cylindrical metal segments is conducted through the metal. Under certain conditions, however, a glow discharge may form along the axis of the tubes conduction occurs along the path which results in the lowest sustaining VO 1tage. The energy losses at insulating and conducting walls in the region of the positive column are investigated theoretically for rare gas discharges at high current densities. The energy losses are found to be greater when the walls are conducting, resulting in a higher axial electric field. A possible criterion for the maximum length of a metal segment is proposed. The maximum length may be defined as the length at which the radial electric field in the positive ion sheath at one end of a metal segment is equal to the breakdown field. A relationship between the maximum length of a metal segment and the tube radius and pressure has been derived. The form of this relationship agrees qualitatively with experimental results. Measurements of the anode-cathode breakdown voltage of a gas in a tube containing metal segments of small bore are presented. The anode-cathode breakdown voltage is found to be approximately equal to the sum of the breakdown voltages of the inter-segment and electrode-segment gaps. When the system is operating on the left of the Paschen minimum, the anode-cathode breakdown voltage may be increased by the addition of metal segments in such away as to reduce the effective gap. Individually the product's of pressure and inter-segment gap (pd) are less than the product (pd) min at the Paschen minimum. Hence, the anode-cathode breakdown voltage will increase as the number of gaps (and segments) increases and pd decreases. Discharges in dual-bore (alternate narrow and wide bore) metal tubes have been investigated. The anode-cathode breakdown voltages and sustaining voltages of low current density discharges, confined by dual-bore tubes of various lengths, are shown to depend upon the system geometry. A laser system using dual-bore metal discharge tubes, has been designed and tested for producing high current densities in rare gas-metal vapour mixtures.
1992-07-01T00:00:00ZRidge, Natalie AnneThis thesis describes experimental and theoretical aspects of low pressure gas discharges confined by metallic walls. Simple considerations predict that the current between the anode and the cathode of a discharge tube which contains isolated cylindrical metal segments is conducted through the metal. Under certain conditions, however, a glow discharge may form along the axis of the tubes conduction occurs along the path which results in the lowest sustaining VO 1tage. The energy losses at insulating and conducting walls in the region of the positive column are investigated theoretically for rare gas discharges at high current densities. The energy losses are found to be greater when the walls are conducting, resulting in a higher axial electric field. A possible criterion for the maximum length of a metal segment is proposed. The maximum length may be defined as the length at which the radial electric field in the positive ion sheath at one end of a metal segment is equal to the breakdown field. A relationship between the maximum length of a metal segment and the tube radius and pressure has been derived. The form of this relationship agrees qualitatively with experimental results. Measurements of the anode-cathode breakdown voltage of a gas in a tube containing metal segments of small bore are presented. The anode-cathode breakdown voltage is found to be approximately equal to the sum of the breakdown voltages of the inter-segment and electrode-segment gaps. When the system is operating on the left of the Paschen minimum, the anode-cathode breakdown voltage may be increased by the addition of metal segments in such away as to reduce the effective gap. Individually the product's of pressure and inter-segment gap (pd) are less than the product (pd) min at the Paschen minimum. Hence, the anode-cathode breakdown voltage will increase as the number of gaps (and segments) increases and pd decreases. Discharges in dual-bore (alternate narrow and wide bore) metal tubes have been investigated. The anode-cathode breakdown voltages and sustaining voltages of low current density discharges, confined by dual-bore tubes of various lengths, are shown to depend upon the system geometry. A laser system using dual-bore metal discharge tubes, has been designed and tested for producing high current densities in rare gas-metal vapour mixtures.Atmospheric guiding of electromagnetic wavesCondon, Brian P.https://hdl.handle.net/10023/140372019-04-01T10:02:30Z1993-07-01T00:00:00ZWe propose to alter the propagation conditions experienced by a microwave beam by the generation of a "laser beam atmospheric waveguide". The waveguide is formed by tailored refractive index changes caused by the absorption of a small part of the energy of an annular laser beam. The objective is to increase the microwave radiation field experienced by a target through improved directionality rather than total radiated power from the source. The equations governing the propagation of high power laser beams in the paraxial limit and their interactions (both linear and non-linear) with an absorbing atmosphere are derived and studied. The mechanisms which lead to the formation of the guide and the effects of the propagation environment are considered in detail and the full paraxial form of the thermal blooming wave equation is derived from first principles. Refractive index changes in air caused by the passage of a 1 kW CW CO2 Gaussian laser beam are studied theoretically and both linear and non-linear cases examined. In the linear case, it is predicted that the laser beam produces a refractive index change of greater than 1 part in 10<super> 5</super> for a 1 second beam exposure. In the non-linear case, the iterative scheme developed predicts self-interaction and beam break-up after less than 0.5 s. For an annular beam, refractive index changes of 3 parts in 10 are predicted for the linear case. The influence of refractive index fluctuations on microwave radiation is modelled using a ray-tracing algorithm to investigate the behaviour of microwave radiation in an atmospheric waveguide. For a step-index guide of 5 cm radius, there is strong guiding so that even with a small perturbation in refractivity, rays with a wide range of launch angles are trapped. In the case of a guide with a quadratic refractive index profile, small changes in refractive index (1 part in 10<super>6</super>) produce weak guiding where only rays with trajectories very close to the optic axis are trapped. As the refractive index change increases, more divergent rays are trapped until a transition to strong guiding occurs at a critical value (changes in refractive index of the order of 1 part in 10<super>4</super>). A number of implementations of the waveguiding concept are proposed and evaluated. For the purposes of an experimental verification, a specially designed Annular Beam Director ("ABD") of an on-axis type is tested. Annular laser beams are propagated over short distances in the laboratory and the results presented. Measurements made with a rotating wire laser beam analyser indicate that the ABD performs well. Experiments designed to measure refractive index changes caused by a 1 kW CW CO2 laser beam of Gaussian profile are described. Measurements are made at wavelengths of 633 nm using a specially configured Michelson interferometer and at 3 mm using a millimetre wave quasi-optical FM noise measurement system. Typical results indicate refractive index changes of the order of 1 part in 106 at both the wavelengths considered. The guiding of microwave radiation is verified using a 1 kW CW CO2 annular laser beam, produced by the ABD, into which microwave energy is injected using a small copper reflector located at the centre of the annulus. In one implementation, the microwave energy is coupled out of the guide with a second copper reflector. In a second implementation, the microwave detector imit is located on the optic axis and the laser beam is terminated in an annular beam dump. The results show clear evidence that the high power laser beam forms a waveguide, increasing the amount of microwave radiation reaching the detector by a factor of 1.5.
1993-07-01T00:00:00ZCondon, Brian P.We propose to alter the propagation conditions experienced by a microwave beam by the generation of a "laser beam atmospheric waveguide". The waveguide is formed by tailored refractive index changes caused by the absorption of a small part of the energy of an annular laser beam. The objective is to increase the microwave radiation field experienced by a target through improved directionality rather than total radiated power from the source. The equations governing the propagation of high power laser beams in the paraxial limit and their interactions (both linear and non-linear) with an absorbing atmosphere are derived and studied. The mechanisms which lead to the formation of the guide and the effects of the propagation environment are considered in detail and the full paraxial form of the thermal blooming wave equation is derived from first principles. Refractive index changes in air caused by the passage of a 1 kW CW CO2 Gaussian laser beam are studied theoretically and both linear and non-linear cases examined. In the linear case, it is predicted that the laser beam produces a refractive index change of greater than 1 part in 10<super> 5</super> for a 1 second beam exposure. In the non-linear case, the iterative scheme developed predicts self-interaction and beam break-up after less than 0.5 s. For an annular beam, refractive index changes of 3 parts in 10 are predicted for the linear case. The influence of refractive index fluctuations on microwave radiation is modelled using a ray-tracing algorithm to investigate the behaviour of microwave radiation in an atmospheric waveguide. For a step-index guide of 5 cm radius, there is strong guiding so that even with a small perturbation in refractivity, rays with a wide range of launch angles are trapped. In the case of a guide with a quadratic refractive index profile, small changes in refractive index (1 part in 10<super>6</super>) produce weak guiding where only rays with trajectories very close to the optic axis are trapped. As the refractive index change increases, more divergent rays are trapped until a transition to strong guiding occurs at a critical value (changes in refractive index of the order of 1 part in 10<super>4</super>). A number of implementations of the waveguiding concept are proposed and evaluated. For the purposes of an experimental verification, a specially designed Annular Beam Director ("ABD") of an on-axis type is tested. Annular laser beams are propagated over short distances in the laboratory and the results presented. Measurements made with a rotating wire laser beam analyser indicate that the ABD performs well. Experiments designed to measure refractive index changes caused by a 1 kW CW CO2 laser beam of Gaussian profile are described. Measurements are made at wavelengths of 633 nm using a specially configured Michelson interferometer and at 3 mm using a millimetre wave quasi-optical FM noise measurement system. Typical results indicate refractive index changes of the order of 1 part in 106 at both the wavelengths considered. The guiding of microwave radiation is verified using a 1 kW CW CO2 annular laser beam, produced by the ABD, into which microwave energy is injected using a small copper reflector located at the centre of the annulus. In one implementation, the microwave energy is coupled out of the guide with a second copper reflector. In a second implementation, the microwave detector imit is located on the optic axis and the laser beam is terminated in an annular beam dump. The results show clear evidence that the high power laser beam forms a waveguide, increasing the amount of microwave radiation reaching the detector by a factor of 1.5.Improving the sensitivity and utility of pulsed
dipolar experiments in EPR at 94 GHzMotion, Claire Louisehttps://hdl.handle.net/10023/139692019-05-11T02:02:52Z2018-06-27T00:00:00ZPulsed dipolar spectroscopy (PDS) is an electron paramagnetic resonance (EPR) technique,
used to conduct long range distance measurements in proteins in the nanometre range. This
thesis presents a number of methodological and instrumental techniques to improve the sensitivity and utility of PDS experiments using a home-built high power pulsed spectrometer,
HiPER, operating at 94 GHz. These include the implementation of phase-modulated composite pulses, which correct for imperfections arising due to inhomogeneity, and offer increased
excitation bandwidth as well as experimental protocols such as annealing and glassing of samples. A theoretical study into the use of matched filtering to reduce echo noise during measurements, has predicted gains of up to a factor of 3 enhancement in signal-to-noise.
Using such techniques we demonstrate sensitivity enhancements of more than 30 on PDS
experiments, between nitroxides and Fe centres, in haem-proteins, corresponding to a reduction in averaging time of almost 1,000, in comparison to standard commercial spectrometers
operating at X-band. The use of composite pulses in PDS experiments on nitroxide biradicals
were also investigated, including their limitations due to intramolecular effects. The thesis
then describes a single frequency dipolar modulation experiment, RIDME, and uses high field
measurements to determine both the distance and relative orientation of a cobalt-nitroxide
system, for the first time. Finally, a design study is conducted to implement frequency and
amplitude modulated pulses on a spectrometer at 9/34 GHz to improve sensitivity.
2018-06-27T00:00:00ZMotion, Claire LouisePulsed dipolar spectroscopy (PDS) is an electron paramagnetic resonance (EPR) technique,
used to conduct long range distance measurements in proteins in the nanometre range. This
thesis presents a number of methodological and instrumental techniques to improve the sensitivity and utility of PDS experiments using a home-built high power pulsed spectrometer,
HiPER, operating at 94 GHz. These include the implementation of phase-modulated composite pulses, which correct for imperfections arising due to inhomogeneity, and offer increased
excitation bandwidth as well as experimental protocols such as annealing and glassing of samples. A theoretical study into the use of matched filtering to reduce echo noise during measurements, has predicted gains of up to a factor of 3 enhancement in signal-to-noise.
Using such techniques we demonstrate sensitivity enhancements of more than 30 on PDS
experiments, between nitroxides and Fe centres, in haem-proteins, corresponding to a reduction in averaging time of almost 1,000, in comparison to standard commercial spectrometers
operating at X-band. The use of composite pulses in PDS experiments on nitroxide biradicals
were also investigated, including their limitations due to intramolecular effects. The thesis
then describes a single frequency dipolar modulation experiment, RIDME, and uses high field
measurements to determine both the distance and relative orientation of a cobalt-nitroxide
system, for the first time. Finally, a design study is conducted to implement frequency and
amplitude modulated pulses on a spectrometer at 9/34 GHz to improve sensitivity.Modelling of the self-sustained transverse-discharge excited xenon chloride laserTurner, Miles Markhttps://hdl.handle.net/10023/139482018-06-12T11:29:15Z1990-07-01T00:00:00ZThis thesis discusses computational techniques for modelling high pressure self-sustained transverse-discharge excited gas lasers, with special reference to the XeCl laser. The laser discharge is modelled in up to two spatial dimensions, including a numerical calculation of the electric fields taking account of the electrode profile and distribution of conductivity within the discharge. A detailed plasma chemistry model for the XeCl laser is developed, and validated by extensive comparisons with experimental data.
1990-07-01T00:00:00ZTurner, Miles MarkThis thesis discusses computational techniques for modelling high pressure self-sustained transverse-discharge excited gas lasers, with special reference to the XeCl laser. The laser discharge is modelled in up to two spatial dimensions, including a numerical calculation of the electric fields taking account of the electrode profile and distribution of conductivity within the discharge. A detailed plasma chemistry model for the XeCl laser is developed, and validated by extensive comparisons with experimental data.The optogalvanic effect in molecular discharges and the stabilization of CO₂ lasersMoffatt, S.https://hdl.handle.net/10023/139282019-05-28T08:58:40Z1983-07-01T00:00:00ZThe optical perturbation of discharge current voltage characteristics (optogalvanic effect or OGE) has been investigated for CO2 and CO laser discharges. A quantitative power perturbation model is constructed and a series of experiments show, a close agreement for changes of all the major operating parameters of the CO2 laser. The theory consists of an evaluation of the microscopic kinetic relaxation processes leading to the changes in thermal balance of the discharge that occur due to the absorption and amplification of the resonant laser radiation. A generation of compact and efficient cw CO2 lasers has been developed which produces higher stable output powers per unit length than previously reported, and these have been actively stabilized by OGE to provide a high degree of frequency stability (<50 kHz) and amplitude stability (< 0.5%) which is a six orders and two orders of magnitude improvement over the passive resonator capability, respectively. New optogalvanic effects have been discovered both at high frequencies (up to 100 kHz) and for sequence (00°2) laser transitions in CO2 and also in the cw CO laser. Preliminary investigation of optogalvanic detection of standing wave saturation resonances in low pressure discharged CO2 have been carried out and some analysis of discharge noise has been necessary to evaluate the ability of such a narrow band detector for laser stabilization. The empirical evidence provided by the temporal response of OGE combined with the gas composition dependence (including N2 free mixtures) proves conclusively that no major ionization mechanism described so far can be responsible for the effect. A thermal explanation of the effect due to modified kinetic cooling of the laser gas has been developed from existing qualitative explanations. This proposed "gas temperature" power perturbation model provides for the first time an accurate (~20%) prediction of perturbations.(amplitude, phase, and frequency) due to the resonant interaction of a CO2 laser beam over a wide range (up to 4 orders of magnitude) of detailed parametric changes, with a CO2 or laser mixture discharge.
1983-07-01T00:00:00ZMoffatt, S.The optical perturbation of discharge current voltage characteristics (optogalvanic effect or OGE) has been investigated for CO2 and CO laser discharges. A quantitative power perturbation model is constructed and a series of experiments show, a close agreement for changes of all the major operating parameters of the CO2 laser. The theory consists of an evaluation of the microscopic kinetic relaxation processes leading to the changes in thermal balance of the discharge that occur due to the absorption and amplification of the resonant laser radiation. A generation of compact and efficient cw CO2 lasers has been developed which produces higher stable output powers per unit length than previously reported, and these have been actively stabilized by OGE to provide a high degree of frequency stability (<50 kHz) and amplitude stability (< 0.5%) which is a six orders and two orders of magnitude improvement over the passive resonator capability, respectively. New optogalvanic effects have been discovered both at high frequencies (up to 100 kHz) and for sequence (00°2) laser transitions in CO2 and also in the cw CO laser. Preliminary investigation of optogalvanic detection of standing wave saturation resonances in low pressure discharged CO2 have been carried out and some analysis of discharge noise has been necessary to evaluate the ability of such a narrow band detector for laser stabilization. The empirical evidence provided by the temporal response of OGE combined with the gas composition dependence (including N2 free mixtures) proves conclusively that no major ionization mechanism described so far can be responsible for the effect. A thermal explanation of the effect due to modified kinetic cooling of the laser gas has been developed from existing qualitative explanations. This proposed "gas temperature" power perturbation model provides for the first time an accurate (~20%) prediction of perturbations.(amplitude, phase, and frequency) due to the resonant interaction of a CO2 laser beam over a wide range (up to 4 orders of magnitude) of detailed parametric changes, with a CO2 or laser mixture discharge.A differential lidar system based on a xenon chloride laserMillington, Roger Bradleyhttps://hdl.handle.net/10023/139272019-04-01T10:09:46Z1985-07-01T00:00:00ZA differential absorption lidar (DIAL) system, based on a xenon chloride exoimer laser, has been developed and applied to measurements of atmospheric sulphur dioxide. This thesis describes the basis of the technique, the construction and operation of the prototype system, and its theoretical and practical sensitivity to sulphur dioxide concentration. The system, comprising the laser, telescope, detector and data handling equipment was developed for robustness in the field, and novel features have been included for this purpose. The Newtonian type telescope was chosen for its small image size and adequate field-of-view. It uses a "Cassegrain" type primary mirror and a visible reflector at the focus, allowing direct viewing of the scattering target. This feature is used in conjunction with direct viewing through the laser cavity, through a dielectric coated mirror, to permit laser/telescope alignment on the target. The laser itself is pumped by transverse discharge after corona/U,V pre-ionization. Laser characteristics have been measured. Those of primary importance to lidar are the energy per pulse, at 5 to 8mJ, the pulse duration of 32ns, the maximum pulse repetition rate, at 20pps, and the number of pulses to half energy, at 18000 per gas fill. The emission spectrum of the laser, with wavelengths at 307.92nm and 308.17nm, has been compared with the absorption spectrum of sulphur dioxide, showing a difference between the respective absorption coefficients. This indicated the suitability of this particular laser to the differential absorption technique. Unique selection between these wavelengths, per pulse, is desirable for optimum sensitivity in DIAU However, a novel method has been devised for modifying the relative wavelength content of one of the pulses, by inserting an absorption cell of sulphur dioxide into the laser optical cavity. The advantages of this over "distinct" wavelength selection are cheapness and robustness. However, theoretical work has shown a subsequent loss of sensitivity to atmospheric sulphur dioxide concentration, by a factor of about 5, A 3-element birefringent filter has been designed in case a more conventional tuning method is required. Specification of the detector and its operating conditions have been closely defined in order to optimise sensitivity to very low levels of backscattered light, whilst reducing the effects of unwanted background and noise. To this end, a solar-blind photomultiplier has been employed in conjunction with a narrow-band interference filter, centred around the laser emission wavelength. A relatively simple signal handling circuit was built to perform the minimum requirement of measuring the intensity of backscattered radiation. A micro-computer is used to control the circuitry in test and data acquisition modes, and to store data, allowing signal averaging and subsequent data analysis. Program algorithms for data analysis were developed from lidar and DIAL theory. A theoretical investigation of atmospheric scattering properties was carried-out to provide scatter coefficients for application in the lidar equation. The validity of the lidar equation was proved when computer-modelled oscilloscope traces of lidar return signals were found to match, closely, experimental traces of return signals from the smoke plume at Methil power station. A theoretical treatment, using the lidar equation, gave expressions for target gas concentrations as a function of return signal intensities. This was done for the case where unique wavelength pulses are transmitted into the atmosphere and was repeated for the case of "mixed wavelength" pulses, applying to the DIAL experiment. Prediction of sensitivity of the system in measuring sulphur dioxide concentration is based on the noise content of the return signals. The analysis has given a detection limit range of about 10ppm.m to 800ppm.m, for the "mixed wavelength" application and an expected range of 2ppm.m to I60ppm.ra if the laser is tuned conventionally, depending on signal strength and number of averaged pulse pairs. Errors expected in measurements of finite sulphur dioxide concentrations are given. The system was applied to measuring sulphur dioxide, emitted under control, into the path of the laser pulse. Measured peaks of about 120ppm were expected and measurements taken successively are in agreement with expected dispersal rates. Fluctuations of measured sulphur dioxide levels, about a mean, are shown to be within the theoretically-evaluated error limits. This close agreement between theory and experiment allows the theoretical detection limits to be treated as realistic.
1985-07-01T00:00:00ZMillington, Roger BradleyA differential absorption lidar (DIAL) system, based on a xenon chloride exoimer laser, has been developed and applied to measurements of atmospheric sulphur dioxide. This thesis describes the basis of the technique, the construction and operation of the prototype system, and its theoretical and practical sensitivity to sulphur dioxide concentration. The system, comprising the laser, telescope, detector and data handling equipment was developed for robustness in the field, and novel features have been included for this purpose. The Newtonian type telescope was chosen for its small image size and adequate field-of-view. It uses a "Cassegrain" type primary mirror and a visible reflector at the focus, allowing direct viewing of the scattering target. This feature is used in conjunction with direct viewing through the laser cavity, through a dielectric coated mirror, to permit laser/telescope alignment on the target. The laser itself is pumped by transverse discharge after corona/U,V pre-ionization. Laser characteristics have been measured. Those of primary importance to lidar are the energy per pulse, at 5 to 8mJ, the pulse duration of 32ns, the maximum pulse repetition rate, at 20pps, and the number of pulses to half energy, at 18000 per gas fill. The emission spectrum of the laser, with wavelengths at 307.92nm and 308.17nm, has been compared with the absorption spectrum of sulphur dioxide, showing a difference between the respective absorption coefficients. This indicated the suitability of this particular laser to the differential absorption technique. Unique selection between these wavelengths, per pulse, is desirable for optimum sensitivity in DIAU However, a novel method has been devised for modifying the relative wavelength content of one of the pulses, by inserting an absorption cell of sulphur dioxide into the laser optical cavity. The advantages of this over "distinct" wavelength selection are cheapness and robustness. However, theoretical work has shown a subsequent loss of sensitivity to atmospheric sulphur dioxide concentration, by a factor of about 5, A 3-element birefringent filter has been designed in case a more conventional tuning method is required. Specification of the detector and its operating conditions have been closely defined in order to optimise sensitivity to very low levels of backscattered light, whilst reducing the effects of unwanted background and noise. To this end, a solar-blind photomultiplier has been employed in conjunction with a narrow-band interference filter, centred around the laser emission wavelength. A relatively simple signal handling circuit was built to perform the minimum requirement of measuring the intensity of backscattered radiation. A micro-computer is used to control the circuitry in test and data acquisition modes, and to store data, allowing signal averaging and subsequent data analysis. Program algorithms for data analysis were developed from lidar and DIAL theory. A theoretical investigation of atmospheric scattering properties was carried-out to provide scatter coefficients for application in the lidar equation. The validity of the lidar equation was proved when computer-modelled oscilloscope traces of lidar return signals were found to match, closely, experimental traces of return signals from the smoke plume at Methil power station. A theoretical treatment, using the lidar equation, gave expressions for target gas concentrations as a function of return signal intensities. This was done for the case where unique wavelength pulses are transmitted into the atmosphere and was repeated for the case of "mixed wavelength" pulses, applying to the DIAL experiment. Prediction of sensitivity of the system in measuring sulphur dioxide concentration is based on the noise content of the return signals. The analysis has given a detection limit range of about 10ppm.m to 800ppm.m, for the "mixed wavelength" application and an expected range of 2ppm.m to I60ppm.ra if the laser is tuned conventionally, depending on signal strength and number of averaged pulse pairs. Errors expected in measurements of finite sulphur dioxide concentrations are given. The system was applied to measuring sulphur dioxide, emitted under control, into the path of the laser pulse. Measured peaks of about 120ppm were expected and measurements taken successively are in agreement with expected dispersal rates. Fluctuations of measured sulphur dioxide levels, about a mean, are shown to be within the theoretically-evaluated error limits. This close agreement between theory and experiment allows the theoretical detection limits to be treated as realistic.Elements of a 200 watt pulsed excimer laserMcDuff, Glenhttps://hdl.handle.net/10023/139262019-04-01T10:08:42Z1998-07-01T00:00:00ZThis thesis describes the theoretical and experimental investigation into many aspects of circuit and component design for high average power short pulse systems. The primary objective of this work is to develop both circuit design criteria and pulsed components for long life rare gas halide lasers. A pulsed system consists of three major components, energy storage, switches, and load. This investigation considers the type of circuit which uses capacitive energy storage in combination with a closing switch to transfer electrical power to a load. Specific loads are not addressed but the implications of load characteristics that affect circuit/component design and life are considered. The investigation reported, describes the physical and electrical characteristics and analysis of phenomena that adversely affect the performance and life of pulse duty components. In the area of capacitive storage, lifetimes of one particular design was improved by 3 orders of magnitude and a means of detecting the imminent failure of oil filled capacitors was devised and patented. In the area of switching, methods are described by which hydrogen thyratrons can be operated in parallel with equal current sharing without the need for inductive or resistive ballasting. Finally, the design and testing of a 200 watt XeCl laser modulator is presented.
1998-07-01T00:00:00ZMcDuff, GlenThis thesis describes the theoretical and experimental investigation into many aspects of circuit and component design for high average power short pulse systems. The primary objective of this work is to develop both circuit design criteria and pulsed components for long life rare gas halide lasers. A pulsed system consists of three major components, energy storage, switches, and load. This investigation considers the type of circuit which uses capacitive energy storage in combination with a closing switch to transfer electrical power to a load. Specific loads are not addressed but the implications of load characteristics that affect circuit/component design and life are considered. The investigation reported, describes the physical and electrical characteristics and analysis of phenomena that adversely affect the performance and life of pulse duty components. In the area of capacitive storage, lifetimes of one particular design was improved by 3 orders of magnitude and a means of detecting the imminent failure of oil filled capacitors was devised and patented. In the area of switching, methods are described by which hydrogen thyratrons can be operated in parallel with equal current sharing without the need for inductive or resistive ballasting. Finally, the design and testing of a 200 watt XeCl laser modulator is presented.Second harmonic generation in sodium vapour induced by a magnetic fieldSinclair, Bruce Davidhttps://hdl.handle.net/10023/139252019-04-01T10:06:53Z1987-07-01T00:00:00ZA XeCl excimer laser system whose output characteristics are suitable for stimulated Raman wavelength shifting, has been developed and used in single-pass conversion experiments in CH4 and H2. The overall system, comprising the laser, Raman focusing optics and Raman cell, has been designed with simplicity and system compactness in mind, suitable for a transportable lidar application. The XeCl laser is a fast discharge pumped device, driven by a C-C transfer circuit with a N2-filled spark gap switch. Various ultraviolet preionisation schemes and cavity component arrangements, employed during the laser development, are described and compared. A general overview of the basic spark preionisation techniques commonly used in rare-gas halide lasers is also presented. Over the course of the development, the available laser output power has been increased by a factor of 100, by methods described. Laser output characteristics relevant to stimulated Raman scattering (SRS), are the emission wavelength (308nm), the output pulse energy (>100mJ), pulse duration (~6.5ns FWHM), peak pulse power (>107 W) and the output beam divergence (U x 15 mrad). The design for a new laser, using a low inductance cavity geometry and an 'automatic-integral' preionisation scheme is described. The problems of poor laser beam quality and focusability which are inherent in rare-gas halide lasers, are overcome by the use of an unstable optical resonator. Experiments with a range of 'continuously-coupled' unstable resonators, in which the laser output coupler is a simple planoconvex lens, are reported. They show that pulse duration, energy and beam divergence are systematically reduced as the round-trip resonator magnification M is increased, with the result that the focal spot area can be up to 2000 times smaller than obtainable with conventional stable-cavity optics. Furthermore, these output diaracteristics are shown to depend not simply on the overall value of M, but also on the individual one-way magnifications M12 and M21 in the resonator, between mirrors 1 and 2. The dependence has produced significant experimental differences in resonators with very similar values of M. This new observation is attributed, in part, to the shortness of the laser pulse duration in this device. Experiments have been performed in high pressure CH4 andH2 to find the pumping geometries necessary to achieve SRS. The most unstable of the range cf resonators studied, having the least divergent output and the highest beam quality, is shown to be the best configuration for the application. In addition, it is found that pump focal power densities greater than 1010W/cm2 do not automatically guarantee coherent scattering if they have been produced as a result of tight focusing, and that a relatively long Raman focusing length (>~50cm) is a more fundamental experimental requirement. Raman scattering in CH4 gas has generated output lines from the first anti-Stokes (AS1) to the fourth Stokes (S4) order, at wavelengths of 283nm, 338nm, 375nm, 421nm and 480nm. The higher Raman gain in H2 has produced a wider range of Raman-shifted orders, extending from the ultraviolet into the infrared, at wavelengths given by 245nm (AS2), 273nm, 353nm, 4l4nm, 500nm, 631nm and (S5). The relative abundance of the competing SRS and four-wave mixing processes in the Raman cell, which determine the range and distribution of energy within the Raman spectrum, has been varied by reducing the feasibility of angular phase matching in the medium. Increasing the pump beam focal length and the Raman cell pressure has enabled the overall energy conversion efficiency into the S<p> order to be optimised at ~29% in H2, for a corresponding pump depletionof ~32%. For the most intense region of the XeCl laser pump beam only, these values were measured to be 56% and 60% respectively.
1987-07-01T00:00:00ZSinclair, Bruce DavidA XeCl excimer laser system whose output characteristics are suitable for stimulated Raman wavelength shifting, has been developed and used in single-pass conversion experiments in CH4 and H2. The overall system, comprising the laser, Raman focusing optics and Raman cell, has been designed with simplicity and system compactness in mind, suitable for a transportable lidar application. The XeCl laser is a fast discharge pumped device, driven by a C-C transfer circuit with a N2-filled spark gap switch. Various ultraviolet preionisation schemes and cavity component arrangements, employed during the laser development, are described and compared. A general overview of the basic spark preionisation techniques commonly used in rare-gas halide lasers is also presented. Over the course of the development, the available laser output power has been increased by a factor of 100, by methods described. Laser output characteristics relevant to stimulated Raman scattering (SRS), are the emission wavelength (308nm), the output pulse energy (>100mJ), pulse duration (~6.5ns FWHM), peak pulse power (>107 W) and the output beam divergence (U x 15 mrad). The design for a new laser, using a low inductance cavity geometry and an 'automatic-integral' preionisation scheme is described. The problems of poor laser beam quality and focusability which are inherent in rare-gas halide lasers, are overcome by the use of an unstable optical resonator. Experiments with a range of 'continuously-coupled' unstable resonators, in which the laser output coupler is a simple planoconvex lens, are reported. They show that pulse duration, energy and beam divergence are systematically reduced as the round-trip resonator magnification M is increased, with the result that the focal spot area can be up to 2000 times smaller than obtainable with conventional stable-cavity optics. Furthermore, these output diaracteristics are shown to depend not simply on the overall value of M, but also on the individual one-way magnifications M12 and M21 in the resonator, between mirrors 1 and 2. The dependence has produced significant experimental differences in resonators with very similar values of M. This new observation is attributed, in part, to the shortness of the laser pulse duration in this device. Experiments have been performed in high pressure CH4 andH2 to find the pumping geometries necessary to achieve SRS. The most unstable of the range cf resonators studied, having the least divergent output and the highest beam quality, is shown to be the best configuration for the application. In addition, it is found that pump focal power densities greater than 1010W/cm2 do not automatically guarantee coherent scattering if they have been produced as a result of tight focusing, and that a relatively long Raman focusing length (>~50cm) is a more fundamental experimental requirement. Raman scattering in CH4 gas has generated output lines from the first anti-Stokes (AS1) to the fourth Stokes (S4) order, at wavelengths of 283nm, 338nm, 375nm, 421nm and 480nm. The higher Raman gain in H2 has produced a wider range of Raman-shifted orders, extending from the ultraviolet into the infrared, at wavelengths given by 245nm (AS2), 273nm, 353nm, 4l4nm, 500nm, 631nm and (S5). The relative abundance of the competing SRS and four-wave mixing processes in the Raman cell, which determine the range and distribution of energy within the Raman spectrum, has been varied by reducing the feasibility of angular phase matching in the medium. Increasing the pump beam focal length and the Raman cell pressure has enabled the overall energy conversion efficiency into the S<p> order to be optimised at ~29% in H2, for a corresponding pump depletionof ~32%. For the most intense region of the XeCl laser pump beam only, these values were measured to be 56% and 60% respectively.Comparative studies of copper bromide lasersLittle, Laurahttps://hdl.handle.net/10023/138952019-04-01T10:03:32Z1998-06-01T00:00:00ZThis thesis reports the first comprehensive comparison of the operating regimes of the three major types of Cu halide laser, which oscillate on the 510.6 nm and 578.2 nm resonance-metastable transitions of atomic Cu in pulsed discharges at 10-50 kHz pulse recurrence frequency. The three lasers had similar active volumes (36.8-43.5 cm3) and bores (12.5-13 mm), were excited using the same power supply and circuit and monitored using the same diagnostic apparatus. The CuBr-Ne laser produced an annular output beam, weighted towards the yellow transition, with a maximum average output power of 3.55 W and a maximum efficiency of 0.71 %. When H2 gas was added to this laser at a level of ~5%, the output beam developed an axial (central) peak in intensity, the beam was less constricted, the balance of green and yellow powers was improved, the output power rose to a maximum of 11.4 W and the maximum efficiency reached 1.47 %. In both of these lasers, the CuBr vapour was generated by heating a sidearm of the discharge tube. The vapour was entrained in a flow of Ne buffer gas to seed the active volume. A Cu hybrid laser, where CuBr is generated in the tube in situ by reaction of the discharge products of a Ne-HBr buffer gas with Cu pieces in the tube, has been compared to the two conventional CuBr lasers. The Cu hybrid laser also produced an output beam with a central maximum, little or no constriction and a good balance of green and yellow powers. Maximum average output power reached 12.8 W and the maximum efficiency was 1.66%. In terms of specific average output power, the hybrid laser was clearly superior to the other two, with values of 82 mW.cm-3 (CuBr), 262 mW.cm-3 (CuBr-H2) and 348 mW.cm-3 (Cu hybrid). The specific output power of the Cu hybrid laser obtained in these studies is a record value for any Cu laser (including elemental Cu lasers) of tube bore ~12.5 mm. This result and the general dependences of output power on buffer gas pressure, additive (H2, HBr) pressure, pulse recurrence frequency and charging voltage and capacitances are discussed in detail in terms of the fundamental processes and chemical reactions. The most important processes responsible for the high powers and efficiencies and the Gaussian-like beam profiles in the presence of hydrogen are dissociative attachment of HBr in the interpulse period and at the beginning of the pulse, and the reduction of CuxBrx polymers and monomers by H2 to free Cu atoms in the active volume. This is the first time that the importance of hydrogen reduction in these lasers has been identified. Without it, the filling in of the annular output beam cannot be explained. The mechanism of Cu seeding of the hybrid laser has also been studied in detail, as it is the most obvious difference between the Cu hybrid and conventional CuBr lasers. The basic reactions of the seeding process are described, and it is found that in addition to Cu3Br3 and Cu4Br4 polymers there must be a substantial amount of CuH in the discharge to account for the large density of Cu atoms in free form and locked up in molecular forms. This is the first time that CuH has been suggested as a major Cu-bearing species. The process of Cu dendrite formation in the tube is also discussed. Finally, the properties of the hybrid laser have been considered from the point of view of scaling to very high average output powers. It has been shown that average output powers of 1 kW are possible using current technology.
1998-06-01T00:00:00ZLittle, LauraThis thesis reports the first comprehensive comparison of the operating regimes of the three major types of Cu halide laser, which oscillate on the 510.6 nm and 578.2 nm resonance-metastable transitions of atomic Cu in pulsed discharges at 10-50 kHz pulse recurrence frequency. The three lasers had similar active volumes (36.8-43.5 cm3) and bores (12.5-13 mm), were excited using the same power supply and circuit and monitored using the same diagnostic apparatus. The CuBr-Ne laser produced an annular output beam, weighted towards the yellow transition, with a maximum average output power of 3.55 W and a maximum efficiency of 0.71 %. When H2 gas was added to this laser at a level of ~5%, the output beam developed an axial (central) peak in intensity, the beam was less constricted, the balance of green and yellow powers was improved, the output power rose to a maximum of 11.4 W and the maximum efficiency reached 1.47 %. In both of these lasers, the CuBr vapour was generated by heating a sidearm of the discharge tube. The vapour was entrained in a flow of Ne buffer gas to seed the active volume. A Cu hybrid laser, where CuBr is generated in the tube in situ by reaction of the discharge products of a Ne-HBr buffer gas with Cu pieces in the tube, has been compared to the two conventional CuBr lasers. The Cu hybrid laser also produced an output beam with a central maximum, little or no constriction and a good balance of green and yellow powers. Maximum average output power reached 12.8 W and the maximum efficiency was 1.66%. In terms of specific average output power, the hybrid laser was clearly superior to the other two, with values of 82 mW.cm-3 (CuBr), 262 mW.cm-3 (CuBr-H2) and 348 mW.cm-3 (Cu hybrid). The specific output power of the Cu hybrid laser obtained in these studies is a record value for any Cu laser (including elemental Cu lasers) of tube bore ~12.5 mm. This result and the general dependences of output power on buffer gas pressure, additive (H2, HBr) pressure, pulse recurrence frequency and charging voltage and capacitances are discussed in detail in terms of the fundamental processes and chemical reactions. The most important processes responsible for the high powers and efficiencies and the Gaussian-like beam profiles in the presence of hydrogen are dissociative attachment of HBr in the interpulse period and at the beginning of the pulse, and the reduction of CuxBrx polymers and monomers by H2 to free Cu atoms in the active volume. This is the first time that the importance of hydrogen reduction in these lasers has been identified. Without it, the filling in of the annular output beam cannot be explained. The mechanism of Cu seeding of the hybrid laser has also been studied in detail, as it is the most obvious difference between the Cu hybrid and conventional CuBr lasers. The basic reactions of the seeding process are described, and it is found that in addition to Cu3Br3 and Cu4Br4 polymers there must be a substantial amount of CuH in the discharge to account for the large density of Cu atoms in free form and locked up in molecular forms. This is the first time that CuH has been suggested as a major Cu-bearing species. The process of Cu dendrite formation in the tube is also discussed. Finally, the properties of the hybrid laser have been considered from the point of view of scaling to very high average output powers. It has been shown that average output powers of 1 kW are possible using current technology.Studies of copper halide lasersLivingstone, E.S.https://hdl.handle.net/10023/138942019-04-01T10:08:34Z1992-07-01T00:00:00ZCopper Halide lasers are discussed and the results of experiments reported. It is found that the presence of small quantities of an electron attaching gas (such as bromine) cause discharge instability. Specially designed electrodes which remove excess bromine cure this problem and yield a stable discharge. A 4W copper bromide laser is operated, sealed-off, for 100 hours. This laser has an apertured discharge tube with side-arm reservoirs to control copper bromide vapour pressure, a feature essential to stable operation. The addition of small amounts of hydrogen changes the beam from an annular to a gaussian-like profile. The estimated lifetime of this laser tube is 1,000 hours. Exploiting the theory of metallic walls for discharge confinement, it is found that metal segments shorter than about 1m can support a stable discharge at high pulse repetition rates (5 - 20kHz). On the basis of this, a novel copper halide laser containing cylindrical copper segments is demonstrated. Neon and halogen gases flow through the tube. The reaction between the halogen and the copper segment walls forms copper halide in-situ. Hydrogen bromide, bromine and chlorine have been used. Hydrogen bromide proves to be the most suitable. A one metre long laser tube of this design produces 40W. Pulsed power supplies for metal vapour lasers are discussed. The conventional form of the capacitor-transfer circuit has the peaking capacitor value around one half of the storage capacitor value. It is found that equal capacitor values produce the best results for the gold vapour laser system we describe. The reliability of a gold vapour laser is improved by replacing the hollow anode thyratron (which has a high latch rate) with a solid anode thyratron. The replacement thyratron, in combination with a saturating charging choke, significantly reduces the latch rate. The laser is used for studies of photodynamic therapy of cancer in a local hospital.
1992-07-01T00:00:00ZLivingstone, E.S.Copper Halide lasers are discussed and the results of experiments reported. It is found that the presence of small quantities of an electron attaching gas (such as bromine) cause discharge instability. Specially designed electrodes which remove excess bromine cure this problem and yield a stable discharge. A 4W copper bromide laser is operated, sealed-off, for 100 hours. This laser has an apertured discharge tube with side-arm reservoirs to control copper bromide vapour pressure, a feature essential to stable operation. The addition of small amounts of hydrogen changes the beam from an annular to a gaussian-like profile. The estimated lifetime of this laser tube is 1,000 hours. Exploiting the theory of metallic walls for discharge confinement, it is found that metal segments shorter than about 1m can support a stable discharge at high pulse repetition rates (5 - 20kHz). On the basis of this, a novel copper halide laser containing cylindrical copper segments is demonstrated. Neon and halogen gases flow through the tube. The reaction between the halogen and the copper segment walls forms copper halide in-situ. Hydrogen bromide, bromine and chlorine have been used. Hydrogen bromide proves to be the most suitable. A one metre long laser tube of this design produces 40W. Pulsed power supplies for metal vapour lasers are discussed. The conventional form of the capacitor-transfer circuit has the peaking capacitor value around one half of the storage capacitor value. It is found that equal capacitor values produce the best results for the gold vapour laser system we describe. The reliability of a gold vapour laser is improved by replacing the hollow anode thyratron (which has a high latch rate) with a solid anode thyratron. The replacement thyratron, in combination with a saturating charging choke, significantly reduces the latch rate. The laser is used for studies of photodynamic therapy of cancer in a local hospital.Gas contamination in discharge excited XeF excimer lasersDuval, A.B.https://hdl.handle.net/10023/138932019-04-01T10:10:32Z1984-07-01T00:00:00ZInfrared, ultraviolet and mass spectrometric techniques have been used to investigate the short gas-life of discharge excited XeF lasers, for He/Xe/NF3 mixtures. Infrared absorption studies provided initial information on the changes which occur in the laser gas composition during pulsing. The information was used to complement those of mass spectrometric studies, in which the chemical composition of laser gas mixtures were determined as a function of the number of laser pulses. Ultraviolet absorption spectroscopy was used to study optical absorption at the laser wavelength in fresh and used gas mixtures. The effects of several contaminants on laser performance were studied by adding small concentrations of these contaminants to the basic gas mixture of He/Xe/NF3. The results provided information on the identity of the main contaminants. Cold traps were used to extend the gas-life, and to identify dominant contaminants. The laser device used in this work is excited by a conventional blumlein circuit, which is triggered by a pressurised spark-gap switch. For a single gas fill of the basic mixture (He/Xe/NF3), the number of laser pulses to half-energy is approximately 150/litre atm. Infrared and mass spectrometric studies show that the fast deterioration of laser performance is due to the depletion of NF3, and to the accumulation of contaminants in the laser. The contaminants have been identified as N2, O2, NO, NO2, N2O, CO, CO2, NF2, N2F2, HF and CF4. Of these, NO2 absorbs at the laser wavelength (351nm), but the absorption coefficient in used gas mixtures is small compared to small signal gains of laser devices similar to the one used in this work. There is strong evidence that water vapour, which is one of the main impurities in fresh gas mixtures, may be the source of oxygen in the formation of oxides of nitrogen (NO, NO2, N2O) and carbon (CO, CO2) For fresh gas mixtures, the laser pulse energy is insensitive to the addition of small concentrations of N2, H2 and CF4. In contrast, the addition of 0.05% of CO2, CO and O2 results in approximately 60, MO and 20% reductions in the laser pulse energy respectively. The estimated change in laser output after 1000 shots due to the accumulation of CO2, O2 and CO is 20, 10 and 5% respectively. The addition of small concentrations ( < 1 torr) of N2 CO2, CO, O2 and H2 results in negligible changes in the gas-life. However, when 2 torr of CF4 is added to the basic mixture of He/Xe/NF3 a threefold increase in the gas-life is observed. The improved gas-life is attributed to lower rates of formation of O2, NO2 and NO. After using He/Xe/NF3/H2 mixtures, the gas-life of the basic mixture increased by a factor of five to about 700 shots/litre atm. Mass spectrometric analysis of the gas mixture before and after lasing shows that the improvement in the gas-life is mainly due to lower levels of O2, NO2 and NO, and to a significant reduction in the rate of depletion of NF3. The eventual deterioration of laser performance is mainly attributed to the accumulation of CO and CO2 in the laser. For the laser device and gas mixture used in this work, the optimum trap temperature lies in the region of -100 to -150°C. For a trap temperature of -150°C, the gas-life is 1500 shots/litre atm for a single gas fill. This is about 2.5 times the best result obtained without the use of cold traps. The eventual termination of laser action is due to NF3 depletion and the accumulation of contaminants in the laser. By boiling-off the contaminants sequentially, CO2 has been identified as a major contaminant.
1984-07-01T00:00:00ZDuval, A.B.Infrared, ultraviolet and mass spectrometric techniques have been used to investigate the short gas-life of discharge excited XeF lasers, for He/Xe/NF3 mixtures. Infrared absorption studies provided initial information on the changes which occur in the laser gas composition during pulsing. The information was used to complement those of mass spectrometric studies, in which the chemical composition of laser gas mixtures were determined as a function of the number of laser pulses. Ultraviolet absorption spectroscopy was used to study optical absorption at the laser wavelength in fresh and used gas mixtures. The effects of several contaminants on laser performance were studied by adding small concentrations of these contaminants to the basic gas mixture of He/Xe/NF3. The results provided information on the identity of the main contaminants. Cold traps were used to extend the gas-life, and to identify dominant contaminants. The laser device used in this work is excited by a conventional blumlein circuit, which is triggered by a pressurised spark-gap switch. For a single gas fill of the basic mixture (He/Xe/NF3), the number of laser pulses to half-energy is approximately 150/litre atm. Infrared and mass spectrometric studies show that the fast deterioration of laser performance is due to the depletion of NF3, and to the accumulation of contaminants in the laser. The contaminants have been identified as N2, O2, NO, NO2, N2O, CO, CO2, NF2, N2F2, HF and CF4. Of these, NO2 absorbs at the laser wavelength (351nm), but the absorption coefficient in used gas mixtures is small compared to small signal gains of laser devices similar to the one used in this work. There is strong evidence that water vapour, which is one of the main impurities in fresh gas mixtures, may be the source of oxygen in the formation of oxides of nitrogen (NO, NO2, N2O) and carbon (CO, CO2) For fresh gas mixtures, the laser pulse energy is insensitive to the addition of small concentrations of N2, H2 and CF4. In contrast, the addition of 0.05% of CO2, CO and O2 results in approximately 60, MO and 20% reductions in the laser pulse energy respectively. The estimated change in laser output after 1000 shots due to the accumulation of CO2, O2 and CO is 20, 10 and 5% respectively. The addition of small concentrations ( < 1 torr) of N2 CO2, CO, O2 and H2 results in negligible changes in the gas-life. However, when 2 torr of CF4 is added to the basic mixture of He/Xe/NF3 a threefold increase in the gas-life is observed. The improved gas-life is attributed to lower rates of formation of O2, NO2 and NO. After using He/Xe/NF3/H2 mixtures, the gas-life of the basic mixture increased by a factor of five to about 700 shots/litre atm. Mass spectrometric analysis of the gas mixture before and after lasing shows that the improvement in the gas-life is mainly due to lower levels of O2, NO2 and NO, and to a significant reduction in the rate of depletion of NF3. The eventual deterioration of laser performance is mainly attributed to the accumulation of CO and CO2 in the laser. For the laser device and gas mixture used in this work, the optimum trap temperature lies in the region of -100 to -150°C. For a trap temperature of -150°C, the gas-life is 1500 shots/litre atm for a single gas fill. This is about 2.5 times the best result obtained without the use of cold traps. The eventual termination of laser action is due to NF3 depletion and the accumulation of contaminants in the laser. By boiling-off the contaminants sequentially, CO2 has been identified as a major contaminant.Investigations into improving the performance of discharge-pumped rare-gas-halide excimer lasersFairlie, S.A.https://hdl.handle.net/10023/138922019-04-01T10:04:08Z1993-07-01T00:00:00ZThe construction and operation of a small active volume, discharge pumped, rare gas halide excimer laser is described. The values of laser output parameters such as pulse duration, pulse energy and overall efficiency presently achieved with such systems are much less that theoretical studies predict. The performance of this laser when pumped using a novel pulsed power modulator design containing nonlinear capacitors to produce a very fast rise time voltage pulse is contrasted with the performance obtained from a conventional driver circuit. The purpose of this was to determine if such circuits could lead to improvements in glow discharge stability and also laser pumping efficiency by quickly achieving the optimum pumping rates predicted by theory. It was concluded that while the rapid establishment of optimum pumping conditions may be beneficial, too fast a rate of rise of discharge current appears to be detrimental to discharge stability, probably due to skin effects. Having established that premature glow discharge collapse is a serious limiting factor in producing long duration excimer laser pulses, a study is carried out of the factors believed to influence discharge stability. While the effects of halogen donors on discharge stability have received most attention in the past the part played by the other constituents of the laser gas mix tends to have been neglected. A theoretical and experimental study of the role of the rare gas partners, xenon, krypton and argon is presented. It is well known that gas mixes using helium as the buffer gas perform less well than with a neon buffer and this is attributed to the driving of discharge instabilities rather than kinetic factors. A comparison of the relative influences of the buffer gases helium, neon and argon on discharge stability is carried out and claims by other workers for improved laser performance using a mixed helium / argon buffer are tested. Finally, in an attempt to inhibit the mechanisms driving glow discharge collapse, the effects of externally applied magnetic fields on discharge stability and laser performance are investigated.
1993-07-01T00:00:00ZFairlie, S.A.The construction and operation of a small active volume, discharge pumped, rare gas halide excimer laser is described. The values of laser output parameters such as pulse duration, pulse energy and overall efficiency presently achieved with such systems are much less that theoretical studies predict. The performance of this laser when pumped using a novel pulsed power modulator design containing nonlinear capacitors to produce a very fast rise time voltage pulse is contrasted with the performance obtained from a conventional driver circuit. The purpose of this was to determine if such circuits could lead to improvements in glow discharge stability and also laser pumping efficiency by quickly achieving the optimum pumping rates predicted by theory. It was concluded that while the rapid establishment of optimum pumping conditions may be beneficial, too fast a rate of rise of discharge current appears to be detrimental to discharge stability, probably due to skin effects. Having established that premature glow discharge collapse is a serious limiting factor in producing long duration excimer laser pulses, a study is carried out of the factors believed to influence discharge stability. While the effects of halogen donors on discharge stability have received most attention in the past the part played by the other constituents of the laser gas mix tends to have been neglected. A theoretical and experimental study of the role of the rare gas partners, xenon, krypton and argon is presented. It is well known that gas mixes using helium as the buffer gas perform less well than with a neon buffer and this is attributed to the driving of discharge instabilities rather than kinetic factors. A comparison of the relative influences of the buffer gases helium, neon and argon on discharge stability is carried out and claims by other workers for improved laser performance using a mixed helium / argon buffer are tested. Finally, in an attempt to inhibit the mechanisms driving glow discharge collapse, the effects of externally applied magnetic fields on discharge stability and laser performance are investigated.Gas contamination in discharge excited KrF and atomic fluorine lasersGovindanunny, T.https://hdl.handle.net/10023/138912019-11-05T15:08:34Z1984-07-01T00:00:00ZThis thesis deals with gas contamination problems in atomic fluorine and KrF lasers. Five different versions of transverse electric discharge lasers were constructed using different materials, geometries, and discharge circuits. Three of these were investigated in detail for lifetime performance of a single fill of He-F2 mix (atomic fluorine laser) and of He-Kr-F2 mix (KrF laser). This was done using an on-line quadrupole mass spectrometer. The evolution of the various gas components appearing in the mix was calculated from the mass spectra of the gas mixture recorded at intervals when the laser was operated at one pulse per second. The impurity or contaminant species (ie species other than He, Kr, F2) were found to be the same in all the lasers, differing only in their concentrations. The major contaminants found were CO2, N2, O2, COF2, CF2, SiF4, HF, CO, NO, SF6, and H2O. Of these, CO2 was identified as the most deleterious impurity for both atomic fluorine and KrF lasers, reducing their output energy and the fluorine content, thereby reducing the single fill lifetime of the mix. A simple cold trap decreased the partial pressures of most of the detected impurities in the gas mixture and resulted in a marked increase in the number of shots to half energy and a decrease in the depletion rate of fluorine by half in both KrF and atomic fluorine lasers. To isolate and quantify the effects of individual contaminant species, they were deliberately added singly to the pure gas mix. These experiments confirmed that CO2 was the most important impurity and that H2 and CF4 were the least harmful. Since the impurities were found to influence arcing of the discharge, they must affect preionization and/or discharge processes. Absorption effects were found to be insignificant in the regions of the laser wavelengths (248 nm, 730 nm). In order to quantify the effect of the impurities an effectiveness constant for each contaminant has been defined and used in a simple model which successfully calculates the laser output energy. This model has circumvented the problems arising from the physical complexity of the system and the lack of data on the various kinetic processes. The calculations show that CO2 impurity depletes the energy output of the KrF laser at the rate of 0.9 mJ/ppt(wrt He).
1984-07-01T00:00:00ZGovindanunny, T.This thesis deals with gas contamination problems in atomic fluorine and KrF lasers. Five different versions of transverse electric discharge lasers were constructed using different materials, geometries, and discharge circuits. Three of these were investigated in detail for lifetime performance of a single fill of He-F2 mix (atomic fluorine laser) and of He-Kr-F2 mix (KrF laser). This was done using an on-line quadrupole mass spectrometer. The evolution of the various gas components appearing in the mix was calculated from the mass spectra of the gas mixture recorded at intervals when the laser was operated at one pulse per second. The impurity or contaminant species (ie species other than He, Kr, F2) were found to be the same in all the lasers, differing only in their concentrations. The major contaminants found were CO2, N2, O2, COF2, CF2, SiF4, HF, CO, NO, SF6, and H2O. Of these, CO2 was identified as the most deleterious impurity for both atomic fluorine and KrF lasers, reducing their output energy and the fluorine content, thereby reducing the single fill lifetime of the mix. A simple cold trap decreased the partial pressures of most of the detected impurities in the gas mixture and resulted in a marked increase in the number of shots to half energy and a decrease in the depletion rate of fluorine by half in both KrF and atomic fluorine lasers. To isolate and quantify the effects of individual contaminant species, they were deliberately added singly to the pure gas mix. These experiments confirmed that CO2 was the most important impurity and that H2 and CF4 were the least harmful. Since the impurities were found to influence arcing of the discharge, they must affect preionization and/or discharge processes. Absorption effects were found to be insignificant in the regions of the laser wavelengths (248 nm, 730 nm). In order to quantify the effect of the impurities an effectiveness constant for each contaminant has been defined and used in a simple model which successfully calculates the laser output energy. This model has circumvented the problems arising from the physical complexity of the system and the lack of data on the various kinetic processes. The calculations show that CO2 impurity depletes the energy output of the KrF laser at the rate of 0.9 mJ/ppt(wrt He).Embedding problems for lie algebras in elementary particle physicsEkins, Judith M.https://hdl.handle.net/10023/138192019-04-01T10:04:57Z1973-01-01T00:00:00Z1973-01-01T00:00:00ZEkins, Judith M.Ultrashort pulse generation and synchronisation in self-modelocked vibronic lasersEvans, Jonathan Michaelhttps://hdl.handle.net/10023/138092018-06-11T08:58:50Z1994-07-01T00:00:00ZThis thesis is concerned with the generation of ultrashort pulses from vibronic lasers. The two laser active materials used in the course of the work were Ti:sapphire and Cr:LiSAF. A self-modelocked Ti:sapphire laser has been described which generated pulses as short as 2ps, tunable over the wavelength range 730 - 850nm. The average output power was 400mW corresponding to a peak pulse power of 1kW. Using a prism sequence to implement intracavity dispersion-compensation resulted in the generation of near-transform limited pulses as short as 53fs with a peak pulse power of ~100kW. Two initiation techniques have been developed for the generally non-self-starting self- modelocking process, based upon intracavity insertion of either a regeneratively driven acousto-optic modulator or a solid-state saturable absorber. A cw Cr:LiSAF laser pumped by the 476.5nm line of the argon-ion laser output, was demonstrated; this generated a maximum output power of 300mW with a slope efficiency of 20% at 825nm. A dispersion-compensated self-modelocked Cr:LiSAF laser has been described that generated pulses as short as 45fs over the tuning range 770-910nm. The peak pulse power generated was 40kW. The phase noise of a modelocked Ti: sapphire laser has been reduced by referencing the cavity frequency to an ultrastable crystal oscillator. The phase noise of the frequency locked laser was 410fs (100-500Hz), 305fs (500Hz-5kHz) and 263fs (5-50kHz). By referencing two modelocked Ti:sapphire lasers to a common crystal oscillator two independently tunable pulse sequences with a relative timing jitter of ~1ps have been generated. A novel laser based upon a single Ti:sapphire gain element generating synchronised pulses at two different wavelengths has been demonstrated. Cross-correlation data recorded between the two output pulse sequences indicated a relative timing jitter of 26fs.
1994-07-01T00:00:00ZEvans, Jonathan MichaelThis thesis is concerned with the generation of ultrashort pulses from vibronic lasers. The two laser active materials used in the course of the work were Ti:sapphire and Cr:LiSAF. A self-modelocked Ti:sapphire laser has been described which generated pulses as short as 2ps, tunable over the wavelength range 730 - 850nm. The average output power was 400mW corresponding to a peak pulse power of 1kW. Using a prism sequence to implement intracavity dispersion-compensation resulted in the generation of near-transform limited pulses as short as 53fs with a peak pulse power of ~100kW. Two initiation techniques have been developed for the generally non-self-starting self- modelocking process, based upon intracavity insertion of either a regeneratively driven acousto-optic modulator or a solid-state saturable absorber. A cw Cr:LiSAF laser pumped by the 476.5nm line of the argon-ion laser output, was demonstrated; this generated a maximum output power of 300mW with a slope efficiency of 20% at 825nm. A dispersion-compensated self-modelocked Cr:LiSAF laser has been described that generated pulses as short as 45fs over the tuning range 770-910nm. The peak pulse power generated was 40kW. The phase noise of a modelocked Ti: sapphire laser has been reduced by referencing the cavity frequency to an ultrastable crystal oscillator. The phase noise of the frequency locked laser was 410fs (100-500Hz), 305fs (500Hz-5kHz) and 263fs (5-50kHz). By referencing two modelocked Ti:sapphire lasers to a common crystal oscillator two independently tunable pulse sequences with a relative timing jitter of ~1ps have been generated. A novel laser based upon a single Ti:sapphire gain element generating synchronised pulses at two different wavelengths has been demonstrated. Cross-correlation data recorded between the two output pulse sequences indicated a relative timing jitter of 26fs.Oscillation phenomena in argon laser dischargesRoss, John Neilhttps://hdl.handle.net/10023/138072019-04-01T10:06:28Z1974-07-01T00:00:00ZIn this thesis instabilities which occur in the plasma tube of an argon ion laser are examined. Coherent oscillations in the discharge potential across the plasma tube are found to be a usual feature of an argon laser. The light output of the laser is shown to be modulated by these oscillations only if the discharge current is modulated. It is found that these oscillations are due to instabilities in the anode zone of the gas discharge. The oscillations occurring within the anode zone of an argon laser have been studied using optical and Langmiuir probe techniques. The oscillations are discussed in terms of a phenomenological model due to Pupp. Many of the observed features of the oscillations can be described in terms of this model. The values of the anode fall that have been observed in the argon laser are substantially less than would be expected on the basis of earlier work on glow discharges in argon. this is discussed and an estimate of the anode fall is made which is closer to the observed value than that predicted by Von Engel's theory of the anode fall. A simple mathematical model of the anode zone is presented and it is shown that under the conditions appropriate to an argon laser the anode fall is not expected to be stable. These instabilities in the anode zone of the plasma tube, while they appear to be a normal feature of the argon laser discharge, do not necessarily modulate the laser light. It is shown that if the power supply for the laser has a high output impedance at the frequency of the oscillations then the modulation of the light may be reduced to a negligible level.
1974-07-01T00:00:00ZRoss, John NeilIn this thesis instabilities which occur in the plasma tube of an argon ion laser are examined. Coherent oscillations in the discharge potential across the plasma tube are found to be a usual feature of an argon laser. The light output of the laser is shown to be modulated by these oscillations only if the discharge current is modulated. It is found that these oscillations are due to instabilities in the anode zone of the gas discharge. The oscillations occurring within the anode zone of an argon laser have been studied using optical and Langmiuir probe techniques. The oscillations are discussed in terms of a phenomenological model due to Pupp. Many of the observed features of the oscillations can be described in terms of this model. The values of the anode fall that have been observed in the argon laser are substantially less than would be expected on the basis of earlier work on glow discharges in argon. this is discussed and an estimate of the anode fall is made which is closer to the observed value than that predicted by Von Engel's theory of the anode fall. A simple mathematical model of the anode zone is presented and it is shown that under the conditions appropriate to an argon laser the anode fall is not expected to be stable. These instabilities in the anode zone of the plasma tube, while they appear to be a normal feature of the argon laser discharge, do not necessarily modulate the laser light. It is shown that if the power supply for the laser has a high output impedance at the frequency of the oscillations then the modulation of the light may be reduced to a negligible level.Ultrafast optical nonlinearities in InGaAsP waveguide devicesRoberts, Peter D.https://hdl.handle.net/10023/138062019-04-01T10:09:07Z1996-07-01T00:00:00ZThis thesis presents an investigation of the enhanced intensity-dependent refractive nonlinearity in optical amplifiers biased to transparency. Nonlinearities in an optical amplifier with a bulk active region and in optical amplifiers containing four, eight and sixteen quantum wells were compared using picosecond and femtosecond duration pulses generated using colour-centre lasers. Measurements of nonlinear absorptive and refractive dynamics in the amplifiers were performed using pump-probe and time-division interferometry techniques. A curve fitting function was used to distinguish nonlinearities from the measured dynamics. Carrier-heating, caused predominantly by free-carrier absorption, and virtual effects were found to be the most important nonlinearities in optical amplifiers biased to transparency. A strong correlation of the magnitude of the carrier-heating effect with the thickness of the amplifier active regions was observed. A novel measurement of the intensity dependence of the current required to bias optical amplifiers to transparency was performed using a technique which monitored the opto-electronic voltage across the amplifiers. The measurement showed that the transparency current increased linearly with intensity in the 4 QW and 8 QW amplifiers as a result of carrier-heating. Measurements performed on the 16 QW and bulk amplifiers showed a nonlinear variation of transparency current with intensity. These measurements were supplemented with a pump-probe investigation which revealed a negative trend in the transmission with a time constant in excess of 200 ps. It was suggested that a saturation effect related to the amount of heat added to the carrier distribution through free-carrier absorption was responsible for both of these effects. All-optical switching of picosecond pulses via the investigated enhanced nonlinearity was demonstrated in a polarisation switch, constructed using the bulk amplifier, and a nonlinear directional coupler. The optical powers required to perform optical switching were 1.6 W and 5.8 W respectively.
1996-07-01T00:00:00ZRoberts, Peter D.This thesis presents an investigation of the enhanced intensity-dependent refractive nonlinearity in optical amplifiers biased to transparency. Nonlinearities in an optical amplifier with a bulk active region and in optical amplifiers containing four, eight and sixteen quantum wells were compared using picosecond and femtosecond duration pulses generated using colour-centre lasers. Measurements of nonlinear absorptive and refractive dynamics in the amplifiers were performed using pump-probe and time-division interferometry techniques. A curve fitting function was used to distinguish nonlinearities from the measured dynamics. Carrier-heating, caused predominantly by free-carrier absorption, and virtual effects were found to be the most important nonlinearities in optical amplifiers biased to transparency. A strong correlation of the magnitude of the carrier-heating effect with the thickness of the amplifier active regions was observed. A novel measurement of the intensity dependence of the current required to bias optical amplifiers to transparency was performed using a technique which monitored the opto-electronic voltage across the amplifiers. The measurement showed that the transparency current increased linearly with intensity in the 4 QW and 8 QW amplifiers as a result of carrier-heating. Measurements performed on the 16 QW and bulk amplifiers showed a nonlinear variation of transparency current with intensity. These measurements were supplemented with a pump-probe investigation which revealed a negative trend in the transmission with a time constant in excess of 200 ps. It was suggested that a saturation effect related to the amount of heat added to the carrier distribution through free-carrier absorption was responsible for both of these effects. All-optical switching of picosecond pulses via the investigated enhanced nonlinearity was demonstrated in a polarisation switch, constructed using the bulk amplifier, and a nonlinear directional coupler. The optical powers required to perform optical switching were 1.6 W and 5.8 W respectively.Modelocked vibronic lasers for the 700nm - 1000nm regionSpence, David E.https://hdl.handle.net/10023/138052019-04-01T10:04:11Z1993-07-01T00:00:00ZThe work in this thesis is concerned with the characterisation and development of modelocked solid-state lasers covering the 700 - 1000 nm region. Results are presented for a passively modelocked LiF:F+2 colour-centre laser, however, most of the work has concentrated on the Ti: sapphire (Ti:Al2O3) laser. In chapter 2, the operation of a cw LiF: F+2 colour-centre laser is discussed. This laser was passively modelocked using the saturable absorber dye, DaQTeC and pulses as short as 170 fs were generated with average output powers of ~10 mW. Pulses as short as 127 fs were generated in a dispersion compensated, colliding-pulse modelocked geometry over a wavelength range of 925 - 950 nm. A nonlinear external cavity was added to the basic laser configuration in an attempt to extend the modelocked tuning range and the saturable absorber dye lifetime. The technique of coupled-cavity modelocking was applied to a Ti:Al2O3 laser and enabled pulses as short as 1.3 ps to be generated. These pulses were frequency chirped and could be directly compressed to 290 fs outside the laser. By using the technique of intracavity dispersion compensation in both the main and coupled cavities, pulses as short as 90 fs were generated, having average powers of ~200 mW and peak powers of more than 20 kW. The simpler technique of self-modelocking is described in chapter 5 and allowed the generation of pulses as short as 60 fs from a dispersion compensated cavity configuration. Average output powers of ~600 mW were measured, which corresponded to peak powers of 110 kW. This laser had a modelocked tuning range which spanned the 750 - 950 nm region. Using fibre/prism pulse compression techniques pulses as short as 45 fs were produced. In chapter 6, the measurement and suppression of phase noise on the self-modelocked Ti:Al2O3 laser are discussed. The technique for noise reduction was also applied to two similar self-modelocked lasers in an attempt to synchronise the two laser pulse sequences.
1993-07-01T00:00:00ZSpence, David E.The work in this thesis is concerned with the characterisation and development of modelocked solid-state lasers covering the 700 - 1000 nm region. Results are presented for a passively modelocked LiF:F+2 colour-centre laser, however, most of the work has concentrated on the Ti: sapphire (Ti:Al2O3) laser. In chapter 2, the operation of a cw LiF: F+2 colour-centre laser is discussed. This laser was passively modelocked using the saturable absorber dye, DaQTeC and pulses as short as 170 fs were generated with average output powers of ~10 mW. Pulses as short as 127 fs were generated in a dispersion compensated, colliding-pulse modelocked geometry over a wavelength range of 925 - 950 nm. A nonlinear external cavity was added to the basic laser configuration in an attempt to extend the modelocked tuning range and the saturable absorber dye lifetime. The technique of coupled-cavity modelocking was applied to a Ti:Al2O3 laser and enabled pulses as short as 1.3 ps to be generated. These pulses were frequency chirped and could be directly compressed to 290 fs outside the laser. By using the technique of intracavity dispersion compensation in both the main and coupled cavities, pulses as short as 90 fs were generated, having average powers of ~200 mW and peak powers of more than 20 kW. The simpler technique of self-modelocking is described in chapter 5 and allowed the generation of pulses as short as 60 fs from a dispersion compensated cavity configuration. Average output powers of ~600 mW were measured, which corresponded to peak powers of 110 kW. This laser had a modelocked tuning range which spanned the 750 - 950 nm region. Using fibre/prism pulse compression techniques pulses as short as 45 fs were produced. In chapter 6, the measurement and suppression of phase noise on the self-modelocked Ti:Al2O3 laser are discussed. The technique for noise reduction was also applied to two similar self-modelocked lasers in an attempt to synchronise the two laser pulse sequences.Ultra short laser pulses and coupled-cavity mode locking in a KCI:TI colour-centre laserZhu, Xiaononghttps://hdl.handle.net/10023/138042019-04-01T10:04:55Z1991-07-01T00:00:00ZThe basic principles of generation, characterisation and propagation of ultrashort laser pulses and related experimental studies of mode locking of a coupled-cavity KC1:Ti colour-centre laser are presented. Extensive experimental investigations of mode locking with a two-cavity arrangement similar to a previously reported soliton laser has indicated that such a scheme is a very general technique for the generation of picosecond- femtosecond pulses. With or without the formation of optical solitons in the control fibre a variety of spectral and temporal features for the laser output pulses can be obtained. This new type of pulse generation technique has thus been named as "coupled-cavity mode locking" and it has been shown in this work that the coupled-cavity mode-locked KC1:Ti colour-centre laser is a very useful femtosecond laser source. Through replacing the traditional electron-optical streak camera tube by an optical Kerr material and using the optical (pulse) beam to scan the pulse(s) to be measured, a novel pulse measurement method have been proposed. For this new measurement scheme no electron-photon transformation and any other electronic systems are involved, and therefore it is expected to have the advantages of easy operation and high resolution. Study of the relationship between the temporal and the spectral features of an optical field suggests that a Fourier transform of a first-order spectral autocorrelation of a femtosecond pulse will give rise to the intensity profile of the pulse. A schematic for the measurement of the first-order spectral autocorrelation is also configurated. A strong similarity between the impact of dispersion on the temporal feature and that of self-phase modulation on the spectral feature of optical pulses has been found. This leads to two important inferences: first, the spectral extension induced by self-phase modulation is linearly proportional to the peak optical intensity only when the associated nonlinear phase shift is sufficiently large, second, if the incident pulses are appropriately chirped a suitable length of SPM-medium can be used to compress the incident spectrum (rather than broaden it). To initiate coupled-cavity mode locking a minimum optical power coupled into the control cavity is required. The specific value of this threshold is not only dependent on the properties of the nonlinear medium such as the length of the fibre, the magnitude of the Kerr coefficient, but also on the main cavity parameters, e. g. the main cavity power level and the effective gain bandwidth. Such a fact simply illustrates that to understand the physical mechanisms involved in the coupled-cavity mode locking the interactive function of various factors in both cavities must be considered. The upper-limit of the nonlinear phase shift, phiNL, experienced by the pulses coupled into the control cavity is found to be phiNL ≤ 2TC. Above this value, further increase of the intrafibre power would lead to the laser operation in an unstable regime. In addition, if the peak optical intensity inside the fibre core is sufficiently high the other nonlinear effects such as modulational instability and self-Raman action can occur besides self-phase modulation. In these cases the process of mode locking enhancement may be frustrated. By incorporating an Er-doped mono-mode optical fibre in the control cavity of such a coupled-cavity mode-locked KCl:Ti laser has led to the generation of 75 fs pulses at 1.5 mum. This result represents the shortest pulses produced with this method in a non-soliton regime.
1991-07-01T00:00:00ZZhu, XiaonongThe basic principles of generation, characterisation and propagation of ultrashort laser pulses and related experimental studies of mode locking of a coupled-cavity KC1:Ti colour-centre laser are presented. Extensive experimental investigations of mode locking with a two-cavity arrangement similar to a previously reported soliton laser has indicated that such a scheme is a very general technique for the generation of picosecond- femtosecond pulses. With or without the formation of optical solitons in the control fibre a variety of spectral and temporal features for the laser output pulses can be obtained. This new type of pulse generation technique has thus been named as "coupled-cavity mode locking" and it has been shown in this work that the coupled-cavity mode-locked KC1:Ti colour-centre laser is a very useful femtosecond laser source. Through replacing the traditional electron-optical streak camera tube by an optical Kerr material and using the optical (pulse) beam to scan the pulse(s) to be measured, a novel pulse measurement method have been proposed. For this new measurement scheme no electron-photon transformation and any other electronic systems are involved, and therefore it is expected to have the advantages of easy operation and high resolution. Study of the relationship between the temporal and the spectral features of an optical field suggests that a Fourier transform of a first-order spectral autocorrelation of a femtosecond pulse will give rise to the intensity profile of the pulse. A schematic for the measurement of the first-order spectral autocorrelation is also configurated. A strong similarity between the impact of dispersion on the temporal feature and that of self-phase modulation on the spectral feature of optical pulses has been found. This leads to two important inferences: first, the spectral extension induced by self-phase modulation is linearly proportional to the peak optical intensity only when the associated nonlinear phase shift is sufficiently large, second, if the incident pulses are appropriately chirped a suitable length of SPM-medium can be used to compress the incident spectrum (rather than broaden it). To initiate coupled-cavity mode locking a minimum optical power coupled into the control cavity is required. The specific value of this threshold is not only dependent on the properties of the nonlinear medium such as the length of the fibre, the magnitude of the Kerr coefficient, but also on the main cavity parameters, e. g. the main cavity power level and the effective gain bandwidth. Such a fact simply illustrates that to understand the physical mechanisms involved in the coupled-cavity mode locking the interactive function of various factors in both cavities must be considered. The upper-limit of the nonlinear phase shift, phiNL, experienced by the pulses coupled into the control cavity is found to be phiNL ≤ 2TC. Above this value, further increase of the intrafibre power would lead to the laser operation in an unstable regime. In addition, if the peak optical intensity inside the fibre core is sufficiently high the other nonlinear effects such as modulational instability and self-Raman action can occur besides self-phase modulation. In these cases the process of mode locking enhancement may be frustrated. By incorporating an Er-doped mono-mode optical fibre in the control cavity of such a coupled-cavity mode-locked KCl:Ti laser has led to the generation of 75 fs pulses at 1.5 mum. This result represents the shortest pulses produced with this method in a non-soliton regime.Studies of copper and gold vapour lasersClark, Graeme Lawrencehttps://hdl.handle.net/10023/138032019-04-01T10:07:02Z1988-07-01T00:00:00ZThe work described in this thesis covers various aspects of pulsed copper and gold vapour lasers. The work is divided into four main parts : a computer model of the kinetics of the copper vapour laser discharge; construction and characterization of a copper vapour laser and a gold vapour laser system (to be used for photodynamic cancer treatment); analysis of the thermal processes occurring in the various forms of thermal insulation used in these lasers; and studies of the use of metal walls to confine a discharge plasma. The results of this work were combined in the design of the first copper vapour laser to use metal rather than an electrically insulating ceramic material for confinement of the discharge plasma. Laser action in copper vapour has been achieved in a number of metal-walled designs, with continuous lengths of metal ranging from 30 mm, in a segmented design, to 400 mm, where the discharge plasma was confined by two molybdenum tubes of this length. A theoretical explanation of the behaviour of plasmas in metal-walled discharge vessels is described.
1988-07-01T00:00:00ZClark, Graeme LawrenceThe work described in this thesis covers various aspects of pulsed copper and gold vapour lasers. The work is divided into four main parts : a computer model of the kinetics of the copper vapour laser discharge; construction and characterization of a copper vapour laser and a gold vapour laser system (to be used for photodynamic cancer treatment); analysis of the thermal processes occurring in the various forms of thermal insulation used in these lasers; and studies of the use of metal walls to confine a discharge plasma. The results of this work were combined in the design of the first copper vapour laser to use metal rather than an electrically insulating ceramic material for confinement of the discharge plasma. Laser action in copper vapour has been achieved in a number of metal-walled designs, with continuous lengths of metal ranging from 30 mm, in a segmented design, to 400 mm, where the discharge plasma was confined by two molybdenum tubes of this length. A theoretical explanation of the behaviour of plasmas in metal-walled discharge vessels is described.Generation and propagation of ultrashort laser pulses using nonlinear waveguidesSu, Zenglihttps://hdl.handle.net/10023/138022019-04-01T10:04:06Z1995-07-01T00:00:00ZThe main objectives in this research project related to the generation of ultrashort laser pulses using a KC1:T1 colour-centre laser and the study of their propagation in optical waveguides. Coupled-cavity mode-locking of the KC1:T1 colour-centre laser using either monomode optical fibre or passive AlGaAs waveguides as the nonlinear element in the control-cavity have been investigated. With the optical fibre as the nonlinear element, pulses as short as 63 fs have been generated. The large nonlinearity of the AlGaAs waveguides illuminated near the half- bandgap energy has been confirmed through the propagation of ultrashort laser pulses, and nonlinear phase shift in excess of 2mu has been observed. By undertaking the studies described here, the measurements have indicated that the waveguides used have a linear loss of 0.74 cm<super> -1</super>, a two-photon-absorption coefficient of about 0.1 cm/GW, and a nonlinear refractive index of 0.8 x 10<super>-13</super> cm2/W. Coupled-cavity mode-locking with passive AlGaAs waveguides as the control-cavity nonlinear element has been achieved, and two different guiding geometries of the AlGaAs waveguides have been used. With a straight waveguide geometry, pulses having duration of ~ 230 fs have been generated. When a curved waveguide geometry was utilised and appropriate dispersion compensation applied then pulses as short as 160 fs have been produced. By employing waveguides having reduced lengths (down to 1.2 mm), some further shortening of the output pulses was achieved and pulses as short as 150 fs have been recorded. By using a gold coating on rear facet of the waveguide, successful coupled-cavity mode-locking has been achieved at an output power level as low as 4 mW. Some applications using the coupled-cavity mode-locked KC1:T1 colour-centre laser have been performed. Femtosecond pulses at 1.3-mum spectral region have been produced through the process of self-phase-modulation mediated four-wave-mixing in an erbium-doped fibre. Measurement on the group-velocity-dispersion of the AlGaAs waveguide has also been made, and a value of D = -1100 ps/nm/km has been deduced.
1995-07-01T00:00:00ZSu, ZengliThe main objectives in this research project related to the generation of ultrashort laser pulses using a KC1:T1 colour-centre laser and the study of their propagation in optical waveguides. Coupled-cavity mode-locking of the KC1:T1 colour-centre laser using either monomode optical fibre or passive AlGaAs waveguides as the nonlinear element in the control-cavity have been investigated. With the optical fibre as the nonlinear element, pulses as short as 63 fs have been generated. The large nonlinearity of the AlGaAs waveguides illuminated near the half- bandgap energy has been confirmed through the propagation of ultrashort laser pulses, and nonlinear phase shift in excess of 2mu has been observed. By undertaking the studies described here, the measurements have indicated that the waveguides used have a linear loss of 0.74 cm<super> -1</super>, a two-photon-absorption coefficient of about 0.1 cm/GW, and a nonlinear refractive index of 0.8 x 10<super>-13</super> cm2/W. Coupled-cavity mode-locking with passive AlGaAs waveguides as the control-cavity nonlinear element has been achieved, and two different guiding geometries of the AlGaAs waveguides have been used. With a straight waveguide geometry, pulses having duration of ~ 230 fs have been generated. When a curved waveguide geometry was utilised and appropriate dispersion compensation applied then pulses as short as 160 fs have been produced. By employing waveguides having reduced lengths (down to 1.2 mm), some further shortening of the output pulses was achieved and pulses as short as 150 fs have been recorded. By using a gold coating on rear facet of the waveguide, successful coupled-cavity mode-locking has been achieved at an output power level as low as 4 mW. Some applications using the coupled-cavity mode-locked KC1:T1 colour-centre laser have been performed. Femtosecond pulses at 1.3-mum spectral region have been produced through the process of self-phase-modulation mediated four-wave-mixing in an erbium-doped fibre. Measurement on the group-velocity-dispersion of the AlGaAs waveguide has also been made, and a value of D = -1100 ps/nm/km has been deduced.Magnetic field induced sum frequency mixing in sodium vapourPoustie, Alistair J.https://hdl.handle.net/10023/137652019-04-01T10:03:41Z1990-07-01T00:00:00ZA study of magnetic field induced sum frequency mixing (SFM) in sodium vapour was carried out using continuous-wave lasers as the sources of the fundamental radiation. The three-wave mixing nonlinear optical process was resonantly enhanced by tuning the laser frequencies close to single and two-photon resonances in the sodium atoms. The coherent ultraviolet radiation at the sum frequency of the two input laser frequencies was emitted by the coherently driven 3S-4D electric-quadrupole, which was rotated by the transverse magnetic field to allow collinear generation of the sum frequency wave. Two single-frequency dye lasers were used to examine in detail the role of the intermediate 3P atomic states in the coherent two-photon absorption. Resonant single photon transitions were investigated for the first time in a nonlinear optical process in an atomic vapour. High resolution SFM line profiles were obtained which illustrated the complicating contributions of hole-burning, velocity selection, optical pumping, saturation and frequency dependent phase mismatching to the three-wave mixing effect. The use of additional single photon resonant enhancement and control over the refractive index of the sodium vapour showed that large effective nonlinear X(2) susceptibilities were possible in atomic vapours which could exceed those of nonlinear crystals. The variation of SFM power with atomic particle density due to bulk phase mismatching reflected the wavelength dependence of the sodium dispersion with the 3P intermediate state off-resonance. Phase shifts of the focused Gaussian laser beams led to an asymmetric behaviour of the phase matching with respect to the sign of the phase mismatch k. Saturation spectroscopy was utilised for the first time to examine the Zeeman spectra of the sodium 3S-3P D line resonances in a transverse magnetic field. A novel experimental method was used to restrict the detrimental effects of velocity changing collisions on the resolution of the nonlinear laser spectroscopic technique. The possibility of using optical pumping with a transversal, resonant light beam to induce the second-order nonlinearity necessary for second harmonic generation in sodium vapour was experimentally investigated.
1990-07-01T00:00:00ZPoustie, Alistair J.A study of magnetic field induced sum frequency mixing (SFM) in sodium vapour was carried out using continuous-wave lasers as the sources of the fundamental radiation. The three-wave mixing nonlinear optical process was resonantly enhanced by tuning the laser frequencies close to single and two-photon resonances in the sodium atoms. The coherent ultraviolet radiation at the sum frequency of the two input laser frequencies was emitted by the coherently driven 3S-4D electric-quadrupole, which was rotated by the transverse magnetic field to allow collinear generation of the sum frequency wave. Two single-frequency dye lasers were used to examine in detail the role of the intermediate 3P atomic states in the coherent two-photon absorption. Resonant single photon transitions were investigated for the first time in a nonlinear optical process in an atomic vapour. High resolution SFM line profiles were obtained which illustrated the complicating contributions of hole-burning, velocity selection, optical pumping, saturation and frequency dependent phase mismatching to the three-wave mixing effect. The use of additional single photon resonant enhancement and control over the refractive index of the sodium vapour showed that large effective nonlinear X(2) susceptibilities were possible in atomic vapours which could exceed those of nonlinear crystals. The variation of SFM power with atomic particle density due to bulk phase mismatching reflected the wavelength dependence of the sodium dispersion with the 3P intermediate state off-resonance. Phase shifts of the focused Gaussian laser beams led to an asymmetric behaviour of the phase matching with respect to the sign of the phase mismatch k. Saturation spectroscopy was utilised for the first time to examine the Zeeman spectra of the sodium 3S-3P D line resonances in a transverse magnetic field. A novel experimental method was used to restrict the detrimental effects of velocity changing collisions on the resolution of the nonlinear laser spectroscopic technique. The possibility of using optical pumping with a transversal, resonant light beam to induce the second-order nonlinearity necessary for second harmonic generation in sodium vapour was experimentally investigated.Investigation into all-solid-state, pulsed vibronic lasers and their pump sourcesMorrison, Garry R.https://hdl.handle.net/10023/137642019-04-01T10:05:27Z1996-07-01T00:00:00ZThis thesis is concerned with the design and construction of an all-solid-state, tuneable, pulsed Cr:LiSAF laser and its evolution into an all-solid-state, single-frequency, pulsed Ti:sapphire laser. The initial investigation into the pulsed operation of the vibronic medium CrrLiSAF led to the development of a novel 1.3mum Nd:YLF pump laser. The development of this diode-pumped, Q-switched, 1.3mum Nd:YLF laser is described in detail. This included a comparison of the stimulated emission cross-sections of the 1.321(alphamum and 1.313(0,m lines, which were shown to be equally strong. The frequency-doubling of the 1.321(mum laser in LBO and KTP is compared and the performance of LBO is shown to be superior in this situation. The resulting 660.5nm laser was used as the pump source for a gain-switched Cr.LiSAF laser. The operation of a tuneable, gain-switched CnLiSAF laser with line-narrowing is described in detail. Specifically, the factors limiting its suitability as a diode-pumped, tuneable, single- frequency, pulsed laser medium are addressed. This was achieved, in part, by the construction of an equivalent gain-switched Ti:sapphire laser which was pumped by a frequency-doubled 1mum Nd:YLF laser. The performance of the two gain-switched vibronic lasers is compared when line-narrowed by interferometric means. Ti:sapphire was established as the superior option in this regard and it was developed further, culminating in an all-solid-state, high repetition rate, gain-switched, single-frequency laser. Single-frequency operation was achieved by the technique of injection- seeding the Ti: sapphire resonator with a single-frequency diode laser. The development of this laser, specifically in its high repetition rate form, is considered to be of great interest and potential.
1996-07-01T00:00:00ZMorrison, Garry R.This thesis is concerned with the design and construction of an all-solid-state, tuneable, pulsed Cr:LiSAF laser and its evolution into an all-solid-state, single-frequency, pulsed Ti:sapphire laser. The initial investigation into the pulsed operation of the vibronic medium CrrLiSAF led to the development of a novel 1.3mum Nd:YLF pump laser. The development of this diode-pumped, Q-switched, 1.3mum Nd:YLF laser is described in detail. This included a comparison of the stimulated emission cross-sections of the 1.321(alphamum and 1.313(0,m lines, which were shown to be equally strong. The frequency-doubling of the 1.321(mum laser in LBO and KTP is compared and the performance of LBO is shown to be superior in this situation. The resulting 660.5nm laser was used as the pump source for a gain-switched Cr.LiSAF laser. The operation of a tuneable, gain-switched CnLiSAF laser with line-narrowing is described in detail. Specifically, the factors limiting its suitability as a diode-pumped, tuneable, single- frequency, pulsed laser medium are addressed. This was achieved, in part, by the construction of an equivalent gain-switched Ti:sapphire laser which was pumped by a frequency-doubled 1mum Nd:YLF laser. The performance of the two gain-switched vibronic lasers is compared when line-narrowed by interferometric means. Ti:sapphire was established as the superior option in this regard and it was developed further, culminating in an all-solid-state, high repetition rate, gain-switched, single-frequency laser. Single-frequency operation was achieved by the technique of injection- seeding the Ti: sapphire resonator with a single-frequency diode laser. The development of this laser, specifically in its high repetition rate form, is considered to be of great interest and potential.Diode-laser pumped, high-repetition-rate, Nd:YLF laser and its frequency conversionRahlff, Christianhttps://hdl.handle.net/10023/137632019-04-01T10:05:19Z1996-07-01T00:00:00ZThis thesis describes the development of a continuous-wave (cw), diode pumped, repetitively Q-switched Nd:YLF laser, and its application as a pump source for non-linear optical frequency converters and optical parametric oscillators (OPO's), with typical pulse widths of several tens of nanoseconds and repetition rates in the kHz region. At the core of this thesis is the proposal and realisation of a novel rhombic Nd:YLF slab geometry, which is pumped by two 20W cw AlGaAs diode arrays, each focused by means of 3 lenses into a folded resonator path. The novelty of this adaptation is that it uses Yttrium Lithium Fluoride (YLF), known for its comparatively low tensile strength, as the Neodymium host crystal, by addressing and solving the thermal stress fracture problem in YLF for pump powers up to 20W per surface in an end-pumped configuration. With the excellent beam quality, high average output power and short pulses achieved, Nd:YLF has been established as an excellent material for high density, diode pumped, Q-switched lasers at 1047nm. With identical slabs coated for 1321nm operation also available, the performance and laser parameters such as crossection, linewidth and thermal lensing for these two transitions were compared. The pronounced change in the thermal lensing of the 1321nm transition led to an investigation of excited state upconversion (ESA) in Nd:YLF at 1047 and 1321nm under cw and Q-switched operation. The excellent access to the pumped volume made not only a time dependent spectral analysis of the visible and near UV fluorescence bands possible, but also permitted a spatial analysis of the different up-conversion areas. This pump density and laser transition dependent loss mechanism has implications on the end-effect governed lensing, upper state lifetime and Q-switched pulse form. The low threshold, high repetition rate conversion schemes realised during this thesis expanded the spectral cover of the 1mum pump laser from the near UV (349nm) at one end of the spectrum over the green (523.5nm) and 1.5mum region up to the near infrared at 3.445mum. The progress in this area lies not in the well known phase match configuration of the materials used such as KTP, KTA and LBO, but in the adaptation of these nonlinear converters and non-critically phase matched (NCPM) OPO's to achieve high conversion efficiencies at low pulse energies (typically less than 1mJ, 40kW). With average output powers in the green of 3W (~60% conversion from the IR at 6kHz repetition rate), 750mW in the UV, IW at 1.54mum and 280mW at 3.44mum it has been demonstrated that the high average output power of the repetitively Q-switched system can be preserved in the up- and down-converted radiation.
1996-07-01T00:00:00ZRahlff, ChristianThis thesis describes the development of a continuous-wave (cw), diode pumped, repetitively Q-switched Nd:YLF laser, and its application as a pump source for non-linear optical frequency converters and optical parametric oscillators (OPO's), with typical pulse widths of several tens of nanoseconds and repetition rates in the kHz region. At the core of this thesis is the proposal and realisation of a novel rhombic Nd:YLF slab geometry, which is pumped by two 20W cw AlGaAs diode arrays, each focused by means of 3 lenses into a folded resonator path. The novelty of this adaptation is that it uses Yttrium Lithium Fluoride (YLF), known for its comparatively low tensile strength, as the Neodymium host crystal, by addressing and solving the thermal stress fracture problem in YLF for pump powers up to 20W per surface in an end-pumped configuration. With the excellent beam quality, high average output power and short pulses achieved, Nd:YLF has been established as an excellent material for high density, diode pumped, Q-switched lasers at 1047nm. With identical slabs coated for 1321nm operation also available, the performance and laser parameters such as crossection, linewidth and thermal lensing for these two transitions were compared. The pronounced change in the thermal lensing of the 1321nm transition led to an investigation of excited state upconversion (ESA) in Nd:YLF at 1047 and 1321nm under cw and Q-switched operation. The excellent access to the pumped volume made not only a time dependent spectral analysis of the visible and near UV fluorescence bands possible, but also permitted a spatial analysis of the different up-conversion areas. This pump density and laser transition dependent loss mechanism has implications on the end-effect governed lensing, upper state lifetime and Q-switched pulse form. The low threshold, high repetition rate conversion schemes realised during this thesis expanded the spectral cover of the 1mum pump laser from the near UV (349nm) at one end of the spectrum over the green (523.5nm) and 1.5mum region up to the near infrared at 3.445mum. The progress in this area lies not in the well known phase match configuration of the materials used such as KTP, KTA and LBO, but in the adaptation of these nonlinear converters and non-critically phase matched (NCPM) OPO's to achieve high conversion efficiencies at low pulse energies (typically less than 1mJ, 40kW). With average output powers in the green of 3W (~60% conversion from the IR at 6kHz repetition rate), 750mW in the UV, IW at 1.54mum and 280mW at 3.44mum it has been demonstrated that the high average output power of the repetitively Q-switched system can be preserved in the up- and down-converted radiation.Novel mode-locking techniques for colour-centre lasersKennedy, Gordon T.https://hdl.handle.net/10023/137572019-04-01T10:02:31Z1994-07-01T00:00:00ZThe work reported in this thesis is primarily concerned with the generation of ultrashort pulses from a NaCl:OH- colour-centre laser. Active mode locking of the NaCl:OH- laser by synchronous pumping and acousto-optic loss modulation was characterised in detail. Synchronously mode-locked pulses of 8 ps duration were compressed to 250 fs in an anomalously-dispersive optical fibre and a novel soliton-effect pulse compressor was constructed from a nonlinear fibre-loop mirror. Using this device, the synchronously mode-locked pulses were compressed to 300 fs with no discernible background radiation. The NaChOH- laser was coupled-cavity-mode locked using both nonlinear Fabry- Perot and Michelson cavity configurations, and pulses of 110 fs duration were obtained. A sawtooth amplitude modulation of the laser output was observed. This modulation, which arose from the beating of the mode-locked Nd:YAG pump laser and NaCl:OH- laser pulse trains, was avoided by frequency-referencing the colour-centre laser to the pump laser. A characterisation of the phase noise of the frequency-referenced, coupled-cavity mode- locked laser was performed. By replacing the frequency synthesiser for the pump laser mode locker with a crystal oscillator, the phase noise of both the Nd:YAG pump laser and the NaCl:OH- colour-centre laser were reduced by two orders of magnitude. The technique of self-mode locking was successfully applied to the NaCl:OH- laser. For this laser, it was necessary to include a rod of high-nonlinearity lead-silicate glass in the laser cavity to achieve sufficient self focusing for self-mode locking. Stable mode-locking was initiated by a regenerative acousto-optic modulation and pulses of 95 fs were obtained. The thesis concludes with descriptions of some experiments performed using a mode-locked NaCl:OH- laser and a KCl:TiO(1) colour-centre laser. Efficient pulsed Raman amplification in an optical fibre was achieved by using an optical fibre thats group-velocity dispersion was the same for both the pump and signal wavelengths. By co-propagating pulse trains from the KCl:TiO(1) and NaCl:OH- lasers through a semiconductor optical amplifier cross phase modulation was observed. Ultrafast all-optical switching using the nonlinearity at half the bandgap was demonstrated for an GaA1As integrated interferometer. This nonlinearity was subsequently used to coupled-cavity mode lock the KCl:TiO(1) colour-centre laser.
1994-07-01T00:00:00ZKennedy, Gordon T.The work reported in this thesis is primarily concerned with the generation of ultrashort pulses from a NaCl:OH- colour-centre laser. Active mode locking of the NaCl:OH- laser by synchronous pumping and acousto-optic loss modulation was characterised in detail. Synchronously mode-locked pulses of 8 ps duration were compressed to 250 fs in an anomalously-dispersive optical fibre and a novel soliton-effect pulse compressor was constructed from a nonlinear fibre-loop mirror. Using this device, the synchronously mode-locked pulses were compressed to 300 fs with no discernible background radiation. The NaChOH- laser was coupled-cavity-mode locked using both nonlinear Fabry- Perot and Michelson cavity configurations, and pulses of 110 fs duration were obtained. A sawtooth amplitude modulation of the laser output was observed. This modulation, which arose from the beating of the mode-locked Nd:YAG pump laser and NaCl:OH- laser pulse trains, was avoided by frequency-referencing the colour-centre laser to the pump laser. A characterisation of the phase noise of the frequency-referenced, coupled-cavity mode- locked laser was performed. By replacing the frequency synthesiser for the pump laser mode locker with a crystal oscillator, the phase noise of both the Nd:YAG pump laser and the NaCl:OH- colour-centre laser were reduced by two orders of magnitude. The technique of self-mode locking was successfully applied to the NaCl:OH- laser. For this laser, it was necessary to include a rod of high-nonlinearity lead-silicate glass in the laser cavity to achieve sufficient self focusing for self-mode locking. Stable mode-locking was initiated by a regenerative acousto-optic modulation and pulses of 95 fs were obtained. The thesis concludes with descriptions of some experiments performed using a mode-locked NaCl:OH- laser and a KCl:TiO(1) colour-centre laser. Efficient pulsed Raman amplification in an optical fibre was achieved by using an optical fibre thats group-velocity dispersion was the same for both the pump and signal wavelengths. By co-propagating pulse trains from the KCl:TiO(1) and NaCl:OH- lasers through a semiconductor optical amplifier cross phase modulation was observed. Ultrafast all-optical switching using the nonlinearity at half the bandgap was demonstrated for an GaA1As integrated interferometer. This nonlinearity was subsequently used to coupled-cavity mode lock the KCl:TiO(1) colour-centre laser.Optical and electrical pumping of colour-centre mediaJohnston, Colin I.https://hdl.handle.net/10023/137562019-04-01T10:08:24Z1991-07-01T00:00:00ZWithin this thesis the exploitation of the large homogeneously broadened bandwidth of the LiF:F+2 colour-centre laser by production of frequency tunable ultrashort optical pulses over the 0.8-1.0mum spectral region has been presented. A synchronously pumped LiF:F+2 colour-centre laser produced pulses of 700fs duration with average powers of 30mW when a colliding-pulse-modelocked travelling-wave cavity was implemented. Passive modelocking of the LiF:F+2 colour-centre laser was achieved over two spectral operating regions centred around 860nm and 930nm when the saturable absorber dyes IR140 and DaQTeC were employed. Pulse durations as short as 180fs and 130fs were obtained at 860nm and 930nm respectively using colliding-pulse-modelocked group-velocity-dispersion compensated resonators. The laser was pumped at a 10% duty cycle throughout. The use of coupled-cavity- modelocking techniques combined with passive modelocking was found to extend both the tuning range of the laser and useful operating lifetime of the saturable dye. A home built NaCl:OH- colour-centre laser which encorporates the stabilised F+2 colour- centre is presented. Output powers of up to 450mW were obtained for input pump powers of 4W and the laser tuned from 1.4-1.8?m. Electroluminescence studies of NaCl, CsI, CsI:Na, CsI:Tl, and KI crystals are also presented in a study to assess the feasibility of obtaining laser action from such materials by electrical excitation. KI is shown to be the favoured laser candidate by this excitation method and evidence of temporal narrowing and signal enhancement of the electroluminescence output is presented.
1991-07-01T00:00:00ZJohnston, Colin I.Within this thesis the exploitation of the large homogeneously broadened bandwidth of the LiF:F+2 colour-centre laser by production of frequency tunable ultrashort optical pulses over the 0.8-1.0mum spectral region has been presented. A synchronously pumped LiF:F+2 colour-centre laser produced pulses of 700fs duration with average powers of 30mW when a colliding-pulse-modelocked travelling-wave cavity was implemented. Passive modelocking of the LiF:F+2 colour-centre laser was achieved over two spectral operating regions centred around 860nm and 930nm when the saturable absorber dyes IR140 and DaQTeC were employed. Pulse durations as short as 180fs and 130fs were obtained at 860nm and 930nm respectively using colliding-pulse-modelocked group-velocity-dispersion compensated resonators. The laser was pumped at a 10% duty cycle throughout. The use of coupled-cavity- modelocking techniques combined with passive modelocking was found to extend both the tuning range of the laser and useful operating lifetime of the saturable dye. A home built NaCl:OH- colour-centre laser which encorporates the stabilised F+2 colour- centre is presented. Output powers of up to 450mW were obtained for input pump powers of 4W and the laser tuned from 1.4-1.8?m. Electroluminescence studies of NaCl, CsI, CsI:Na, CsI:Tl, and KI crystals are also presented in a study to assess the feasibility of obtaining laser action from such materials by electrical excitation. KI is shown to be the favoured laser candidate by this excitation method and evidence of temporal narrowing and signal enhancement of the electroluminescence output is presented.Holosteric Nd : YAG lasersNorrie, Callumhttps://hdl.handle.net/10023/137552019-04-01T10:10:51Z1991-07-01T00:00:00ZA miniature Nd:YAG laser that was transversely pumped by a quasi-cw laser-diode array has been designed and constructed. This laser was injection seeded by a continuous wave single-frequency Nd:YAG laser that was also pumped by a laser- diode array. This was the first reported holosteric, or all-solid-state, laser that was capable of generating single frequency pulses in a high quality single transverse mode beam which it achieved at peak powers up to 7 kW. Two different types of laser-diode array were used in this work, and both have been characterised with respect to their use as pump sources for solid-state lasers. A fibre-coupled type SDL-2430-H2 laser-diode array, which emitted 100 mW from the end of a 100 mum core diameter fibre at the Nd:YAG absorption wavelength of 809 nm, was used to longitudinally pump a continuous wave Nd:YAG laser. Spatial hole-burning encouraged this laser to operate on several longitudinal modes, with an output power of up to 31 mW. With the addition of an etalon and a Brewster angled plate to the cavity of this laser, single longiuidinal mode operation was achieved at an output power level of 10 mW. The frequency from this laser was stabilised against thermal drift by phase sensitive locking to the resonance of an external reference cavity. The pump source for the transversely pumped Nd:YAG laser was an SDL-922-J quasi-cw laser-diode bar, which emitted from its 1 cm wide aperture pulses of 200 mus duration with energies up to 5 mJ at reperition rates between 10 - 100 pps. The characteristics of this laser-bar that were measured include a frequency chirp of 5 nm through the pulse, which was found to have a significant effect on the pumping of the Nd:YAG medium. An analysis of the pump-rate distribution throughout the Nd:YAG laser-rod was undertaken with reference to the transverse mode structure of this laser. This was used as the basis of a full rate-equation-based model of the transversely pumped laser. Fundamental transverse mode pulses with energies of 1.1 mJ were generated when the aperture provided by the Nd:YAG rod itself was used to suppress higher order modes. Upon Q-switching the transversely pumped laser using an acousto-optic modulator, single transverse mode pulses with peak power of up to 13 kW were produced. This laser was induced to operate on a single longitudinal mode by injection seeding with the single frequency diode-pumped laser. Operating at a peak power of 7 kW, this holosteric laser was particularly "well-behaved" with a pulse-to-pulse intensity stability of 6% over 100 pulses. A LiNbO3 electro-optic Q-switch configuration was implemented as optical damage to the acousto-optic component had limited the intracavity flux intensity. Development of these lasers towards higher peak power microlaser systems for applications including range-finding and surgery is continuing.
1991-07-01T00:00:00ZNorrie, CallumA miniature Nd:YAG laser that was transversely pumped by a quasi-cw laser-diode array has been designed and constructed. This laser was injection seeded by a continuous wave single-frequency Nd:YAG laser that was also pumped by a laser- diode array. This was the first reported holosteric, or all-solid-state, laser that was capable of generating single frequency pulses in a high quality single transverse mode beam which it achieved at peak powers up to 7 kW. Two different types of laser-diode array were used in this work, and both have been characterised with respect to their use as pump sources for solid-state lasers. A fibre-coupled type SDL-2430-H2 laser-diode array, which emitted 100 mW from the end of a 100 mum core diameter fibre at the Nd:YAG absorption wavelength of 809 nm, was used to longitudinally pump a continuous wave Nd:YAG laser. Spatial hole-burning encouraged this laser to operate on several longitudinal modes, with an output power of up to 31 mW. With the addition of an etalon and a Brewster angled plate to the cavity of this laser, single longiuidinal mode operation was achieved at an output power level of 10 mW. The frequency from this laser was stabilised against thermal drift by phase sensitive locking to the resonance of an external reference cavity. The pump source for the transversely pumped Nd:YAG laser was an SDL-922-J quasi-cw laser-diode bar, which emitted from its 1 cm wide aperture pulses of 200 mus duration with energies up to 5 mJ at reperition rates between 10 - 100 pps. The characteristics of this laser-bar that were measured include a frequency chirp of 5 nm through the pulse, which was found to have a significant effect on the pumping of the Nd:YAG medium. An analysis of the pump-rate distribution throughout the Nd:YAG laser-rod was undertaken with reference to the transverse mode structure of this laser. This was used as the basis of a full rate-equation-based model of the transversely pumped laser. Fundamental transverse mode pulses with energies of 1.1 mJ were generated when the aperture provided by the Nd:YAG rod itself was used to suppress higher order modes. Upon Q-switching the transversely pumped laser using an acousto-optic modulator, single transverse mode pulses with peak power of up to 13 kW were produced. This laser was induced to operate on a single longitudinal mode by injection seeding with the single frequency diode-pumped laser. Operating at a peak power of 7 kW, this holosteric laser was particularly "well-behaved" with a pulse-to-pulse intensity stability of 6% over 100 pulses. A LiNbO3 electro-optic Q-switch configuration was implemented as optical damage to the acousto-optic component had limited the intracavity flux intensity. Development of these lasers towards higher peak power microlaser systems for applications including range-finding and surgery is continuing.Optical and mass spectrometric studies of microwave dischargesHewitt, Janet A.https://hdl.handle.net/10023/137542019-04-01T10:08:01Z1986-07-01T00:00:00ZThe structure, operation and performance characteristics of the TR cell and its role in microwave duplexing in a radar system are discussed. Theory of the microwave discharge is discussed, and the mathematics of microwave transmission along a waveguide examined. Two computer models are established; one to model the transfer of heat from the microwave discharge in the cell to the cell window, and one to model the operation of the TR cell, in terms of the reaction kinetics of the gases within. The results of both models are compared with experimental observations. Finally, the gas in the cell is analysed throughout the manufacturing procedure of the cell and during its operation. Study of the mathematics of microwave transmission along a waveguide leads to expressions for the conductivity, and reflection and transmission coefficients for an ionized gas, resulting in calculations of the electron density in the ionized gas as a function of input power. A computer program to model the heat transfer from the microwave discharge in the TR cell to the cell window has been written. Good agreement with experimental results has been obtained. The temperatures at selected points on the TR cell window, frame and flange are calculated, using the finite difference method. The power incident on the window is input to the program together with the window dimensions and materials selected. The output from the program is in the form of temperatures at selected points across the TR cell window, frame and flange. The temperature at which a window is likely to fail is estimated from the results of the program. Three different techniques are used in the analysis of the gas in the TR cell during its manufacture and operation. They are the study of relative changes in peak heights in the microwave-excited optical emission spectrum of the gas, using an optical spectrum analyser with a recording facility, measurement of the performance of the cell when subject to high power microwave pulses and finally, mass spectrometry of the gases in the cell. Several batches of cells were studied. Using the results from these different analytical techniques, the manufacturing procedure and the operation of the cell are discussed. The batch of cells analysed using mass spectrometry contained traces of oxides of carbon and nitrogen, which were shown to have a negative influence on the performance of the cells. Finally, a computer model of the operation of the TR cell is established. The reaction rates and cross sections of the species likely to be present are calculated from the available literature. The model predicts the number densities of the species present as a function of the operating time of the cell and is used to predict the useful lifetime of the cell. The partial success of the model is due in part to the scarcity of reaction rate data for the microwave discharge.
1986-07-01T00:00:00ZHewitt, Janet A.The structure, operation and performance characteristics of the TR cell and its role in microwave duplexing in a radar system are discussed. Theory of the microwave discharge is discussed, and the mathematics of microwave transmission along a waveguide examined. Two computer models are established; one to model the transfer of heat from the microwave discharge in the cell to the cell window, and one to model the operation of the TR cell, in terms of the reaction kinetics of the gases within. The results of both models are compared with experimental observations. Finally, the gas in the cell is analysed throughout the manufacturing procedure of the cell and during its operation. Study of the mathematics of microwave transmission along a waveguide leads to expressions for the conductivity, and reflection and transmission coefficients for an ionized gas, resulting in calculations of the electron density in the ionized gas as a function of input power. A computer program to model the heat transfer from the microwave discharge in the TR cell to the cell window has been written. Good agreement with experimental results has been obtained. The temperatures at selected points on the TR cell window, frame and flange are calculated, using the finite difference method. The power incident on the window is input to the program together with the window dimensions and materials selected. The output from the program is in the form of temperatures at selected points across the TR cell window, frame and flange. The temperature at which a window is likely to fail is estimated from the results of the program. Three different techniques are used in the analysis of the gas in the TR cell during its manufacture and operation. They are the study of relative changes in peak heights in the microwave-excited optical emission spectrum of the gas, using an optical spectrum analyser with a recording facility, measurement of the performance of the cell when subject to high power microwave pulses and finally, mass spectrometry of the gases in the cell. Several batches of cells were studied. Using the results from these different analytical techniques, the manufacturing procedure and the operation of the cell are discussed. The batch of cells analysed using mass spectrometry contained traces of oxides of carbon and nitrogen, which were shown to have a negative influence on the performance of the cells. Finally, a computer model of the operation of the TR cell is established. The reaction rates and cross sections of the species likely to be present are calculated from the available literature. The model predicts the number densities of the species present as a function of the operating time of the cell and is used to predict the useful lifetime of the cell. The partial success of the model is due in part to the scarcity of reaction rate data for the microwave discharge.Sum frequency mixing and quantum interference in three-level atomsMoseley, Richard R.https://hdl.handle.net/10023/137532019-04-01T10:03:02Z1995-07-01T00:00:00ZThis work contains theoretical and experimental studies of the properties of three-level atoms subjected to two single-frequency, continuous-wave laser sources with special emphasis on magnetic-field-induced sum frequency mixing (SFM) and electromagnetically-induced transparency (EIT). In sodium vapour, two resonant-enhancement routes for SFM were experimentally studied. In one route (3S-3P-3D) the output is generated on a quadrupole-allowed transition and on the other (3S-3P-4P), a dipole-allowed transition. Phase matching conditions are studied in detail. On the second route, the two contributions to the phase mismatch can combine to heavily distort the spectroscopic lineshapes observed. Doubly-resonant SFM is considered and two paths for excitation of the output coherence are identified. Control of their relative strength, via the input laser strengths, is predicted, as well as interference between them. These are both experimentally observed on the 3S1/2-3P1/2-3D3/2 transition scheme in sodium. Theory is presented to show how the use of a strong upper laser in resonant SFM can, by EIT, greatly reduce the absorption on the lower transition without similarly reducing the nonlinear process. A significant increase in conversion efficiency is predicted. Theory is also presented on the inclusion of the local field correction, which is relevant for high vapour densities, in the Bloch equations for the three-level atom. Nonlinear coherence cross-coupling terms arise and the generalised equations are then used for two case studies; one concerning EIT, the other SFM. EIT is experimentally studied using continuous-wave lasers in rubidium vapour. Focusing and de-focusing were observed on the probe laser beam as it tunes through the transparency window at close, but separate, detunings. This is attributed to the radial intensity profile of the coupling laser which imposes a spatial refractive index modulation on the medium at the probe laser wavelength.
1995-07-01T00:00:00ZMoseley, Richard R.This work contains theoretical and experimental studies of the properties of three-level atoms subjected to two single-frequency, continuous-wave laser sources with special emphasis on magnetic-field-induced sum frequency mixing (SFM) and electromagnetically-induced transparency (EIT). In sodium vapour, two resonant-enhancement routes for SFM were experimentally studied. In one route (3S-3P-3D) the output is generated on a quadrupole-allowed transition and on the other (3S-3P-4P), a dipole-allowed transition. Phase matching conditions are studied in detail. On the second route, the two contributions to the phase mismatch can combine to heavily distort the spectroscopic lineshapes observed. Doubly-resonant SFM is considered and two paths for excitation of the output coherence are identified. Control of their relative strength, via the input laser strengths, is predicted, as well as interference between them. These are both experimentally observed on the 3S1/2-3P1/2-3D3/2 transition scheme in sodium. Theory is presented to show how the use of a strong upper laser in resonant SFM can, by EIT, greatly reduce the absorption on the lower transition without similarly reducing the nonlinear process. A significant increase in conversion efficiency is predicted. Theory is also presented on the inclusion of the local field correction, which is relevant for high vapour densities, in the Bloch equations for the three-level atom. Nonlinear coherence cross-coupling terms arise and the generalised equations are then used for two case studies; one concerning EIT, the other SFM. EIT is experimentally studied using continuous-wave lasers in rubidium vapour. Focusing and de-focusing were observed on the probe laser beam as it tunes through the transparency window at close, but separate, detunings. This is attributed to the radial intensity profile of the coupling laser which imposes a spatial refractive index modulation on the medium at the probe laser wavelength.Mode-locked colour-centre lasers and their applicationGrant, Robert S.https://hdl.handle.net/10023/137522019-04-01T10:05:35Z1992-07-01T00:00:00ZThe passive and coupled-cavity mode locking of a LiF:F2+ colour-centre laser was studied. Pulses of less than 180 fs were obtained in the 0.85 mum spectral region by passive mode-locking using the saturable absorber dye IR140. Coupled-cavity mode locking, where a length of optical fibre was incorporated in an external control cavity, resulted in the generation of 1.5 ps duration pulses. Active and coupled-cavity techniques were applied to a KCl:TiO(1) colour-centre laser operating in the 1.5 mum spectral region. Coupled-cavity mode locking was obtained using either Fabry-Perot or Michelson interferometer arrangements. Although either arrangement generated similar pulse durations, the Michelson scheme was found to be more stable. Pulse durations of less than 100 fs were routinely obtained from either arrangement by using small-core fibre in which the dispersion minimum coincided with the laser operating wavelength. The KCl:TiO(1) laser was used to study various absorptive and refractive nonlinearities in a 1.5 mum InGaAsP optical amplifier. Pulse distortion caused by gain saturation and loss saturation was studied in the temporal and spectral domains. In addition, cross-phase modulation related to gain saturation was investigated. Self-phase modulation caused by an ultrafast refractive nonlinearity has been observed for the first time, and its nonlinear coefficient n2 was deduced to be -2 x 10<super>-11</super>cm2W-1. Coupled-cavity mode locking of the KCl:TiO(1) laser was also obtained when the InGaAsP amplifier was used, with pulses as short as 280 fs. Self-starting operation was achieved by eliminating parasitic optical feedback from the device facets. Mode locking was observed for amplifier drive currents either above or below transparency, suggesting that saturable gain and saturable loss respectively were the dominant nonlinearities exploited. The synchronous and coupled-cavity mode locking of a NaCl:OH- laser operating near 1.57 mum was investigated. Coupled-cavity mode locking using an optical fibre was achieved, but was accompanied by a sawtooth modulation in power output (or pulse duration) related to the short gain storage time of NaC1;OH-. This was circumvented by substituting the InGaAsP amplifier for the fibre to obtain self-starting mode locking.
1992-07-01T00:00:00ZGrant, Robert S.The passive and coupled-cavity mode locking of a LiF:F2+ colour-centre laser was studied. Pulses of less than 180 fs were obtained in the 0.85 mum spectral region by passive mode-locking using the saturable absorber dye IR140. Coupled-cavity mode locking, where a length of optical fibre was incorporated in an external control cavity, resulted in the generation of 1.5 ps duration pulses. Active and coupled-cavity techniques were applied to a KCl:TiO(1) colour-centre laser operating in the 1.5 mum spectral region. Coupled-cavity mode locking was obtained using either Fabry-Perot or Michelson interferometer arrangements. Although either arrangement generated similar pulse durations, the Michelson scheme was found to be more stable. Pulse durations of less than 100 fs were routinely obtained from either arrangement by using small-core fibre in which the dispersion minimum coincided with the laser operating wavelength. The KCl:TiO(1) laser was used to study various absorptive and refractive nonlinearities in a 1.5 mum InGaAsP optical amplifier. Pulse distortion caused by gain saturation and loss saturation was studied in the temporal and spectral domains. In addition, cross-phase modulation related to gain saturation was investigated. Self-phase modulation caused by an ultrafast refractive nonlinearity has been observed for the first time, and its nonlinear coefficient n2 was deduced to be -2 x 10<super>-11</super>cm2W-1. Coupled-cavity mode locking of the KCl:TiO(1) laser was also obtained when the InGaAsP amplifier was used, with pulses as short as 280 fs. Self-starting operation was achieved by eliminating parasitic optical feedback from the device facets. Mode locking was observed for amplifier drive currents either above or below transparency, suggesting that saturable gain and saturable loss respectively were the dominant nonlinearities exploited. The synchronous and coupled-cavity mode locking of a NaCl:OH- laser operating near 1.57 mum was investigated. Coupled-cavity mode locking using an optical fibre was achieved, but was accompanied by a sawtooth modulation in power output (or pulse duration) related to the short gain storage time of NaC1;OH-. This was circumvented by substituting the InGaAsP amplifier for the fibre to obtain self-starting mode locking.Microchip lasers : an investigation of transverse mode definition, spectral selectivity and novel frequency modulation/up-conversion techniquesMacKinnon, Neilhttps://hdl.handle.net/10023/137512019-04-01T10:06:50Z1994-07-01T00:00:00ZIn this thesis describes experimental and theoretical work is described for a class of solid-state lasers which are classified by the term "microchip". The work presented here may be grouped into two parts. Chapters 2 and 5 describe work directed towards novel frequency modulation and up-conversion and are therefore of a more developmental flavour. The work presented in chapters 3 and 4 are of a more fundamental nature and pertains to transverse mode definition and spectral selectivity in these devices. In chapter 2 a laser diode array pumped, electro-optically tunable microchip laser is described. Based on the combined gain/non-linear material neodymium doped magnesium oxide lithium niobate (Nd:MgO:LiNbO3) tuning of the six oscillating axial modes was achieved at a sensitivity of 8.9 MHz V-1 mm. Chapter 3 describes the mechanisms observed to define the transverse mode of operation in lithium neodymium tetraphosphate, LiNdP4O12, or LNP, microchip lasers. This material was of interest in this particular study in that the dn/dT of the material was negative and would clearly preclude the guiding mechanisms postulated for the Nd:MgO:LiNbO3 microchip laser as described in chapter 2. The LNP microchip laser was excited by a Ti:sapphire laser and a HeNe based Fizeau interferometer was used to map out the pump induced changes in optical thickness between the cavity mirrors. Further investigation using a modified interferometer revealed that definition of the transverse mode of operation was primarily due to pump induced input surface deformation. The single frequency properties of a laser diode pumped LNP device, as a function of gain length, are the subject of chapter 4. The diode pumped LNP device output exhibited the same excellent spatial characteristics of operation as the Ti:sapphire pumped device. Fourteen milliwatts of single frequency output was obtained. A model, based on spatial dephasing of longitudinal modes, was modified appropriately to predict the single frequency performance of the LNP device as a function of gain length within the resonator. Chapter 5 deals with developmental work on a laser diode array pumped, composite material microchip laser for green output. The device consisted of a neodymium doped yttrium vanadate (Nd:YVO4) gain section in intimate optical contact with a non-linear material (potassium titanyl phosphate, or KTP) for frequency doubling of the 1.064 ?m line in Nd:YVO4. Initial experiments on the device were performed using a laser diode and coupling optics. Six milliwatts of C.W. green power were obtained for 150 mW of incident pump radiation. The device operated in a near diffraction limited spatial mode (M2=1.1) at all the incident pump powers. The oscillating waist within the device was found to be pump power dependent. The conversion of the fundamental into the green was observed to be only slightly reduced by the variable elliptical eigenpolarisation within the cavity. (Abstract shortened by ProQuest.)
1994-07-01T00:00:00ZMacKinnon, NeilIn this thesis describes experimental and theoretical work is described for a class of solid-state lasers which are classified by the term "microchip". The work presented here may be grouped into two parts. Chapters 2 and 5 describe work directed towards novel frequency modulation and up-conversion and are therefore of a more developmental flavour. The work presented in chapters 3 and 4 are of a more fundamental nature and pertains to transverse mode definition and spectral selectivity in these devices. In chapter 2 a laser diode array pumped, electro-optically tunable microchip laser is described. Based on the combined gain/non-linear material neodymium doped magnesium oxide lithium niobate (Nd:MgO:LiNbO3) tuning of the six oscillating axial modes was achieved at a sensitivity of 8.9 MHz V-1 mm. Chapter 3 describes the mechanisms observed to define the transverse mode of operation in lithium neodymium tetraphosphate, LiNdP4O12, or LNP, microchip lasers. This material was of interest in this particular study in that the dn/dT of the material was negative and would clearly preclude the guiding mechanisms postulated for the Nd:MgO:LiNbO3 microchip laser as described in chapter 2. The LNP microchip laser was excited by a Ti:sapphire laser and a HeNe based Fizeau interferometer was used to map out the pump induced changes in optical thickness between the cavity mirrors. Further investigation using a modified interferometer revealed that definition of the transverse mode of operation was primarily due to pump induced input surface deformation. The single frequency properties of a laser diode pumped LNP device, as a function of gain length, are the subject of chapter 4. The diode pumped LNP device output exhibited the same excellent spatial characteristics of operation as the Ti:sapphire pumped device. Fourteen milliwatts of single frequency output was obtained. A model, based on spatial dephasing of longitudinal modes, was modified appropriately to predict the single frequency performance of the LNP device as a function of gain length within the resonator. Chapter 5 deals with developmental work on a laser diode array pumped, composite material microchip laser for green output. The device consisted of a neodymium doped yttrium vanadate (Nd:YVO4) gain section in intimate optical contact with a non-linear material (potassium titanyl phosphate, or KTP) for frequency doubling of the 1.064 ?m line in Nd:YVO4. Initial experiments on the device were performed using a laser diode and coupling optics. Six milliwatts of C.W. green power were obtained for 150 mW of incident pump radiation. The device operated in a near diffraction limited spatial mode (M2=1.1) at all the incident pump powers. The oscillating waist within the device was found to be pump power dependent. The conversion of the fundamental into the green was observed to be only slightly reduced by the variable elliptical eigenpolarisation within the cavity. (Abstract shortened by ProQuest.)Nonlinear frequency conversion of a continuous-wave, laser diode-pumped Nd:YLF laserZhang, Junhttps://hdl.handle.net/10023/137182019-11-05T15:07:35Z1996-07-01T00:00:00ZA continuous-wave, frequency-doubled, diode-pumped Nd:YLF laser, capable of generating 1-W of single-frequency radiation at 523.5 nm, with a linewidth of 10 kHz and frequency tunability of 9 GHz has been developed. By using the tunable green laser as pump source, 4.5 GHz continuous smooth tuning in the range of 10<super>20</super> - 10<super>70</super> nm has been demonstrated in a low threshold doubly-resonant optical parametric oscillator. The investigation of thermal effect in the end-pump Nd:YLF laser crystal and the consideration of diode array end-pump geometry have led to an optimum folded-cavity design. Such an optical resonator can eliminate the astigmatism in the laser output, which is induced by the cavity folding on a curved mirror and the anisotropic thermal effect in Nd:YLF, resulting in a circular fundamental laser mode. In addition, a tightly focused beam waist is produced inside the cavity so that efficient intracavity SHG can be achieved. Over 30% optical conversion efficiency from diode to TEM00 10<super>47</super> nm laser output and 10% conversion efficiency from diode to single-frequency SHG green radiation has been demonstrated. A novel intracavity birefringent filter frequency selection technique has been applied in the standing-wave laser resonator to achieve the single-frequency operation. The performances of the laser in the fundamental wave and in second harmonic generation are investigated in detail in this thesis. The tuning behaviour and stability requirements of type-I and type-II CW doubly-resonant OPO have been compared by using the above green laser as the pump source and LBO, KTP as nonlinear crystals. Through utilisation of a simple cavity length servo, type-II phase matching allows single signal-idler mode-pair operation. By means of pump frequency tuning, and cavity length servo control, the output of signal and idler frequency can be tuned continuously over the KTP crystal phase-matching range. Theoretical analyses on diode-end-pumped Nd:YLF lasers, intracavity frequency doubling, the novel intracavity birefringent filter, and the timing behaviour and stability requirements of single cavity DRO are also presented in this work.
1996-07-01T00:00:00ZZhang, JunA continuous-wave, frequency-doubled, diode-pumped Nd:YLF laser, capable of generating 1-W of single-frequency radiation at 523.5 nm, with a linewidth of 10 kHz and frequency tunability of 9 GHz has been developed. By using the tunable green laser as pump source, 4.5 GHz continuous smooth tuning in the range of 10<super>20</super> - 10<super>70</super> nm has been demonstrated in a low threshold doubly-resonant optical parametric oscillator. The investigation of thermal effect in the end-pump Nd:YLF laser crystal and the consideration of diode array end-pump geometry have led to an optimum folded-cavity design. Such an optical resonator can eliminate the astigmatism in the laser output, which is induced by the cavity folding on a curved mirror and the anisotropic thermal effect in Nd:YLF, resulting in a circular fundamental laser mode. In addition, a tightly focused beam waist is produced inside the cavity so that efficient intracavity SHG can be achieved. Over 30% optical conversion efficiency from diode to TEM00 10<super>47</super> nm laser output and 10% conversion efficiency from diode to single-frequency SHG green radiation has been demonstrated. A novel intracavity birefringent filter frequency selection technique has been applied in the standing-wave laser resonator to achieve the single-frequency operation. The performances of the laser in the fundamental wave and in second harmonic generation are investigated in detail in this thesis. The tuning behaviour and stability requirements of type-I and type-II CW doubly-resonant OPO have been compared by using the above green laser as the pump source and LBO, KTP as nonlinear crystals. Through utilisation of a simple cavity length servo, type-II phase matching allows single signal-idler mode-pair operation. By means of pump frequency tuning, and cavity length servo control, the output of signal and idler frequency can be tuned continuously over the KTP crystal phase-matching range. Theoretical analyses on diode-end-pumped Nd:YLF lasers, intracavity frequency doubling, the novel intracavity birefringent filter, and the timing behaviour and stability requirements of single cavity DRO are also presented in this work.Narrow linewidth, diode laser pumped, solid state lasersGallaher, Nigel R.https://hdl.handle.net/10023/137172019-04-01T10:05:07Z1994-07-01T00:00:00ZThe design, construction, evaluation and development of an all solid state, narrow linewidth laser source is presented. The narrow linewidth laser system was based on a miniature standing wave Nd:YAG laser cavity, end-pumped with 100mW of 809nm light from a fibre coupled GaAlAs diode laser array. This basic CW laser generated up to 30mW at 1064nm in a single, diffraction limited transverse mode (TEM00) but multi-longitudinal mode output beam. The laser had a pump power threshold of 24mW and an optical to optical slope efficiency of 39%. A simple rate equation based numerical model of this laser was developed to allow various design parameters such as length of Nd:YAG gain medium and amount of output coupling to be optimised. Excellent agreement between the numerical model predictions of the output power as a function of input pump power and experimental data from the optimised multi-longitudinal mode laser was obtained. To restrict this laser to operate on a single longitudinal mode, twisted cavity mode and intracavity etalon, mode selecting techniques were investigated. Both methods were found to produce reliable single mode laser operation and resulted in output powers at the 10mW level. The relative free running frequency stability between a pair of single longitudinal mode diode laser pumped Nd:YAG lasers was investigated. By isolating these lasers from environmental noise using a small, custom built anechoic chamber the linewidth of the optical heterodyne signal between the two free running lasers was reduced from tens of megahertz to around 10kHz measured on a millisecond time scale. Further improvement in linewidth was achieved by actively locking the laser frequency to a novel ultra high finesse (F~12,500, free spectral range ~500MHz) spherical mirror Fabry-Perot reference interferometer using the technique of Pound-Drever locking. The locked laser displayed a maximum frequency deviation of only 1kHz from the centre of the reference cavity transmission and a frequency noise spectral density of ~20Hz/ √Hz at 1kHz. In one of the first reported demonstrations of an all solid state injection seeded laser system, this single frequency laser was used to injection seed a diode laser array, transversely pumped, Q-switched Nd:YAG laser to produce 0.25mJ, 35ns pulses in a single longitudinal, single transverse mode beam. Preliminary results on injection locking between two single frequency diode laser pumped Nd:YAG laser are also reported. A novel frequency stabilisation scheme based on resonant optical feedback locking iproposed and some preliminary experimental work on this technique is presented.
1994-07-01T00:00:00ZGallaher, Nigel R.The design, construction, evaluation and development of an all solid state, narrow linewidth laser source is presented. The narrow linewidth laser system was based on a miniature standing wave Nd:YAG laser cavity, end-pumped with 100mW of 809nm light from a fibre coupled GaAlAs diode laser array. This basic CW laser generated up to 30mW at 1064nm in a single, diffraction limited transverse mode (TEM00) but multi-longitudinal mode output beam. The laser had a pump power threshold of 24mW and an optical to optical slope efficiency of 39%. A simple rate equation based numerical model of this laser was developed to allow various design parameters such as length of Nd:YAG gain medium and amount of output coupling to be optimised. Excellent agreement between the numerical model predictions of the output power as a function of input pump power and experimental data from the optimised multi-longitudinal mode laser was obtained. To restrict this laser to operate on a single longitudinal mode, twisted cavity mode and intracavity etalon, mode selecting techniques were investigated. Both methods were found to produce reliable single mode laser operation and resulted in output powers at the 10mW level. The relative free running frequency stability between a pair of single longitudinal mode diode laser pumped Nd:YAG lasers was investigated. By isolating these lasers from environmental noise using a small, custom built anechoic chamber the linewidth of the optical heterodyne signal between the two free running lasers was reduced from tens of megahertz to around 10kHz measured on a millisecond time scale. Further improvement in linewidth was achieved by actively locking the laser frequency to a novel ultra high finesse (F~12,500, free spectral range ~500MHz) spherical mirror Fabry-Perot reference interferometer using the technique of Pound-Drever locking. The locked laser displayed a maximum frequency deviation of only 1kHz from the centre of the reference cavity transmission and a frequency noise spectral density of ~20Hz/ √Hz at 1kHz. In one of the first reported demonstrations of an all solid state injection seeded laser system, this single frequency laser was used to injection seed a diode laser array, transversely pumped, Q-switched Nd:YAG laser to produce 0.25mJ, 35ns pulses in a single longitudinal, single transverse mode beam. Preliminary results on injection locking between two single frequency diode laser pumped Nd:YAG laser are also reported. A novel frequency stabilisation scheme based on resonant optical feedback locking iproposed and some preliminary experimental work on this technique is presented.The formation of excimer molecules in microwave dischargesFrank, Timothy Grahamhttps://hdl.handle.net/10023/137162019-04-01T10:03:50Z1984-07-01T00:00:00ZExperimental and theoretical aspects of the formation of excimer molecules in gas discharges are described. Experimental studies are of the emission spectrum of rare-gas halide molecules, produced in a microwave discharge, and cover aspects relevant to rare-gas halide lasers and to the possibility of pumping these lasers by microwave discharge. The experimental work consists mainly of an extensive programme testing mixtures of a rare-gas (Xe, Kr, or Ar) with a halogon donor (SF6, NF3, or HCl) and a buffer gas (He, Ne, or Ar). The majority of work concerns KrF and XeF but some results are also presented for ArF, XeCl, and KrCl. The spectrum of XeO is observed. The main variables are mixture ratio and total pressure and the principal measurements are available either from integrated spectra or from temporal records of the intensity at the spectrum peak. The results form an empirical optimisation study. One of the aims of this study was to find the mixture ratios and total pressures for which the fluorescent emission, for any given excimer system, was a maximum. These ratios and pressures could then be used as a starting point in attempts to obtain lasing. Considerable use was made of SF6, as the donor because it is a cheap alternative to NF3 and has not been studied in detail as a donor elsewhere. Results are extracted from the main body of results in order to compare the performance of these two donors and to compare the performance of the three buffers. It is shown conclusively that greater lifetimes are obtainable with SF6 but at the cost of lower intensities. In the buffer comparision much lower fluorescent emission is observed when Ar is used, in both XeF and KrF mixtures, and that discharges are only formed below about 100Torr. Below the optimum dilutions the results for He and Ne aresimilar. However, at higher dilutions in XeF and KrF mixtures, fluorescent intensities are significantly greater, and discharges may be obtained at greater total pressure, with Ne buffer. In XeF mixtures it is found that gas lives are 4 to 8 times as long with Ne buffer as they are with He buffer. The temporal records provide gas life information on a scale ranging from 300 to 10<p>6 pulses. To reveal behaviourover fewer pulses and within single pulses a high speed system (5ns response time) was developed using a monochromator, fast photomultiplier, and storage oscilloscope. Because of the very small ratio of the pulse width to the inter-pulse period a blanking system was devised to remove the over-bright base line occurring during the inter-pulse period. The high repetition rate of 1100pps and much reduced volume enabled gas life studies to be carried out in times which are orders of magnitude smaller than those required using the 10pps of a conventional excimer laser system with much greater volume. (Abstract shortened by ProQuest.)
1984-07-01T00:00:00ZFrank, Timothy GrahamExperimental and theoretical aspects of the formation of excimer molecules in gas discharges are described. Experimental studies are of the emission spectrum of rare-gas halide molecules, produced in a microwave discharge, and cover aspects relevant to rare-gas halide lasers and to the possibility of pumping these lasers by microwave discharge. The experimental work consists mainly of an extensive programme testing mixtures of a rare-gas (Xe, Kr, or Ar) with a halogon donor (SF6, NF3, or HCl) and a buffer gas (He, Ne, or Ar). The majority of work concerns KrF and XeF but some results are also presented for ArF, XeCl, and KrCl. The spectrum of XeO is observed. The main variables are mixture ratio and total pressure and the principal measurements are available either from integrated spectra or from temporal records of the intensity at the spectrum peak. The results form an empirical optimisation study. One of the aims of this study was to find the mixture ratios and total pressures for which the fluorescent emission, for any given excimer system, was a maximum. These ratios and pressures could then be used as a starting point in attempts to obtain lasing. Considerable use was made of SF6, as the donor because it is a cheap alternative to NF3 and has not been studied in detail as a donor elsewhere. Results are extracted from the main body of results in order to compare the performance of these two donors and to compare the performance of the three buffers. It is shown conclusively that greater lifetimes are obtainable with SF6 but at the cost of lower intensities. In the buffer comparision much lower fluorescent emission is observed when Ar is used, in both XeF and KrF mixtures, and that discharges are only formed below about 100Torr. Below the optimum dilutions the results for He and Ne aresimilar. However, at higher dilutions in XeF and KrF mixtures, fluorescent intensities are significantly greater, and discharges may be obtained at greater total pressure, with Ne buffer. In XeF mixtures it is found that gas lives are 4 to 8 times as long with Ne buffer as they are with He buffer. The temporal records provide gas life information on a scale ranging from 300 to 10<p>6 pulses. To reveal behaviourover fewer pulses and within single pulses a high speed system (5ns response time) was developed using a monochromator, fast photomultiplier, and storage oscilloscope. Because of the very small ratio of the pulse width to the inter-pulse period a blanking system was devised to remove the over-bright base line occurring during the inter-pulse period. The high repetition rate of 1100pps and much reduced volume enabled gas life studies to be carried out in times which are orders of magnitude smaller than those required using the 10pps of a conventional excimer laser system with much greater volume. (Abstract shortened by ProQuest.)Diode laser pumped Nd : YAG laser for 946-nm and its frequency doublingHong, Junhuahttps://hdl.handle.net/10023/137152019-04-01T10:07:34Z1992-07-01T00:00:00ZThe development of a pulsed all-solid-state blue laser system is described in this thesis. The laser system was constructed on the basis of the latest diode laser pumping technology and the availability of the nonlinear material, potassium niobate, in single crystals of high optical quality. With the advantages of diode laser pumping, the 946-nm line in Nd:YAG was successfully made to lase at room temperature, both in cw and Q-switched operation. By frequency doubling the Q-switched 946-nm pulses in a KNbO3 nonlinear crystal, pulses of coherent blue light at a wavelength of 473 nm were obtained with high conversion efficiency. The 946 nm laser line differs from the more usual transitions used in the Nd:YAG laser in having a lower laser level in the ground state manifold. As a result it experiences significant reabsorption loss due to the (thermally excited) lower level population. Because of this reabsorption loss, the length of the crystal is an important parameter, and as a result of a trade-off between the pumping efficiency and the reabsorption loss, there is an optimum crystal length for every pump power level. A theoretical model was developed to describe this system, and with output coupler transmission, pump beam size and cavity mode size as parameters, optimum conditions for laser oscillation were obtained numerically. The theoretical model was tested with both a diode laser and a Ti:Sapphire laser as a pump source for the 946-nm laser. The theory was found to be in excellent agreement with the results. When pumping with the Ti:Sapphire laser, a slope efficiency of 58 % was obtained with a 2-mm long Nd:YAG crystal, but this dropped to about 11 % with diode laser pumping, which shows the advantage of pumping with a beam of high optical quality.The Q-switched version of the 946-nm Nd:YAG laser was also studied theoretically, and a modified four level model was developed to describe the dynamic behaviour of the laser in the presence of lower state reabsorption. The relation of this quasi-three level model to the ideal four level model used in the absence of such absorption was explored. The experiments on Q-switching described here were the first to be reported on the application of this technique to a diode laser pumped 946-nm Nd:YAG laser. Q-switching was achieved by a Brewster-angled acoustic-optic Q-switch, which also acted as a linear polariser. When the laser was pumped by a 0.5 W diode-laser array, pulses with an energy of 4.9 muJ and pulse duration of 68.5 ns were observed at room temperature. Cooling the Nd:YAG crystal to 5 °C improved the laser performance to give pulses of 5.6 muJ pulse energy and 62 ns pulse duration, corresponding to peak powers of 76 W. The pulse repetition rate was kept at 1.5 kHz throughout. Blue light was generated by passing the 946-nm laser pulses along the a-axis of a 5.1-mm long potassium niobate nonlinear crystal, which was kept at 185 °C to obtain noncritical phase matching. The nonlinear coefficient used in this case was d32 (~19.5 pm/V 946 nm). Problems connected with crystal depoling were successfully solved both by the use of a uniform and well controlled temperature environment for the crystal and by providing a poling electric field during heating/cooling. Pulses of coherent blue light at 473 nm with 42 ns duration and 22 W peak power were generated from the output of the 946 nm laser described above, corresponding to an internal peak-power conversion efficiency of 40%, which was in excellent agreement with the second-harmonic-generation theory.
1992-07-01T00:00:00ZHong, JunhuaThe development of a pulsed all-solid-state blue laser system is described in this thesis. The laser system was constructed on the basis of the latest diode laser pumping technology and the availability of the nonlinear material, potassium niobate, in single crystals of high optical quality. With the advantages of diode laser pumping, the 946-nm line in Nd:YAG was successfully made to lase at room temperature, both in cw and Q-switched operation. By frequency doubling the Q-switched 946-nm pulses in a KNbO3 nonlinear crystal, pulses of coherent blue light at a wavelength of 473 nm were obtained with high conversion efficiency. The 946 nm laser line differs from the more usual transitions used in the Nd:YAG laser in having a lower laser level in the ground state manifold. As a result it experiences significant reabsorption loss due to the (thermally excited) lower level population. Because of this reabsorption loss, the length of the crystal is an important parameter, and as a result of a trade-off between the pumping efficiency and the reabsorption loss, there is an optimum crystal length for every pump power level. A theoretical model was developed to describe this system, and with output coupler transmission, pump beam size and cavity mode size as parameters, optimum conditions for laser oscillation were obtained numerically. The theoretical model was tested with both a diode laser and a Ti:Sapphire laser as a pump source for the 946-nm laser. The theory was found to be in excellent agreement with the results. When pumping with the Ti:Sapphire laser, a slope efficiency of 58 % was obtained with a 2-mm long Nd:YAG crystal, but this dropped to about 11 % with diode laser pumping, which shows the advantage of pumping with a beam of high optical quality.The Q-switched version of the 946-nm Nd:YAG laser was also studied theoretically, and a modified four level model was developed to describe the dynamic behaviour of the laser in the presence of lower state reabsorption. The relation of this quasi-three level model to the ideal four level model used in the absence of such absorption was explored. The experiments on Q-switching described here were the first to be reported on the application of this technique to a diode laser pumped 946-nm Nd:YAG laser. Q-switching was achieved by a Brewster-angled acoustic-optic Q-switch, which also acted as a linear polariser. When the laser was pumped by a 0.5 W diode-laser array, pulses with an energy of 4.9 muJ and pulse duration of 68.5 ns were observed at room temperature. Cooling the Nd:YAG crystal to 5 °C improved the laser performance to give pulses of 5.6 muJ pulse energy and 62 ns pulse duration, corresponding to peak powers of 76 W. The pulse repetition rate was kept at 1.5 kHz throughout. Blue light was generated by passing the 946-nm laser pulses along the a-axis of a 5.1-mm long potassium niobate nonlinear crystal, which was kept at 185 °C to obtain noncritical phase matching. The nonlinear coefficient used in this case was d32 (~19.5 pm/V 946 nm). Problems connected with crystal depoling were successfully solved both by the use of a uniform and well controlled temperature environment for the crystal and by providing a poling electric field during heating/cooling. Pulses of coherent blue light at 473 nm with 42 ns duration and 22 W peak power were generated from the output of the 946 nm laser described above, corresponding to an internal peak-power conversion efficiency of 40%, which was in excellent agreement with the second-harmonic-generation theory.All-solid-state femtosecond Cr:LiSAF and Cr:LiSGaF lasersCritten, Matthew Peterhttps://hdl.handle.net/10023/137142019-04-01T10:06:09Z1997-06-01T00:00:00ZThis thesis describes the development of all-solid-state self-modelocked Cr:LiSAF and Cr:LiSGaF lasers. Diode-pumped colquiriite lasers compare favourably with mainframe-pumped Ti:sapphire lasers, but the conflicting requirements of self-modelocking and pumping by broad-area diodes can cause problems. Two alternatives were investigated. Firstly, high Cr3+ doping permitted pumping in the Cr:LiSAF green absorption trough. Frequency-doubled Nd:YLF and Nd:YAG minilasers offer high-power, diffraction-limited beams in the green. The 'green problem' plagued both minilasers, however, and an Ar-ion pump laser was therefore also used. A laser utilising a 10%-doped AR-coated Cr:LiSAF crystal produced 30 mW output power at 320 mW pump, and 90 fs pulses at 859 nm and 86 MHz repetition rate. The self-modelocking threshold was 280 mW. A laser utilising a 22%-doped Brewster-angled crystal produced 120 mW output power at 1.1 W pump, and 72 fs pulses at 76 mW output. The self-modelocking threshold was 360 mW. 160 mW output was obtained by pumping the crystal at both ends to overcome thermal problems. Secondly, factors affecting CW and self-modelocking thresholds were considered; in particular, the effects of waist size, beam brightness, cavity configuration and intracavity dispersion. A Cr:LiSAF laser, pumped by a self-injection-locked diode, produced sub-100-fs pulses for just 73 mW pump. Its tuning curve was modulated by birefringence effects. The excellent free-running noise properties of the laser permitted a sub-picosecond streak camera evaluation. Two low-threshold lasers utilising low-loss Cr:LiSGaF were also developed. 87 fs pulses at 11.5 mW output power were produced from a Z-cavity laser for 170 mW pump. A laser utilising a compact 'retroreflector' three-minor cavity produced 84- fs pulses at 173 MHz repetition rate for 116 mW pump. Subsequent optimisation led to the production of sub-100-fs pulses for 40 mW pump, with self-modelocked operation demonstrated at pump powers as low as 21 mW.
1997-06-01T00:00:00ZCritten, Matthew PeterThis thesis describes the development of all-solid-state self-modelocked Cr:LiSAF and Cr:LiSGaF lasers. Diode-pumped colquiriite lasers compare favourably with mainframe-pumped Ti:sapphire lasers, but the conflicting requirements of self-modelocking and pumping by broad-area diodes can cause problems. Two alternatives were investigated. Firstly, high Cr3+ doping permitted pumping in the Cr:LiSAF green absorption trough. Frequency-doubled Nd:YLF and Nd:YAG minilasers offer high-power, diffraction-limited beams in the green. The 'green problem' plagued both minilasers, however, and an Ar-ion pump laser was therefore also used. A laser utilising a 10%-doped AR-coated Cr:LiSAF crystal produced 30 mW output power at 320 mW pump, and 90 fs pulses at 859 nm and 86 MHz repetition rate. The self-modelocking threshold was 280 mW. A laser utilising a 22%-doped Brewster-angled crystal produced 120 mW output power at 1.1 W pump, and 72 fs pulses at 76 mW output. The self-modelocking threshold was 360 mW. 160 mW output was obtained by pumping the crystal at both ends to overcome thermal problems. Secondly, factors affecting CW and self-modelocking thresholds were considered; in particular, the effects of waist size, beam brightness, cavity configuration and intracavity dispersion. A Cr:LiSAF laser, pumped by a self-injection-locked diode, produced sub-100-fs pulses for just 73 mW pump. Its tuning curve was modulated by birefringence effects. The excellent free-running noise properties of the laser permitted a sub-picosecond streak camera evaluation. Two low-threshold lasers utilising low-loss Cr:LiSGaF were also developed. 87 fs pulses at 11.5 mW output power were produced from a Z-cavity laser for 170 mW pump. A laser utilising a compact 'retroreflector' three-minor cavity produced 84- fs pulses at 173 MHz repetition rate for 116 mW pump. Subsequent optimisation led to the production of sub-100-fs pulses for 40 mW pump, with self-modelocked operation demonstrated at pump powers as low as 21 mW.A study of the production and application of nonlinear materials for use in frequency doubled dye lasersBastow, Stephen Johnhttps://hdl.handle.net/10023/137122019-04-01T10:05:18Z1986-07-01T00:00:00ZA selection of nonlinear materials for use in various frequency-doubled dye lasers is studied. Lithium Formate Monohydrate is operated intracavity in a Coumarin 102 dye laser for the first time. A maximum of 70 microwatts of tunable ultraviolet light was generated over the range 237nm to 24Snm, representing an improvement of an order of magnitude on previously reported work with this material. A pulsed xenon ion laser of high efficiency has been built and the design optimised, giving an output power of 185W. The laser is used to pump a Rhodamine 6G dye laser containing a crystal of Ammonium Dihydrogen Arsenate, to generate 280 milliwatts of tunable UV over the range 291nm to 297nm and this is the first time that a detailed study of this type of laser has been reported. A crystal growing system for water based crystals is described. A crystal of Urea grown in the apparatus has been cut for use in a dye laser and tested with the beam from an argon ion laser. A value of the nonlinear coefficient d14 of 0.90pm/V is reported.
1986-07-01T00:00:00ZBastow, Stephen JohnA selection of nonlinear materials for use in various frequency-doubled dye lasers is studied. Lithium Formate Monohydrate is operated intracavity in a Coumarin 102 dye laser for the first time. A maximum of 70 microwatts of tunable ultraviolet light was generated over the range 237nm to 24Snm, representing an improvement of an order of magnitude on previously reported work with this material. A pulsed xenon ion laser of high efficiency has been built and the design optimised, giving an output power of 185W. The laser is used to pump a Rhodamine 6G dye laser containing a crystal of Ammonium Dihydrogen Arsenate, to generate 280 milliwatts of tunable UV over the range 291nm to 297nm and this is the first time that a detailed study of this type of laser has been reported. A crystal growing system for water based crystals is described. A crystal of Urea grown in the apparatus has been cut for use in a dye laser and tested with the beam from an argon ion laser. A value of the nonlinear coefficient d14 of 0.90pm/V is reported.High-precision time-domain astrophysics in crowded star-fields with ground based telescopes : globular clusters and the mitigation of the atmospheric turbulenceFiguera Jaimes, Roberto Josehttps://hdl.handle.net/10023/136732019-04-01T10:08:28Z2018-05-04T00:00:00ZWe carried out a three year (2013-2015) observational campaign at the Danish 1.54-m Telescope at the ESO observatory at La Silla in Chile in which we obtained ~1000 astronomical images in the field of 11 Galactic
globular clusters. The selection of these stellar systems was focused mainly on the visibility of the targets and
their relevant physical properties available in the catalogues, among them were considered the density, variable
stars known, colour-magnitude diagrams, and luminosity.
The telescope was equipped with an electron-multiplying CCD (EMCCD) with the aim of taking very short exposure-time
images. The camera was configured to take 10 frames per second. Due to this, the brighter stars
observed were not affected by saturation, it helped to give higher signal to noise ratio to the fainter stars and, importantly,
it minimised the effects of the atmospheric turbulence such as blending between stars in the crowded
fields. To produce normal-exposure-time images (minutes) we implemented the shift-and-add technique that
also enabled us to produce images with better angular resolution than previously achieved with conventional
CCDs on ground-based telescopes, and even enabled us to produce images with angular resolution close to that
obtained with space telescopes.
The detection of the stars in each of the globular clusters and the photometry was performed via difference
image analysis by using the DanDIA pipeline whose procedures and mathematical techniques have been demonstrated
to produce high-precision time-series photometry of very crowded stellar regions.
We produced time-series photometry for ~15000 stars in the fields observed which were statistically analysed
in order to automatically extract variable stars. Our aim is to complete, or improve, the census of the
variable star population in the globular clusters. In NGC 6715, we found light curves for 17 previously known
variable stars near the edges of our reference image (16 RR Lyrae and 1 semi-regular) and we discovered 67
new variables (30 RR Lyrae, 21 long-period irregular, 3 semi-regular, 1 W Virginis, 1 eclipsing binary, and 11
unclassified). This cluster was particularly interesting because apart from the results obtained, it shows the benefits
of using the EMCCD cameras and the shift-and-add technique. It is a cluster studied several times including
data obtained with the OGLE survey and also with the Hubble Space Telescope and our discoveries were still new. Our new RR Lyrae star discoveries help confirm that NGC 6715 is of intermediate Oosterhoff type.
In the other 10 globular clusters, we obtained light curves for 31 previously known variable stars (3 L, 2
SR, 20 RR Lyrae, 1 SX Phe, 3 cataclysmic variables, 1 EW and 1 NC) and we discovered 30 new variables
(16 L, 7 SR, 4 RR Lyrae, 1 SX Phe and 2 NC). In NGC 6093, we analysed the famous case of the 1860 Nova, for which no observations of the Nova in outburst have been made until the present study.
Ephemerides and photometric measurements for the variable stars are available in electronic form through
the Strasbourg Astronomical Data Centre.
2018-05-04T00:00:00ZFiguera Jaimes, Roberto JoseWe carried out a three year (2013-2015) observational campaign at the Danish 1.54-m Telescope at the ESO observatory at La Silla in Chile in which we obtained ~1000 astronomical images in the field of 11 Galactic
globular clusters. The selection of these stellar systems was focused mainly on the visibility of the targets and
their relevant physical properties available in the catalogues, among them were considered the density, variable
stars known, colour-magnitude diagrams, and luminosity.
The telescope was equipped with an electron-multiplying CCD (EMCCD) with the aim of taking very short exposure-time
images. The camera was configured to take 10 frames per second. Due to this, the brighter stars
observed were not affected by saturation, it helped to give higher signal to noise ratio to the fainter stars and, importantly,
it minimised the effects of the atmospheric turbulence such as blending between stars in the crowded
fields. To produce normal-exposure-time images (minutes) we implemented the shift-and-add technique that
also enabled us to produce images with better angular resolution than previously achieved with conventional
CCDs on ground-based telescopes, and even enabled us to produce images with angular resolution close to that
obtained with space telescopes.
The detection of the stars in each of the globular clusters and the photometry was performed via difference
image analysis by using the DanDIA pipeline whose procedures and mathematical techniques have been demonstrated
to produce high-precision time-series photometry of very crowded stellar regions.
We produced time-series photometry for ~15000 stars in the fields observed which were statistically analysed
in order to automatically extract variable stars. Our aim is to complete, or improve, the census of the
variable star population in the globular clusters. In NGC 6715, we found light curves for 17 previously known
variable stars near the edges of our reference image (16 RR Lyrae and 1 semi-regular) and we discovered 67
new variables (30 RR Lyrae, 21 long-period irregular, 3 semi-regular, 1 W Virginis, 1 eclipsing binary, and 11
unclassified). This cluster was particularly interesting because apart from the results obtained, it shows the benefits
of using the EMCCD cameras and the shift-and-add technique. It is a cluster studied several times including
data obtained with the OGLE survey and also with the Hubble Space Telescope and our discoveries were still new. Our new RR Lyrae star discoveries help confirm that NGC 6715 is of intermediate Oosterhoff type.
In the other 10 globular clusters, we obtained light curves for 31 previously known variable stars (3 L, 2
SR, 20 RR Lyrae, 1 SX Phe, 3 cataclysmic variables, 1 EW and 1 NC) and we discovered 30 new variables
(16 L, 7 SR, 4 RR Lyrae, 1 SX Phe and 2 NC). In NGC 6093, we analysed the famous case of the 1860 Nova, for which no observations of the Nova in outburst have been made until the present study.
Ephemerides and photometric measurements for the variable stars are available in electronic form through
the Strasbourg Astronomical Data Centre.Matrix metalloproteinase (MMP)-7 in Barrett’s esophagus and esophageal adenocarcinoma : expression, metabolism and functional significanceGaralla, Hanan M.Lertkowit, NantapornTiszlavicz, LaszloHolmberg, ChrisBeynon, RobSimpson, DeborahVarga, AkosKumar, Jothi DineshDodd, StevenPritchard, David MarkMoore, Andrew R.Rosztoczy, Andras I.Wittman, TiborSimpson, AlecDockray, Graham J.Varro, Andreahttps://hdl.handle.net/10023/136472023-04-25T23:52:20Z2018-05-20T00:00:00ZMatrix metalloproteinase (MMP)‐7, unlike many MMPs, is typically expressed in epithelial cells. It has been linked to epithelial responses to infection, injury, and tissue remodeling including the progression of a number of cancers. We have now examined how MMP‐7 expression changes in the progression to esophageal adenocarcinoma (EAC), and have studied mechanisms regulating its expression and its functional significance. Immunohistochemistry revealed that MMP‐7 was weakly expressed in normal squamous epithelium adjacent to EAC but was abundant in epithelial cells in both preneoplastic lesions of Barrett's esophagus and EAC particularly at the invasive front. In the stroma, putative myofibroblasts expressing MMP‐7 were abundant at the invasive front but were scarce or absent in adjacent tissue. Western blot and ELISA revealed high constitutive secretion of proMMP‐7 in an EAC cell line (OE33) that was inhibited by the phosphatidylinositol (PI) 3‐kinase inhibitor LY294002 but not by inhibitors of protein kinase C, or MAP kinase activation. There was detectable proMMP‐7 in cultured esophageal myofibroblasts but it was undetectable in media. Possible metabolism of MMP‐7 by myofibroblasts studied by proteomic analysis indicated degradation via extensive endopeptidase, followed by amino‐ and carboxpeptidase, cleavages. Myofibroblasts exhibited increased migration and invasion in response to conditioned media from OE33 cells that was reduced by MMP‐7 knockdown and immunoneutralization. Thus, MMP‑7 expression increases at the invasive front in EAC which may be partly attributable to activation of PI 3‐kinase. Secreted MMP‐7 may modify the tumor microenvironment by stimulating stromal cell migration and invasion.
Supported by grants from North West Cancer Research (Grant number: CR945), The Wellcome Trust (Grant number: 074287/Z/03/Z) and a research studentship (HG) from the Libyan Government.
2018-05-20T00:00:00ZGaralla, Hanan M.Lertkowit, NantapornTiszlavicz, LaszloHolmberg, ChrisBeynon, RobSimpson, DeborahVarga, AkosKumar, Jothi DineshDodd, StevenPritchard, David MarkMoore, Andrew R.Rosztoczy, Andras I.Wittman, TiborSimpson, AlecDockray, Graham J.Varro, AndreaMatrix metalloproteinase (MMP)‐7, unlike many MMPs, is typically expressed in epithelial cells. It has been linked to epithelial responses to infection, injury, and tissue remodeling including the progression of a number of cancers. We have now examined how MMP‐7 expression changes in the progression to esophageal adenocarcinoma (EAC), and have studied mechanisms regulating its expression and its functional significance. Immunohistochemistry revealed that MMP‐7 was weakly expressed in normal squamous epithelium adjacent to EAC but was abundant in epithelial cells in both preneoplastic lesions of Barrett's esophagus and EAC particularly at the invasive front. In the stroma, putative myofibroblasts expressing MMP‐7 were abundant at the invasive front but were scarce or absent in adjacent tissue. Western blot and ELISA revealed high constitutive secretion of proMMP‐7 in an EAC cell line (OE33) that was inhibited by the phosphatidylinositol (PI) 3‐kinase inhibitor LY294002 but not by inhibitors of protein kinase C, or MAP kinase activation. There was detectable proMMP‐7 in cultured esophageal myofibroblasts but it was undetectable in media. Possible metabolism of MMP‐7 by myofibroblasts studied by proteomic analysis indicated degradation via extensive endopeptidase, followed by amino‐ and carboxpeptidase, cleavages. Myofibroblasts exhibited increased migration and invasion in response to conditioned media from OE33 cells that was reduced by MMP‐7 knockdown and immunoneutralization. Thus, MMP‑7 expression increases at the invasive front in EAC which may be partly attributable to activation of PI 3‐kinase. Secreted MMP‐7 may modify the tumor microenvironment by stimulating stromal cell migration and invasion.Low pressure plasmas for high power microwave sourcesHirst, Peter Frankhttps://hdl.handle.net/10023/136132019-04-01T10:04:12Z1992-07-01T00:00:00ZThis thesis describes an investigation of the use of low pressure plasmas for the generation of high power microwaves. Previous research has shown that the efficiency of a high power microwave ("HPM") source such as a BWO is enhanced by the introduction of a low pressure plasma into the oscillator cavity. The principle aim of this thesis is to extend the use of low pressure plasmas to the whole HPM system. Electron beams with current densities of the order of 20 A cm-2 can be generated in a cold cathode glow discharge at low gas pressures. Results are presented which show the effects of magnetic fields and electrode spacing on the I-V characteristics of a DC glow discharge electron gun. A glow discharge electron gun with an operating voltage of 350 kV has been designed and tested. A new kind of RP plasma cathode is proposed in which electrons are drawn from an RF discharge in a low pressure gas. An analysis of the production of an annular RF plasma cathode using a microwave-excited helical slow-wave structure is presented. Experimental results show that the RF plasma cathode yields electron current densities an order of magnitude higher than does a solid cathode. Examples of the implementation of the RF plasma cathode in a number of components of an HPM system are given. The propagation of electromagnetic waves in plasma-loaded waveguides of circular cross-section has been modelled. Numerical solutions are presented for the case of slow-waves in a longitudinally-magnetised plasma waveguide. Propagation below the cut-off frequency of the waveguide is generally possible and, according to the configuration, the propagating waves may be used for plasma generation or for RF power transmission. A new kind of high power microwave waveguide switch, based on the properties of plasma waveguides, is proposed. The design of new kind of magnetron, the "Glow Discharge Inverted Magnetron" ("GDIM"), is presented. The GDIM is an inverted magnetron with the resonant structure located on the cathode. The resonant cavities are used as a source of glow discharge electron beams, which gives high power operation without requiring relativistic voltages.
1992-07-01T00:00:00ZHirst, Peter FrankThis thesis describes an investigation of the use of low pressure plasmas for the generation of high power microwaves. Previous research has shown that the efficiency of a high power microwave ("HPM") source such as a BWO is enhanced by the introduction of a low pressure plasma into the oscillator cavity. The principle aim of this thesis is to extend the use of low pressure plasmas to the whole HPM system. Electron beams with current densities of the order of 20 A cm-2 can be generated in a cold cathode glow discharge at low gas pressures. Results are presented which show the effects of magnetic fields and electrode spacing on the I-V characteristics of a DC glow discharge electron gun. A glow discharge electron gun with an operating voltage of 350 kV has been designed and tested. A new kind of RP plasma cathode is proposed in which electrons are drawn from an RF discharge in a low pressure gas. An analysis of the production of an annular RF plasma cathode using a microwave-excited helical slow-wave structure is presented. Experimental results show that the RF plasma cathode yields electron current densities an order of magnitude higher than does a solid cathode. Examples of the implementation of the RF plasma cathode in a number of components of an HPM system are given. The propagation of electromagnetic waves in plasma-loaded waveguides of circular cross-section has been modelled. Numerical solutions are presented for the case of slow-waves in a longitudinally-magnetised plasma waveguide. Propagation below the cut-off frequency of the waveguide is generally possible and, according to the configuration, the propagating waves may be used for plasma generation or for RF power transmission. A new kind of high power microwave waveguide switch, based on the properties of plasma waveguides, is proposed. The design of new kind of magnetron, the "Glow Discharge Inverted Magnetron" ("GDIM"), is presented. The GDIM is an inverted magnetron with the resonant structure located on the cathode. The resonant cavities are used as a source of glow discharge electron beams, which gives high power operation without requiring relativistic voltages.Periodic dielectric disturbances and novel opticsFindlay, Ewan Donaldhttps://hdl.handle.net/10023/136122019-04-01T10:09:59Z1995-07-01T00:00:00ZSeveral possible methods for creating permanent and temporary periodic dielectric disturbances and their application to optics have been examined. The coupled mode theory is used to describe the travelling wave electro-optic diffractor (TWEOD) proposed by Dawber et al. A basic theoretical model of the TWEOD is presented and developed to give the design parameters for the construction of a working barium titanate TWEOD operating at 10 GHz. The coupled mode theory is also used to describe the dielectric disturbances created by pump beam interference patterns within a laser medium. Pump architectures which yield a periodic disturbance and thus create temporary diffraction gratings are analysed. Permanent dielectric disturbances can be generated in glass heated to temperatures above the transformation point. Some possible methods for imposing both uniform and periodic dielectric disturbances on glass, which are generated by imposing stresses on the glass whilst it is at its transformation temperature are proposed. Uniformly pre-stressed glass can be used to improve the angular response of glass acousto-optic diffractors. Periodically prestressed glass forms a permanent acousto-optic diffraction grating. Acoustic standing waves may also be used to induce periodic structural change in other transparent media and on the surface of etchable substrates. Such acoustically induced structures may be used as permanent diffraction gratings and holograms. A potential application for some of the novel devices described in this work is illustrated in the implementiation of an optical frequency division multiplexer ("optiplexer") for video signals (3 dB Bandwidth of 6 MHz) using acousto-optic frequency shifting techniques. This is a times six improvement on the channel bandwidth of 1 MHz for the optiplexer achieved by Dawber et al.
1995-07-01T00:00:00ZFindlay, Ewan DonaldSeveral possible methods for creating permanent and temporary periodic dielectric disturbances and their application to optics have been examined. The coupled mode theory is used to describe the travelling wave electro-optic diffractor (TWEOD) proposed by Dawber et al. A basic theoretical model of the TWEOD is presented and developed to give the design parameters for the construction of a working barium titanate TWEOD operating at 10 GHz. The coupled mode theory is also used to describe the dielectric disturbances created by pump beam interference patterns within a laser medium. Pump architectures which yield a periodic disturbance and thus create temporary diffraction gratings are analysed. Permanent dielectric disturbances can be generated in glass heated to temperatures above the transformation point. Some possible methods for imposing both uniform and periodic dielectric disturbances on glass, which are generated by imposing stresses on the glass whilst it is at its transformation temperature are proposed. Uniformly pre-stressed glass can be used to improve the angular response of glass acousto-optic diffractors. Periodically prestressed glass forms a permanent acousto-optic diffraction grating. Acoustic standing waves may also be used to induce periodic structural change in other transparent media and on the surface of etchable substrates. Such acoustically induced structures may be used as permanent diffraction gratings and holograms. A potential application for some of the novel devices described in this work is illustrated in the implementiation of an optical frequency division multiplexer ("optiplexer") for video signals (3 dB Bandwidth of 6 MHz) using acousto-optic frequency shifting techniques. This is a times six improvement on the channel bandwidth of 1 MHz for the optiplexer achieved by Dawber et al.Dye lasers with induced Bragg gratingsMcIntyre, Iain A.https://hdl.handle.net/10023/136112019-04-01T10:10:21Z1985-07-01T00:00:00ZThis thesis describes a study of dye lasers which contain an induced Bragg gain grating within the active medium. These lasers fall into two categories - the distributed feedback dye laser and a novel configuration of a conventional cavity dye laser which is tuned by an induced grating (TING dye laser). After introducing the two different types of laser, a review of distributed feedback systems is made, discussing the coupled-wave theory of distributed feedback, including saturation and linewidth effects, and summarising the characteristics of previous experimental systems. This is followed by an analysis of the TING laser using multimode semi-classical laser theory. It is shown that the presence of the induced Bragg gain grating can have significant tuning and line-narrowing properties. In particular, it is shown that, if the induced grating fills a sufficiently large proportion of the cavity, the TING laser is able to oscillate on a single cavity mode in a stable fashion. Experiments on distributed feedback and TING lasers, both pumped by a frequency doubled, Q-switched Nd:YAG laser are then described. The distributed feedback system is completely characterised for efficiency, beam divergence, tuning range and amplified spontaneous emission. The linewidth of the radiation produced by this laser is investigated in particular detail. Contributions to the linewidth due to finite linewidth of pumping radiation, pump beam divergence and thermal effects in the dye are assessed experimentally and methods of minimising their influence explored. This last effect, due to refractive index change through solvent heating by the pump radiation, is shown to produce a frequency chirping in the output pulse from the dye laser. Careful choice of solvent enables this to be reduced. A further process limiting the ultimate linewidth achievable in dynamic distributed feedback dye lasers is identified. This is the changing dispersion, due to the changing saturated gain, in the active medium during the course of a pumping pulse. A model of this dispersion is used to predict correctly the temporal behaviour of the central laser frequency during a pulse. Having identified the processes limiting the ultimate linewidths achievable in conventional distributed feedback dye lasers, a novel technique for further line narrowing, that of incorporating frequency selective feedback, is explored. By using an etalon/mirror combination to feed radiation back into the dye laser at one end, linewidths have been reduced from around 0.3 A, the linewidth of the conventional DFB dye laser at reasonable pump powers, to 0.024 A with frequency selective feedback. Finally, a dual wavelength dye laser system is described in which the laser experiences feedback due to a conventional grazing incidence grating cavity and distributed feedback simultaneously. The two wavelengths obtained are independently tunable and this laser is characterised especially with respect to levels of amplified spontaneous emission and to gain competition between the two oscillating frequencies.
1985-07-01T00:00:00ZMcIntyre, Iain A.This thesis describes a study of dye lasers which contain an induced Bragg gain grating within the active medium. These lasers fall into two categories - the distributed feedback dye laser and a novel configuration of a conventional cavity dye laser which is tuned by an induced grating (TING dye laser). After introducing the two different types of laser, a review of distributed feedback systems is made, discussing the coupled-wave theory of distributed feedback, including saturation and linewidth effects, and summarising the characteristics of previous experimental systems. This is followed by an analysis of the TING laser using multimode semi-classical laser theory. It is shown that the presence of the induced Bragg gain grating can have significant tuning and line-narrowing properties. In particular, it is shown that, if the induced grating fills a sufficiently large proportion of the cavity, the TING laser is able to oscillate on a single cavity mode in a stable fashion. Experiments on distributed feedback and TING lasers, both pumped by a frequency doubled, Q-switched Nd:YAG laser are then described. The distributed feedback system is completely characterised for efficiency, beam divergence, tuning range and amplified spontaneous emission. The linewidth of the radiation produced by this laser is investigated in particular detail. Contributions to the linewidth due to finite linewidth of pumping radiation, pump beam divergence and thermal effects in the dye are assessed experimentally and methods of minimising their influence explored. This last effect, due to refractive index change through solvent heating by the pump radiation, is shown to produce a frequency chirping in the output pulse from the dye laser. Careful choice of solvent enables this to be reduced. A further process limiting the ultimate linewidth achievable in dynamic distributed feedback dye lasers is identified. This is the changing dispersion, due to the changing saturated gain, in the active medium during the course of a pumping pulse. A model of this dispersion is used to predict correctly the temporal behaviour of the central laser frequency during a pulse. Having identified the processes limiting the ultimate linewidths achievable in conventional distributed feedback dye lasers, a novel technique for further line narrowing, that of incorporating frequency selective feedback, is explored. By using an etalon/mirror combination to feed radiation back into the dye laser at one end, linewidths have been reduced from around 0.3 A, the linewidth of the conventional DFB dye laser at reasonable pump powers, to 0.024 A with frequency selective feedback. Finally, a dual wavelength dye laser system is described in which the laser experiences feedback due to a conventional grazing incidence grating cavity and distributed feedback simultaneously. The two wavelengths obtained are independently tunable and this laser is characterised especially with respect to levels of amplified spontaneous emission and to gain competition between the two oscillating frequencies.Passive ranging of near field targets in the audio frequency bandDennis, Ian Robert Howardhttps://hdl.handle.net/10023/136102019-04-01T10:06:12Z1997-12-01T00:00:00ZThe use of passive ranging techniques for imaging targets situated in the near field for the audio frequency band is presented in this thesis. A three port interferometer used in millimetre waves is introduced and it is shown that with some approximations, it can be used to passively image targets in the medium-far field. The geometry of this system is then adapted for use in the near field in the audio frequency band, without the need for making any approximations. A model is then constructed to demonstrate the theory. The design, construction and testing of the new system for use in an anechoic chamber is then presented. This thesis also introduces a number of digital signal processing techniques that take full advantage of the recent increase in computing power and decrease in costs. These techniques are then used to perform frequency analysis on real data obtained experimentally from the new three port system. A number of different experiments and results are then presented. A loudspeaker is used as a test source and it is shown that the two separate drive units can be distinguished very accurately with the system. The whole system is then fully characterized and the optimal system setup conditions are found. Finally some ideas for future work and possible applications are presented. It is shown that the system would make an ideal tool for testing the spatial coherence properties of loudspeakers.
1997-12-01T00:00:00ZDennis, Ian Robert HowardThe use of passive ranging techniques for imaging targets situated in the near field for the audio frequency band is presented in this thesis. A three port interferometer used in millimetre waves is introduced and it is shown that with some approximations, it can be used to passively image targets in the medium-far field. The geometry of this system is then adapted for use in the near field in the audio frequency band, without the need for making any approximations. A model is then constructed to demonstrate the theory. The design, construction and testing of the new system for use in an anechoic chamber is then presented. This thesis also introduces a number of digital signal processing techniques that take full advantage of the recent increase in computing power and decrease in costs. These techniques are then used to perform frequency analysis on real data obtained experimentally from the new three port system. A number of different experiments and results are then presented. A loudspeaker is used as a test source and it is shown that the two separate drive units can be distinguished very accurately with the system. The whole system is then fully characterized and the optimal system setup conditions are found. Finally some ideas for future work and possible applications are presented. It is shown that the system would make an ideal tool for testing the spatial coherence properties of loudspeakers.Millimetre wave quasi-optical signal processing and spread spectrum techniquesRobertson, Duncan Alexanderhttps://hdl.handle.net/10023/136042019-04-01T10:05:38Z1995-07-01T00:00:00ZThe use of quasi-optical techniques for signal processing at millimetre wave frequencies and their application to spread spectrum systems is presented in this thesis. Millimetre waves offer a number of advantages when compared to microwave and optical signals and as a result they are finding an increasing number of applications in the area of communications. Traditionally, millimetre wave research has been centred on scientific and experimental interests. In recent years, however, practical communications systems have been emerging at these frequencies. New technologies are being developed which aim to exploit the potential advantages of millimetric signals and the work undertaken here relates to one particular avenue - that of quasi-optics. This thesis introduces measurement methods for assessing millimetric components and quasi-optical systems, followed by an appraisal of Gunn oscillators and their associated power supplies as suitable signal sources for communications. A type of Gunn oscillator displaying semi-chaotic behaviour is described which may have potential as an inherently broadband source suitable for spread spectrum applications. The application of quasi-optical signal processing methods to a prototype spread spectrum millimetre wave system is presented. The system has been shown to operate satisfactorily in a number of demonstrations. Theoretical models of the quasi-optical circuit show good agreement with experiment and an analysis of the signal-to-noise behaviour predicts the potential receiver performance. Some ideas for future work, building on that presented here, are suggested.
1995-07-01T00:00:00ZRobertson, Duncan AlexanderThe use of quasi-optical techniques for signal processing at millimetre wave frequencies and their application to spread spectrum systems is presented in this thesis. Millimetre waves offer a number of advantages when compared to microwave and optical signals and as a result they are finding an increasing number of applications in the area of communications. Traditionally, millimetre wave research has been centred on scientific and experimental interests. In recent years, however, practical communications systems have been emerging at these frequencies. New technologies are being developed which aim to exploit the potential advantages of millimetric signals and the work undertaken here relates to one particular avenue - that of quasi-optics. This thesis introduces measurement methods for assessing millimetric components and quasi-optical systems, followed by an appraisal of Gunn oscillators and their associated power supplies as suitable signal sources for communications. A type of Gunn oscillator displaying semi-chaotic behaviour is described which may have potential as an inherently broadband source suitable for spread spectrum applications. The application of quasi-optical signal processing methods to a prototype spread spectrum millimetre wave system is presented. The system has been shown to operate satisfactorily in a number of demonstrations. Theoretical models of the quasi-optical circuit show good agreement with experiment and an analysis of the signal-to-noise behaviour predicts the potential receiver performance. Some ideas for future work, building on that presented here, are suggested.The design and development of fast pulsed power supplies using transmission line transformersWilson, Colin Richardhttps://hdl.handle.net/10023/136002019-04-01T10:02:39Z1992-07-01T00:00:00ZThis thesis is concerned with the development of Transmission Line Transfomer (TLT) pulsed power supplies and the generation of fast risetime (>50ns), good quality, high repetition-rate voltage pulses for flash x-ray preionisors and other applications. It explains the principle of the TLT and reports on two TLT pulsed power supplies that have been built. The first, or prototype, produced output voltage pulses with risetimes of 50ns and durations of 200ns and was used to power a flash x-ray preionisation source for a mercury bromide laser. The second, a 50kV, 100? device, was built as part of a wider research program concerned with the development of space based pulsed power supplies. The development of ceramic tile technology is also described and the relevant electrical, mechanical and thermal properties of some barium titanate tiles given; it is then shown how ceramic tiles can be used to construct compact pulse generators for TLT systems. Finally, the subject of nonlinear dielectric pulse sharpening is introduced and pulse sharpening in a delay line ladder network containing air-core inductors and non-linear capacitors is demonstrated. It is then explained how these lines can be used to improve the output risetime of a TLT.
1992-07-01T00:00:00ZWilson, Colin RichardThis thesis is concerned with the development of Transmission Line Transfomer (TLT) pulsed power supplies and the generation of fast risetime (>50ns), good quality, high repetition-rate voltage pulses for flash x-ray preionisors and other applications. It explains the principle of the TLT and reports on two TLT pulsed power supplies that have been built. The first, or prototype, produced output voltage pulses with risetimes of 50ns and durations of 200ns and was used to power a flash x-ray preionisation source for a mercury bromide laser. The second, a 50kV, 100? device, was built as part of a wider research program concerned with the development of space based pulsed power supplies. The development of ceramic tile technology is also described and the relevant electrical, mechanical and thermal properties of some barium titanate tiles given; it is then shown how ceramic tiles can be used to construct compact pulse generators for TLT systems. Finally, the subject of nonlinear dielectric pulse sharpening is introduced and pulse sharpening in a delay line ladder network containing air-core inductors and non-linear capacitors is demonstrated. It is then explained how these lines can be used to improve the output risetime of a TLT.Optical fibres in pre-detector signal processingFlinn, A.R.https://hdl.handle.net/10023/135992019-04-01T10:06:18Z1989-07-01T00:00:00ZThe basic form of conventional electro-optic sensors is described. The main drawback of these sensors is their inability to deal with the background radiation which usually accompanies the signal. This 'clutter' limits the sensors performance long before other noise such as 'shot' noise. Pre-detector signal processing using the complex amplitude of the light is introduced as a means to discriminate between the signal and 'clutter'. Further improvements to predetector signal processors can be made by the inclusion of optical fibres allowing radiation to be used with greater efficiency and enabling certain signal processing tasks to be carried out with an ease unequalled by any other method. The theory of optical waveguides and their application in sensors, interferometers, and signal processors is reviewed. Geometrical aspects of the formation of linear and circular interference fringes are described along with temporal and spatial coherence theory and their relationship to Michelson's visibility function. The requirements for efficient coupling of a source into singlemode and multimode fibres are given. We describe interference experiments between beams of light emitted from a few metres of two or more, singlemode or multimode, optical fibres. Fresnel's equation is used to obtain expressions for Fresnel and Fraunhofer diffraction patterns which enable electro-optic (E-0) sensors to be analysed by Fourier optics. Image formation is considered when the aperture plane of an E-0 sensor is illuminated with partially coherent light. This allows sensors to be designed using optical transfer functions which are sensitive to the spatial coherence of the illuminating light. Spatial coherence sensors which use gratings as aperture plane reticles are discussed. By using fibre arrays, spatial coherence processing enables E-0 sensors to discriminate between a spatially coherent source and an incoherent background. The sensors enable the position and wavelength of the source to be determined. Experiments are described which use optical fibre arrays as masks for correlation with spatial distributions of light in image planes of E-0 sensors. Correlations between laser light from different points in a scene is investigated by interfering the light emitted from an array of fibres, placed in the image plane of a sensor, with each other. Temporal signal processing experiments show that the visibility of interference fringes gives information about path differences in a scene or through an optical system. Most E-0 sensors employ wavelength filtering of the detected radiation to improve their discrimination and this is shown to be less selective than temporal coherence filtering which is sensitive to spectral bandwidth. Experiments using fibre interferometers to discriminate between red and blue laser light by their bandwidths are described. In most cases the path difference need only be a few tens of centimetres. We consider spatial and temporal coherence in fibres. We show that high visibility interference fringes can be produced by red and blue laser light transmitted through over 100 metres of singlemode or multimode fibre. The effect of detector size, relative to speckle size, is considered for fringes produced by multimode fibres. The effect of dispersion on the coherence of the light emitted from fibres is considered in terms of correlation and interference between modes. We describe experiments using a spatial light modulator called SIGHT-MOD. The device is used in various systems as a fibre optic switch and as a programmable aperture plane reticle. The contrast of the device is measured using red and green, HeNe, sources. Fourier transform images of patterns on the SIGHT-MOD are obtained and used to demonstrate the geometrical manipulation of images using 2D fibre arrays. Correlation of Fourier transform images of the SIGHT-MOD with 2D fibre arrays is demonstrated.
1989-07-01T00:00:00ZFlinn, A.R.The basic form of conventional electro-optic sensors is described. The main drawback of these sensors is their inability to deal with the background radiation which usually accompanies the signal. This 'clutter' limits the sensors performance long before other noise such as 'shot' noise. Pre-detector signal processing using the complex amplitude of the light is introduced as a means to discriminate between the signal and 'clutter'. Further improvements to predetector signal processors can be made by the inclusion of optical fibres allowing radiation to be used with greater efficiency and enabling certain signal processing tasks to be carried out with an ease unequalled by any other method. The theory of optical waveguides and their application in sensors, interferometers, and signal processors is reviewed. Geometrical aspects of the formation of linear and circular interference fringes are described along with temporal and spatial coherence theory and their relationship to Michelson's visibility function. The requirements for efficient coupling of a source into singlemode and multimode fibres are given. We describe interference experiments between beams of light emitted from a few metres of two or more, singlemode or multimode, optical fibres. Fresnel's equation is used to obtain expressions for Fresnel and Fraunhofer diffraction patterns which enable electro-optic (E-0) sensors to be analysed by Fourier optics. Image formation is considered when the aperture plane of an E-0 sensor is illuminated with partially coherent light. This allows sensors to be designed using optical transfer functions which are sensitive to the spatial coherence of the illuminating light. Spatial coherence sensors which use gratings as aperture plane reticles are discussed. By using fibre arrays, spatial coherence processing enables E-0 sensors to discriminate between a spatially coherent source and an incoherent background. The sensors enable the position and wavelength of the source to be determined. Experiments are described which use optical fibre arrays as masks for correlation with spatial distributions of light in image planes of E-0 sensors. Correlations between laser light from different points in a scene is investigated by interfering the light emitted from an array of fibres, placed in the image plane of a sensor, with each other. Temporal signal processing experiments show that the visibility of interference fringes gives information about path differences in a scene or through an optical system. Most E-0 sensors employ wavelength filtering of the detected radiation to improve their discrimination and this is shown to be less selective than temporal coherence filtering which is sensitive to spectral bandwidth. Experiments using fibre interferometers to discriminate between red and blue laser light by their bandwidths are described. In most cases the path difference need only be a few tens of centimetres. We consider spatial and temporal coherence in fibres. We show that high visibility interference fringes can be produced by red and blue laser light transmitted through over 100 metres of singlemode or multimode fibre. The effect of detector size, relative to speckle size, is considered for fringes produced by multimode fibres. The effect of dispersion on the coherence of the light emitted from fibres is considered in terms of correlation and interference between modes. We describe experiments using a spatial light modulator called SIGHT-MOD. The device is used in various systems as a fibre optic switch and as a programmable aperture plane reticle. The contrast of the device is measured using red and green, HeNe, sources. Fourier transform images of patterns on the SIGHT-MOD are obtained and used to demonstrate the geometrical manipulation of images using 2D fibre arrays. Correlation of Fourier transform images of the SIGHT-MOD with 2D fibre arrays is demonstrated.3-2: Invited paper: Color on Demand - Color-tunable OLEDs for lighting and displaysFröbel, MarkusSchwab, TobiasKliem, MonaLenk, SimoneLeo, KarlReineke, SebastianGather, Malte C.https://hdl.handle.net/10023/132742024-03-27T00:42:37Z2017-01-01T00:00:00ZA device concept for highly efficient OLEDs is introduced that allows to tune the emission color of the device over a broad range of the CIE color gamut. The approach exploits the different polarities of the positive and negative half-cycles of an alternating current driving signal to independently address two vertically stacked emission units with complementary color. Ultrathin metal electrodes fabricated by a wetting layer approach are used to achieve good electrical contact to each stack with minimal impact on optical performance.
2017-01-01T00:00:00ZFröbel, MarkusSchwab, TobiasKliem, MonaLenk, SimoneLeo, KarlReineke, SebastianGather, Malte C.A device concept for highly efficient OLEDs is introduced that allows to tune the emission color of the device over a broad range of the CIE color gamut. The approach exploits the different polarities of the positive and negative half-cycles of an alternating current driving signal to independently address two vertically stacked emission units with complementary color. Ultrathin metal electrodes fabricated by a wetting layer approach are used to achieve good electrical contact to each stack with minimal impact on optical performance.Formation of stars and stellar clusters in galactic environmentSmilgys, Romashttps://hdl.handle.net/10023/132292019-04-01T10:07:19Z2018-03-19T00:00:00ZStar and stellar cluster formation in spiral galaxies is one of the biggest questions of astrophysics. In this thesis, I study how star formation, and the formation of stellar clusters, proceeds using SPH simulations. These simulations model a region of 400 pc and 10⁷ solar masses. Star formation is modelled through the use of sink particles which represent small groups of stars. Star formation occurs in high density regions, created by galactic spiral arm passage. The spiral shock compresses the gas and generates high density regions. Once these regions attain sufficiently high density, self-gravity becomes dominant and drives collapse and star formation. The regions fragment hierarchically, forming local small groups of stars. These fall together to form clusters, which grow through subsequent mergers and large scale gas infall. As the individual star formation occurs over large distances before forming a stellar cluster, this process can result in significant age spreads of 1-2 Myrs. One protocluster is found to fail to merge due to the large scale tidal forces from the nearby regions, and instead expands forming a dispersed population of young stars such as an OB association.
2018-03-19T00:00:00ZSmilgys, RomasStar and stellar cluster formation in spiral galaxies is one of the biggest questions of astrophysics. In this thesis, I study how star formation, and the formation of stellar clusters, proceeds using SPH simulations. These simulations model a region of 400 pc and 10⁷ solar masses. Star formation is modelled through the use of sink particles which represent small groups of stars. Star formation occurs in high density regions, created by galactic spiral arm passage. The spiral shock compresses the gas and generates high density regions. Once these regions attain sufficiently high density, self-gravity becomes dominant and drives collapse and star formation. The regions fragment hierarchically, forming local small groups of stars. These fall together to form clusters, which grow through subsequent mergers and large scale gas infall. As the individual star formation occurs over large distances before forming a stellar cluster, this process can result in significant age spreads of 1-2 Myrs. One protocluster is found to fail to merge due to the large scale tidal forces from the nearby regions, and instead expands forming a dispersed population of young stars such as an OB association.Light-tissue interactions for developing portable and wearable optoelectronic devices for sensing of tissue condition, diagnostics and treatment in Photodynamic therapy (PDT)Kulyk, Olenahttps://hdl.handle.net/10023/131992019-04-01T10:10:26Z2016-06-25T00:00:00ZThis thesis presents the development and in-vivo applications of wearable and portable devices for the investigation of light interaction with tissue involved in Photodynamic therapy (PDT) and during contraction of muscles. A hand-held device and a clinical method were developed for time course in-vivo imaging of the fluorescence of the photosensitizer Protoporphyrin IX (PpIX) in healthy and diseased skin with the aim to guide improvement of PDT protocols. The device was used in a small clinical study on 11 healthy volunteers and 13 patients diagnosed with non-melanoma skin cancer (NMSC). Two types of PpIX precursors were administered: Ameluz gel and Metvix® cream. The fluorescence was imaged with a 10 minute time step over three hours which was the recommended metabolism time before commencing PDT treatment at Ninewells Hospital, Dundee. The fluorescence time course was calculated by integrating the areas with the highest intensity. The fluorescence continued to grow in all subjects during the three hours. The time course varied between individuals. There was no statistical significance between either healthy volunteers or patients in Ameluz vs Metvix® groups; nor was there statistical difference between the three lesions groups (Actinic keratosis (AK) Ameluz vs AK Metvix® vs Basal cell carcinoma (BCC) Metvix®). The p-value was larger than 0.05 in a two sample t-test with unequal variances for all the groups. However, there was strong body site dependence between the head & neck compared to the lower leg & feet, or the trunk & hands body site groups (p-value<0.01). One of the possible explanations for this was temperature and vasculature variation in skin at different body sites: the temperature is higher and the vasculature structure is denser at the head and the neck compared to the lower leg or the trunk. The temperature was not measured during the study. So in order to support this hypothesis, typical skin temperatures at the lesion sites were taken from the IR thermal images of healthy skin available in literature. PpIX fluorescence had a positive correlation to temperature. If this hypothesis is true, it will be highly important to PDT treatment. Increasing the temperature could speed up the metabolism and reduce the waiting time before starting the treatment; ambient temperature should be taken into account for daylight PDT; cooling air as pain management should be administered with caution. Potential improvements for wearable PDT light sources were investigated by modelling light transport in skin for the current LED-based Ambulight PDT device, a commercial OLED for future devices and a directional OLED developed in the group. The optical models were implemented in commercial optical software (with intrinsic Monte Carlo ray tracing and Henyey-Greenstein scattering approximation) which was validated on diffuse reflectance and transmittance measurements using in-house made tissue phantoms. The modelling was applied to investigate the benefits from diffusive and forward scattering properties of skin on light transmission in treatment light sources. 1 mm thick skin can only compensate approximately 10% of non-uniform irradiance. It means that uniform illumination is crucial for the treatment light sources. Forward scattering in skin showed a 10% improved light transmission from a collimated emission compared to a wide angle Lambertian emission. However, depth-dependent transmission measurements of directional vs Lambertian emission from organic light emitting films (a nano-imprinted grating was fabricated to provide directional emission in one of the films), collimated vs diffused HeNe laser light through fresh porcine skin did not show the expected improvement. This could be explained by skin roughness which was previously found to change the optical properties and may also affect light coupling. The modelling was applied to guide an optical design of another wearable device – a muscle contraction sensor. Muscle is fibrous and because of that scatters light differently in different directions. The sensor detects the change in backscattered light in parallel and perpendicular directions with respect to muscle fibres. The sensor was implemented on a wearable bandage on fully flexible substrate with flexible OLED and organic photodiodes. The major advantages of organic optoelectronic sensing compared to conventional electromyography (EMG) sensors are the ability to distinguish two types of contractions (isotonic and isometric), insensitivity to electromagnetic interference and the absence of an immune response due to non-invasive electrode-free sensing. Optical modelling was performed to understand the operation of the sensor. A 3D anisotropic optical model of scattering in muscle was created by geometrical manipulations with the standard Henyey-Greenstein scattering volumes. The penetration depth from the Super Yellow OLED was found to be 20-25 mm; the optimal separation between the source and the detector was found to be 20 mm. This distance provided a still detectable signal along with the best discrimination between the two backscatterings. When a 2 mm thick layer of skin and a 2 mm thick layer of adipose tissue were added to the model, the signal was hugely diffused. The discrimination between the two backscatterings decreased by three orders of magnitude, the penetration depth in muscle was reduced, and the intensity of the signal dropped down but was still detectable. With 5 mm thick adipose tissue and 2 mm thick skin the signal was too diffused and interacted with very shallow layers of muscle which approached the limits of the optical sensing of muscle activity.
2016-06-25T00:00:00ZKulyk, OlenaThis thesis presents the development and in-vivo applications of wearable and portable devices for the investigation of light interaction with tissue involved in Photodynamic therapy (PDT) and during contraction of muscles. A hand-held device and a clinical method were developed for time course in-vivo imaging of the fluorescence of the photosensitizer Protoporphyrin IX (PpIX) in healthy and diseased skin with the aim to guide improvement of PDT protocols. The device was used in a small clinical study on 11 healthy volunteers and 13 patients diagnosed with non-melanoma skin cancer (NMSC). Two types of PpIX precursors were administered: Ameluz gel and Metvix® cream. The fluorescence was imaged with a 10 minute time step over three hours which was the recommended metabolism time before commencing PDT treatment at Ninewells Hospital, Dundee. The fluorescence time course was calculated by integrating the areas with the highest intensity. The fluorescence continued to grow in all subjects during the three hours. The time course varied between individuals. There was no statistical significance between either healthy volunteers or patients in Ameluz vs Metvix® groups; nor was there statistical difference between the three lesions groups (Actinic keratosis (AK) Ameluz vs AK Metvix® vs Basal cell carcinoma (BCC) Metvix®). The p-value was larger than 0.05 in a two sample t-test with unequal variances for all the groups. However, there was strong body site dependence between the head & neck compared to the lower leg & feet, or the trunk & hands body site groups (p-value<0.01). One of the possible explanations for this was temperature and vasculature variation in skin at different body sites: the temperature is higher and the vasculature structure is denser at the head and the neck compared to the lower leg or the trunk. The temperature was not measured during the study. So in order to support this hypothesis, typical skin temperatures at the lesion sites were taken from the IR thermal images of healthy skin available in literature. PpIX fluorescence had a positive correlation to temperature. If this hypothesis is true, it will be highly important to PDT treatment. Increasing the temperature could speed up the metabolism and reduce the waiting time before starting the treatment; ambient temperature should be taken into account for daylight PDT; cooling air as pain management should be administered with caution. Potential improvements for wearable PDT light sources were investigated by modelling light transport in skin for the current LED-based Ambulight PDT device, a commercial OLED for future devices and a directional OLED developed in the group. The optical models were implemented in commercial optical software (with intrinsic Monte Carlo ray tracing and Henyey-Greenstein scattering approximation) which was validated on diffuse reflectance and transmittance measurements using in-house made tissue phantoms. The modelling was applied to investigate the benefits from diffusive and forward scattering properties of skin on light transmission in treatment light sources. 1 mm thick skin can only compensate approximately 10% of non-uniform irradiance. It means that uniform illumination is crucial for the treatment light sources. Forward scattering in skin showed a 10% improved light transmission from a collimated emission compared to a wide angle Lambertian emission. However, depth-dependent transmission measurements of directional vs Lambertian emission from organic light emitting films (a nano-imprinted grating was fabricated to provide directional emission in one of the films), collimated vs diffused HeNe laser light through fresh porcine skin did not show the expected improvement. This could be explained by skin roughness which was previously found to change the optical properties and may also affect light coupling. The modelling was applied to guide an optical design of another wearable device – a muscle contraction sensor. Muscle is fibrous and because of that scatters light differently in different directions. The sensor detects the change in backscattered light in parallel and perpendicular directions with respect to muscle fibres. The sensor was implemented on a wearable bandage on fully flexible substrate with flexible OLED and organic photodiodes. The major advantages of organic optoelectronic sensing compared to conventional electromyography (EMG) sensors are the ability to distinguish two types of contractions (isotonic and isometric), insensitivity to electromagnetic interference and the absence of an immune response due to non-invasive electrode-free sensing. Optical modelling was performed to understand the operation of the sensor. A 3D anisotropic optical model of scattering in muscle was created by geometrical manipulations with the standard Henyey-Greenstein scattering volumes. The penetration depth from the Super Yellow OLED was found to be 20-25 mm; the optimal separation between the source and the detector was found to be 20 mm. This distance provided a still detectable signal along with the best discrimination between the two backscatterings. When a 2 mm thick layer of skin and a 2 mm thick layer of adipose tissue were added to the model, the signal was hugely diffused. The discrimination between the two backscatterings decreased by three orders of magnitude, the penetration depth in muscle was reduced, and the intensity of the signal dropped down but was still detectable. With 5 mm thick adipose tissue and 2 mm thick skin the signal was too diffused and interacted with very shallow layers of muscle which approached the limits of the optical sensing of muscle activity.Nonlinear frequency conversion in isotropic semiconductor waveguidesMoutzouris, Konstantinoshttps://hdl.handle.net/10023/131172019-04-01T10:07:01Z2003-01-01T00:00:00ZThis thesis describes an experimental investigation of optical frequency conversion in
isotropic semiconductor waveguides by use of several phase-matching approaches.
Efficient, type I second harmonic generation of femtosecond pulses is reported in
birefringently-phase-matched GaAs/Alox waveguides pumped at 2.01 µm. Practical
second harmonic average powers of up to ~ 650 µW are obtained, for an average
launched pump power of ~ 5 mW. This corresponds to a waveguide conversion
efficiency of ~ 20 % and a normalized conversion efficiency of greater than 1000 %
W⁻¹cm⁻². Pump depletion of more than 80 % is recorded.
Second harmonic generation by type I, third order quasi-phase-matching in a GaAs-
AlAs superlattice waveguide is reported for fundamental wavelengths from ~1480 to
1520 nm. Quasi-phase-matching is achieved through modulation of the nonlinear
coefficient χ[sub](zxy)⁽²⁾, which is realised by periodically tuning the superlattice bandgap. An average output power of ~25 nW is obtained for a launched pump power of <2.3 mW.
Type I second harmonic generation by use of first order quasi-phase-matching in a
GaAs/AlAs symmetric superlattice waveguide is also reported, with femtosecond
fundamental pulses at 1.55 µm. A periodic spatial modulation of the bulk-like second-
order susceptibility χ[sub](zxy)⁽²⁾ is realized using quantum well intermixing by As⁺ ion
implantation. A practical second harmonic average power of ~1.5 µW is detected, for
a coupled pump power of ~11 mW.
Second harmonic generation through modal-phase-matching in GaAs/AlGaAs
semiconductor waveguides is reported. Using femtosecond pulses, both type I and
type II second harmonic conversion is demonstrated for fundamental wavelengths
near 1.55 µm. An average second harmonic power of ~10.3 µW is collected at the
waveguide output for a coupled pump power of <20 mW.
For a complete characterisation, the optical loss is measured in these nonlinear GaAs-
based waveguides over the spectral range 1.3-2.1 µm in the infrared, by deploying a
femtosecond scattering technique. Typical losses of ~5-10 dB/cm are measured for
the best of the waveguides, while a systematic intensity and wavelength dependent
study revealed the contribution of Rayleigh scattering and two photon absorption in
the overall transmission loss.
2003-01-01T00:00:00ZMoutzouris, KonstantinosThis thesis describes an experimental investigation of optical frequency conversion in
isotropic semiconductor waveguides by use of several phase-matching approaches.
Efficient, type I second harmonic generation of femtosecond pulses is reported in
birefringently-phase-matched GaAs/Alox waveguides pumped at 2.01 µm. Practical
second harmonic average powers of up to ~ 650 µW are obtained, for an average
launched pump power of ~ 5 mW. This corresponds to a waveguide conversion
efficiency of ~ 20 % and a normalized conversion efficiency of greater than 1000 %
W⁻¹cm⁻². Pump depletion of more than 80 % is recorded.
Second harmonic generation by type I, third order quasi-phase-matching in a GaAs-
AlAs superlattice waveguide is reported for fundamental wavelengths from ~1480 to
1520 nm. Quasi-phase-matching is achieved through modulation of the nonlinear
coefficient χ[sub](zxy)⁽²⁾, which is realised by periodically tuning the superlattice bandgap. An average output power of ~25 nW is obtained for a launched pump power of <2.3 mW.
Type I second harmonic generation by use of first order quasi-phase-matching in a
GaAs/AlAs symmetric superlattice waveguide is also reported, with femtosecond
fundamental pulses at 1.55 µm. A periodic spatial modulation of the bulk-like second-
order susceptibility χ[sub](zxy)⁽²⁾ is realized using quantum well intermixing by As⁺ ion
implantation. A practical second harmonic average power of ~1.5 µW is detected, for
a coupled pump power of ~11 mW.
Second harmonic generation through modal-phase-matching in GaAs/AlGaAs
semiconductor waveguides is reported. Using femtosecond pulses, both type I and
type II second harmonic conversion is demonstrated for fundamental wavelengths
near 1.55 µm. An average second harmonic power of ~10.3 µW is collected at the
waveguide output for a coupled pump power of <20 mW.
For a complete characterisation, the optical loss is measured in these nonlinear GaAs-
based waveguides over the spectral range 1.3-2.1 µm in the infrared, by deploying a
femtosecond scattering technique. Typical losses of ~5-10 dB/cm are measured for
the best of the waveguides, while a systematic intensity and wavelength dependent
study revealed the contribution of Rayleigh scattering and two photon absorption in
the overall transmission loss.Scanning tunnelling microscopy and spectroscopy of quantum materialsNeat, Matthew Jameshttps://hdl.handle.net/10023/130082023-03-10T03:03:03Z2018-06-27T00:00:00ZScanning tunnelling microscopy and spectroscopy (STM/STS) of heavy fermion materials
and superconductors has enabled the high energy and momentum resolution probing
of their electronic and many-body interactions. Heavy fermion superconductivity and
its links to unconventional pairing mechanisms remains not fully understood; and the
potential role for topology in governing the surface properties of these materials is an
exciting new topic in condensed matter physics.
In this thesis I will present STM/STS investigations into four quantum materials with
heavy fermion and/or superconducting properties. In UPt3 the heavy fermion behaviour
is investigated, resulting in the visualisation of the Kondo lattice. Signatures of inelastic
tunnelling were seen, consistent with known phonon excitations; as well as a small
gap-like structure above the Fermi level. No superconducting gap was detected at the
surface of UPt3 and quasiparticle interference (QPI) revealed light bands at the Fermi
level. The spectroscopy of PdTe2 is then investigated, revealing sharp peaks in conductance
consistent with band extrema seen in ARPES; as well as conventional type-II BCS
superconductivity. Strong QPI signatures were revealed to be consistent with scattering
of quasiparticles from the topological surface states seen in ARPES, with a chiral spin
texture dictating the q-space structure of the QPI imaging. The superconductivity at
high magnetic eld in FeSe was then investigated, revealing a two-band character gap
structure with signi cant anisotropy. The superconducting gap persisted up to 14 T;
suggesting that FeSe remains a superconductor up to even higher magnetic elds. Finally
QPI measurements on SmB6 were compared to a bulk tight binding model, which
demonstrated that the surface band structure and QPI is well described by a bulk model
and in particular f-states and their hybridisation. Evidence for topological properties
are not obvious but in-gap conduction is seen, therefore providing some metallic surface
states of some kind.
2018-06-27T00:00:00ZNeat, Matthew JamesScanning tunnelling microscopy and spectroscopy (STM/STS) of heavy fermion materials
and superconductors has enabled the high energy and momentum resolution probing
of their electronic and many-body interactions. Heavy fermion superconductivity and
its links to unconventional pairing mechanisms remains not fully understood; and the
potential role for topology in governing the surface properties of these materials is an
exciting new topic in condensed matter physics.
In this thesis I will present STM/STS investigations into four quantum materials with
heavy fermion and/or superconducting properties. In UPt3 the heavy fermion behaviour
is investigated, resulting in the visualisation of the Kondo lattice. Signatures of inelastic
tunnelling were seen, consistent with known phonon excitations; as well as a small
gap-like structure above the Fermi level. No superconducting gap was detected at the
surface of UPt3 and quasiparticle interference (QPI) revealed light bands at the Fermi
level. The spectroscopy of PdTe2 is then investigated, revealing sharp peaks in conductance
consistent with band extrema seen in ARPES; as well as conventional type-II BCS
superconductivity. Strong QPI signatures were revealed to be consistent with scattering
of quasiparticles from the topological surface states seen in ARPES, with a chiral spin
texture dictating the q-space structure of the QPI imaging. The superconductivity at
high magnetic eld in FeSe was then investigated, revealing a two-band character gap
structure with signi cant anisotropy. The superconducting gap persisted up to 14 T;
suggesting that FeSe remains a superconductor up to even higher magnetic elds. Finally
QPI measurements on SmB6 were compared to a bulk tight binding model, which
demonstrated that the surface band structure and QPI is well described by a bulk model
and in particular f-states and their hybridisation. Evidence for topological properties
are not obvious but in-gap conduction is seen, therefore providing some metallic surface
states of some kind.Single-molecule studies of surface-immobilised
and freely diffusing RNA structuresShaw, Euan Stuarthttps://hdl.handle.net/10023/129742023-12-01T03:03:32Z2016-06-22T00:00:00ZDuring the process of transcription, a messenger RNA (mRNA) copy of the genetic
information encoded in the DNA is built. As mRNA is constructed, secondary and
tertiary motifs may form, which combine into intricate structures through a
process called RNA folding, enabling RNA to perform biological functions beyond
transporting genetic information, including gene regulation and catalytic self-
cleaving processes. Facilitating RNA folding are divalent ions, located site-
specifically within the structure, and monovalent ions which bind non-specifically
to the phosphate backbone, shielding the negative charge and allowing the motifs
to move into close proximity and interact. Three-way RNA junctions are among the
smallest biologically-active RNA structures and are known to mediate both gene
regulatory and catalytic processes. In the first part of this thesis, I use single-
molecule total-internal reflection fluorescence microscopy with Förster resonance
energy transfer (TIRFM-FRET) to characterise the folding and function of two of
these structures: the adenine riboswitch and the hammerhead ribozyme. Using
single-molecule Förster resonance energy transfer (sm-FRET), I extract
information on both the prevalent conformations of these molecules at specific
chemical conditions and kinetic information on structural rearrangements which
occurr on both the molecular and global levels. Building on this knowledge built up
of the folding pathway of the adenine riboswitch induced by monovalent ions, I
moved to develop a method in which the competing interplay between monovalent
ions and urea, an unfolding reagent, is exploited to isolate and overpopulate a
transient intermediate state identified on the folding pathway. Although chemical denaturants are commonly used to investigate the structures of proteins, their
application to RNA folding is still in its infancy. For the first time, I demonstrate
that this approach allows the manipulation of the folding dynamics of RNA, forcing
the structure into a state which is ordinarily poorly-populated. I speculate that this
could enable a detailed characterisation of these states by NMR and other high
resolution ensemble techniques. Finally, I move on to expanding the range of
single-molecule techniques available in St Andrews. Despite the power of single-
molecule TIRFM-FRET, it requires surface immobilisation, which can compromise
biological function through further modifications to the natural form of the sample
under investigation. To overcome this problem, I implement single-molecule
fluorescence correlation spectroscopy (sm-FCS), which probes freely diffusing
samples in solution. After testing this sm-FCS system various test structures, I
upgrade it for dual-colour fluorescence cross-correlation spectroscopy (sm-FCCS),
and finally to multi-parameter fluorescence detection (sm-MFD), where the
fluorescence lifetime of the sample is also returned. The capabilities of these three
techniques are tested by examining protein-DNA interactions, RNA structure and
vesicle morphology.
2016-06-22T00:00:00ZShaw, Euan StuartDuring the process of transcription, a messenger RNA (mRNA) copy of the genetic
information encoded in the DNA is built. As mRNA is constructed, secondary and
tertiary motifs may form, which combine into intricate structures through a
process called RNA folding, enabling RNA to perform biological functions beyond
transporting genetic information, including gene regulation and catalytic self-
cleaving processes. Facilitating RNA folding are divalent ions, located site-
specifically within the structure, and monovalent ions which bind non-specifically
to the phosphate backbone, shielding the negative charge and allowing the motifs
to move into close proximity and interact. Three-way RNA junctions are among the
smallest biologically-active RNA structures and are known to mediate both gene
regulatory and catalytic processes. In the first part of this thesis, I use single-
molecule total-internal reflection fluorescence microscopy with Förster resonance
energy transfer (TIRFM-FRET) to characterise the folding and function of two of
these structures: the adenine riboswitch and the hammerhead ribozyme. Using
single-molecule Förster resonance energy transfer (sm-FRET), I extract
information on both the prevalent conformations of these molecules at specific
chemical conditions and kinetic information on structural rearrangements which
occurr on both the molecular and global levels. Building on this knowledge built up
of the folding pathway of the adenine riboswitch induced by monovalent ions, I
moved to develop a method in which the competing interplay between monovalent
ions and urea, an unfolding reagent, is exploited to isolate and overpopulate a
transient intermediate state identified on the folding pathway. Although chemical denaturants are commonly used to investigate the structures of proteins, their
application to RNA folding is still in its infancy. For the first time, I demonstrate
that this approach allows the manipulation of the folding dynamics of RNA, forcing
the structure into a state which is ordinarily poorly-populated. I speculate that this
could enable a detailed characterisation of these states by NMR and other high
resolution ensemble techniques. Finally, I move on to expanding the range of
single-molecule techniques available in St Andrews. Despite the power of single-
molecule TIRFM-FRET, it requires surface immobilisation, which can compromise
biological function through further modifications to the natural form of the sample
under investigation. To overcome this problem, I implement single-molecule
fluorescence correlation spectroscopy (sm-FCS), which probes freely diffusing
samples in solution. After testing this sm-FCS system various test structures, I
upgrade it for dual-colour fluorescence cross-correlation spectroscopy (sm-FCCS),
and finally to multi-parameter fluorescence detection (sm-MFD), where the
fluorescence lifetime of the sample is also returned. The capabilities of these three
techniques are tested by examining protein-DNA interactions, RNA structure and
vesicle morphology.The onset of gravitational collapse in molecular cloudsClark, Paul Campbellhttps://hdl.handle.net/10023/129452019-04-01T10:10:03Z2005-01-01T00:00:00ZWe conduct an investigation into the role that turbulence plays in the formation of stars. In small clouds, with masses of ~ 30 Mʘ and where the turbulence is only injected at the start, we find that the turbulence does not trigger star formation. Instead, the dissipation of the kinetic energy allows the mean Jeans mass of the cloud to control the formation of stars. The equipartition of the kinetic and thermal energies in the final stages before star formation, allows the pre-protostellar clumps to fragment. Binary and multiple systems are thus a natural product of star formation in a turbulent environment. We find that globally unbound clouds can be the sites of star formation. Furthermore the star formation efficiency is naturally less than 100%, thus in part providing an explanation for the low efficiency in star forming regions. Globally unbound GMCs not only form stars, and naturally disperse, within a few crossing times, but also provide a mechanism for the formation of OB associations.
2005-01-01T00:00:00ZClark, Paul CampbellWe conduct an investigation into the role that turbulence plays in the formation of stars. In small clouds, with masses of ~ 30 Mʘ and where the turbulence is only injected at the start, we find that the turbulence does not trigger star formation. Instead, the dissipation of the kinetic energy allows the mean Jeans mass of the cloud to control the formation of stars. The equipartition of the kinetic and thermal energies in the final stages before star formation, allows the pre-protostellar clumps to fragment. Binary and multiple systems are thus a natural product of star formation in a turbulent environment. We find that globally unbound clouds can be the sites of star formation. Furthermore the star formation efficiency is naturally less than 100%, thus in part providing an explanation for the low efficiency in star forming regions. Globally unbound GMCs not only form stars, and naturally disperse, within a few crossing times, but also provide a mechanism for the formation of OB associations.Transiting extra-solar planets in the field of open cluster NGC 7789Bramich, Daniel Martynhttps://hdl.handle.net/10023/129442019-04-01T10:02:51Z2005-01-01T00:00:00ZWe present results from 30 nights of observations of the intermediate-age Solar-metallicity open cluster NGC 7789 with the WFC camera on the INT telescope in La Palma. From ~900 epochs, we obtained lightcurves and Sloan r' - i' colours for ~33000 stars, with ~2400 stars with better than 1% precision. We find 24 transit candidates, 14 of which we can assign a period. We rule out the transiting planet model for 21 of these candidates using various robust arguments. For 2 candidates we are unable to decide on their nature, although it seems most likely that they are eclipsing binaries as well. We have one candidate exhibiting a single eclipse for which we derive a radius of 1.81+/0.09- Three candidates remain that require follow-up observations in order to determine their nature. Monte Carlo simulations reveal that we expected to detect ~2 transiting 3d to 5d hot Jupiter planets from all the stars in our sample if 1% of stars host such a companion and that a typical hot Jupiter radius is similar to that of HD 209458b. Our failure to find good transiting hot Jupiter candidates allows us to place an upper limit on the 3d to 5d hot Jupiter fraction of 2.6% for all stars at the 1% significance level, and similar useful limits on the hot Jupiter fraction of the different star types in our sample.
2005-01-01T00:00:00ZBramich, Daniel MartynWe present results from 30 nights of observations of the intermediate-age Solar-metallicity open cluster NGC 7789 with the WFC camera on the INT telescope in La Palma. From ~900 epochs, we obtained lightcurves and Sloan r' - i' colours for ~33000 stars, with ~2400 stars with better than 1% precision. We find 24 transit candidates, 14 of which we can assign a period. We rule out the transiting planet model for 21 of these candidates using various robust arguments. For 2 candidates we are unable to decide on their nature, although it seems most likely that they are eclipsing binaries as well. We have one candidate exhibiting a single eclipse for which we derive a radius of 1.81+/0.09- Three candidates remain that require follow-up observations in order to determine their nature. Monte Carlo simulations reveal that we expected to detect ~2 transiting 3d to 5d hot Jupiter planets from all the stars in our sample if 1% of stars host such a companion and that a typical hot Jupiter radius is similar to that of HD 209458b. Our failure to find good transiting hot Jupiter candidates allows us to place an upper limit on the 3d to 5d hot Jupiter fraction of 2.6% for all stars at the 1% significance level, and similar useful limits on the hot Jupiter fraction of the different star types in our sample.The detection and characterisation of extrasolar planetsLeigh, Christopherhttps://hdl.handle.net/10023/129432019-04-01T10:05:13Z2004-01-01T00:00:00ZSince the discovery of 51 Pegasi b in 1995, continued observations have indirectly identified over 110 planetary objects. These Jupiter-mass objects cause their host star to "wobble" slightly about the common centre-of-mass of the system, which is detectable as radial motion in high-precision Doppler spectroscopy. Of the known planets, approximately 20% are found to orbit within 0.1 AU of the star, whilst the transit of HD209458 has inferred the gas-giant nature of these close-in extrasolar giant planets (CEGPs). The discovery of CEGPs has produced a wave of speculative theory as to the exact nature of these objects, and how they came to exist so close to their parent star. Our spectroscopic technique provides a method of achieving the direct detection of a CEGP atmosphere, the results of which will allow us to test emerging models that aim to predict the atmospheric nature of CEGPs and may provide additional information on the orbital inclination and mass of the planet.
We start with a historical review of the field of extrasolar planets, followed by an introduction to the fundamental concepts which underpin the reflection of starlight from a planet's surface. We then investigate the prospects of detecting such a reflection, before detailing the technique we have devised and applied here to two known CEGP hosts.
In the first instance, r Bootis, we combined observations at the 4.2-m William Herschel telescope in 1998, 1999 and 2000. The dataset comprised 893 high-resolution échelle spectra with a total integration time of 75hr 32min spanning 17 nights. We establish an upper limit on the planet's geometric albedo p < 0.39 (at the 99.9% significance level) at the most probable orbital inclination i ~ 36 deg, assuming a grey albedo, a Venus-like phase function and a planetary radius Rp - 1.2RJup. Although a weak candidate signal appears near to the most probable radial velocity amplitude, its statistical significance is insufficient for us to claim a detection with any confidence. In the second instance, HD75289, 4 nights of VLT(UT2)/UVES observations were secured in 2003 Jan, yielding 684 high-resolution spectra with a total integration time of 26 hours. We establish an upper limit on the planet's geometric albedo p < 0.12 (to the 99.9% significance level) at the most probable orbital inclination i ~/= 60 deg, assuming a grey albedo, a Venus-like phase function and a planetary radius Rp = 1.6RJup. In both cases, we are able to rule out some combinations of the predicted planetary radius and atmospheric albedo models with high, reflective cloud decks.
2004-01-01T00:00:00ZLeigh, ChristopherSince the discovery of 51 Pegasi b in 1995, continued observations have indirectly identified over 110 planetary objects. These Jupiter-mass objects cause their host star to "wobble" slightly about the common centre-of-mass of the system, which is detectable as radial motion in high-precision Doppler spectroscopy. Of the known planets, approximately 20% are found to orbit within 0.1 AU of the star, whilst the transit of HD209458 has inferred the gas-giant nature of these close-in extrasolar giant planets (CEGPs). The discovery of CEGPs has produced a wave of speculative theory as to the exact nature of these objects, and how they came to exist so close to their parent star. Our spectroscopic technique provides a method of achieving the direct detection of a CEGP atmosphere, the results of which will allow us to test emerging models that aim to predict the atmospheric nature of CEGPs and may provide additional information on the orbital inclination and mass of the planet.
We start with a historical review of the field of extrasolar planets, followed by an introduction to the fundamental concepts which underpin the reflection of starlight from a planet's surface. We then investigate the prospects of detecting such a reflection, before detailing the technique we have devised and applied here to two known CEGP hosts.
In the first instance, r Bootis, we combined observations at the 4.2-m William Herschel telescope in 1998, 1999 and 2000. The dataset comprised 893 high-resolution échelle spectra with a total integration time of 75hr 32min spanning 17 nights. We establish an upper limit on the planet's geometric albedo p < 0.39 (at the 99.9% significance level) at the most probable orbital inclination i ~ 36 deg, assuming a grey albedo, a Venus-like phase function and a planetary radius Rp - 1.2RJup. Although a weak candidate signal appears near to the most probable radial velocity amplitude, its statistical significance is insufficient for us to claim a detection with any confidence. In the second instance, HD75289, 4 nights of VLT(UT2)/UVES observations were secured in 2003 Jan, yielding 684 high-resolution spectra with a total integration time of 26 hours. We establish an upper limit on the planet's geometric albedo p < 0.12 (to the 99.9% significance level) at the most probable orbital inclination i ~/= 60 deg, assuming a grey albedo, a Venus-like phase function and a planetary radius Rp = 1.6RJup. In both cases, we are able to rule out some combinations of the predicted planetary radius and atmospheric albedo models with high, reflective cloud decks.Surface brightness distributions of late-type starsJeffers, Sandra Victoriahttps://hdl.handle.net/10023/129412019-04-01T10:10:40Z2005-01-01T00:00:00ZThe aim of this work has been to increase our understanding of the surface brightness distributions of late-type stars through Doppler imaging and eclipse mapping techniques. Combining spectroscopic and photometric observations with the technique of Doppler Imaging, I have reconstructed surface images of the G2V star He 699 (for 08 October 2000), which show high latitude and polar structures. In the case of the KOV star AB Dor, the Doppler images for January 1992 and November 1993 show a large polar cap with small dark features also present at intermediate to high latitudes. As the phase sampling of the observations was insufficient to apply the sheared-image method it was not possible to detect any differential rotation.
In the second part of my thesis I determine the surface brightness distribution of the primary component of the RS CVn eclipsing binary SV Cam. I have used extrapolated size distributions of sunspots to an active star to synthesize images of stellar photospheres with high spot filling factors. The resulting surface images, reconstructed with the Maximum Entropy eclipse mapping technique, show large spurious spot features at the quadrature points. It is concluded that two-spot modelling or chi-squared minimisation techniques are more susceptible to spurious structures being generated by systematic errors, arising from incorrect assumptions about photospheric surface brightness, than simple Fourier analysis of the light-curves.
Spectrophotometric data from 9 HST orbits, observed in November 2001, have been used to eclipse-map the primary component of SV Cam. In combination with its HIPPAR- COS parallax it is found that the surface flux in the eclipsed low-latitude region is about 30% lower than computed from the best fitting PHOENIX model atmosphere. This flux deficit can only be accounted for if about a third of the primary's surface is covered with unresolved spots. However, when the spottedness from the eclipsed region is applied to the entire surface of the primary star, there still remains an unaccounted flux deficit. The remaining flux deficit is explained by the presence of a large polar spot extending down to latitude 48+/-6 deg. When the Maximum Entropy eclipse mapping technique is used to fit SV Cam's lightcurve, the observed minus computed residuals show strong spurious peaks at the quadrature points. It is only possible to reduce these peaks with the addition of a polar cap and the reduction of the primary star's temperature, to account for the star being peppered with unresolvable spots. Motivated by this result we investigate the limb darkening of the primary component of SV Cam. The wavelength dependence of the limb darkening is analysed by sub-dividing the HST lightcurve into 10 bands of equal emission flux. Flux variations between the first and fourth contact of the primary eclipse indicate that the limb darkening decreases towards longer wavelengths, in accordance with published limb darkening laws. Comparing fits of ATLAS and PHOENIX model atmospheres we find a wavelength dependence of the best fitting model. Due to its smooth cutoff at the stellar limb, the spherical geometry of the PHOENIX model atmosphere gives the best fit during partial eclipse. Between the second and third contact the difference between spherical and plane-parallel geometry is less important.
2005-01-01T00:00:00ZJeffers, Sandra VictoriaThe aim of this work has been to increase our understanding of the surface brightness distributions of late-type stars through Doppler imaging and eclipse mapping techniques. Combining spectroscopic and photometric observations with the technique of Doppler Imaging, I have reconstructed surface images of the G2V star He 699 (for 08 October 2000), which show high latitude and polar structures. In the case of the KOV star AB Dor, the Doppler images for January 1992 and November 1993 show a large polar cap with small dark features also present at intermediate to high latitudes. As the phase sampling of the observations was insufficient to apply the sheared-image method it was not possible to detect any differential rotation.
In the second part of my thesis I determine the surface brightness distribution of the primary component of the RS CVn eclipsing binary SV Cam. I have used extrapolated size distributions of sunspots to an active star to synthesize images of stellar photospheres with high spot filling factors. The resulting surface images, reconstructed with the Maximum Entropy eclipse mapping technique, show large spurious spot features at the quadrature points. It is concluded that two-spot modelling or chi-squared minimisation techniques are more susceptible to spurious structures being generated by systematic errors, arising from incorrect assumptions about photospheric surface brightness, than simple Fourier analysis of the light-curves.
Spectrophotometric data from 9 HST orbits, observed in November 2001, have been used to eclipse-map the primary component of SV Cam. In combination with its HIPPAR- COS parallax it is found that the surface flux in the eclipsed low-latitude region is about 30% lower than computed from the best fitting PHOENIX model atmosphere. This flux deficit can only be accounted for if about a third of the primary's surface is covered with unresolved spots. However, when the spottedness from the eclipsed region is applied to the entire surface of the primary star, there still remains an unaccounted flux deficit. The remaining flux deficit is explained by the presence of a large polar spot extending down to latitude 48+/-6 deg. When the Maximum Entropy eclipse mapping technique is used to fit SV Cam's lightcurve, the observed minus computed residuals show strong spurious peaks at the quadrature points. It is only possible to reduce these peaks with the addition of a polar cap and the reduction of the primary star's temperature, to account for the star being peppered with unresolvable spots. Motivated by this result we investigate the limb darkening of the primary component of SV Cam. The wavelength dependence of the limb darkening is analysed by sub-dividing the HST lightcurve into 10 bands of equal emission flux. Flux variations between the first and fourth contact of the primary eclipse indicate that the limb darkening decreases towards longer wavelengths, in accordance with published limb darkening laws. Comparing fits of ATLAS and PHOENIX model atmospheres we find a wavelength dependence of the best fitting model. Due to its smooth cutoff at the stellar limb, the spherical geometry of the PHOENIX model atmosphere gives the best fit during partial eclipse. Between the second and third contact the difference between spherical and plane-parallel geometry is less important.The magnetic field of AB DoradûsPointer, Graham Richardhttps://hdl.handle.net/10023/129402019-04-01T10:03:54Z2001-01-01T00:00:00ZObservations of AB Doradus, a nearby, rapidly-rotating K0 dwarf are analysed, and the surface magnetic field is shown to be approximated by a potential field. Evolving the surface magnetic field according to diffusion and the observed differential rotation still yields good correlation between the calculated and observed radial field after 30 days, contradictory to the results of Barnes et al. (1998), leading to the conclusion that there is an additional cause for the evolution of the magnetic field. The chromospheric magnetic field is modelled as a potential field with a source surface. Using the stability criteria g.B = 0 and B.V(g.B) < 0, places where prominences can be stable are investigated. For agreement with the results of Donati et al. (2000)- that prominences form preferentially near the equatorial plane and at and beyond corotation- it is necessary to add a quasidipolar field of maximum strength ~20G.
2001-01-01T00:00:00ZPointer, Graham RichardObservations of AB Doradus, a nearby, rapidly-rotating K0 dwarf are analysed, and the surface magnetic field is shown to be approximated by a potential field. Evolving the surface magnetic field according to diffusion and the observed differential rotation still yields good correlation between the calculated and observed radial field after 30 days, contradictory to the results of Barnes et al. (1998), leading to the conclusion that there is an additional cause for the evolution of the magnetic field. The chromospheric magnetic field is modelled as a potential field with a source surface. Using the stability criteria g.B = 0 and B.V(g.B) < 0, places where prominences can be stable are investigated. For agreement with the results of Donati et al. (2000)- that prominences form preferentially near the equatorial plane and at and beyond corotation- it is necessary to add a quasidipolar field of maximum strength ~20G.A search for extra-solar planetary transits in the field of open cluster NGC 6819Street, Rachelhttps://hdl.handle.net/10023/129392019-04-01T10:05:21Z2002-01-01T00:00:00ZThe technique of searching for extra-solar planetary transits is investigated. This technique, which relies on detecting the brief, shallow eclipses caused by planets passing across the line of sight to the primary star, requires high-precision time-series photometry of large numbers of stars in order to detect these statistically rare events. Observations of ~ 18000 stars in the field including the intermediate-age open cluster NGC 6819 are presented. This target field contrasts with the stellar environment surveyed by the radial velocity technique, which concentrates on the Solar neighbourhood. I present the data-reduction techniques used to obtain high-precision photometry in a semi-automated fashion for tens of thousands of stars at a time, together with an algorithm designed to search the resulting lightcurves for the transit signatures of hot Jupiter type planets. I describe simulations designed to test the detection efficiency of this algorithm and, for comparison, predict the number of transits expected from this data, assuming that hot Jupiter planets similar to HD 209458 are as common in the field of NGC 6819 as they are in the Solar neighbourhood. While no planetary transits have yet been identified, the detection of several very low amplitude eclipses by stellar companions demonstrates the effectiveness of the method. This study also indicates that stellar activity and particularly blending are significant causes of false detections. A useful additional consequence of studying this time-series photometry is the census it provides of some of the variable stars in the field. I report on the discovery of a variety of newly-discovered variables, including Algol-type detached eclipsing binaries which are likely to consist of M-dwarf stars. Further study of these stars is strongly recommended in order to help constrain models of stellar structure at the very low mass end. I conclude with a summary of this work in the context of other efforts being made in this field and recommend promising avenues of further study.
2002-01-01T00:00:00ZStreet, RachelThe technique of searching for extra-solar planetary transits is investigated. This technique, which relies on detecting the brief, shallow eclipses caused by planets passing across the line of sight to the primary star, requires high-precision time-series photometry of large numbers of stars in order to detect these statistically rare events. Observations of ~ 18000 stars in the field including the intermediate-age open cluster NGC 6819 are presented. This target field contrasts with the stellar environment surveyed by the radial velocity technique, which concentrates on the Solar neighbourhood. I present the data-reduction techniques used to obtain high-precision photometry in a semi-automated fashion for tens of thousands of stars at a time, together with an algorithm designed to search the resulting lightcurves for the transit signatures of hot Jupiter type planets. I describe simulations designed to test the detection efficiency of this algorithm and, for comparison, predict the number of transits expected from this data, assuming that hot Jupiter planets similar to HD 209458 are as common in the field of NGC 6819 as they are in the Solar neighbourhood. While no planetary transits have yet been identified, the detection of several very low amplitude eclipses by stellar companions demonstrates the effectiveness of the method. This study also indicates that stellar activity and particularly blending are significant causes of false detections. A useful additional consequence of studying this time-series photometry is the census it provides of some of the variable stars in the field. I report on the discovery of a variety of newly-discovered variables, including Algol-type detached eclipsing binaries which are likely to consist of M-dwarf stars. Further study of these stars is strongly recommended in order to help constrain models of stellar structure at the very low mass end. I conclude with a summary of this work in the context of other efforts being made in this field and recommend promising avenues of further study.The local universe as seen by the Millennium Galaxy CatalogueLemon, D. J.https://hdl.handle.net/10023/129362019-04-01T10:03:57Z2003-01-01T00:00:00ZIn this thesis we construct a B-band catalogue of the local universe which we call the Millennium Galaxy Catalogue (MGC). The MGC is photometrically and astrometrically accurate to +/-0.03 mags and +/-0.08" respectively and covers an area of~ 36 sq deg in the NGP. Colour and redshift information are available from overlapping regions of the two-degree Field Galaxy Redshift Survey (2dFGRS) and the Sloan Digital Sky Survey Early Data Release (SDSS-EDR). With an exposure time of 750s and an isophotal surface brightness limit of 26 mags/sq arcsecs, the MGC is the largest and deepest photometric survey of the local universe to-date.
As well as containing photometric information, by making use of a newly developed software package-Galaxy Image 2D (GIM2D), and assuming a standard de Vaucouleurs and exponential galaxy profile, the MGC also contains structural parameters (half light radius and (B/T) etc) to all galaxies in the magnitude range 16 < BMGC < 20.
By making use of the information in the MGC we are able to classify our galaxies into three morphological types (E/SO, Sabc and Sd/Irr) using (B/T) cuts. In doing this we find that 30.52% of the galaxies form a "new" galaxy population identified by having a (B/T) of exactly 0. After visual inspection this population is found to be a mixture of early and late type star-forming Spirals and Irregular galaxies, all of which have a disk component that is flatter than an exponential. After visually redistributing these galaxies we find that the local galaxy population consists of 28.1% E/SO, 39.64% Sabc and 31.37% Sd/Irr.
From the redshift information contained within the MGC we are able to look at the galaxy distribution in terms of physical parameters. We find that the galaxies are distributed in absolute magnitude, surface brightness and half light radii in a manner that is consistent with the hierarchical formation scenario.
We derive total and morphological galaxy number counts and, after combining them via a step wise maximum likelihood (SWML) technique, we arrive at morphological LFs. From examining the galaxy counts we find that there is no steep rise in the bright end, eradicating the need for strong local evolution. Also one does not need to renormalise the morphological or total galaxy counts in order for them to be consistent with faitner counts. The Sabc and Sd/Irr LFs are surprisingly similar with both showing a mild faint end slope.
We look at the properties of bulges and disks of galaxies. We find that as galaxies become more diskey there bulges move away from the Kormendy law for Elliptical galaxies.
Due to the similarity of the Spiral and Irregular population LFs, as well as apparent and intrinsic parameter distributions, we are forced to conclude that one can only reliably distinguish between the Ellipticals and the general galaxy population, and that investigating the bulge and disk components of a galaxy may be more useful than Hubble types.
2003-01-01T00:00:00ZLemon, D. J.In this thesis we construct a B-band catalogue of the local universe which we call the Millennium Galaxy Catalogue (MGC). The MGC is photometrically and astrometrically accurate to +/-0.03 mags and +/-0.08" respectively and covers an area of~ 36 sq deg in the NGP. Colour and redshift information are available from overlapping regions of the two-degree Field Galaxy Redshift Survey (2dFGRS) and the Sloan Digital Sky Survey Early Data Release (SDSS-EDR). With an exposure time of 750s and an isophotal surface brightness limit of 26 mags/sq arcsecs, the MGC is the largest and deepest photometric survey of the local universe to-date.
As well as containing photometric information, by making use of a newly developed software package-Galaxy Image 2D (GIM2D), and assuming a standard de Vaucouleurs and exponential galaxy profile, the MGC also contains structural parameters (half light radius and (B/T) etc) to all galaxies in the magnitude range 16 < BMGC < 20.
By making use of the information in the MGC we are able to classify our galaxies into three morphological types (E/SO, Sabc and Sd/Irr) using (B/T) cuts. In doing this we find that 30.52% of the galaxies form a "new" galaxy population identified by having a (B/T) of exactly 0. After visual inspection this population is found to be a mixture of early and late type star-forming Spirals and Irregular galaxies, all of which have a disk component that is flatter than an exponential. After visually redistributing these galaxies we find that the local galaxy population consists of 28.1% E/SO, 39.64% Sabc and 31.37% Sd/Irr.
From the redshift information contained within the MGC we are able to look at the galaxy distribution in terms of physical parameters. We find that the galaxies are distributed in absolute magnitude, surface brightness and half light radii in a manner that is consistent with the hierarchical formation scenario.
We derive total and morphological galaxy number counts and, after combining them via a step wise maximum likelihood (SWML) technique, we arrive at morphological LFs. From examining the galaxy counts we find that there is no steep rise in the bright end, eradicating the need for strong local evolution. Also one does not need to renormalise the morphological or total galaxy counts in order for them to be consistent with faitner counts. The Sabc and Sd/Irr LFs are surprisingly similar with both showing a mild faint end slope.
We look at the properties of bulges and disks of galaxies. We find that as galaxies become more diskey there bulges move away from the Kormendy law for Elliptical galaxies.
Due to the similarity of the Spiral and Irregular population LFs, as well as apparent and intrinsic parameter distributions, we are forced to conclude that one can only reliably distinguish between the Ellipticals and the general galaxy population, and that investigating the bulge and disk components of a galaxy may be more useful than Hubble types.The bivariate space density of galaxiesCross, Nicholas James Gerainthttps://hdl.handle.net/10023/129352019-04-01T10:09:49Z2002-01-01T00:00:00ZThe luminosity function of galaxies, the measurement of the space density as a function of luminosity, is an important test of cosmology, galaxy formation and evolution. Unfortunately, there is a factor of two variation in recent measurements of the luminosity function. Most of this variation is due to systematic errors, caused by various selection effects. With two large new surveys, the Two degree Field Galaxy Redshift Survey and the Sloan Digital Sky Survey, underway it is important to recognise and eliminate these selection effects if we are going to improve our measurement of the luminosity function and fully utilise these surveys. By measuring the space density of galaxies as a function of surface brightness as well as luminosity, a bivariate brightness distribution, we can comprehend many of the selection effects such as light loss, incompleteness and the visibility of galaxies. Since galaxies have a variety of shapes and sizes, a distribution in luminosity and surface brightness helps to separate out different types of galaxy. Correlations between the luminosity and surface brightness place extra constraints on models of galaxy formation and evolution. When we analyse our results, we find that recent surveys that have not taken into account surface brightness selection effects underestimate the luminosity of the bright end by 5-10%. Using the bivariate brightness distribution, we can constrain the luminosity density to a range that varies by < 20% rather than by a factor of 2. We find that the luminosity function is flat over the range -19.5 < M < -17 and then rises sharply as late-type spiral galaxies begin to dominate. The space density does not vary with surface brightness with the result that low surface brightness galaxies are at least as common as normal galaxies. However, low surface brightness galaxies are also intrinsically faint, following the luminosity-surface brightness correlation for spirals, so they do not contribute significantly to the luminosity density.
2002-01-01T00:00:00ZCross, Nicholas James GeraintThe luminosity function of galaxies, the measurement of the space density as a function of luminosity, is an important test of cosmology, galaxy formation and evolution. Unfortunately, there is a factor of two variation in recent measurements of the luminosity function. Most of this variation is due to systematic errors, caused by various selection effects. With two large new surveys, the Two degree Field Galaxy Redshift Survey and the Sloan Digital Sky Survey, underway it is important to recognise and eliminate these selection effects if we are going to improve our measurement of the luminosity function and fully utilise these surveys. By measuring the space density of galaxies as a function of surface brightness as well as luminosity, a bivariate brightness distribution, we can comprehend many of the selection effects such as light loss, incompleteness and the visibility of galaxies. Since galaxies have a variety of shapes and sizes, a distribution in luminosity and surface brightness helps to separate out different types of galaxy. Correlations between the luminosity and surface brightness place extra constraints on models of galaxy formation and evolution. When we analyse our results, we find that recent surveys that have not taken into account surface brightness selection effects underestimate the luminosity of the bright end by 5-10%. Using the bivariate brightness distribution, we can constrain the luminosity density to a range that varies by < 20% rather than by a factor of 2. We find that the luminosity function is flat over the range -19.5 < M < -17 and then rises sharply as late-type spiral galaxies begin to dominate. The space density does not vary with surface brightness with the result that low surface brightness galaxies are at least as common as normal galaxies. However, low surface brightness galaxies are also intrinsically faint, following the luminosity-surface brightness correlation for spirals, so they do not contribute significantly to the luminosity density.Experimental studies of cold atom guiding using hollow light beamsRhodes, Daniel Paulhttps://hdl.handle.net/10023/129322019-04-01T10:02:47Z2005-01-01T00:00:00ZThis thesis is concerned with the guiding of cold atoms using optical forces, which is of great importance in the field of atom optics. Atomic beams can be used for precision sensor equipment, building nano-scale structures, construction of quantum computers and to further the understanding of the properties of atoms. Atoms are guided along light beams using the dipole force; there are two regimes under which this force works. Typically red-detuned guides are used (atoms are attracted towards the light) such guides, however, require large detuning and high powers. In this thesis we investigate the use of blue-detuned (atoms are repelled from the light) hollow light beams of moderate power (a few hundred mW) and confine atoms in the dark centre of the beams. Several magneto-optical traps (MOTs) have been constructed to exploit different guiding geometries. Hollow beams have been generated using a variety of methods; in particular the use of a computer controlled spatial light modulator (SLM) has provided great versatility and simplicity to the experimental arrangements. First, experiments were performed with a low-velocity intense source (LVIS) of atoms. A co-linear LG beam significantly enhances the observed flux, however, considerable difficulties are encountered loading atoms into oblique guides. Imaging a hole in the walls of the light tube was used to improve the loading efficiency. Second, guiding a free-falling atom cloud is performed using a non-diffracting Bessel beam. It is found that while the potential of the Bessel beam is steeper than equivalent LG beams the power distribution across the beam severely limits its usefulness. The next study investigated higher-order LG guide beams generated with an SLM. High order modes have a narrower profile so confine the atoms with less interaction with the guide beam, leading to a more natural guide (as opposed to a pushing force). Finally the SLM was used to create non-trivial beam shapes for beam splitters and interferometers.
2005-01-01T00:00:00ZRhodes, Daniel PaulThis thesis is concerned with the guiding of cold atoms using optical forces, which is of great importance in the field of atom optics. Atomic beams can be used for precision sensor equipment, building nano-scale structures, construction of quantum computers and to further the understanding of the properties of atoms. Atoms are guided along light beams using the dipole force; there are two regimes under which this force works. Typically red-detuned guides are used (atoms are attracted towards the light) such guides, however, require large detuning and high powers. In this thesis we investigate the use of blue-detuned (atoms are repelled from the light) hollow light beams of moderate power (a few hundred mW) and confine atoms in the dark centre of the beams. Several magneto-optical traps (MOTs) have been constructed to exploit different guiding geometries. Hollow beams have been generated using a variety of methods; in particular the use of a computer controlled spatial light modulator (SLM) has provided great versatility and simplicity to the experimental arrangements. First, experiments were performed with a low-velocity intense source (LVIS) of atoms. A co-linear LG beam significantly enhances the observed flux, however, considerable difficulties are encountered loading atoms into oblique guides. Imaging a hole in the walls of the light tube was used to improve the loading efficiency. Second, guiding a free-falling atom cloud is performed using a non-diffracting Bessel beam. It is found that while the potential of the Bessel beam is steeper than equivalent LG beams the power distribution across the beam severely limits its usefulness. The next study investigated higher-order LG guide beams generated with an SLM. High order modes have a narrower profile so confine the atoms with less interaction with the guide beam, leading to a more natural guide (as opposed to a pushing force). Finally the SLM was used to create non-trivial beam shapes for beam splitters and interferometers.Electron microscopy of solidsBlackley, Ross A.https://hdl.handle.net/10023/129302019-04-01T10:08:40Z2005-01-01T00:00:00ZA series of compounds with general composition Ba1+/-xBi1-/+xO3 were prepared (x=0.05, 0.1, 0.2, 0.4, 0.5). The compounds were then studied by means of selected area electron diffraction and high resolution transmission electron microscopy. It was found that for a small change in the cation composition there was little or no overall change to the basic perovskite unit cell structure, whereas for the compounds with more marked changes in cation composition subtle changes to the basic unit cell were recorded. The evidence for transformation is presented by changes in the diffraction patterns and HRTEM imaging. These changes are compared with the results expected and observed for the basic perovskite unit cell for the standard BaBiO3 sample. It is thought that these changes in structure are brought about by the ordering of the Bi cation into much more complicated structures. This method of structural determination can indeed be applied to other complex materials. It has been shown that in the duration of this project that high resolution electron microscopy is invaluable in the structural determination of both microporous and mesoporous materials. The identification of pore diameters ranging from 2-20nm is of particular interest to many workers in this research field. It is shown that direct observation of these systems at sub-nanometer resolution and of loaded nanoparticles within is potentially of great value in the understanding and therefore the manipulation of these structures. It was concluded that the observation of certain nanoparticles directly loaded into the porous channels could lead to problems, however HRTEM (High Resolution Transmission Electron Microscopy) had sufficient resolution to allow the determination of the size and location of these particles. Chemical analysis using EDS was used to identify and quantify the loaded metal within the pore channels or in many cases out with the target region in these materials. It was shown that scanning electron microscopy is a more than valuable tool in the study of morphology and elemental analysis from many solids.
2005-01-01T00:00:00ZBlackley, Ross A.A series of compounds with general composition Ba1+/-xBi1-/+xO3 were prepared (x=0.05, 0.1, 0.2, 0.4, 0.5). The compounds were then studied by means of selected area electron diffraction and high resolution transmission electron microscopy. It was found that for a small change in the cation composition there was little or no overall change to the basic perovskite unit cell structure, whereas for the compounds with more marked changes in cation composition subtle changes to the basic unit cell were recorded. The evidence for transformation is presented by changes in the diffraction patterns and HRTEM imaging. These changes are compared with the results expected and observed for the basic perovskite unit cell for the standard BaBiO3 sample. It is thought that these changes in structure are brought about by the ordering of the Bi cation into much more complicated structures. This method of structural determination can indeed be applied to other complex materials. It has been shown that in the duration of this project that high resolution electron microscopy is invaluable in the structural determination of both microporous and mesoporous materials. The identification of pore diameters ranging from 2-20nm is of particular interest to many workers in this research field. It is shown that direct observation of these systems at sub-nanometer resolution and of loaded nanoparticles within is potentially of great value in the understanding and therefore the manipulation of these structures. It was concluded that the observation of certain nanoparticles directly loaded into the porous channels could lead to problems, however HRTEM (High Resolution Transmission Electron Microscopy) had sufficient resolution to allow the determination of the size and location of these particles. Chemical analysis using EDS was used to identify and quantify the loaded metal within the pore channels or in many cases out with the target region in these materials. It was shown that scanning electron microscopy is a more than valuable tool in the study of morphology and elemental analysis from many solids.Magnetic studies of cobalt based granular thin filmsOates, Colin Johnhttps://hdl.handle.net/10023/129282019-04-01T10:05:30Z2002-01-01T00:00:00ZThe magnetic recording media used for hard disks in laptops and PC's is constantly being improved, leading to rapid increases in data rate and storage density. However, by the year 2010, it is predicted that the superparamagnetic limit will be reached, which is potentially insufficient for data storage. At the beginning of this century, CoCr -based alloys are used in longitudinal media since cobalt has a high magnetocrystalline anisotropy. In this thesis, the static and dynamic properties of longitudinal recording thin films were investigated in order to explain and correlate their magnetic characteristics to their recording properties. The samples in question were test samples and some were in commercial use. Magnetic techniques such as high field ferromagnetic resonance and torque magnetometry were used to determine accurately the crystalline anisotropy field. High field ferromagnetic resonance is an ideal tool to determine the crystalline anisotropy, magnetisation, Lande g-factor and the gyromagnetic damping factor. In contrast to previous work, there are no FMR simulations and so all the relevant parameters were determined directly from measurement. Ideally, there should be no exchange interactions between the neighbouring cobalt grains; however, interactions between the grains within the CoCr-alloy recording layer exist. Previous work on the measurements of interactions in recording media involves measuring the sample's magnetisation. In this thesis, an alternative novel method involves torque magnetometry. Another technique that was used in this thesis is small angle neutron scattering, which aims to determine the size of the magnetic grains and compare that with the physical size determined from TEM, by Seagate. There is an extended section on CoxAg1-x granular thin films, which involves determining the sample's g-factor, effective anisotropy, grain size, exchange constant and comparing the FMR lineshapes at 9.5 and 92GHz.
2002-01-01T00:00:00ZOates, Colin JohnThe magnetic recording media used for hard disks in laptops and PC's is constantly being improved, leading to rapid increases in data rate and storage density. However, by the year 2010, it is predicted that the superparamagnetic limit will be reached, which is potentially insufficient for data storage. At the beginning of this century, CoCr -based alloys are used in longitudinal media since cobalt has a high magnetocrystalline anisotropy. In this thesis, the static and dynamic properties of longitudinal recording thin films were investigated in order to explain and correlate their magnetic characteristics to their recording properties. The samples in question were test samples and some were in commercial use. Magnetic techniques such as high field ferromagnetic resonance and torque magnetometry were used to determine accurately the crystalline anisotropy field. High field ferromagnetic resonance is an ideal tool to determine the crystalline anisotropy, magnetisation, Lande g-factor and the gyromagnetic damping factor. In contrast to previous work, there are no FMR simulations and so all the relevant parameters were determined directly from measurement. Ideally, there should be no exchange interactions between the neighbouring cobalt grains; however, interactions between the grains within the CoCr-alloy recording layer exist. Previous work on the measurements of interactions in recording media involves measuring the sample's magnetisation. In this thesis, an alternative novel method involves torque magnetometry. Another technique that was used in this thesis is small angle neutron scattering, which aims to determine the size of the magnetic grains and compare that with the physical size determined from TEM, by Seagate. There is an extended section on CoxAg1-x granular thin films, which involves determining the sample's g-factor, effective anisotropy, grain size, exchange constant and comparing the FMR lineshapes at 9.5 and 92GHz.Electron beam lithography and induced deposition for nanoplasmonic applicationsHeyerick, Alinehttps://hdl.handle.net/10023/128912022-10-07T09:03:09Z2018-06-27T00:00:00ZNanoplasmonics concerns the study of light-metal interactions on a subwavelength scale, exhibiting behaviour able to achieve arbitrary control and manipulation of light at the nanoscale, an important goal for the further development of nanophotonic devices. As the field of plasmonics advances, research is looking beyond the materials and fabrication techniques traditionally employed. This thesis discusses the design, fabrication methods, and characterisation of nanoplasmonic structures. A comparison between two electron beam based fabrication techniques, electron beam lithography (EBL) and electron beam induced deposition (EBID), is presented. The discussion covers both the fabrication technology and the properties of the resulting nanoscale structures: gold nanoscale features obtained via standard EBL, and tungsten structures achieved through EBID. As a mature technology, the well-understood characteristics and reliable fabrication procedures of electron beam lithography are weighed up against the limitations of top-down planar fabrication. Electron beam induced deposition is presented as an alternative technology, able to achieve nanoscale fabrication resolution with a point-and-shoot bottom-up deposition technique, but constrained by the lack of optimised fabrication settings as a result of incomplete understanding of a complex set of patterning parameters. Direct-write EBID technology offers to overcome several of the limitations and challenges of electron beam technology, including three-dimensional and greyscale patterning, and precise alignment and orientation of nanoscale features on arbitrary substrate patterns. This thesis also presents the discussion of chirped plasmonic diffraction gratings as a specific application in nanoplasmonics. Their theoretical design based on Fourier analysis and simulation-based design, their fabrication using both electron beam lithography and induced deposition, as well as the characterisation of their far-field diffraction pattern are discussed in detail. As part of the characterisation step, a Fourier microscopy setup for the measurement of the far-field diffraction patterns of nanophotonic structures in both reflection and transmission was constructed.
2018-06-27T00:00:00ZHeyerick, AlineNanoplasmonics concerns the study of light-metal interactions on a subwavelength scale, exhibiting behaviour able to achieve arbitrary control and manipulation of light at the nanoscale, an important goal for the further development of nanophotonic devices. As the field of plasmonics advances, research is looking beyond the materials and fabrication techniques traditionally employed. This thesis discusses the design, fabrication methods, and characterisation of nanoplasmonic structures. A comparison between two electron beam based fabrication techniques, electron beam lithography (EBL) and electron beam induced deposition (EBID), is presented. The discussion covers both the fabrication technology and the properties of the resulting nanoscale structures: gold nanoscale features obtained via standard EBL, and tungsten structures achieved through EBID. As a mature technology, the well-understood characteristics and reliable fabrication procedures of electron beam lithography are weighed up against the limitations of top-down planar fabrication. Electron beam induced deposition is presented as an alternative technology, able to achieve nanoscale fabrication resolution with a point-and-shoot bottom-up deposition technique, but constrained by the lack of optimised fabrication settings as a result of incomplete understanding of a complex set of patterning parameters. Direct-write EBID technology offers to overcome several of the limitations and challenges of electron beam technology, including three-dimensional and greyscale patterning, and precise alignment and orientation of nanoscale features on arbitrary substrate patterns. This thesis also presents the discussion of chirped plasmonic diffraction gratings as a specific application in nanoplasmonics. Their theoretical design based on Fourier analysis and simulation-based design, their fabrication using both electron beam lithography and induced deposition, as well as the characterisation of their far-field diffraction pattern are discussed in detail. As part of the characterisation step, a Fourier microscopy setup for the measurement of the far-field diffraction patterns of nanophotonic structures in both reflection and transmission was constructed.Title redactedMatheson, Andrew Barclayhttps://hdl.handle.net/10023/128792018-03-08T10:53:58Z2016-06-01T00:00:00Z2016-06-01T00:00:00ZMatheson, Andrew BarclayCSI 2264 : accretion process in classical T Tauri stars in the young cluster NGC 2264Sousa, A. P.Alencar, S. H. P.Bouvier, J.Stauffer, J.Venuti, L.Hillenbrand, L.Cody, A. M.Teixeira, P. S.Guimarães, M. M.McGinnis, P. T.Rebull, L.Flaccomio, E.Fürész, G.Micela, G.Gameiro, J. F.https://hdl.handle.net/10023/128712022-04-14T20:35:54Z2016-02-01T00:00:00ZContext. NGC 2264 is a young stellar cluster (~3 Myr) with hundreds of low-mass accreting stars that allow a detailed analysis of the accretion process taking place in the pre-main sequence. Aims. Our goal is to relate the photometric and spectroscopic variability of classical T Tauri stars to the physical processes acting in the stellar and circumstellar environment, within a few stellar radii from the star. Methods. NGC 2264 was the target of a multiwavelength observational campaign with CoRoT, MOST, Spitzer, and Chandra satellites and photometric and spectroscopic observations from the ground. We classified the CoRoT light curves of accreting systems according to their morphology and compared our classification to several accretion diagnostics and disk parameters. Results. The morphology of the CoRoT light curve reflects the evolution of the accretion process and of the inner disk region. Accretion burst stars present high mass-accretion rates and optically thick inner disks. AA Tau-like systems, whose light curves are dominated by circumstellar dust obscuration, show intermediate mass-accretion rates and are located in the transition of thick to anemic disks. Classical T Tauri stars with spot-like light curves correspond mostly to systems with a low mass-accretion rate and low mid-IR excess. About 30% of the classical T Tauri stars observed inthe 2008 and 2011 CoRoT runs changed their light-curve morphology.Transitions from AA Tau-like and spot-like to a periodic light curves and vice versa were common. The analysis of the Hα emission line variability of 58 accreting stars showed that 8 presented a periodicity that in a few cases was coincident with the photometric period. The blue and red wings of the Hα line profiles often do not correlate with each other, indicating that they are strongly influenced by different physical processes. Classical T Tauri stars have a dynamic stellar and circumstellar environment that can be explained by magnetospheric accretion and outflow models, including variations from stable to unstable accretion regimes on timescales of a few years.
APS and SHPA acknowledge support from CNPq, CAPES and Fapemig. JFG acknowledges support from FCT ref project UID/FIS/04434/2013.
2016-02-01T00:00:00ZSousa, A. P.Alencar, S. H. P.Bouvier, J.Stauffer, J.Venuti, L.Hillenbrand, L.Cody, A. M.Teixeira, P. S.Guimarães, M. M.McGinnis, P. T.Rebull, L.Flaccomio, E.Fürész, G.Micela, G.Gameiro, J. F.Context. NGC 2264 is a young stellar cluster (~3 Myr) with hundreds of low-mass accreting stars that allow a detailed analysis of the accretion process taking place in the pre-main sequence. Aims. Our goal is to relate the photometric and spectroscopic variability of classical T Tauri stars to the physical processes acting in the stellar and circumstellar environment, within a few stellar radii from the star. Methods. NGC 2264 was the target of a multiwavelength observational campaign with CoRoT, MOST, Spitzer, and Chandra satellites and photometric and spectroscopic observations from the ground. We classified the CoRoT light curves of accreting systems according to their morphology and compared our classification to several accretion diagnostics and disk parameters. Results. The morphology of the CoRoT light curve reflects the evolution of the accretion process and of the inner disk region. Accretion burst stars present high mass-accretion rates and optically thick inner disks. AA Tau-like systems, whose light curves are dominated by circumstellar dust obscuration, show intermediate mass-accretion rates and are located in the transition of thick to anemic disks. Classical T Tauri stars with spot-like light curves correspond mostly to systems with a low mass-accretion rate and low mid-IR excess. About 30% of the classical T Tauri stars observed inthe 2008 and 2011 CoRoT runs changed their light-curve morphology.Transitions from AA Tau-like and spot-like to a periodic light curves and vice versa were common. The analysis of the Hα emission line variability of 58 accreting stars showed that 8 presented a periodicity that in a few cases was coincident with the photometric period. The blue and red wings of the Hα line profiles often do not correlate with each other, indicating that they are strongly influenced by different physical processes. Classical T Tauri stars have a dynamic stellar and circumstellar environment that can be explained by magnetospheric accretion and outflow models, including variations from stable to unstable accretion regimes on timescales of a few years.Title redactedLong, Yunhttps://hdl.handle.net/10023/128062022-10-05T13:45:17Z2018-06-27T00:00:00Z2018-06-27T00:00:00ZLong, YunDriven-dissipative spin chain model based on exciton-polariton condensatesSigurdsson, H.Ramsay, A. J.Ohadi, H.Rubo, Y. G.Liew, T. C.H.Baumberg, J. J.Shelykh, I. A.https://hdl.handle.net/10023/126782023-04-18T23:44:48Z2017-10-15T00:00:00ZAn infinite chain of driven-dissipative condensate spins with uniform nearest-neighbor coherent coupling is solved analytically and investigated numerically. Above a critical occupation threshold the condensates undergo spontaneous spin bifurcation (becoming magnetized) forming a binary chain of spin-up or spin-down states. Minimization of the bifurcation threshold determines the magnetic order as a function of the coupling strength. This allows control of multiple magnetic orders via adiabatic (slow ramping of) pumping. In addition to ferromagnetic and antiferromagnetic ordered states we show the formation of a paired-spin ordered state |···↑↑↓↓···⟩ as a consequence of the phase degree of freedom between condensates.
This work was supported by the Research Fund of the University of Iceland, The Icelandic Research Fund, Grant No. 163082-051, Grant No. EPSRC EP/L027151/1, Grant No. ERC LINASS 320503, the Mexican Conacyt Grant No. 251808, and the Singaporean MOE GrantsNo. 2015-T2-1-055 and No. 2016-T1-1-084. I.A.S. acknowledges support from a mega-grant No. 14.Y26.31.0015 and GOSZADANIE No.3.2614.2017/4.6 of the Ministry of Education and Science of Russian Federation.
2017-10-15T00:00:00ZSigurdsson, H.Ramsay, A. J.Ohadi, H.Rubo, Y. G.Liew, T. C.H.Baumberg, J. J.Shelykh, I. A.An infinite chain of driven-dissipative condensate spins with uniform nearest-neighbor coherent coupling is solved analytically and investigated numerically. Above a critical occupation threshold the condensates undergo spontaneous spin bifurcation (becoming magnetized) forming a binary chain of spin-up or spin-down states. Minimization of the bifurcation threshold determines the magnetic order as a function of the coupling strength. This allows control of multiple magnetic orders via adiabatic (slow ramping of) pumping. In addition to ferromagnetic and antiferromagnetic ordered states we show the formation of a paired-spin ordered state |···↑↑↓↓···⟩ as a consequence of the phase degree of freedom between condensates.Strain-assisted optomechanical coupling of polariton condensate spin to a micromechanical resonatorBe'Er, O.Ohadi, H.Del Valle-Inclan Redondo, Y.Ramsay, A. J.Tsintzos, S. I.Hatzopoulos, Z.Savvidis, P. G.Baumberg, J. J.https://hdl.handle.net/10023/126772024-03-22T00:42:34Z2017-12-27T00:00:00ZWe report spin and intensity coupling of an exciton-polariton condensate to the mechanical vibrations of a circular membrane microcavity. We optically drive the microcavity resonator at the lowest mechanical resonance frequency while creating an optically trapped spin-polarized polariton condensate in different locations on the microcavity and observe spin and intensity oscillations of the condensate at the vibration frequency of the resonator. Spin oscillations are induced by vibrational strain driving, whilst the modulation of the optical trap due to the displacement of the membrane causes intensity oscillations in the condensate emission. Our results demonstrate spin-phonon coupling in a macroscopically coherent condensate.
We acknowledge Grant Nos. EPSRC EP/L027151/1, ERC LINASS 320503, and Leverhulme Trust Grant No. VP1-2013-011. P.S. acknowledges support from ITMO Fellowship Program and megaGrant No. 14.Y26.31.0015 of the Ministry of Education and Science of Russian Federation. A.J.R. acknowledges support of Horizon 2020 programme (No. FETPROACT-2016 732894-HOT). Supporting research data can be found at: http://doi.org/10.17863/CAM.16887.
2017-12-27T00:00:00ZBe'Er, O.Ohadi, H.Del Valle-Inclan Redondo, Y.Ramsay, A. J.Tsintzos, S. I.Hatzopoulos, Z.Savvidis, P. G.Baumberg, J. J.We report spin and intensity coupling of an exciton-polariton condensate to the mechanical vibrations of a circular membrane microcavity. We optically drive the microcavity resonator at the lowest mechanical resonance frequency while creating an optically trapped spin-polarized polariton condensate in different locations on the microcavity and observe spin and intensity oscillations of the condensate at the vibration frequency of the resonator. Spin oscillations are induced by vibrational strain driving, whilst the modulation of the optical trap due to the displacement of the membrane causes intensity oscillations in the condensate emission. Our results demonstrate spin-phonon coupling in a macroscopically coherent condensate.Surface flux patterns on planets in circumbinary systems and potential for photosynthesisForgan, Duncan H.Mead, AlexanderCockell, Charles S.Raven, John A.https://hdl.handle.net/10023/126122023-04-18T23:44:57Z2015-07-01T00:00:00ZRecently, the Kepler Space Telescope has detected several planets inorbit around a close binary star system. These so-called circumbinary planets will experience non-trivial spatial and temporal distributions of radiative flux on their surfaces, with features not seen in their single-star orbiting counterparts. Earth-like circumbinary planets inhabited by photosynthetic organisms will be forced to adapt to these unusual flux patterns. We map the flux received by putative Earth-like planets (as a function of surface latitude/longitude and time) orbiting the binary star systems Kepler-16 and Kepler-47, two star systems which already boast circumbinary exoplanet detections. The longitudinal and latitudinal distribution of flux is sensitive to the centre-of-mass motion of the binary, and the relative orbital phases of the binary and planet. Total eclipses of the secondary by the primary, as well as partial eclipses of the primary by the secondary add an extra forcing term to the system. We also find that the patterns of darkness on the surface are equally unique. Beyond the planet's polar circles, the surface spends a significantly longer time in darkness than latitudes around the equator, due to the stars' motions delaying the first sunrise of spring (or hastening the last sunset of autumn). In the case of Kepler-47, we also find a weak longitudinal dependence for darkness, but this effect tends to average out if considered over many orbits. In the light of these flux and darkness patterns, we consider and discuss the prospects and challenges for photosynthetic organisms, using terrestrial analogues as a guide.
2015-07-01T00:00:00ZForgan, Duncan H.Mead, AlexanderCockell, Charles S.Raven, John A.Recently, the Kepler Space Telescope has detected several planets inorbit around a close binary star system. These so-called circumbinary planets will experience non-trivial spatial and temporal distributions of radiative flux on their surfaces, with features not seen in their single-star orbiting counterparts. Earth-like circumbinary planets inhabited by photosynthetic organisms will be forced to adapt to these unusual flux patterns. We map the flux received by putative Earth-like planets (as a function of surface latitude/longitude and time) orbiting the binary star systems Kepler-16 and Kepler-47, two star systems which already boast circumbinary exoplanet detections. The longitudinal and latitudinal distribution of flux is sensitive to the centre-of-mass motion of the binary, and the relative orbital phases of the binary and planet. Total eclipses of the secondary by the primary, as well as partial eclipses of the primary by the secondary add an extra forcing term to the system. We also find that the patterns of darkness on the surface are equally unique. Beyond the planet's polar circles, the surface spends a significantly longer time in darkness than latitudes around the equator, due to the stars' motions delaying the first sunrise of spring (or hastening the last sunset of autumn). In the case of Kepler-47, we also find a weak longitudinal dependence for darkness, but this effect tends to average out if considered over many orbits. In the light of these flux and darkness patterns, we consider and discuss the prospects and challenges for photosynthetic organisms, using terrestrial analogues as a guide.Galaxy And Mass Assembly (GAMA) : a 'No Smoking' zone for giant elliptical galaxies?Khosroshahi, Habib G.Raouf, MojtabaMiraghaei, HalimeBrough, SarahCroton, Darren J.Driver, SimonGraham, AlisterBaldry, IvanBrown, MichaelPrescott, MattWang, Lingyuhttps://hdl.handle.net/10023/125632023-04-18T23:44:34Z2017-06-20T00:00:00ZWe study the radio emission of the most massive galaxies in a sample of dynamically relaxed and unrelaxed galaxy groups from the Galaxy and Mass Assembly survey. The dynamical state of the group is defined by the stellar dominance of the brightest group galaxy (BGG), e.g., the luminosity gap between the two most luminous members, and the offset between the position of the BGG and the luminosity centroid of the group. We find that the radio luminosity of the largest galaxy in the group strongly depends on its environment, such that the BGGs in dynamically young (evolving) groups are an order of magnitude more luminous in the radio than those with a similar stellar mass but residing in dynamically old (relaxed) groups. This observation has been successfully reproduced by a newly developed semi-analytic model that allows us to explore the various causes of these findings. We find that the fraction of radio-loud BGGs in the observed dynamically young groups is ~2 times that of the dynamically old groups. We discuss the implications of this observational constraint on the central galaxy properties in the context of galaxy mergers and the super massive black hole accretion rate.
2017-06-20T00:00:00ZKhosroshahi, Habib G.Raouf, MojtabaMiraghaei, HalimeBrough, SarahCroton, Darren J.Driver, SimonGraham, AlisterBaldry, IvanBrown, MichaelPrescott, MattWang, LingyuWe study the radio emission of the most massive galaxies in a sample of dynamically relaxed and unrelaxed galaxy groups from the Galaxy and Mass Assembly survey. The dynamical state of the group is defined by the stellar dominance of the brightest group galaxy (BGG), e.g., the luminosity gap between the two most luminous members, and the offset between the position of the BGG and the luminosity centroid of the group. We find that the radio luminosity of the largest galaxy in the group strongly depends on its environment, such that the BGGs in dynamically young (evolving) groups are an order of magnitude more luminous in the radio than those with a similar stellar mass but residing in dynamically old (relaxed) groups. This observation has been successfully reproduced by a newly developed semi-analytic model that allows us to explore the various causes of these findings. We find that the fraction of radio-loud BGGs in the observed dynamically young groups is ~2 times that of the dynamically old groups. We discuss the implications of this observational constraint on the central galaxy properties in the context of galaxy mergers and the super massive black hole accretion rate.Architecture of the Andromeda galaxy : a quantitative analysis of clustering in the inner stellar haloKafle, P. R.Sharma, S.Robotham, A. S. G.Lewis, G. F.Driver, S. P.https://hdl.handle.net/10023/125522023-04-18T23:44:36Z2017-02-01T00:00:00ZWe present a quantitative measurement of the amount of clustering present in the inner ∼30 kpc of the stellar halo of the Andromeda galaxy (M31). For this we analyse the angular positions and radial velocities of the carefully selected planetary nebulae in the M31 stellar halo. We study the cumulative distribution of pairwise distances in angular position and line-of-sight velocity space, and find that the M31 stellar halo contains substantially more stars in the form of close pairs as compared to that of a featureless smooth halo. In comparison to a smoothed/scrambled distribution, we estimate that the clustering excess in the M31 inner halo is roughly 40 per cent at maximum and on average ∼20 per cent. Importantly, comparing against the 11 stellar halo models of Bullock & Johnston, which were simulated within the context of the ΛCDM (Λ cold dark matter) cosmological paradigm, we find that the amount of substructures in the M31 stellar halo closely resembles that of a typical ΛCDM halo.
2017-02-01T00:00:00ZKafle, P. R.Sharma, S.Robotham, A. S. G.Lewis, G. F.Driver, S. P.We present a quantitative measurement of the amount of clustering present in the inner ∼30 kpc of the stellar halo of the Andromeda galaxy (M31). For this we analyse the angular positions and radial velocities of the carefully selected planetary nebulae in the M31 stellar halo. We study the cumulative distribution of pairwise distances in angular position and line-of-sight velocity space, and find that the M31 stellar halo contains substantially more stars in the form of close pairs as compared to that of a featureless smooth halo. In comparison to a smoothed/scrambled distribution, we estimate that the clustering excess in the M31 inner halo is roughly 40 per cent at maximum and on average ∼20 per cent. Importantly, comparing against the 11 stellar halo models of Bullock & Johnston, which were simulated within the context of the ΛCDM (Λ cold dark matter) cosmological paradigm, we find that the amount of substructures in the M31 stellar halo closely resembles that of a typical ΛCDM halo.The SAMI galaxy survey : kinematics of dusty early-type galaxiesBassett, RobertBekki, K.Cortese, L.Couch, W. J.Sansom, A. E.Sande, J. van deBryant, J. J.Foster, C.Croom, S. M.Brough, S.Sweet, S. M.Medling, A. M.Owers, M. S.Driver, S. P.Davies, L. J. M.Wong, O. I.Groves, B. A.Bland-Hawthorn, J.Richards, S. N.Goodwin, M.Konstantopoulos, I. S.Lawrence, J. S.https://hdl.handle.net/10023/125392023-04-18T23:44:32Z2017-09-01T00:00:00ZRecently, large samples of visually classified early-type galaxies (ETGs) containing dust have been identified using space-based infrared observations with the Herschel Space Telescope. The presence of large quantities of dust in massive ETGs is peculiar as X-ray haloes of these galaxies are expected to destroy dust in ∼107 yr (or less). This has sparked a debate regarding the origin of the dust: Is it internally produced by asymptotic giant branch stars, or is it accreted externally through mergers? We examine the 2D stellar and ionized gas kinematics of dusty ETGs using integral field spectroscopy observations from the SAMI Galaxy Survey, and integrated star formation rates, stellar masses and dust masses from the GAMA survey. Only 8 per cent (4/49) of visually classified ETGs are kinematically consistent with being dispersion-supported systems. These ‘dispersion-dominated galaxies’ exhibit discrepancies between stellar and ionized gas kinematics, either offsets in the kinematic position angle or large differences in the rotational velocity, and are outliers in star formation rate at a fixed dust mass compared to normal star-forming galaxies. These properties are suggestive of recent merger activity. The remaining ∼90 per cent of dusty ETGs have low velocity dispersions and/or large circular velocities, typical of ‘rotation-dominated galaxies’. These results, along with the general evidence of published works on X-ray emission in ETGs, suggest that they are unlikely to host hot, X-ray gas consistent with their low M* when compared to dispersion-dominated galaxies. This means that dust will be long-lived and thus these galaxies do not require external scenarios for the origin of their dust content.
2017-09-01T00:00:00ZBassett, RobertBekki, K.Cortese, L.Couch, W. J.Sansom, A. E.Sande, J. van deBryant, J. J.Foster, C.Croom, S. M.Brough, S.Sweet, S. M.Medling, A. M.Owers, M. S.Driver, S. P.Davies, L. J. M.Wong, O. I.Groves, B. A.Bland-Hawthorn, J.Richards, S. N.Goodwin, M.Konstantopoulos, I. S.Lawrence, J. S.Recently, large samples of visually classified early-type galaxies (ETGs) containing dust have been identified using space-based infrared observations with the Herschel Space Telescope. The presence of large quantities of dust in massive ETGs is peculiar as X-ray haloes of these galaxies are expected to destroy dust in ∼107 yr (or less). This has sparked a debate regarding the origin of the dust: Is it internally produced by asymptotic giant branch stars, or is it accreted externally through mergers? We examine the 2D stellar and ionized gas kinematics of dusty ETGs using integral field spectroscopy observations from the SAMI Galaxy Survey, and integrated star formation rates, stellar masses and dust masses from the GAMA survey. Only 8 per cent (4/49) of visually classified ETGs are kinematically consistent with being dispersion-supported systems. These ‘dispersion-dominated galaxies’ exhibit discrepancies between stellar and ionized gas kinematics, either offsets in the kinematic position angle or large differences in the rotational velocity, and are outliers in star formation rate at a fixed dust mass compared to normal star-forming galaxies. These properties are suggestive of recent merger activity. The remaining ∼90 per cent of dusty ETGs have low velocity dispersions and/or large circular velocities, typical of ‘rotation-dominated galaxies’. These results, along with the general evidence of published works on X-ray emission in ETGs, suggest that they are unlikely to host hot, X-ray gas consistent with their low M* when compared to dispersion-dominated galaxies. This means that dust will be long-lived and thus these galaxies do not require external scenarios for the origin of their dust content.VALES I : the molecular gas content in star-forming dusty H-ATLAS galaxies up to z=0.35Villanueva, V.Ibar, E.Hughes, T. M.Lara-López, M. A.Dunne, L.Eales, S.Ivison, R. J.Aravena, M.Baes, M.Bourne, N.Cassata, P.Cooray, A.Dannerbauer, H.Davies, L. J. M.Driver, S. P.Dye, S.Furlanetto, C.Herrera-Camus, R.Maddox, S. J.Michalowski, M. J.Molina, J.Riechers, D.Sansom, A. E.Smith, M. W. L.Rodighiero, G.Valiante, E.Werf, P. van derhttps://hdl.handle.net/10023/125382023-04-18T23:44:32Z2017-10-01T00:00:00ZWe present an extragalactic survey using observations from the Atacama Large Millimeter/submillimeter Array (ALMA) to characterize galaxy populations up to z = 0.35: the Valparaíso ALMA Line Emission Survey (VALES). We use ALMA Band-3 CO(1–0) observations to study the molecular gas content in a sample of 67 dusty normal star-forming galaxies selected from the Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS). We have spectrally detected 49 galaxies at >5σ significance and 12 others are seen at low significance in stacked spectra. CO luminosities are in the range of (0.03–1.31) × 1010 K km s−1 pc2, equivalent to log(Mgas/M⊙)=8.9--10.9 assuming an αCO = 4.6 (K km s−1 pc2)−1, which perfectly complements the parameter space previously explored with local and high-z normal galaxies. We compute the optical to CO size ratio for 21 galaxies resolved by ALMA at ∼3.5 arcsec resolution (6.5 kpc), finding that the molecular gas is on average ∼ 0.6 times more compact than the stellar component. We obtain a global Schmidt–Kennicutt relation, given by log[ΣSFR/(M⊙yr−1kpc−2)]=(1.26±0.02)×log[ΣMH2/(M⊙pc−2)]−(3.6±0.2). We find a significant fraction of galaxies lying at ‘intermediate efficiencies’ between a long-standing mode of star formation activity and a starburst, specially at LIR = 1011–12 L⊙. Combining our observations with data taken from the literature, we propose that star formation efficiencies can be parametrized by log[SFR/MH2]=0.19×(logLIR−11.45)−8.26−0.41 × arctan[−4.84(logLIR−11.45)]. Within the redshift range we explore (z < 0.35), we identify a rapid increase of the gas content as a function of redshift.
2017-10-01T00:00:00ZVillanueva, V.Ibar, E.Hughes, T. M.Lara-López, M. A.Dunne, L.Eales, S.Ivison, R. J.Aravena, M.Baes, M.Bourne, N.Cassata, P.Cooray, A.Dannerbauer, H.Davies, L. J. M.Driver, S. P.Dye, S.Furlanetto, C.Herrera-Camus, R.Maddox, S. J.Michalowski, M. J.Molina, J.Riechers, D.Sansom, A. E.Smith, M. W. L.Rodighiero, G.Valiante, E.Werf, P. van derWe present an extragalactic survey using observations from the Atacama Large Millimeter/submillimeter Array (ALMA) to characterize galaxy populations up to z = 0.35: the Valparaíso ALMA Line Emission Survey (VALES). We use ALMA Band-3 CO(1–0) observations to study the molecular gas content in a sample of 67 dusty normal star-forming galaxies selected from the Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS). We have spectrally detected 49 galaxies at >5σ significance and 12 others are seen at low significance in stacked spectra. CO luminosities are in the range of (0.03–1.31) × 1010 K km s−1 pc2, equivalent to log(Mgas/M⊙)=8.9--10.9 assuming an αCO = 4.6 (K km s−1 pc2)−1, which perfectly complements the parameter space previously explored with local and high-z normal galaxies. We compute the optical to CO size ratio for 21 galaxies resolved by ALMA at ∼3.5 arcsec resolution (6.5 kpc), finding that the molecular gas is on average ∼ 0.6 times more compact than the stellar component. We obtain a global Schmidt–Kennicutt relation, given by log[ΣSFR/(M⊙yr−1kpc−2)]=(1.26±0.02)×log[ΣMH2/(M⊙pc−2)]−(3.6±0.2). We find a significant fraction of galaxies lying at ‘intermediate efficiencies’ between a long-standing mode of star formation activity and a starburst, specially at LIR = 1011–12 L⊙. Combining our observations with data taken from the literature, we propose that star formation efficiencies can be parametrized by log[SFR/MH2]=0.19×(logLIR−11.45)−8.26−0.41 × arctan[−4.84(logLIR−11.45)]. Within the redshift range we explore (z < 0.35), we identify a rapid increase of the gas content as a function of redshift.Carpe lucem: harnessing organic light sources for optogeneticsMorton, AndrewMurawski, CarolineGather, Malte Christianhttps://hdl.handle.net/10023/122362023-04-19T00:45:49Z2016-12-01T00:00:00ZWith the advent of optogenetics, numerous functions in cells have been rendered responsive to the experimental delivery of light. The most common implementation of this technique features neurons genetically modified to express light-sensitive ion channel proteins, which open specifically in response to pulses of blue light, triggering electrical impulses in neurons. Optogenetics now has matured to a point where in addition to answering fundamental questions about the function of the brain, scientists begin to consider clinical applications. Further progress in this field however will require new ways of delivering light. One of these involves the use of organic light-emitting diodes (OLEDs), a display technology increasingly common in modern-day smart phones, for the optical stimulation of cells.
2016-12-01T00:00:00ZMorton, AndrewMurawski, CarolineGather, Malte ChristianWith the advent of optogenetics, numerous functions in cells have been rendered responsive to the experimental delivery of light. The most common implementation of this technique features neurons genetically modified to express light-sensitive ion channel proteins, which open specifically in response to pulses of blue light, triggering electrical impulses in neurons. Optogenetics now has matured to a point where in addition to answering fundamental questions about the function of the brain, scientists begin to consider clinical applications. Further progress in this field however will require new ways of delivering light. One of these involves the use of organic light-emitting diodes (OLEDs), a display technology increasingly common in modern-day smart phones, for the optical stimulation of cells.Advanced multimodal methods in biomedicine : Raman spectroscopy and digital holographic microscopyMcReynolds, Naomihttps://hdl.handle.net/10023/121292020-03-31T09:29:26Z2017-12-07T00:00:00ZMoving towards label-free technologies is essential for many clinical and research
applications. Raman spectroscopy is a powerful tool in the field of biomedicine
for label-free cell characterisation and disease diagnosis, owing to its high chemical specificity. However, Raman scattering is a relatively weak process and can require long acquisition times, thus hampering its integration to clinical technologies.
Multimodal analysis is currently pushing the boundaries in biomedicine, obtaining
more information than would be possible using a single mode and overcoming any
limitations specific to a single technique. Digital holographic microscopy (DHM) is a
rapid and label-free quantitative phase imaging modality, providing complementary
information to Raman spectroscopy, and is thus an ideal candidate for combination
in a multimodal system.
Firstly, this thesis explores the use of wavelength modulated Raman spectroscopy (WMRS), for the classification of immune cell subsets. Following this a
multimodal approach, combining Raman spectroscopy and DHM, is demonstrated,
where each technique is considered individually and in combination. The complementary modalities provide a wealth of information (both chemical and morphological) for cell characterisation, which is a step towards achieving a label-free technology for the identification of human immune cells. The suitability of WMRS to
discriminate between closely related neuronal cell types is also explored.
Furthermore optical spectroscopic techniques are useful for the analysis of food
and beverages. The use of Raman and fluorescence spectroscopy to successfully discriminate between various whisky and extra-virgin olive oil brands is demonstrated,
which may aid the detection of counterfeit or adulterated samples. The use of a compact Raman device is utilised, demonstrating the potential for in-field analysis.
Finally, monodisperse and highly spherical nanoparticles are synthesised. A
short study demonstrates the potential for these nanoparticles to benefit the techniques of surface enhanced Raman spectroscopy and optical trapping, by way of
minimising variability.
2017-12-07T00:00:00ZMcReynolds, NaomiMoving towards label-free technologies is essential for many clinical and research
applications. Raman spectroscopy is a powerful tool in the field of biomedicine
for label-free cell characterisation and disease diagnosis, owing to its high chemical specificity. However, Raman scattering is a relatively weak process and can require long acquisition times, thus hampering its integration to clinical technologies.
Multimodal analysis is currently pushing the boundaries in biomedicine, obtaining
more information than would be possible using a single mode and overcoming any
limitations specific to a single technique. Digital holographic microscopy (DHM) is a
rapid and label-free quantitative phase imaging modality, providing complementary
information to Raman spectroscopy, and is thus an ideal candidate for combination
in a multimodal system.
Firstly, this thesis explores the use of wavelength modulated Raman spectroscopy (WMRS), for the classification of immune cell subsets. Following this a
multimodal approach, combining Raman spectroscopy and DHM, is demonstrated,
where each technique is considered individually and in combination. The complementary modalities provide a wealth of information (both chemical and morphological) for cell characterisation, which is a step towards achieving a label-free technology for the identification of human immune cells. The suitability of WMRS to
discriminate between closely related neuronal cell types is also explored.
Furthermore optical spectroscopic techniques are useful for the analysis of food
and beverages. The use of Raman and fluorescence spectroscopy to successfully discriminate between various whisky and extra-virgin olive oil brands is demonstrated,
which may aid the detection of counterfeit or adulterated samples. The use of a compact Raman device is utilised, demonstrating the potential for in-field analysis.
Finally, monodisperse and highly spherical nanoparticles are synthesised. A
short study demonstrates the potential for these nanoparticles to benefit the techniques of surface enhanced Raman spectroscopy and optical trapping, by way of
minimising variability.Dirac solitons in general relativity and conformal gravityDorkenoo Leggat, Alasdairhttps://hdl.handle.net/10023/121272019-04-01T10:08:04Z2017-06-27T00:00:00ZStatic, spherically-symmetric particle-like solutions to the coupled Einstein-Dirac and Einstein-Dirac-Maxwell equations have been studied by Finster, Smoller and Yau (FSY). In their work, FSY left the fermion mass as a parameter set to ±1. This thesis generalises these equations to include the Higgs field, letting the fermion mass become a function through coupling, μ. We discuss the dynamics associated with the Higgs field and find that there exist qualitatively similar solutions to those found by FSY, with well behaved, non-divergent metric components and electrostatic potential, close to the origin, going over to the point-particle solutions for large r; the Schwarzschild or Reissner-Nordström metric, and the Coulomb potential. We then go on to discuss an alternative gravity theory, conformal gravity, (CG), and look for solutions of the CG equations of motion coupled to the Dirac, Higgs and Maxwell equations. We obtain asymptotically nonvanishing, yet fully normalisable Dirac spinor components, resembling those of FSY, and, in the case where charge is included, non-divergent electrostatic potential close to the origin, matching onto the Coulomb potential for large r.
2017-06-27T00:00:00ZDorkenoo Leggat, AlasdairStatic, spherically-symmetric particle-like solutions to the coupled Einstein-Dirac and Einstein-Dirac-Maxwell equations have been studied by Finster, Smoller and Yau (FSY). In their work, FSY left the fermion mass as a parameter set to ±1. This thesis generalises these equations to include the Higgs field, letting the fermion mass become a function through coupling, μ. We discuss the dynamics associated with the Higgs field and find that there exist qualitatively similar solutions to those found by FSY, with well behaved, non-divergent metric components and electrostatic potential, close to the origin, going over to the point-particle solutions for large r; the Schwarzschild or Reissner-Nordström metric, and the Coulomb potential. We then go on to discuss an alternative gravity theory, conformal gravity, (CG), and look for solutions of the CG equations of motion coupled to the Dirac, Higgs and Maxwell equations. We obtain asymptotically nonvanishing, yet fully normalisable Dirac spinor components, resembling those of FSY, and, in the case where charge is included, non-divergent electrostatic potential close to the origin, matching onto the Coulomb potential for large r.SDSS-IV MaNGA : constraints on the conditions for star formation in galaxy discsStark, David V.Bundy, Kevin A.Orr, Matthew E.Hopkins, Philip F.Westfall, KyleBershady, MatthewLi, ChengBizyaev, DmitryMasters, Karen L.Weijmans, Anne-MarieLacerna, IvanThomas, DanielDrory, NivYan, RenbinZhang, Kaihttps://hdl.handle.net/10023/120852023-04-18T23:41:51Z2017-11-13T00:00:00ZRegions of disc galaxies with widespread star formation tend to be both gravitationally unstable and self-shielded against ionizing radiation, whereas extended outer discs with little or no star formation tend to be stable and unshielded on average. We explore what drives the transition between these two regimes, specifically whether discs first meet the conditions for self-shielding (parameterized by dust optical depth, τ) or gravitational instability (parameterized by a modified version of Toomre’s instability parameters, Qthermal, which quantifies the stability of a gas disc that is thermally supported at T = 104 K). We first introduce a new metric formed by the product of these quantities, Qthermalτ, which indicates whether the conditions for disk instability or self-shielding are easier to meet in a given region of a galaxy, and we discuss how Qthermalτ can be constrained even in the absence of direct gas information. We then analyse a sample of 13 galaxies with resolved gas measurements and find that on average galaxies will reach the threshold for disk instabilities (Qthermal < 1) before reaching the threshold for self-shielding (τ > 1). Using integral field spectroscopic observations of a sample of 236 galaxies from the MaNGA survey, we find that the value of Qthermalτ in star-forming discs is consistent with similar behavior. These results support a scenario where disc fragmentation and collapse occurs before self-shielding, suggesting that gravitational instabilities are the primary condition for widespread star formation in galaxy discs. Our results support similar conclusions based on recent galaxy simulations.
Funding: Leverhulme Trust Early Career Fellowship (AW)
2017-11-13T00:00:00ZStark, David V.Bundy, Kevin A.Orr, Matthew E.Hopkins, Philip F.Westfall, KyleBershady, MatthewLi, ChengBizyaev, DmitryMasters, Karen L.Weijmans, Anne-MarieLacerna, IvanThomas, DanielDrory, NivYan, RenbinZhang, KaiRegions of disc galaxies with widespread star formation tend to be both gravitationally unstable and self-shielded against ionizing radiation, whereas extended outer discs with little or no star formation tend to be stable and unshielded on average. We explore what drives the transition between these two regimes, specifically whether discs first meet the conditions for self-shielding (parameterized by dust optical depth, τ) or gravitational instability (parameterized by a modified version of Toomre’s instability parameters, Qthermal, which quantifies the stability of a gas disc that is thermally supported at T = 104 K). We first introduce a new metric formed by the product of these quantities, Qthermalτ, which indicates whether the conditions for disk instability or self-shielding are easier to meet in a given region of a galaxy, and we discuss how Qthermalτ can be constrained even in the absence of direct gas information. We then analyse a sample of 13 galaxies with resolved gas measurements and find that on average galaxies will reach the threshold for disk instabilities (Qthermal < 1) before reaching the threshold for self-shielding (τ > 1). Using integral field spectroscopic observations of a sample of 236 galaxies from the MaNGA survey, we find that the value of Qthermalτ in star-forming discs is consistent with similar behavior. These results support a scenario where disc fragmentation and collapse occurs before self-shielding, suggesting that gravitational instabilities are the primary condition for widespread star formation in galaxy discs. Our results support similar conclusions based on recent galaxy simulations.Hybrid photonic crystal cavity based lasersLiles, Alexandros Athanasioshttps://hdl.handle.net/10023/120812019-04-01T10:07:09Z2017-12-07T00:00:00ZIn recent years, Silicon Photonics has emerged as a promising technology for cost-effective fabrication of photonic components and integrated circuits, the application of which is recently expanding in technological fields beyond tele- and data-communications, such as sensing and biophotonics. Compact, energy-efficient laser sources with precise wavelength control are crucial for the aforementioned applications. However, practical, efficient, electrically-pumped lasers on Silicon or other group IV elements are still absent, owing to the indirect bandgap of those materials. Consequently, the integration of III-V compounds on Silicon currently appears to be the most viable route to the realization of such lasers.
In this thesis, I present and explore the potential of an External Cavity (EC) hybrid III-V/Silicon laser design, comprising a III-V-based Reflective Semiconductor Optical Amplifier (RSOA) and a Silicon reflector chip, based on a two-dimensional Photonic Crystal (PhC) cavity vertically coupled to a low-refractive-index dielectric waveguide. The vertically coupled system functions as a wavelength-selective reflector, determining the lasing wavelength. Based on this architecture mW-level continuous-wave (CW) lasing at room temperature was shown both in a fiber-based long cavity scheme and die-based short cavity scheme, with SMSR of > 25 dB and > 40 dB, respectively.
Furthermore, by electrically modulating the refractive index of the PhC cavity in the reflector chip, tuning of the emitted wavelength was achieved in the die-based short cavity EC laser configuration. In this way, I demonstrated the suitability of the examined EC configuration for direct frequency modulation. The proposed scheme eliminates the need for wavelength matching between the laser source and a resonant modulator, and reveals the potential of employing low-power-consumption resonant modulation in practical Silicon Photonics applications.
2017-12-07T00:00:00ZLiles, Alexandros AthanasiosIn recent years, Silicon Photonics has emerged as a promising technology for cost-effective fabrication of photonic components and integrated circuits, the application of which is recently expanding in technological fields beyond tele- and data-communications, such as sensing and biophotonics. Compact, energy-efficient laser sources with precise wavelength control are crucial for the aforementioned applications. However, practical, efficient, electrically-pumped lasers on Silicon or other group IV elements are still absent, owing to the indirect bandgap of those materials. Consequently, the integration of III-V compounds on Silicon currently appears to be the most viable route to the realization of such lasers.
In this thesis, I present and explore the potential of an External Cavity (EC) hybrid III-V/Silicon laser design, comprising a III-V-based Reflective Semiconductor Optical Amplifier (RSOA) and a Silicon reflector chip, based on a two-dimensional Photonic Crystal (PhC) cavity vertically coupled to a low-refractive-index dielectric waveguide. The vertically coupled system functions as a wavelength-selective reflector, determining the lasing wavelength. Based on this architecture mW-level continuous-wave (CW) lasing at room temperature was shown both in a fiber-based long cavity scheme and die-based short cavity scheme, with SMSR of > 25 dB and > 40 dB, respectively.
Furthermore, by electrically modulating the refractive index of the PhC cavity in the reflector chip, tuning of the emitted wavelength was achieved in the die-based short cavity EC laser configuration. In this way, I demonstrated the suitability of the examined EC configuration for direct frequency modulation. The proposed scheme eliminates the need for wavelength matching between the laser source and a resonant modulator, and reveals the potential of employing low-power-consumption resonant modulation in practical Silicon Photonics applications.Lightning on exoplanets and brown dwarfsHodosán, Gabriellahttps://hdl.handle.net/10023/120792019-04-01T10:03:01Z2017-12-07T00:00:00ZLightning is an important electrical phenomenon, known to exist in several Solar System planets. Amongst others, it carries information on convection and cloud formation, and may be important for pre-biotic chemistry. Exoplanets and brown dwarfs have been shown to host environments appropriate for the initiation of lightning discharges. In this PhD project, I aim to determine if lightning on exoplanets and brown dwarfs can be more energetic than it is known from Solar System planets, what are the most promising signatures to look for, and if these "exo-lightning" signatures can be detected from Earth.
This thesis focuses on three major topics. First I discuss a lightning climatology study of Earth, Jupiter, Saturn, and Venus. I apply the obtained lightning statistics to extrasolar planets in order to give a first estimate on lightning occurrence on exoplanets and brown dwarfs. Next, I introduce a short study of potential lightning activity on the exoplanet HAT-P-11b, based on previous radio observations. Related to this, I discuss a first estimate of observability of lightning from close brown dwarfs, with the optical Danish Telescope. The final part of my project focuses on a lightning radio model, which is applied to study the energy and radio power released from lightning discharges in hot giant gas planetary and brown dwarf atmospheres. The released energy determines the observability of signatures, and the effect lightning has on the local atmosphere of the object.
This work combines knowledge obtained from planetary and earth sciences and uses that to learn more about extrasolar systems. My main results show that lightning on exoplanets may be more energetic than in the Solar System, supporting the possibility of future observations and detection of lightning activity on an extrasolar body. My work provides the base for future radio, optical, and infrared search for "exo-lightning".
2017-12-07T00:00:00ZHodosán, GabriellaLightning is an important electrical phenomenon, known to exist in several Solar System planets. Amongst others, it carries information on convection and cloud formation, and may be important for pre-biotic chemistry. Exoplanets and brown dwarfs have been shown to host environments appropriate for the initiation of lightning discharges. In this PhD project, I aim to determine if lightning on exoplanets and brown dwarfs can be more energetic than it is known from Solar System planets, what are the most promising signatures to look for, and if these "exo-lightning" signatures can be detected from Earth.
This thesis focuses on three major topics. First I discuss a lightning climatology study of Earth, Jupiter, Saturn, and Venus. I apply the obtained lightning statistics to extrasolar planets in order to give a first estimate on lightning occurrence on exoplanets and brown dwarfs. Next, I introduce a short study of potential lightning activity on the exoplanet HAT-P-11b, based on previous radio observations. Related to this, I discuss a first estimate of observability of lightning from close brown dwarfs, with the optical Danish Telescope. The final part of my project focuses on a lightning radio model, which is applied to study the energy and radio power released from lightning discharges in hot giant gas planetary and brown dwarf atmospheres. The released energy determines the observability of signatures, and the effect lightning has on the local atmosphere of the object.
This work combines knowledge obtained from planetary and earth sciences and uses that to learn more about extrasolar systems. My main results show that lightning on exoplanets may be more energetic than in the Solar System, supporting the possibility of future observations and detection of lightning activity on an extrasolar body. My work provides the base for future radio, optical, and infrared search for "exo-lightning".A spin- and angle-resolved photoemission study of coupled spin-orbital textures driven by global and local inversion symmetry breakingBawden, Lewishttps://hdl.handle.net/10023/120492019-08-09T08:05:33Z2017-09-21T00:00:00ZThe effect of spin-orbit coupling had once been thought to be a minor perturbation to the low energy band structure that could be ignored. Instead, a surge in recent theoretical and experimental efforts have shown spin-orbit interactions to have significant consequences. The main objective of this thesis is to investigate the role of the orbital sector and crystal symmetries in governing the spin texture in materials that have strong spin-orbit interactions. This can be accessed through a combination of spin- and angle-resolved photoemission spectroscopy (ARPES and spin-ARPES), both of which are powerful techniques for probing the one-electron band structure plus interactions, and supported by density functional theory calculations (DFT).
We focus first on a globally inversion asymmetric material, the layered semiconductor BiTeI, which hosts a giant spin-splitting of its bulk bands. We show that these spin-split bands develop a previously undiscovered, momentum-space ordering of the atomic orbitals. We demonstrate this orbital texture to be atomic element specific by exploiting resonant enhancements in ARPES. These orbital textures drive a hierarchy of spin textures that are then tied to the constituent atomic layers. This opens routes to controlling the spin-splitting through manipulation of the atomic orbitals.
This is contrasted against a material where inversion symmetry is globally upheld but locally broken within each monolayer of a two layer unit cell. Through our ARPES and spin-ARPES measurements of 2H-NbSe₂, we discover the first experimental evidence for a strong out-of-plane spin polarisation that persists up to the Fermi surface in this globally inversion symmetric material. This is found to be intrinsically linked to the orbital character and dimensionality of the underlying bands. So far, previous theories underpinning this (and related) materials’ collective phases assume a spin-degenerate Fermi sea. We therefore expect this spin-polarisation to play a role in determining the underlying mechanism for the charge density wave phase and superconductivity.
Through these studies, this thesis then develops the importance of global versus local inversion symmetry breaking and uncovers how this is intricately tied to the underlying atomic orbital configuration.
2017-09-21T00:00:00ZBawden, LewisThe effect of spin-orbit coupling had once been thought to be a minor perturbation to the low energy band structure that could be ignored. Instead, a surge in recent theoretical and experimental efforts have shown spin-orbit interactions to have significant consequences. The main objective of this thesis is to investigate the role of the orbital sector and crystal symmetries in governing the spin texture in materials that have strong spin-orbit interactions. This can be accessed through a combination of spin- and angle-resolved photoemission spectroscopy (ARPES and spin-ARPES), both of which are powerful techniques for probing the one-electron band structure plus interactions, and supported by density functional theory calculations (DFT).
We focus first on a globally inversion asymmetric material, the layered semiconductor BiTeI, which hosts a giant spin-splitting of its bulk bands. We show that these spin-split bands develop a previously undiscovered, momentum-space ordering of the atomic orbitals. We demonstrate this orbital texture to be atomic element specific by exploiting resonant enhancements in ARPES. These orbital textures drive a hierarchy of spin textures that are then tied to the constituent atomic layers. This opens routes to controlling the spin-splitting through manipulation of the atomic orbitals.
This is contrasted against a material where inversion symmetry is globally upheld but locally broken within each monolayer of a two layer unit cell. Through our ARPES and spin-ARPES measurements of 2H-NbSe₂, we discover the first experimental evidence for a strong out-of-plane spin polarisation that persists up to the Fermi surface in this globally inversion symmetric material. This is found to be intrinsically linked to the orbital character and dimensionality of the underlying bands. So far, previous theories underpinning this (and related) materials’ collective phases assume a spin-degenerate Fermi sea. We therefore expect this spin-polarisation to play a role in determining the underlying mechanism for the charge density wave phase and superconductivity.
Through these studies, this thesis then develops the importance of global versus local inversion symmetry breaking and uncovers how this is intricately tied to the underlying atomic orbital configuration.Spectroscopic imaging STM study of the interplay between magnetism and superconductivity in iron-based superconductorsAluru, Rama K. P.https://hdl.handle.net/10023/120482018-07-24T10:42:14Z2017-12-07T00:00:00ZThe discovery of high-temperature superconductivity in 1986 in copper-oxide materials have
opened up new avenues to investigate new families of quantum materials that were previously
not known. Understanding the mechanism of superconductivity in high-T[sub]c superconductors
has been an important research theme in condensed matter physics, as it is believed to be
essential to realize the next generation engineered materials that become superconducting at
room temperature. Discovered in 2006, iron based superconductors are a new addition to
the family of high-T[sub]c superconductors, these materials exhibit several interesting properties
and show some vivid similarities with cuprates and other families of high-temperature
superconductors.
In this thesis, I will present the spin-polarized scanning tunneling microscopy (SPSTM) study carried out on the parent compound of iron chalcogenide high temperature superconductor Fe[sub](1+y)Te to investigate the bi-collinear antiferromagnetic order. Magnetic tips in this work are prepared using a novel preparation technique by picking up excess iron atoms and clusters of FeTe from the surface of the sample. Next, I will present the SP-STM results obtained in the spin glass phase of Fe[sub](1+y)SeₓTe₁₋ₓ visualizing the interplay between the short ranged bi-directional bi-collinear antiferromagnetic order and superconductivity at the atomic scale.
In this thesis, I will also present the scanning tunneling microscopy and spectroscopy (STM/STS) study of the native and engineered defect bound states in the iron-pnictide superconductor LiFeAs. This study addresses the pairing symmetry of the superconducting order parameter and understanding of dip-hump features seen in STM spectra outside the superconducting gap in iron pnictide superconductor LiFeAs.
2017-12-07T00:00:00ZAluru, Rama K. P.The discovery of high-temperature superconductivity in 1986 in copper-oxide materials have
opened up new avenues to investigate new families of quantum materials that were previously
not known. Understanding the mechanism of superconductivity in high-T[sub]c superconductors
has been an important research theme in condensed matter physics, as it is believed to be
essential to realize the next generation engineered materials that become superconducting at
room temperature. Discovered in 2006, iron based superconductors are a new addition to
the family of high-T[sub]c superconductors, these materials exhibit several interesting properties
and show some vivid similarities with cuprates and other families of high-temperature
superconductors.
In this thesis, I will present the spin-polarized scanning tunneling microscopy (SPSTM) study carried out on the parent compound of iron chalcogenide high temperature superconductor Fe[sub](1+y)Te to investigate the bi-collinear antiferromagnetic order. Magnetic tips in this work are prepared using a novel preparation technique by picking up excess iron atoms and clusters of FeTe from the surface of the sample. Next, I will present the SP-STM results obtained in the spin glass phase of Fe[sub](1+y)SeₓTe₁₋ₓ visualizing the interplay between the short ranged bi-directional bi-collinear antiferromagnetic order and superconductivity at the atomic scale.
In this thesis, I will also present the scanning tunneling microscopy and spectroscopy (STM/STS) study of the native and engineered defect bound states in the iron-pnictide superconductor LiFeAs. This study addresses the pairing symmetry of the superconducting order parameter and understanding of dip-hump features seen in STM spectra outside the superconducting gap in iron pnictide superconductor LiFeAs.Single-molecule studies of nucleic acid folding and nucleic acid-protein interactionsPérez González, Daniel Cibránhttps://hdl.handle.net/10023/120392023-04-12T11:45:35Z2017-12-07T00:00:00ZNucleic acids and proteins, some of the building blocks of life, are not static structures but highly dynamic entities that need to interact with one another to meet cellular demands. The work presented in this thesis focuses on the application of highly sensitive fluorescence methods, both at ensemble and single-molecule level, to determine the dynamics and structure of specific biomolecular interactions with nanometer resolution and in temporal scales from nanoseconds to minutes, which includes most biologically relevant processes. The main aims of my PhD can be classified in three areas: i) exploring new fluorescent sensors with increased specificity for certain nucleic acid structures; ii) understanding how some of these nucleic acids sense the presence of small molecules in the cellular environment and trigger gene regulation by altering their structure; and iii) understanding how certain molecular machines, such as helicase proteins, are able to unwind the DNA double helix by using chemical energy in the form of ATP hydrolysis.
2017-12-07T00:00:00ZPérez González, Daniel CibránNucleic acids and proteins, some of the building blocks of life, are not static structures but highly dynamic entities that need to interact with one another to meet cellular demands. The work presented in this thesis focuses on the application of highly sensitive fluorescence methods, both at ensemble and single-molecule level, to determine the dynamics and structure of specific biomolecular interactions with nanometer resolution and in temporal scales from nanoseconds to minutes, which includes most biologically relevant processes. The main aims of my PhD can be classified in three areas: i) exploring new fluorescent sensors with increased specificity for certain nucleic acid structures; ii) understanding how some of these nucleic acids sense the presence of small molecules in the cellular environment and trigger gene regulation by altering their structure; and iii) understanding how certain molecular machines, such as helicase proteins, are able to unwind the DNA double helix by using chemical energy in the form of ATP hydrolysis.Nano-engineered solution processed solid-state semiconductor lasersWhitworth, Guy Lukehttps://hdl.handle.net/10023/120282018-07-25T13:59:03Z2016-11-01T00:00:00ZThis thesis describes various methods for patterning the electronic and physical
structure of conjugated polymers, ranging from molecular to sub-wavelength
scales. This was done to improve the operation, fabrication and application of
conjugated polymer distributed feedback lasers and additionally new gain
materials were explored to overcome to some of the limitations.
Organic lasers are currently limited to pulsed operation and unable to
achieve continuous wave operation. The effects of triplet states preventing
continuous wave operation is explored here, and a triplet management
scheme based on selectively quenching guest molecules was used to structure
the excited states. Using excited state spectroscopy this scheme was optimised
and then utilised to extend the conjugated polymer lifetime by a factor of ~3.
CH₃NH₃PbI₃ perovskite waveguides were also fabricated and then
nanostructured using nanoimprinted substrates to make one of the first
perovskite distributed feedback lasers. Perovskite semiconductors share many of
the same properties as organic semiconductors with the potential not to suffer
from triplet-state interactions.
The physical engineering of conjugated polymers was also explored with
the development of a new solvent based nanoimprinting method for the nano-structuring of polymer films. This processed allowed for the imprinting of sub-wavelength scale gratings directly into conjugated polymers. The structured
polymer films were subsequently explored as data transmitters as well as
distributed feedback lasers. Lastly, the engineering moved to the molecular
scale. Using cocaine molecules, a specially synthesised molecularly imprinted
polymer used to develop the first “laser turn-on” detection system, combing
both physical and electronic structuring themes of the thesis.
2016-11-01T00:00:00ZWhitworth, Guy LukeThis thesis describes various methods for patterning the electronic and physical
structure of conjugated polymers, ranging from molecular to sub-wavelength
scales. This was done to improve the operation, fabrication and application of
conjugated polymer distributed feedback lasers and additionally new gain
materials were explored to overcome to some of the limitations.
Organic lasers are currently limited to pulsed operation and unable to
achieve continuous wave operation. The effects of triplet states preventing
continuous wave operation is explored here, and a triplet management
scheme based on selectively quenching guest molecules was used to structure
the excited states. Using excited state spectroscopy this scheme was optimised
and then utilised to extend the conjugated polymer lifetime by a factor of ~3.
CH₃NH₃PbI₃ perovskite waveguides were also fabricated and then
nanostructured using nanoimprinted substrates to make one of the first
perovskite distributed feedback lasers. Perovskite semiconductors share many of
the same properties as organic semiconductors with the potential not to suffer
from triplet-state interactions.
The physical engineering of conjugated polymers was also explored with
the development of a new solvent based nanoimprinting method for the nano-structuring of polymer films. This processed allowed for the imprinting of sub-wavelength scale gratings directly into conjugated polymers. The structured
polymer films were subsequently explored as data transmitters as well as
distributed feedback lasers. Lastly, the engineering moved to the molecular
scale. Using cocaine molecules, a specially synthesised molecularly imprinted
polymer used to develop the first “laser turn-on” detection system, combing
both physical and electronic structuring themes of the thesis.A non-Newtonian perspective of gravity : testing modified gravity theories in galaxies and galaxy clustersHodson, Alistairhttps://hdl.handle.net/10023/120162019-04-01T10:06:50Z2017-12-07T00:00:00ZThis thesis attempts to test several frameworks of non-Newtonian gravity in the context of galaxies and galaxy clusters. The theory most extensively discussed was that of Modified Newtonian Dynamics (MOND) with Galileon gravity, Emergent Gravity (EG) and Modified Gravity (MOG) mentioned to a lesser extent. Specifically, the main focus of this thesis was to determine whether MOND and MOND-like theories were compatible with galaxy cluster data, without the need to include cold dark matter. To do this, the paradigms of Extended MOND (EMOND), Generalised MOND (GMOND) and superfluid dark matter were investigated. The theories were outlined and applied to galaxy cluster data. The main findings of this were that EMOND and GMOND had some success with explaining galaxy cluster mass profiles, without requiring an additional dark matter component. The superfluid paradigm also enjoyed some success in galaxy clusters, which was expected as it behaves in a similar manner to the standard cold dark matter paradigm in cluster environments. However, the superfluid paradigm may have issues in the very centre of galaxy clusters due to the theory predicting constant density cores, whereas the cold dark matter paradigm predicts density cores which are cuspier.
The EMOND paradigm was also tested against ultra-diffuse galaxy (UDGs) data as they appear in cluster environments, where EMOND becomes important. It was found that EMOND can reproduce the inferred mass of the UDGs, assuming they lie on the fundamental manifold (FM). The validity of the assumptions used to model the UDGs are discussed in the text.
A two-body problem was also conducted in the Galileon gravity framework. The amount of additional gravitational force, compared to Newtonian was determined for a small galaxy at the edge of a galaxy cluster.
2017-12-07T00:00:00ZHodson, AlistairThis thesis attempts to test several frameworks of non-Newtonian gravity in the context of galaxies and galaxy clusters. The theory most extensively discussed was that of Modified Newtonian Dynamics (MOND) with Galileon gravity, Emergent Gravity (EG) and Modified Gravity (MOG) mentioned to a lesser extent. Specifically, the main focus of this thesis was to determine whether MOND and MOND-like theories were compatible with galaxy cluster data, without the need to include cold dark matter. To do this, the paradigms of Extended MOND (EMOND), Generalised MOND (GMOND) and superfluid dark matter were investigated. The theories were outlined and applied to galaxy cluster data. The main findings of this were that EMOND and GMOND had some success with explaining galaxy cluster mass profiles, without requiring an additional dark matter component. The superfluid paradigm also enjoyed some success in galaxy clusters, which was expected as it behaves in a similar manner to the standard cold dark matter paradigm in cluster environments. However, the superfluid paradigm may have issues in the very centre of galaxy clusters due to the theory predicting constant density cores, whereas the cold dark matter paradigm predicts density cores which are cuspier.
The EMOND paradigm was also tested against ultra-diffuse galaxy (UDGs) data as they appear in cluster environments, where EMOND becomes important. It was found that EMOND can reproduce the inferred mass of the UDGs, assuming they lie on the fundamental manifold (FM). The validity of the assumptions used to model the UDGs are discussed in the text.
A two-body problem was also conducted in the Galileon gravity framework. The amount of additional gravitational force, compared to Newtonian was determined for a small galaxy at the edge of a galaxy cluster.TOYS - time-domain observations of young starsBozhinova, Innahttps://hdl.handle.net/10023/120142019-04-01T10:02:37Z2017-12-07T00:00:00ZStars form inside clouds of molecular gas and dust. In the early stages of stellar evolution the remainders of the initial cloud form a circumstellar disk. For the next few million years the disk will slowly dissipate via accretion, outflows, photoevaporation and planet growth while the star makes its way onto the Main Sequence. This stage of a star’s life is referred to as the T Tauri phase and is characterised by high-level spectrophotometric variability. This thesis aims to study and map out the environments of T Tauri stars down to the very low mass regime by the means of time-domain monitoring.
Different physical processes in the system manifest themselves as variability on different time- scales as well as produce characteristic spectroscopic and photometric features at various wave- lengths. In order to study young stellar objects in depth, the observing campaigns presented in this work were designed to cover a large range of time-scales - minutes, hours, days and months. Combining all the data, this thesis establishes a baseline of over a decade for some objects. The observations also cover a wide range of wavelengths from the optical to the mid-infrared part of the spectrum.
The star RW Aur experienced two long-lasting dimming events in 2010 and 2014. This thesis presents a large collection of spectral and photometric measurements carried out just before and during the 2014 event. Spectral accretion signatures indicate no change in the accretion activity of the system. Photometry indicates that parallel to the dimming in the optical the star becomes brighter in the mid-infrared. The observations in this work combined with literature data suggest that the origin of the 2014 event is most likely obscuration of the star by hot dust from the disk being lifted into the disk wind.
Very low mass stars (<0.4 M⊙) are the most common type of star in the Galaxy. In order to understand the early stages of stellar evolution we must study young very low mass stars. This work investigates the photometric and spectroscopic variability of seven brown dwarfs in star forming regions near σ Ori and ε Ori. All targets exhibit optical photometric variability between from 0.1 to over 1.0 magnitude that persists on a time-scale of at least one decade. Despite the photometric variability no change in the spectral type is measured. In the cases where the stars are accreting, modelling of the spectral changes suggest the accretion flow is more homogeneous and less funnelled compared to Sun-like T Tauri stars. The non-accreting variables are more plausibly explained by obscuration by circumstellar material, possibly a ring made out of multiple clouds of dust grains and pebbles with varying optical depths.
The star-disk systems studied in this thesis have some broader implications for star and planet formation theory. The case-study of RW Aur has unambiguously demonstrated that the planet- forming environment is very dynamic and can change dramatically on short time-scales, which in turn would have implications for the diversity of planetary systems found in the Galaxy. The Orion stars have shown that the current theory for the T Tauri stage of stellar evolution is valid down to the very low mass regime. The seven dwarfs are a good example for the evolutionary path of circumstellar disks, showing the transition from gas-high, flared accretion disks (σ Ori) to dust-dominated, depleted, structured debris disks (ε Ori).
2017-12-07T00:00:00ZBozhinova, InnaStars form inside clouds of molecular gas and dust. In the early stages of stellar evolution the remainders of the initial cloud form a circumstellar disk. For the next few million years the disk will slowly dissipate via accretion, outflows, photoevaporation and planet growth while the star makes its way onto the Main Sequence. This stage of a star’s life is referred to as the T Tauri phase and is characterised by high-level spectrophotometric variability. This thesis aims to study and map out the environments of T Tauri stars down to the very low mass regime by the means of time-domain monitoring.
Different physical processes in the system manifest themselves as variability on different time- scales as well as produce characteristic spectroscopic and photometric features at various wave- lengths. In order to study young stellar objects in depth, the observing campaigns presented in this work were designed to cover a large range of time-scales - minutes, hours, days and months. Combining all the data, this thesis establishes a baseline of over a decade for some objects. The observations also cover a wide range of wavelengths from the optical to the mid-infrared part of the spectrum.
The star RW Aur experienced two long-lasting dimming events in 2010 and 2014. This thesis presents a large collection of spectral and photometric measurements carried out just before and during the 2014 event. Spectral accretion signatures indicate no change in the accretion activity of the system. Photometry indicates that parallel to the dimming in the optical the star becomes brighter in the mid-infrared. The observations in this work combined with literature data suggest that the origin of the 2014 event is most likely obscuration of the star by hot dust from the disk being lifted into the disk wind.
Very low mass stars (<0.4 M⊙) are the most common type of star in the Galaxy. In order to understand the early stages of stellar evolution we must study young very low mass stars. This work investigates the photometric and spectroscopic variability of seven brown dwarfs in star forming regions near σ Ori and ε Ori. All targets exhibit optical photometric variability between from 0.1 to over 1.0 magnitude that persists on a time-scale of at least one decade. Despite the photometric variability no change in the spectral type is measured. In the cases where the stars are accreting, modelling of the spectral changes suggest the accretion flow is more homogeneous and less funnelled compared to Sun-like T Tauri stars. The non-accreting variables are more plausibly explained by obscuration by circumstellar material, possibly a ring made out of multiple clouds of dust grains and pebbles with varying optical depths.
The star-disk systems studied in this thesis have some broader implications for star and planet formation theory. The case-study of RW Aur has unambiguously demonstrated that the planet- forming environment is very dynamic and can change dramatically on short time-scales, which in turn would have implications for the diversity of planetary systems found in the Galaxy. The Orion stars have shown that the current theory for the T Tauri stage of stellar evolution is valid down to the very low mass regime. The seven dwarfs are a good example for the evolutionary path of circumstellar disks, showing the transition from gas-high, flared accretion disks (σ Ori) to dust-dominated, depleted, structured debris disks (ε Ori).A continuous wave, single frequency intracavity optical parametric oscillator based upon the twisted-mode techniqueHeering, Lea Christinahttps://hdl.handle.net/10023/119512019-04-01T10:04:38Z2014-03-01T00:00:00ZThis document summarises a study into the elimination of spatial hole burning in the pump field of a Nd:YAG and PPLN based Intracavity Optical Parametric Oscillator (ICOPO), using the so-called ‘twisted-mode technique’. This approach relies on the manipulation of circulating field polarisation to achieve a spatially uniform electric field intensity throughout the laser gain material, thus removing the root of multi-longitudinal-mode oscillation in homogeneously broadened systems. During the course of this investigation the ‘twisted-mode’ technique was first applied to a basic 1μm Nd:YAG laser, where it proved very successful in facilitating single frequency operation. The laser cavity was then extended to accommodate the ICOPO, which delivered 93.9mW of mid-IR output for 3W of diode pump power. The implementation of the ‘twisted-mode’ technique was not found to have any significant impact on power performance of the device and once again offered substantial improvements in pump field spectral quality. Although the insertion of an etalon was necessary to achieve good frequency control, the associated drop in power was minimal compared to typical losses observed in etalon-narrowed standing-wave configurations. This can be attributed to both the low finesse that was required as well as the absence of gain unsaturated regions within the active medium, as would be observed in a spatially hole burnt system. Whilst further work will have to be carried out to optimise both the frequency behaviour and power performance of the device, the ‘twisted-mode’ technique shows clear potential as an approach to the development of a reliable, narrow line-width mid-IR spectroscopic source based on Intracavity OPO technology.
2014-03-01T00:00:00ZHeering, Lea ChristinaThis document summarises a study into the elimination of spatial hole burning in the pump field of a Nd:YAG and PPLN based Intracavity Optical Parametric Oscillator (ICOPO), using the so-called ‘twisted-mode technique’. This approach relies on the manipulation of circulating field polarisation to achieve a spatially uniform electric field intensity throughout the laser gain material, thus removing the root of multi-longitudinal-mode oscillation in homogeneously broadened systems. During the course of this investigation the ‘twisted-mode’ technique was first applied to a basic 1μm Nd:YAG laser, where it proved very successful in facilitating single frequency operation. The laser cavity was then extended to accommodate the ICOPO, which delivered 93.9mW of mid-IR output for 3W of diode pump power. The implementation of the ‘twisted-mode’ technique was not found to have any significant impact on power performance of the device and once again offered substantial improvements in pump field spectral quality. Although the insertion of an etalon was necessary to achieve good frequency control, the associated drop in power was minimal compared to typical losses observed in etalon-narrowed standing-wave configurations. This can be attributed to both the low finesse that was required as well as the absence of gain unsaturated regions within the active medium, as would be observed in a spatially hole burnt system. Whilst further work will have to be carried out to optimise both the frequency behaviour and power performance of the device, the ‘twisted-mode’ technique shows clear potential as an approach to the development of a reliable, narrow line-width mid-IR spectroscopic source based on Intracavity OPO technology.Using time-resolved fluorescence to investigate exciton harvesting in organic photovoltaic blendsWard, Alexander J.https://hdl.handle.net/10023/119452019-04-01T10:06:16Z2014-09-01T00:00:00ZThis thesis is an investigation of the photophysical processes that occur in organic photovoltaic blends in the time between light being absorbed and free charges being generated. The purpose of all solar cells is to generate a photocurrent. The free charges, as they flow out of the device, make up the photocurrent, so understanding the processes by which they are created is vitally important to organic photovoltaic research. The main experimental method used was time-resolved fluorescence spectroscopy. This technique was used to probe the exciton population with respect to time for a variety of blends of organic semiconductors, including the high performance photovoltaic materials PCDTBT, PTB7, C71-PCBM and P3HT. The main goal of the work was to characterise the exciton diffusion lengths of these materials by developing a technique called volume quenching. Volume quenching involves blending a small quantity of quenching material into a thin film of semiconducting material. These introduced quenching sites render excitons unemissive on contact. Thus, from the drop in fluorescence compared with the ‘unquenched’ material, it was possible to work out what proportion of the initial excitons have encountered a quenching site in the blends. The results can then be fitted to quantify how diffusive the excitons are -i.e. how far they move. By looking at the rate constant of the quenching process and how it varies with respect to time, quencher concentration and quencher type, it was possible to generate a wealth of additional information, not just about exciton diffusion, but about all the inter-related processes that contribute to exciton harvesting. These processes included the measurement of long-range energy transfer from the donor to the acceptor, electron transfer at the interface with the acceptor and the understanding of nanomorphology of donor-acceptor heterojunctions.
2014-09-01T00:00:00ZWard, Alexander J.This thesis is an investigation of the photophysical processes that occur in organic photovoltaic blends in the time between light being absorbed and free charges being generated. The purpose of all solar cells is to generate a photocurrent. The free charges, as they flow out of the device, make up the photocurrent, so understanding the processes by which they are created is vitally important to organic photovoltaic research. The main experimental method used was time-resolved fluorescence spectroscopy. This technique was used to probe the exciton population with respect to time for a variety of blends of organic semiconductors, including the high performance photovoltaic materials PCDTBT, PTB7, C71-PCBM and P3HT. The main goal of the work was to characterise the exciton diffusion lengths of these materials by developing a technique called volume quenching. Volume quenching involves blending a small quantity of quenching material into a thin film of semiconducting material. These introduced quenching sites render excitons unemissive on contact. Thus, from the drop in fluorescence compared with the ‘unquenched’ material, it was possible to work out what proportion of the initial excitons have encountered a quenching site in the blends. The results can then be fitted to quantify how diffusive the excitons are -i.e. how far they move. By looking at the rate constant of the quenching process and how it varies with respect to time, quencher concentration and quencher type, it was possible to generate a wealth of additional information, not just about exciton diffusion, but about all the inter-related processes that contribute to exciton harvesting. These processes included the measurement of long-range energy transfer from the donor to the acceptor, electron transfer at the interface with the acceptor and the understanding of nanomorphology of donor-acceptor heterojunctions.Negative frequency at the horizon : scattering of light at a refractive index frontJacquet, Maxime J.https://hdl.handle.net/10023/117802021-08-18T02:01:48Z2017-12-07T00:00:00ZThis thesis considers the problem of calculating and observing the mixing of modes of positive and negative frequency in inhomogeneous, dispersive media. Scattering of vacuum modes of the electromagnetic field at a moving interface in the refractive index of a dielectric medium is discussed. Kinematics arguments are used to demonstrate that this interface may, in a regime of linear dispersion, act as the analogue of the event horizon of a black hole to modes of the field. Furthermore, a study of the dispersion of the dielectric shows that five distinct configurations of modes of the inhomogeneous medium at the interface exist as a function of frequency. Thus it is shown that the interface is simultaneously a black- and white-hole horizon-like and horizonless emitter. The role, and importance, of negative-frequency modes of the field in mode conversion at the horizon is established and yields a calculation of the spontaneous photonic flux at the interface. An algorithm to calculate the scattering of vacuum modes at the interface is introduced. Spectra of the photonic flux in the moving and laboratory frame, for all modes and all realisable increase in the refractive index at the interface are computed. As a result of the various mode configurations, the spectra are highly structured in intervals with black-hole, white-hole and no horizon. The spectra are dominated by a negative-frequency mode, which is the partner in any Hawking-type emission. An experiment in which an incoming positive-frequency wave is populated with photons is assembled to observe the transfer of energy to outgoing waves of positive and negative frequency at the horizon. The effect of mode conversion at the interface is clearly shown to be a feature of horizon physics. This is a classical version of the quantum experiment that aims at validating the mechanism of Hawking radiation.
2017-12-07T00:00:00ZJacquet, Maxime J.This thesis considers the problem of calculating and observing the mixing of modes of positive and negative frequency in inhomogeneous, dispersive media. Scattering of vacuum modes of the electromagnetic field at a moving interface in the refractive index of a dielectric medium is discussed. Kinematics arguments are used to demonstrate that this interface may, in a regime of linear dispersion, act as the analogue of the event horizon of a black hole to modes of the field. Furthermore, a study of the dispersion of the dielectric shows that five distinct configurations of modes of the inhomogeneous medium at the interface exist as a function of frequency. Thus it is shown that the interface is simultaneously a black- and white-hole horizon-like and horizonless emitter. The role, and importance, of negative-frequency modes of the field in mode conversion at the horizon is established and yields a calculation of the spontaneous photonic flux at the interface. An algorithm to calculate the scattering of vacuum modes at the interface is introduced. Spectra of the photonic flux in the moving and laboratory frame, for all modes and all realisable increase in the refractive index at the interface are computed. As a result of the various mode configurations, the spectra are highly structured in intervals with black-hole, white-hole and no horizon. The spectra are dominated by a negative-frequency mode, which is the partner in any Hawking-type emission. An experiment in which an incoming positive-frequency wave is populated with photons is assembled to observe the transfer of energy to outgoing waves of positive and negative frequency at the horizon. The effect of mode conversion at the interface is clearly shown to be a feature of horizon physics. This is a classical version of the quantum experiment that aims at validating the mechanism of Hawking radiation.Signatures of charge noise and its impact on exciton qubitsPurohit, Vishalhttps://hdl.handle.net/10023/117532019-04-01T10:08:52Z2016-06-01T00:00:00ZThe research contained within this thesis concerns the detection, identification and
effect of charge noise on quantum dot systems.
In the first research chapter we study the cross correlation between pairs of exciton
qubits subject to a common fluctuating charge environment, whose dynamics
are solved using a transfer matrix approach. Our results show that we are able to
discern features showing whether or not the charges interact with both quantum
dots simultaneously i.e., form a correlated noise source. We find that qubits in a
common charge environment display photon bunching, if both dots are driven on
resonance or if the laser detunings are equal in both qubits and anitibunching if the
laser detunings are in opposite directions.
In the second research chapter we study the auto-correlation function of a single
optically driven exciton qubit interacting with an environment consisting of 1/f
noise and a fluctuating charge. We again use the transfer matrix method and a
sum of Lorentzian distributions to approximate 1/f noise. Our simulations show
that signatures of 1/f noise do exist in photon correlation measurements. From such
measurements we are also able to determine a minimum cut-off frequency of the 1/f
noise, in the case that there is such a cut-off. In addition we also show that a 1/f
and a single fluctuator can be distinguished using the auto-correlation.
In the final research chapter we study a pair of quantum dots, each with a low
lying electron spin qubit and one higher lying level that can be selectively optically
excited from one of the two spin states. Entanglement between the two spins can be
achieved through path erasure. We look at the effect of a single fluctuating charge
of the entanglement between these two `L' shaped electronic structures.
2016-06-01T00:00:00ZPurohit, VishalThe research contained within this thesis concerns the detection, identification and
effect of charge noise on quantum dot systems.
In the first research chapter we study the cross correlation between pairs of exciton
qubits subject to a common fluctuating charge environment, whose dynamics
are solved using a transfer matrix approach. Our results show that we are able to
discern features showing whether or not the charges interact with both quantum
dots simultaneously i.e., form a correlated noise source. We find that qubits in a
common charge environment display photon bunching, if both dots are driven on
resonance or if the laser detunings are equal in both qubits and anitibunching if the
laser detunings are in opposite directions.
In the second research chapter we study the auto-correlation function of a single
optically driven exciton qubit interacting with an environment consisting of 1/f
noise and a fluctuating charge. We again use the transfer matrix method and a
sum of Lorentzian distributions to approximate 1/f noise. Our simulations show
that signatures of 1/f noise do exist in photon correlation measurements. From such
measurements we are also able to determine a minimum cut-off frequency of the 1/f
noise, in the case that there is such a cut-off. In addition we also show that a 1/f
and a single fluctuator can be distinguished using the auto-correlation.
In the final research chapter we study a pair of quantum dots, each with a low
lying electron spin qubit and one higher lying level that can be selectively optically
excited from one of the two spin states. Entanglement between the two spins can be
achieved through path erasure. We look at the effect of a single fluctuating charge
of the entanglement between these two `L' shaped electronic structures.Glass rain : modelling the formation, dynamics and radiative-transport of cloud particles in hot Jupiter exoplanet atmospheresLee, Graham Kim Huathttps://hdl.handle.net/10023/117402019-04-01T10:03:12Z2017-12-07T00:00:00ZThe atmospheres of exoplanets are being characterised in increasing detail by observational facilities and will be examined with even greater clarity with upcoming space based missions such as the James Webb Space Telescope (JWST) and the Wide Field InfraRed Survey Telescope (WFIRST). A major component of exoplanet atmospheres is the presence of cloud particles which produce characteristic observational signatures in transit spectra and influence the geometric albedo of exoplanets. Despite a decade of observational evidence, the formation, dynamics and radiative-transport of exoplanet atmospheric cloud particles remains an open question in the exoplanet community.
In this thesis, we investigate the kinetic chemistry of cloud formation in hot Jupiter exoplanets, their effect on the atmospheric dynamics and observable properties. We use a static 1D cloud formation code to investigate the cloud formation properties of the hot Jupiter HD 189733b. We couple a time-dependent kinetic cloud formation to a 3D radiative-hydrodynamic simulation of the atmosphere of HD 189733b and investigate the dynamical properties of cloud particles in the atmosphere. We develop a 3D multiple-scattering Monte Carlo radiative-transfer code to post-process the results of the cloudy HD 189733b RHD simulation and compare the results to observational results.
We find that the cloud structures of the hot Jupiter HD 189733b are likely to be highly inhomogeneous, with differences in cloud particle sizes, number density and composition with longitude, latitude and depth. Cloud structures are most divergent between the dayside and nightside faces of the planet due to the instability of silicate materials on the hotter dayside.
We find that the HD 189733b simulation in post-processing is consistent with geometric albedo observations of the planet.
Due to the scattering properties of the cloud particles we predict that HD 189733b will be brighter in the upcoming space missions CHaracterising ExOPlanet Satellite (CHEOPS) bandpass compared to the Transiting Exoplanet Space Survey (TESS) bandpass.
2017-12-07T00:00:00ZLee, Graham Kim HuatThe atmospheres of exoplanets are being characterised in increasing detail by observational facilities and will be examined with even greater clarity with upcoming space based missions such as the James Webb Space Telescope (JWST) and the Wide Field InfraRed Survey Telescope (WFIRST). A major component of exoplanet atmospheres is the presence of cloud particles which produce characteristic observational signatures in transit spectra and influence the geometric albedo of exoplanets. Despite a decade of observational evidence, the formation, dynamics and radiative-transport of exoplanet atmospheric cloud particles remains an open question in the exoplanet community.
In this thesis, we investigate the kinetic chemistry of cloud formation in hot Jupiter exoplanets, their effect on the atmospheric dynamics and observable properties. We use a static 1D cloud formation code to investigate the cloud formation properties of the hot Jupiter HD 189733b. We couple a time-dependent kinetic cloud formation to a 3D radiative-hydrodynamic simulation of the atmosphere of HD 189733b and investigate the dynamical properties of cloud particles in the atmosphere. We develop a 3D multiple-scattering Monte Carlo radiative-transfer code to post-process the results of the cloudy HD 189733b RHD simulation and compare the results to observational results.
We find that the cloud structures of the hot Jupiter HD 189733b are likely to be highly inhomogeneous, with differences in cloud particle sizes, number density and composition with longitude, latitude and depth. Cloud structures are most divergent between the dayside and nightside faces of the planet due to the instability of silicate materials on the hotter dayside.
We find that the HD 189733b simulation in post-processing is consistent with geometric albedo observations of the planet.
Due to the scattering properties of the cloud particles we predict that HD 189733b will be brighter in the upcoming space missions CHaracterising ExOPlanet Satellite (CHEOPS) bandpass compared to the Transiting Exoplanet Space Survey (TESS) bandpass.Time but no space : resolving the structure and dynamics of active galactic nuclei using time domain astronomyStarkey, David Andrewhttps://hdl.handle.net/10023/115642019-04-01T10:07:16Z2017-08-01T00:00:00ZThis thesis presents a study of the sub-light year regions of Active Galactic Nuclei (AGN). These environments contain accretion discs that orbit a central super-massive black hole. The luminosity of the AGN inner regions varies over time across all wavelengths with variability at longer wavelengths lagging behind that at shorter wavelengths. Since the AGN themselves are too remote and too compact to resolve directly, I exploit these time lags to infer the physical characteristics of the accretion disc and surrounding gas clouds that emit broad emission lines. These characteristics include the inclination and temperature profile of the accretion disc, and the shape (or light curve) of the luminosity fluctuations that drive the accretion disc variability.
This thesis details the work in the first author papers of Starkey et al. (2016, 2017), in which I detail the statistical code, CREAM (Continuum REverberting AGN Markov Chain Monte Carlo), that I developed to analyse AGN accretion disc variability. I apply the code to a set of AGN light curve observations of the Seyfert 1 galaxy NGC 5548 by the AGN STORM collaboration (De Rosa et al., 2015; Edelson et al., 2015; Fausnaugh et al., 2016a; Goad et al., 2016; Starkey et al., 2017). I also present work detailing my variability analysis of the Seyfert galaxies NGC 6814, NGC 2617, MCG 08-11-11 and NGC 4151. This work has contributed to the analysis presented in (Troyer et al. 2016, Fausnaugh et al. submitted). I also investigate the implications of a twin accretion disc structure (Nealon et al., 2015) on the disc time lag measurements across near UV and optical wavelengths. I finish by detailing a modification to CREAM that allows it to merge continuum light curves observed in a common filter, but taken by multiple telescopes with different calibration and instrumental effects to consider.
2017-08-01T00:00:00ZStarkey, David AndrewThis thesis presents a study of the sub-light year regions of Active Galactic Nuclei (AGN). These environments contain accretion discs that orbit a central super-massive black hole. The luminosity of the AGN inner regions varies over time across all wavelengths with variability at longer wavelengths lagging behind that at shorter wavelengths. Since the AGN themselves are too remote and too compact to resolve directly, I exploit these time lags to infer the physical characteristics of the accretion disc and surrounding gas clouds that emit broad emission lines. These characteristics include the inclination and temperature profile of the accretion disc, and the shape (or light curve) of the luminosity fluctuations that drive the accretion disc variability.
This thesis details the work in the first author papers of Starkey et al. (2016, 2017), in which I detail the statistical code, CREAM (Continuum REverberting AGN Markov Chain Monte Carlo), that I developed to analyse AGN accretion disc variability. I apply the code to a set of AGN light curve observations of the Seyfert 1 galaxy NGC 5548 by the AGN STORM collaboration (De Rosa et al., 2015; Edelson et al., 2015; Fausnaugh et al., 2016a; Goad et al., 2016; Starkey et al., 2017). I also present work detailing my variability analysis of the Seyfert galaxies NGC 6814, NGC 2617, MCG 08-11-11 and NGC 4151. This work has contributed to the analysis presented in (Troyer et al. 2016, Fausnaugh et al. submitted). I also investigate the implications of a twin accretion disc structure (Nealon et al., 2015) on the disc time lag measurements across near UV and optical wavelengths. I finish by detailing a modification to CREAM that allows it to merge continuum light curves observed in a common filter, but taken by multiple telescopes with different calibration and instrumental effects to consider.Ionization in atmospheres of brown dwarfs and giant gas planetsRodriguez-Barrera, Maria Isabelhttps://hdl.handle.net/10023/113632019-04-01T10:02:29Z2017-01-01T00:00:00ZRecent observations of ultra-cool objects suggest that the gas in their uppermost atmospheres is heated, ionised and magnetised to levels that radio and X-ray emission is possible. The aim
of this work is to identify which low mass objects are most susceptible to plasma and magnetic
processes by the effect of thermal ionisation and Lyman continuum photoionisation from the
irradiation of external sources in different environments. I focus my work on very low mass,
ultra-cool objects (late M-dwarfs, brown dwarfs and giant gas planets: T[sub]eff = 1000 . . . 3000 K)
to analyse the electrostatic and magnetic character of these atmosphere. This work has been
carried out in two distinct parts:
a) A reference study to identify which ultra-cool objects are most susceptible to plasma and
magnetic processes considering the thermal ionisation as the only mechanism to ionise
the atmospheric gas. This presents a theoretical framework using a set of fundamental
parameters to analyse the ionisation and magnetic coupling state of objects with
ultra-cool atmospheres. The DRIFT-PHOENIX model atmosphere simulations are used to
determine the local gas properties T[sub]gas [K], p[sub]gas [bar], pₑ [bar] of the atmospheres
structures from the global parameters T[sub]eff [K], log(g) [cm s⁻²] and [M/H]. Electrostatics
interactions dominance over electron-neutral interactions increases as T[sub]eff increases:
throughout the M-dwarf atmospheres, almost for the whole brown dwarf atmospheres
and only giant gas planets with T[sub]eff =1200 K, log(g)=3.0, [M/H]=+0.3 fulfil that criterion
at the most deeper atmosphere. A magnetised gas is found for M-dwarfs, brown
dwarfs and giant gas planet atmospheres even for those with a small degree of ionisation
except to the most deeper atmospheric regions. Hence, the upper atmosphere of
all of studied objects can be magnetically coupled. A large fraction of the atmospheric
volume of M-dwarfs and of early spectral subtypes of brown dwarfs is found to occur
plasma processes and magnetised gas, and are therefore the best candidates to emit in
Hα and radio wavelengths.
b) An analysis of how the Lyman continuum external irradiation effect the plasma and magnetic
state of an ultra-cool atmospheres in additional thermal ionisation. The Monte
Carlo radiative transfer code simulations provide the atmospheric ionisation structure
due to photoionisation. Three different cases are studied to see the effect of the Lyman
continuum irradiation given by an external source: free-floating ultra-cool objects
irradiated by Lyman continuum photons from the interstellar medium (dominated by
Galactic O and B stars), brown dwarfs in star forming regions irradiated by a nearby
(few parsecs) O star and binary systems where a brown dwarf atmosphere is photoionised
by a companion white dwarf. The effect of Lyman continuum irradiation from
external sources greatly increases the level of ionisation in the uppermost atmospheric
regions. Brown dwarfs in star forming regions and brown dwarfs in binary systems with
a companion white dwarf have upper atmospheres that are close to being fully ionised.
Adopting temperatures typical for a chromosphere or a corona, the resulting free-free
X-ray luminosities are found to be comparable with those observed from brown dwarfs
in star forming regions. The results of this study show that it is not unreasonable to
expect powerful energy emissions from brown dwarf atmospheres.
The conclusions of this study are that the thermal ionisation allows the establishment of a magnetised plasma in brown dwarf atmospheres as in particular the rarefied upper parts of the atmospheres despite having low degrees of thermal gas ionisation and that the Lyman continuum photoionisation allows to argue for a chromosphere/corona on brown dwarfs and for X-ray emissions from ultra-cool objects. However, other mechanisms, like Alfvén wave heating, are needed to occur which then lead to rise the local gas temperature in the chromosphere/corona on brown dwarfs.
2017-01-01T00:00:00ZRodriguez-Barrera, Maria IsabelRecent observations of ultra-cool objects suggest that the gas in their uppermost atmospheres is heated, ionised and magnetised to levels that radio and X-ray emission is possible. The aim
of this work is to identify which low mass objects are most susceptible to plasma and magnetic
processes by the effect of thermal ionisation and Lyman continuum photoionisation from the
irradiation of external sources in different environments. I focus my work on very low mass,
ultra-cool objects (late M-dwarfs, brown dwarfs and giant gas planets: T[sub]eff = 1000 . . . 3000 K)
to analyse the electrostatic and magnetic character of these atmosphere. This work has been
carried out in two distinct parts:
a) A reference study to identify which ultra-cool objects are most susceptible to plasma and
magnetic processes considering the thermal ionisation as the only mechanism to ionise
the atmospheric gas. This presents a theoretical framework using a set of fundamental
parameters to analyse the ionisation and magnetic coupling state of objects with
ultra-cool atmospheres. The DRIFT-PHOENIX model atmosphere simulations are used to
determine the local gas properties T[sub]gas [K], p[sub]gas [bar], pₑ [bar] of the atmospheres
structures from the global parameters T[sub]eff [K], log(g) [cm s⁻²] and [M/H]. Electrostatics
interactions dominance over electron-neutral interactions increases as T[sub]eff increases:
throughout the M-dwarf atmospheres, almost for the whole brown dwarf atmospheres
and only giant gas planets with T[sub]eff =1200 K, log(g)=3.0, [M/H]=+0.3 fulfil that criterion
at the most deeper atmosphere. A magnetised gas is found for M-dwarfs, brown
dwarfs and giant gas planet atmospheres even for those with a small degree of ionisation
except to the most deeper atmospheric regions. Hence, the upper atmosphere of
all of studied objects can be magnetically coupled. A large fraction of the atmospheric
volume of M-dwarfs and of early spectral subtypes of brown dwarfs is found to occur
plasma processes and magnetised gas, and are therefore the best candidates to emit in
Hα and radio wavelengths.
b) An analysis of how the Lyman continuum external irradiation effect the plasma and magnetic
state of an ultra-cool atmospheres in additional thermal ionisation. The Monte
Carlo radiative transfer code simulations provide the atmospheric ionisation structure
due to photoionisation. Three different cases are studied to see the effect of the Lyman
continuum irradiation given by an external source: free-floating ultra-cool objects
irradiated by Lyman continuum photons from the interstellar medium (dominated by
Galactic O and B stars), brown dwarfs in star forming regions irradiated by a nearby
(few parsecs) O star and binary systems where a brown dwarf atmosphere is photoionised
by a companion white dwarf. The effect of Lyman continuum irradiation from
external sources greatly increases the level of ionisation in the uppermost atmospheric
regions. Brown dwarfs in star forming regions and brown dwarfs in binary systems with
a companion white dwarf have upper atmospheres that are close to being fully ionised.
Adopting temperatures typical for a chromosphere or a corona, the resulting free-free
X-ray luminosities are found to be comparable with those observed from brown dwarfs
in star forming regions. The results of this study show that it is not unreasonable to
expect powerful energy emissions from brown dwarf atmospheres.
The conclusions of this study are that the thermal ionisation allows the establishment of a magnetised plasma in brown dwarf atmospheres as in particular the rarefied upper parts of the atmospheres despite having low degrees of thermal gas ionisation and that the Lyman continuum photoionisation allows to argue for a chromosphere/corona on brown dwarfs and for X-ray emissions from ultra-cool objects. However, other mechanisms, like Alfvén wave heating, are needed to occur which then lead to rise the local gas temperature in the chromosphere/corona on brown dwarfs.Elements of solar activity : particle acceleration and filament formationWood, Paul D.https://hdl.handle.net/10023/113092019-04-22T08:46:44Z2005-01-01T00:00:00ZThis thesis studies the acceleration of particles to super-thermal energies in explosive solar events as well as the magnetic changes in connectivity that may be responsible for changes in the morphology of quiescent filaments. Firstly a review of some of the observations of solar flare dynamics is given, as well as an introduction to the competing theories attempting to explain both particle acceleration and filament formation. An explanation of the numerical FORTRAN code that is used to calculate the trajectories of particle distribution functions in prescribed electromagnetic fields is given. Examples of known fields are used to test the accuracy of the code and the simple example of the well-known Litvinenko current sheet field is investigated. The results of charged particle orbit calculations in prescribed electric and magnetic fields motivated by magnetic reconnection models are then presented. The electromagnetic fields are chosen to resemble a current sheet with a localised reconnection region. The dependence of the model on the important physical parameters is considered. An introduction to the mathematical formulation of a collapsing magnetic trap is given. The same numerical code is used to calculate single electron orbits in this more complicated time dependent electromagnetic field. Consideration of important previous work is given before describing the best attempts to model the movement of flare loops in a realistic fashion. Finally the process of flux cancellation and filament formation is studied using a range of data including ground-based Hα and SoHO MDI magnetograms. It is found that the cancellation occurs at the ends of Hα sections of the filament and is accompanied by a noticeable increase in the Hα intensity and linkage of the sections. Measurements of the amount of flux cancelled at each site show it is in agreement with an estimate of the axial flux contained in the filament.
2005-01-01T00:00:00ZWood, Paul D.This thesis studies the acceleration of particles to super-thermal energies in explosive solar events as well as the magnetic changes in connectivity that may be responsible for changes in the morphology of quiescent filaments. Firstly a review of some of the observations of solar flare dynamics is given, as well as an introduction to the competing theories attempting to explain both particle acceleration and filament formation. An explanation of the numerical FORTRAN code that is used to calculate the trajectories of particle distribution functions in prescribed electromagnetic fields is given. Examples of known fields are used to test the accuracy of the code and the simple example of the well-known Litvinenko current sheet field is investigated. The results of charged particle orbit calculations in prescribed electric and magnetic fields motivated by magnetic reconnection models are then presented. The electromagnetic fields are chosen to resemble a current sheet with a localised reconnection region. The dependence of the model on the important physical parameters is considered. An introduction to the mathematical formulation of a collapsing magnetic trap is given. The same numerical code is used to calculate single electron orbits in this more complicated time dependent electromagnetic field. Consideration of important previous work is given before describing the best attempts to model the movement of flare loops in a realistic fashion. Finally the process of flux cancellation and filament formation is studied using a range of data including ground-based Hα and SoHO MDI magnetograms. It is found that the cancellation occurs at the ends of Hα sections of the filament and is accompanied by a noticeable increase in the Hα intensity and linkage of the sections. Measurements of the amount of flux cancelled at each site show it is in agreement with an estimate of the axial flux contained in the filament.Investigation of the magnetic properties of copper, aluminium and iron-copper alloysHutchison, T. S. (Thomas S.)https://hdl.handle.net/10023/112192019-04-01T10:06:48Z1949-01-01T00:00:00ZSeveral workers have previously investigated the magnetic properties of copper and aluminium, but in most cases too small regard has been paid to the changes in magnetic susceptibility brought about by mechanical and thermal treatments previous to measurements. In the present work I have attempted to correlate changes in the magnetic susceptibilities of copper and aluminium with metallurgical changes in the metal lattices. A somewhat tentative theory is outlined covering this correlation. Previous investigations by Kussmann and Seeman led to the conclusion that iron impurity in the pure copper was the sole cause of the various magnetic changes recorded. This it is hoped to disapprove. To get some insight into the changes caused by such ferromagnetic impurity a series of magnetic measurements were carried out on low iron content iron-copper alloys, very kindly supplied by Imperial Chemical Industries Ltd., “Metals Division”. The thesis falls naturally into four main parts, and appendices are added at the end to avoid interpolation of the work with definitions etc. Throughout the thesis (Parts III and IV) only a few experimental values, typical of the many results obtained, are quoted.
1949-01-01T00:00:00ZHutchison, T. S. (Thomas S.)Several workers have previously investigated the magnetic properties of copper and aluminium, but in most cases too small regard has been paid to the changes in magnetic susceptibility brought about by mechanical and thermal treatments previous to measurements. In the present work I have attempted to correlate changes in the magnetic susceptibilities of copper and aluminium with metallurgical changes in the metal lattices. A somewhat tentative theory is outlined covering this correlation. Previous investigations by Kussmann and Seeman led to the conclusion that iron impurity in the pure copper was the sole cause of the various magnetic changes recorded. This it is hoped to disapprove. To get some insight into the changes caused by such ferromagnetic impurity a series of magnetic measurements were carried out on low iron content iron-copper alloys, very kindly supplied by Imperial Chemical Industries Ltd., “Metals Division”. The thesis falls naturally into four main parts, and appendices are added at the end to avoid interpolation of the work with definitions etc. Throughout the thesis (Parts III and IV) only a few experimental values, typical of the many results obtained, are quoted.Electron scattering in gasesBaines, Gordon Owenhttps://hdl.handle.net/10023/112182019-04-01T10:09:39Z1935-01-01T00:00:00ZThis thesis is divided into two parts; the first containing theoretical work and discussions; and the second the experimental work. Section 1 contains a discussion of the stationary states in an atom and the kinetic theory expression for the mean free path of an electron in a gas, leading up to the formula for the passage of a beam of electrons through a gas. In section 2, the previous methods of obtaining cross sections, both total and inelastic, are dealt with and the results discussed. An outline of the wave mechanical theory of collision processes is given in section 3, with indications of the agreement obtained with experiment by different methods. The results of some approximate theoretical calculations of phases and an angular scattering curve for krypton are given in section 4. The results of this section have already been published by Dr. F. L. Arnot and the author. Section 5, the first of Part II, contains a description of a new apparatus designed to obtain measurements of the total, elastic and inelastic cross section in gases. The method of using the apparatus and a number of tests of its working are described in section 6. The last section, 7, contains the results obtained with the apparatus for the total, elastic and inelastic cross sections of the mercury atom, along with the cross sections for the ionisation, 2¹P₁, 2³ P₁, 3¹D₂ states. It concludes with an interesting test of the working of the apparatus and a discussion of the possible errors of the method.
1935-01-01T00:00:00ZBaines, Gordon OwenThis thesis is divided into two parts; the first containing theoretical work and discussions; and the second the experimental work. Section 1 contains a discussion of the stationary states in an atom and the kinetic theory expression for the mean free path of an electron in a gas, leading up to the formula for the passage of a beam of electrons through a gas. In section 2, the previous methods of obtaining cross sections, both total and inelastic, are dealt with and the results discussed. An outline of the wave mechanical theory of collision processes is given in section 3, with indications of the agreement obtained with experiment by different methods. The results of some approximate theoretical calculations of phases and an angular scattering curve for krypton are given in section 4. The results of this section have already been published by Dr. F. L. Arnot and the author. Section 5, the first of Part II, contains a description of a new apparatus designed to obtain measurements of the total, elastic and inelastic cross section in gases. The method of using the apparatus and a number of tests of its working are described in section 6. The last section, 7, contains the results obtained with the apparatus for the total, elastic and inelastic cross sections of the mercury atom, along with the cross sections for the ionisation, 2¹P₁, 2³ P₁, 3¹D₂ states. It concludes with an interesting test of the working of the apparatus and a discussion of the possible errors of the method.Ionisation in mercury vapour and heliumM'Ewen, Marjorie B.https://hdl.handle.net/10023/112142019-04-01T10:05:53Z1938-01-01T00:00:00ZThe greater part of the experimental research, described in this thesis, has been devoted to an investigation, by direct electrical methods, of the formation of ionised molecules in mercury vapour and helium. […] This thesis is divided into four parts. The first contains a brief outline of the quantum theory of the atom and the molecule, a summary of our present knowledge of the interchange of energy in various collision processes, and a discussion of the theory of the thermionic emission from a heated filament and its applications. Particular reference for the development of the experimental research described in Parts 2, 3 and 4.
1938-01-01T00:00:00ZM'Ewen, Marjorie B.The greater part of the experimental research, described in this thesis, has been devoted to an investigation, by direct electrical methods, of the formation of ionised molecules in mercury vapour and helium. […] This thesis is divided into four parts. The first contains a brief outline of the quantum theory of the atom and the molecule, a summary of our present knowledge of the interchange of energy in various collision processes, and a discussion of the theory of the thermionic emission from a heated filament and its applications. Particular reference for the development of the experimental research described in Parts 2, 3 and 4.The influence of foreign gases on gaseous spectraRoy, Angus S.https://hdl.handle.net/10023/112132019-04-01T10:08:20Z1933-01-01T00:00:00ZThis thesis is divided into two parts; Part I consisting of the theory and Part II the experimental work. Part I is divided into three sections; in Section 1 a summary is given of collision processes and how they have been used to explain various experimental results; this is followed in Section 2 by the author’s interpretation of some previous experimental observations; a summary of the wave mechanical theory of collisions is given in Section 3. Part II consists of four sections; In Section 1an outline is given of the various ways of exciting spectra and this is followed by a description of the apparatus used by the author in Section 2; the results of the experiments with hydrogen and oxygen are given in Section 3, and the theoretical interpretation of these; the last Section deals with the experiments carried out with hydrogen and helium, and the facts which may be inferred from these experimental results.
1933-01-01T00:00:00ZRoy, Angus S.This thesis is divided into two parts; Part I consisting of the theory and Part II the experimental work. Part I is divided into three sections; in Section 1 a summary is given of collision processes and how they have been used to explain various experimental results; this is followed in Section 2 by the author’s interpretation of some previous experimental observations; a summary of the wave mechanical theory of collisions is given in Section 3. Part II consists of four sections; In Section 1an outline is given of the various ways of exciting spectra and this is followed by a description of the apparatus used by the author in Section 2; the results of the experiments with hydrogen and oxygen are given in Section 3, and the theoretical interpretation of these; the last Section deals with the experiments carried out with hydrogen and helium, and the facts which may be inferred from these experimental results.A study of the mathematics of supersymmetry theoriesWilliams, Denishttps://hdl.handle.net/10023/111252019-04-01T10:09:44Z1986-01-01T00:00:00ZThis Thesis consists of three parts. In the first part a theory of integration is constructed for supermanifolds and supergroups. With this theory expressions for the invariant integral on several Lie supergroups are obtained including the super Poincaré group and supers pace. The unitary irreducible representations of the super Poincaré group are
examined by considering the unitary irreducible representations of a certain set of Lie groups equivalent to the super Poincaré group. These irreducible representations contain, at most, particles of a single spin.
In the second part a detailed examination of the massive representations of the super Poincaré algebra is undertaken. Supermultiplets of second quantized fields are constructed for each of the. massive representations, which allows an understanding of the auxiliary fields of supersymmetry theories.
In the third part super Poincaré invariant superfields on superspace are constructed from the supermultiplets of the second part. This enables a connection between the representations of part one and those of part two to be established. An examination of action integrals on superspace is made enabling the relationship between the integration theory constructed in part one and the Berezin integral to be established.
1986-01-01T00:00:00ZWilliams, DenisThis Thesis consists of three parts. In the first part a theory of integration is constructed for supermanifolds and supergroups. With this theory expressions for the invariant integral on several Lie supergroups are obtained including the super Poincaré group and supers pace. The unitary irreducible representations of the super Poincaré group are
examined by considering the unitary irreducible representations of a certain set of Lie groups equivalent to the super Poincaré group. These irreducible representations contain, at most, particles of a single spin.
In the second part a detailed examination of the massive representations of the super Poincaré algebra is undertaken. Supermultiplets of second quantized fields are constructed for each of the. massive representations, which allows an understanding of the auxiliary fields of supersymmetry theories.
In the third part super Poincaré invariant superfields on superspace are constructed from the supermultiplets of the second part. This enables a connection between the representations of part one and those of part two to be established. An examination of action integrals on superspace is made enabling the relationship between the integration theory constructed in part one and the Berezin integral to be established.Transferred electron oscillators at MM wave frequencies and their characterisation using quasi-optical techniquesSmith, Graham Murrayhttps://hdl.handle.net/10023/111062019-04-01T10:03:36Z1990-01-01T00:00:00ZA study of high frequency millimetre wave oscillators is performed operating at W- band and above, using test bench equipment designed and constructed in St. Andrews. Octave tuneable oscillators have been designed, constructed, and used to characterise developmental Gunn devices, as well as to provide ideal oscillators for test bench measurement systems. These oscillators have been sold to many millimetre-wave laboratories throughout Britain. The operation, optimisation and characterisation of these oscillators is described in detail, and various non-linear effects are explained and modelled successfully. The wideband tuneability and matching has also allowed evaluation of new developmental Gunn devices to accurately determine the optimum operating frequency range of the devices. This was part of a developmental program by GEC Hirst and MEDL which has now produced state of the art GaAs Gunn oscillators at 94GHz.
Much of the characterisation of the oscillators is performed using novel quasi-optical techniques, which has allowed low loss accurate performance at these very high frequencies. Several quasi-optical techniques are described and the design, manufacture and evaluation of many optical components are given. In particular, the frequency and harmonic content of the oscillators was determined using a Martin-Puplett Interferometer which utilised a frequency counting technique. This enabled easy wideband measurements to be performed with much greater accuracy than traditional cavity wavemeters. In addition, a state of the art noise bench has been designed and constructed for operation at W -band and above, that utilises a novel open resonator to effect a very high Q suppression filter. The system has been shown to make noise measurements at much lower power levels and with greater sensitivity than comparable systems.
1990-01-01T00:00:00ZSmith, Graham MurrayA study of high frequency millimetre wave oscillators is performed operating at W- band and above, using test bench equipment designed and constructed in St. Andrews. Octave tuneable oscillators have been designed, constructed, and used to characterise developmental Gunn devices, as well as to provide ideal oscillators for test bench measurement systems. These oscillators have been sold to many millimetre-wave laboratories throughout Britain. The operation, optimisation and characterisation of these oscillators is described in detail, and various non-linear effects are explained and modelled successfully. The wideband tuneability and matching has also allowed evaluation of new developmental Gunn devices to accurately determine the optimum operating frequency range of the devices. This was part of a developmental program by GEC Hirst and MEDL which has now produced state of the art GaAs Gunn oscillators at 94GHz.
Much of the characterisation of the oscillators is performed using novel quasi-optical techniques, which has allowed low loss accurate performance at these very high frequencies. Several quasi-optical techniques are described and the design, manufacture and evaluation of many optical components are given. In particular, the frequency and harmonic content of the oscillators was determined using a Martin-Puplett Interferometer which utilised a frequency counting technique. This enabled easy wideband measurements to be performed with much greater accuracy than traditional cavity wavemeters. In addition, a state of the art noise bench has been designed and constructed for operation at W -band and above, that utilises a novel open resonator to effect a very high Q suppression filter. The system has been shown to make noise measurements at much lower power levels and with greater sensitivity than comparable systems.Fourier analysis of unequally-spaced time series : with applications to the study of helium stars and binary systemsSkillen, W. J. I. (Ian)https://hdl.handle.net/10023/111052019-04-01T10:06:44Z1986-01-01T00:00:00ZThe application of the discrete Fourier transform to the determination of the frequency content of unevenly-sampled astronomical time series is discussed, and an interactive computer package which incorporates a variety of power-spectrum and time-domain techniques is described.
A frequency analysis of the light curves of two hot, extreme helium stars, BD-9°4395 and HD160641, shows that their photometric variability is caused by non-radial pulsation. Spectroscopic evidence in support of non-uniform mass loss is presented for BD-9°4395.
Spectroscopic and photometric observations of two early-type eclipsing binary systems, AL Sculptoris and DM Persei, have been analysed to yield their absolute dimensions. AL Scl is found to be a detached system in which both components rotate faster than synchronism. The origin of distortions in its light curve is unclear. DH Per is shown to be part of a triple system in which the third component is most probably a late-B star in a 98-day orbit with a semi-major axis of 0.9 A.U. The binary system is confirmed to be semi-detached and to have evolved through a phase of rapid mass transfer. DH Per joins a small group of massive, semi-detached systems whose characteristics differ significantly from the classical Algols, and which may result from case-A, mass-transfer processes.
Spectroscopic and photometric observations of the F4V star HD123058 do not support the hypothesis that it is a binary system. Broad lines in its spectrum are attributed to a somewhat enhanced rotation rate, and the star is shown to be essentially unevolved. The derivation of the equation of condition in Sterne's rigorous
method for the analysis of the spectroscopic elements of binary systems, and its modification for incorporating observed times of minimum light into the adjustment of the elements, are outlined. A computer code for the determination of orbital elements according to this scheme is described.
1986-01-01T00:00:00ZSkillen, W. J. I. (Ian)The application of the discrete Fourier transform to the determination of the frequency content of unevenly-sampled astronomical time series is discussed, and an interactive computer package which incorporates a variety of power-spectrum and time-domain techniques is described.
A frequency analysis of the light curves of two hot, extreme helium stars, BD-9°4395 and HD160641, shows that their photometric variability is caused by non-radial pulsation. Spectroscopic evidence in support of non-uniform mass loss is presented for BD-9°4395.
Spectroscopic and photometric observations of two early-type eclipsing binary systems, AL Sculptoris and DM Persei, have been analysed to yield their absolute dimensions. AL Scl is found to be a detached system in which both components rotate faster than synchronism. The origin of distortions in its light curve is unclear. DH Per is shown to be part of a triple system in which the third component is most probably a late-B star in a 98-day orbit with a semi-major axis of 0.9 A.U. The binary system is confirmed to be semi-detached and to have evolved through a phase of rapid mass transfer. DH Per joins a small group of massive, semi-detached systems whose characteristics differ significantly from the classical Algols, and which may result from case-A, mass-transfer processes.
Spectroscopic and photometric observations of the F4V star HD123058 do not support the hypothesis that it is a binary system. Broad lines in its spectrum are attributed to a somewhat enhanced rotation rate, and the star is shown to be essentially unevolved. The derivation of the equation of condition in Sterne's rigorous
method for the analysis of the spectroscopic elements of binary systems, and its modification for incorporating observed times of minimum light into the adjustment of the elements, are outlined. A computer code for the determination of orbital elements according to this scheme is described.An investigation of molecular opacities for late-type starsSharp, Christopher Martinhttps://hdl.handle.net/10023/111032019-04-01T10:04:07Z1981-01-01T00:00:00ZThis work investigates molecular opacities under conditions appropriate to the atmospheres of late-type stars. Given a specified initial abundance of the chemical elements, the equations of atomic ionization and molecular dissociation are solved by iteration for various temperatures and densities. With the abundances of the atomic and molecular species so determined at a particular temperature and density, the continuous absorption coefficient due to a number of processes is first computed, then the molecular band absorption coefficient is computed line-by-line superimposed for diatomic and triatomic molecules considered separately. The band calculations for the diatomic molecules include all possible isotopic variants, but
computing resources did not permit this degree of detail for triatomic molecules, nor diatomic and triatomic molecules to be considered together. These calculations are performed for a number of different temperatures and densities.
The theory for calculating the spectral lines due to molecules is discussed in some detail, with various methods being investigated for computing opacities including the contributions from these spectral lines. With the computing resources available, the best method for the calculation of the opacity due to the molecular absorption is
found to be that of opacity sampling, sampling is adequate, provided the frequency of sampling is adequate.
From the results presented in this thesis, a number of important conclusions can be drawn. At low temperatures and high densities, pressure-induced absorption due to molecular hydrogen is an important source of continuous opacity. Isotopic variants of at least diatomic molecules pave to be considered in any calculations, as they can
significantly affect the contribution that diatomic molecules make to the Rosseland mean opacities. Also at low temperatures, water is a major source of absorption. Finally, tables of some thermodynamic quantities for the molecular gas are given for a number of temperatures and densities.
1981-01-01T00:00:00ZSharp, Christopher MartinThis work investigates molecular opacities under conditions appropriate to the atmospheres of late-type stars. Given a specified initial abundance of the chemical elements, the equations of atomic ionization and molecular dissociation are solved by iteration for various temperatures and densities. With the abundances of the atomic and molecular species so determined at a particular temperature and density, the continuous absorption coefficient due to a number of processes is first computed, then the molecular band absorption coefficient is computed line-by-line superimposed for diatomic and triatomic molecules considered separately. The band calculations for the diatomic molecules include all possible isotopic variants, but
computing resources did not permit this degree of detail for triatomic molecules, nor diatomic and triatomic molecules to be considered together. These calculations are performed for a number of different temperatures and densities.
The theory for calculating the spectral lines due to molecules is discussed in some detail, with various methods being investigated for computing opacities including the contributions from these spectral lines. With the computing resources available, the best method for the calculation of the opacity due to the molecular absorption is
found to be that of opacity sampling, sampling is adequate, provided the frequency of sampling is adequate.
From the results presented in this thesis, a number of important conclusions can be drawn. At low temperatures and high densities, pressure-induced absorption due to molecular hydrogen is an important source of continuous opacity. Isotopic variants of at least diatomic molecules pave to be considered in any calculations, as they can
significantly affect the contribution that diatomic molecules make to the Rosseland mean opacities. Also at low temperatures, water is a major source of absorption. Finally, tables of some thermodynamic quantities for the molecular gas are given for a number of temperatures and densities.The evolutionary status of the hot R coronae borealis starsPollacco, D.https://hdl.handle.net/10023/110692019-04-01T10:03:43Z1989-01-01T00:00:00ZThe evolutionary status of the hot R CrB stars has long remained a matter of conjecture,
primarily because of the relative dearth of relevant observational material. Previously the group had been thought to occupy a position intermediate in status between the EHe and R CrB classes as they have (at least at first glance) observational properties in common with both types of object.
The photosphere of DY Cen has been quantitatively confirmed to be hydrogen-deficient
and photometric variations suggest the star undergoes short period pulsations. Using a
period-temperature relation applicable to hydrogen-deficient stars it would appear that this object has similar physical properties to both the EHe and R CrB stars.
Narrow band imaging of V348 Sgr has shown that the associated nebula exhibits a bipolar structure and therefore must be closely related to planetary nebulae rather than H II regions. Spectroscopic observations have proved that the star in its present evolutionary state is incapable of ionising the nebula. Several scenarios for this behaviour are briefly discussed. The large helium enrichment found in the nebula indicates that processed material must have been ejected during the last major episode of mass loss. Evidence is presented that suggests a strong hydrogen abundance gradient exists within the nebula.
A novel technique has been developed for determining reddening distances. Its main advantage over other similar methods is that both early and late-type stars may be used to establish the reddening-distance relationship. With more development this technique may prove to be an important tool in distance determinations for objects such as planetary nebulae etc. This technique was used to derive a distance of (4. 7±1.0) kpc for V348 Sgr. Using the core-mass relation for hydrogen-deficient stars implies that both V348 Sgr and MV Sgr are lower mass and luminosity objects than EHe and R CrB stars.
The evidence presented in this thesis indicates that the hot R CrB group is not a homogeneous one. DY Cen is much more luminous and massive than the other members. The mass and luminosity of V348 Sgr and MV Sgr are consistent with the scenario that both have recently suffered a thermal pulse (causing re-ignition of a helium burning shell) and are currently looping back to the R CrB domain of the H R diagram.
1989-01-01T00:00:00ZPollacco, D.The evolutionary status of the hot R CrB stars has long remained a matter of conjecture,
primarily because of the relative dearth of relevant observational material. Previously the group had been thought to occupy a position intermediate in status between the EHe and R CrB classes as they have (at least at first glance) observational properties in common with both types of object.
The photosphere of DY Cen has been quantitatively confirmed to be hydrogen-deficient
and photometric variations suggest the star undergoes short period pulsations. Using a
period-temperature relation applicable to hydrogen-deficient stars it would appear that this object has similar physical properties to both the EHe and R CrB stars.
Narrow band imaging of V348 Sgr has shown that the associated nebula exhibits a bipolar structure and therefore must be closely related to planetary nebulae rather than H II regions. Spectroscopic observations have proved that the star in its present evolutionary state is incapable of ionising the nebula. Several scenarios for this behaviour are briefly discussed. The large helium enrichment found in the nebula indicates that processed material must have been ejected during the last major episode of mass loss. Evidence is presented that suggests a strong hydrogen abundance gradient exists within the nebula.
A novel technique has been developed for determining reddening distances. Its main advantage over other similar methods is that both early and late-type stars may be used to establish the reddening-distance relationship. With more development this technique may prove to be an important tool in distance determinations for objects such as planetary nebulae etc. This technique was used to derive a distance of (4. 7±1.0) kpc for V348 Sgr. Using the core-mass relation for hydrogen-deficient stars implies that both V348 Sgr and MV Sgr are lower mass and luminosity objects than EHe and R CrB stars.
The evidence presented in this thesis indicates that the hot R CrB group is not a homogeneous one. DY Cen is much more luminous and massive than the other members. The mass and luminosity of V348 Sgr and MV Sgr are consistent with the scenario that both have recently suffered a thermal pulse (causing re-ignition of a helium burning shell) and are currently looping back to the R CrB domain of the H R diagram.A 220 GHz 3D imaging radar with sub-cm3 voxel resolution for security applicationsRobertson, Duncan AlexanderMacFarlane, David GrahamBryllert, Tomashttps://hdl.handle.net/10023/110372022-05-03T15:31:07Z2017-01-19T00:00:00ZRadars operating at high millimetre and sub-millimetre wave frequencies are promising candidates for security applications such as people screening since they offer the possibility to form 3D images through clothing with sufficient resolution to detect concealed objects. High spatial resolution of order 1 cm can be achieved using practically sized antennas and high range resolution can be achieved using wideband FMCW chirps, e.g. 30 GHz, to yield 0.5 cm range bins. We present a 220 GHz test-bed ‘Pathfinder’ radar which achieves sub-cm3 voxel resolution with very high signal fidelity. The radar is used to de-risk technology under development for next generation people screening systems.
2017-01-19T00:00:00ZRobertson, Duncan AlexanderMacFarlane, David GrahamBryllert, TomasRadars operating at high millimetre and sub-millimetre wave frequencies are promising candidates for security applications such as people screening since they offer the possibility to form 3D images through clothing with sufficient resolution to detect concealed objects. High spatial resolution of order 1 cm can be achieved using practically sized antennas and high range resolution can be achieved using wideband FMCW chirps, e.g. 30 GHz, to yield 0.5 cm range bins. We present a 220 GHz test-bed ‘Pathfinder’ radar which achieves sub-cm3 voxel resolution with very high signal fidelity. The radar is used to de-risk technology under development for next generation people screening systems.All-optical manipulation of photonic membranesKirkpatrick, Blair Connellhttps://hdl.handle.net/10023/110312019-04-01T10:03:43Z2017-06-21T00:00:00ZOptical tweezers have allowed us to harness the momentum of light to trap, move, and
manipulate microscopic particles with Angstrom-level precision. Position and force feedback
systems grant us the ability to feel the microscopic world. As a tool, optical tweezers have allowed
us to study a variety of biological systems, from the mechanical properties of red blood cells to the
quantised motion of motor-molecules such as kinesin. They have been applied, with similar impact,
to the manipulation of gases, atoms, and Bose-Einstein condensates. There are, however, limits to
their applicability. Historically speaking, optical tweezers have only been used to trap relatively
simple structures such as spheres or cylinders.
This thesis is concerned with the development of a fabricational and optical manipulation
protocol that allows holographical optical tweezers to trap photonic membranes. Photonic membranes
are thin, flexible membranes, that are capable of supporting nanoplasmonic features. These
features can be patterned to function as metamaterials, granting the photonic membrane the ability
to function as almost any optical device. It is highly desirable to take advantage of these tools in a
microfluidic environment, however, their extreme aspect ratios mean that they are not traditionally
compatible with the primary technology of microfluidic manipulation: optical tweezers.
In line with recent developments in optical manipulation, an holistic approach to optical
trapping is used to overcome these limitations. Full six-degree-of-freedom control over a photonic
membrane is demonstrated through the use of holographical optical tweezers. Furthermore,
a photonic membrane (PM)-based surface-enhanced Raman spectroscopy sensor is presented
which is capable of detecting rhodamine dye from a topologically undulating sample. This work
moves towards marrying these technologies such that photonic membranes, designed for bespoke
applications, can be readily deployed into a microfluidic environment. Extending the range of tools
available in the microfluidic setting helps pave the way toward the next set of advances in the field
of optical manipulation.
2017-06-21T00:00:00ZKirkpatrick, Blair ConnellOptical tweezers have allowed us to harness the momentum of light to trap, move, and
manipulate microscopic particles with Angstrom-level precision. Position and force feedback
systems grant us the ability to feel the microscopic world. As a tool, optical tweezers have allowed
us to study a variety of biological systems, from the mechanical properties of red blood cells to the
quantised motion of motor-molecules such as kinesin. They have been applied, with similar impact,
to the manipulation of gases, atoms, and Bose-Einstein condensates. There are, however, limits to
their applicability. Historically speaking, optical tweezers have only been used to trap relatively
simple structures such as spheres or cylinders.
This thesis is concerned with the development of a fabricational and optical manipulation
protocol that allows holographical optical tweezers to trap photonic membranes. Photonic membranes
are thin, flexible membranes, that are capable of supporting nanoplasmonic features. These
features can be patterned to function as metamaterials, granting the photonic membrane the ability
to function as almost any optical device. It is highly desirable to take advantage of these tools in a
microfluidic environment, however, their extreme aspect ratios mean that they are not traditionally
compatible with the primary technology of microfluidic manipulation: optical tweezers.
In line with recent developments in optical manipulation, an holistic approach to optical
trapping is used to overcome these limitations. Full six-degree-of-freedom control over a photonic
membrane is demonstrated through the use of holographical optical tweezers. Furthermore,
a photonic membrane (PM)-based surface-enhanced Raman spectroscopy sensor is presented
which is capable of detecting rhodamine dye from a topologically undulating sample. This work
moves towards marrying these technologies such that photonic membranes, designed for bespoke
applications, can be readily deployed into a microfluidic environment. Extending the range of tools
available in the microfluidic setting helps pave the way toward the next set of advances in the field
of optical manipulation.An investigation of contact and non-contact binary systemsMcFarlane, Thomas M.https://hdl.handle.net/10023/110082019-04-01T10:03:33Z1986-01-01T00:00:00ZSpectroscopic and photometric observations of six late-type contact and near-contact binaries are presented, and the results of the analyses discussed. Absolute dimensions have been deduced for three systems: YY Ceti. CX Aquarii and RV Corvi.
YY Ceti, which has an orbital period of 0.79 days, is found to be composed of a normal main-sequence star of spectral type A8 and an evolved G-type star which completely fills its Roche lobe. We argue that the system will evolve into contact within approximately 10 9 years, either by the nuclear evolution of the primary component, or by the loss of angular momentum via magnetic braking. We also propose that this may represent a route to the A-type contact binaries which does not involve a passage through the W-type phase.
The binary CX Aquarii, which has an orbital period of 0.56 days is found to have a similar configuration to YY Ceti, except that its primary component is a main-sequence star of spectral type F5 with an evolved companion of spectral type G9. We argue that the system should achieve contact by loss of angular momentum via magnetic braking within approximately 108 years if the
atmosphere of the primary component is convective, or within approximately 5 x 108 years if the primary possesses a radiative atmosphere.
RV Corvi is found to consist of an unevolved F2 primary component with a K-type companion which has a much larger radius and luminosity than expected for its mass. The system has an orbital period of 0.75 days. Although the binary may be semi-detached, with the primary component completely filling its Roche lobe, it is most probably in a state of marginal contact. In order to obtain a solution to the photometric data it was necessary to treat the secondary component albedo as a free parameter, yielding a value greater than unity. This result implies that there is an abnormal distribution of luminosity on the surface of the binary, which may be interpreted either as an excess of light on the facing hemisphere of one or both of the components, or as a deficit of light on the averted hemisphere of the secondary.
The three remaining binaries, EZ Hydrae, AD Phoenicis and RS Columbae, were all identified as contact systems: EZ Hydrae as a W-type system of orbital period 0.45 days, AD Phoenicis as an A-type system of period 0.38 days, and RS Columbae as an A-type system with a period of 0 .67 days. Although a value for the mass ratio of EZ Hydrae had been obtained from spectroscopic observations, no photometric solution could found because of severe 'disturbances' in its light curve. From our sample of six systems, AD llioenicis and RS Columbae were the only two for which there were no spectroscopic observations. Although photometric solutions were obtained for both systems, these were very insensitive to the value of mass ratio,
which tended toward the physically unrealistic value of unity.
The results for YY Ceti, CX Aquarii and RV Corvi are combined with the published masses, radii and luminosities of 21 other contact and near-contact binaries. It is shown that the primary
components of all contact and near-contact systems are normal main-sequence stars with radii and luminosities appropriate to their masses. The secondary components of the B-type systems and the W-type systems are all overluminous for their masses, due mostly to the radii being increased by a factor of ~1.7. However, these secondary components are segregated on the H-R diagram, where the W-type secondaries appear to the left of the main sequence band due to luminosity transfer from the primary components. The secondary components of the A-type contact systems all have radii and luminosities substantially larger than expected for their masses.
1986-01-01T00:00:00ZMcFarlane, Thomas M.Spectroscopic and photometric observations of six late-type contact and near-contact binaries are presented, and the results of the analyses discussed. Absolute dimensions have been deduced for three systems: YY Ceti. CX Aquarii and RV Corvi.
YY Ceti, which has an orbital period of 0.79 days, is found to be composed of a normal main-sequence star of spectral type A8 and an evolved G-type star which completely fills its Roche lobe. We argue that the system will evolve into contact within approximately 10 9 years, either by the nuclear evolution of the primary component, or by the loss of angular momentum via magnetic braking. We also propose that this may represent a route to the A-type contact binaries which does not involve a passage through the W-type phase.
The binary CX Aquarii, which has an orbital period of 0.56 days is found to have a similar configuration to YY Ceti, except that its primary component is a main-sequence star of spectral type F5 with an evolved companion of spectral type G9. We argue that the system should achieve contact by loss of angular momentum via magnetic braking within approximately 108 years if the
atmosphere of the primary component is convective, or within approximately 5 x 108 years if the primary possesses a radiative atmosphere.
RV Corvi is found to consist of an unevolved F2 primary component with a K-type companion which has a much larger radius and luminosity than expected for its mass. The system has an orbital period of 0.75 days. Although the binary may be semi-detached, with the primary component completely filling its Roche lobe, it is most probably in a state of marginal contact. In order to obtain a solution to the photometric data it was necessary to treat the secondary component albedo as a free parameter, yielding a value greater than unity. This result implies that there is an abnormal distribution of luminosity on the surface of the binary, which may be interpreted either as an excess of light on the facing hemisphere of one or both of the components, or as a deficit of light on the averted hemisphere of the secondary.
The three remaining binaries, EZ Hydrae, AD Phoenicis and RS Columbae, were all identified as contact systems: EZ Hydrae as a W-type system of orbital period 0.45 days, AD Phoenicis as an A-type system of period 0.38 days, and RS Columbae as an A-type system with a period of 0 .67 days. Although a value for the mass ratio of EZ Hydrae had been obtained from spectroscopic observations, no photometric solution could found because of severe 'disturbances' in its light curve. From our sample of six systems, AD llioenicis and RS Columbae were the only two for which there were no spectroscopic observations. Although photometric solutions were obtained for both systems, these were very insensitive to the value of mass ratio,
which tended toward the physically unrealistic value of unity.
The results for YY Ceti, CX Aquarii and RV Corvi are combined with the published masses, radii and luminosities of 21 other contact and near-contact binaries. It is shown that the primary
components of all contact and near-contact systems are normal main-sequence stars with radii and luminosities appropriate to their masses. The secondary components of the B-type systems and the W-type systems are all overluminous for their masses, due mostly to the radii being increased by a factor of ~1.7. However, these secondary components are segregated on the H-R diagram, where the W-type secondaries appear to the left of the main sequence band due to luminosity transfer from the primary components. The secondary components of the A-type contact systems all have radii and luminosities substantially larger than expected for their masses.Design and operation of a 45 μs repetitively pulsed 12 mw electron beam for a CO₂ laserCurry, Randy D.https://hdl.handle.net/10023/109722019-04-01T10:02:38Z1992-01-01T00:00:00ZThe parameter space over which electron beams operate for pumping CO₂ lasers significantly extended. A repetitive pulsed power/electron beam system for the generation of 45 µs, 220 keV, electron beams for CO₂ lasers has been designed and implemented which can operate over a 6:1 impedance range. The system consists of a 150 kV – 250 kV modulator and a large area, (2.500 cm²) 5-20 mλ /cm² hot-cathode electron-beam gun. The system is designed and fabricated to be used in conjunction with an electron beam sustained laser and is capable of operating continuously at up to 10 pps.
The following thesis examines, in detail, the experimental and theoretical investigation of the modulator/electron beam subsystems which comprise the electron beam system. Parameters of the CO₂ laser which determine the electron beam parameter space, and thus the modulator characteristics, are reviewed. Both cold-cathode and hot-cathode pulsed current parameters are reviewed, and the cathode characteristics upon which the
hot-cathode was selected are discussed. The 132 thoriated tungsten filament, grid-controlled, hot-cathode gun used to generate the required 5-20 mλ/cm², 250 cm length source of electron beam with a spatial flatness of ± 10% is shown.
Based on the hot-cathode electron beam gun design, the theoretical and practical design characteristics of the hollow-cathode, thyratron switched, 1800 J, 10 pps, 250 kV modulator are shown. Design equations which allow selection of the modulator operation range, and selection of the modulator matching resistors used to match the fixed impedance modulator to the 6:1 electron beam impedance range with a pulse droop of less then 0.7% and a pulse flatness of ± 10% are derived. Moreover, a detailed discussion of the circuit simulations used to both tune and optimize the modulator's impedance tapered, Transformer-coupled Type E PFN to the four electron beam operating ranges are shown. Practical layout and design and the grounding and shielding principles employed in the system. Component spacings required to insure reliable modulator operation at 250kV are also considered.
The design and implementation of the 45 µs, SCR switched, transformer-coupled PFN based grid-pulser are discussed. The grid pulser characteristics including the 105-650 V, 28-166 A, pulse required to control the 5-20 mλ / cm²,hot-cathode current density are examined. Theoretical design equations are derived to allow, the Miller capacitance, and thus the time varying impedance of the grid-cathode region to be predicted. Circuit simulations showing the grid-pulser circuit interaction with the constant perveance, Miller capacitance dominated, grid-cathode load are discussed. Final layout of the SCR-switched, grid-pulser including design of the spiral inductors used for the PFN is reported. Specific layout details are shown which allow the grid pulser to operate reliably while floating at the 220 kV electron beam acceleration pulse.
A discussion of the subsystem experimental integration phase conducted at Pulse Sciences, Inc. is also presented in the following thesis. Test results of the modulator and grid-pulser into their respective electron beam loads are compared with the circuit simulations. Relevant vacuum flashover physics issues found during the experimental integration of the modulator, grid-pulser, and electron-beam gun are considered. Specific discussions of the vacuum, gap and vacuum insulator physics which effect the operation of the large area electron beam gun are reported. Included in this discussion is the experimental cold- cathode conditioning procedure used to condition the electron-beam gun to 250 kV. In-situ, hot-cathode filament carburization and activation procedures developed during the integration phase of the program are also presented, with a short discussion of the background mass spectra measured during the experiments. Finally, the experimental characterization. of the spatial uniformity of the electron beam is reported along with a short summary of the investigation results.
1992-01-01T00:00:00ZCurry, Randy D.The parameter space over which electron beams operate for pumping CO₂ lasers significantly extended. A repetitive pulsed power/electron beam system for the generation of 45 µs, 220 keV, electron beams for CO₂ lasers has been designed and implemented which can operate over a 6:1 impedance range. The system consists of a 150 kV – 250 kV modulator and a large area, (2.500 cm²) 5-20 mλ /cm² hot-cathode electron-beam gun. The system is designed and fabricated to be used in conjunction with an electron beam sustained laser and is capable of operating continuously at up to 10 pps.
The following thesis examines, in detail, the experimental and theoretical investigation of the modulator/electron beam subsystems which comprise the electron beam system. Parameters of the CO₂ laser which determine the electron beam parameter space, and thus the modulator characteristics, are reviewed. Both cold-cathode and hot-cathode pulsed current parameters are reviewed, and the cathode characteristics upon which the
hot-cathode was selected are discussed. The 132 thoriated tungsten filament, grid-controlled, hot-cathode gun used to generate the required 5-20 mλ/cm², 250 cm length source of electron beam with a spatial flatness of ± 10% is shown.
Based on the hot-cathode electron beam gun design, the theoretical and practical design characteristics of the hollow-cathode, thyratron switched, 1800 J, 10 pps, 250 kV modulator are shown. Design equations which allow selection of the modulator operation range, and selection of the modulator matching resistors used to match the fixed impedance modulator to the 6:1 electron beam impedance range with a pulse droop of less then 0.7% and a pulse flatness of ± 10% are derived. Moreover, a detailed discussion of the circuit simulations used to both tune and optimize the modulator's impedance tapered, Transformer-coupled Type E PFN to the four electron beam operating ranges are shown. Practical layout and design and the grounding and shielding principles employed in the system. Component spacings required to insure reliable modulator operation at 250kV are also considered.
The design and implementation of the 45 µs, SCR switched, transformer-coupled PFN based grid-pulser are discussed. The grid pulser characteristics including the 105-650 V, 28-166 A, pulse required to control the 5-20 mλ / cm²,hot-cathode current density are examined. Theoretical design equations are derived to allow, the Miller capacitance, and thus the time varying impedance of the grid-cathode region to be predicted. Circuit simulations showing the grid-pulser circuit interaction with the constant perveance, Miller capacitance dominated, grid-cathode load are discussed. Final layout of the SCR-switched, grid-pulser including design of the spiral inductors used for the PFN is reported. Specific layout details are shown which allow the grid pulser to operate reliably while floating at the 220 kV electron beam acceleration pulse.
A discussion of the subsystem experimental integration phase conducted at Pulse Sciences, Inc. is also presented in the following thesis. Test results of the modulator and grid-pulser into their respective electron beam loads are compared with the circuit simulations. Relevant vacuum flashover physics issues found during the experimental integration of the modulator, grid-pulser, and electron-beam gun are considered. Specific discussions of the vacuum, gap and vacuum insulator physics which effect the operation of the large area electron beam gun are reported. Included in this discussion is the experimental cold- cathode conditioning procedure used to condition the electron-beam gun to 250 kV. In-situ, hot-cathode filament carburization and activation procedures developed during the integration phase of the program are also presented, with a short discussion of the background mass spectra measured during the experiments. Finally, the experimental characterization. of the spatial uniformity of the electron beam is reported along with a short summary of the investigation results.Advanced light-sheet and structured illumination microscopy techniques for neuroscience and disease diagnosisNylk, Jonathanhttps://hdl.handle.net/10023/108422020-11-17T03:08:22Z2016-11-30T00:00:00Z; Optical microscopy is a cornerstone of biomedical research. Advances in optical techniques enable specific, high resolution, sterile, and biologically compatible imaging. In particular, beam shaping has been used to tailor microscopy techniques to enhance microscope performance. The aim of this Thesis is to investigate the use of novel beam shaping techniques in emerging optical microscopy methods, and to apply these methods in biomedicine.
To overcome the challenges associated with high resolution imaging of large specimens, the use of Airy beams and related techniques are applied to light-sheet microscopy. This approach increases the field-of-view that can be imaged at high resolution by over an order of magnitude compared to standard Gaussian beam based light-sheet microscopy, has reduced phototoxicity, and can be implemented with a low-cost optical system. Advanced implementations show promise for imaging at depth within turbid tissue, in particular for neuroscience.
Super-resolution microscopy techniques enhance the spatial resolution of optical methods. Structured illumination microscopy is investigated as an alternative for electron microscopy in disease diagnosis, capable of visualising pathologically relevant features of kidney disease. Separately, compact optical manipulation methods are developed with the aim of adding functionality to super-resolution techniques.
2016-11-30T00:00:00ZNylk, JonathanOptical microscopy is a cornerstone of biomedical research. Advances in optical techniques enable specific, high resolution, sterile, and biologically compatible imaging. In particular, beam shaping has been used to tailor microscopy techniques to enhance microscope performance. The aim of this Thesis is to investigate the use of novel beam shaping techniques in emerging optical microscopy methods, and to apply these methods in biomedicine.
To overcome the challenges associated with high resolution imaging of large specimens, the use of Airy beams and related techniques are applied to light-sheet microscopy. This approach increases the field-of-view that can be imaged at high resolution by over an order of magnitude compared to standard Gaussian beam based light-sheet microscopy, has reduced phototoxicity, and can be implemented with a low-cost optical system. Advanced implementations show promise for imaging at depth within turbid tissue, in particular for neuroscience.
Super-resolution microscopy techniques enhance the spatial resolution of optical methods. Structured illumination microscopy is investigated as an alternative for electron microscopy in disease diagnosis, capable of visualising pathologically relevant features of kidney disease. Separately, compact optical manipulation methods are developed with the aim of adding functionality to super-resolution techniques.Elastic-resonator-interference-stress-microscopy (ERISM)Liehm, Philipphttps://hdl.handle.net/10023/108242021-03-12T03:02:21Z2017-06-23T00:00:00ZThe forces biological cells apply to their environment are recognised to be critical during
processes like migration, division, wound healing, and stem cell differentiation. Methods to
measure these forces have been extremely valuable in contributing to our understanding of
cell-substrate and cell-cell interactions. However, existing force sensing techniques struggle
to measure forces cells apply perpendicular to the plane of their substrate although these
out-of-plane forces have been demonstrated to be important in many processes. In addition,
most currently used force sensing techniques require fluorescence imaging which can lead to
photo-toxic effects if high frame rates are required. Finally, many methods require detaching
of cells after the measurement which prevents measuring the same cells repeatedly or using
immunostaining, which is an important tool for linking biomechanical and biochemical
observations.
In this thesis, we introduce a novel high-throughput and low-light-intensity force sensing
technique which is inherently well suited to measure vertical forces. Elastic-Resonator-Interference-Stress-Microscopy (ERISM) measures the spatially resolved reflectance of an
elastic micro-cavity. With fully automated hyperspectral imaging and data analysis supported
by transfer-matrix modelling, this allows tracking of nanometre thickness changes
across a large area of the cavity. By combining Atomic-Force-Microscopy with a Finite-Element-Method,
we extract basic material properties of the micro-cavities to calculate
stress and force.
Using the example of different neural cells, we provide experimental evidence that ERISM
measurements can be performed over hours at high frame rates or repeatedly over weeks
for the same sample to investigate a variety of cellular processes like cell spreading, growth cone migration, and stem cell differentiation. We perform immunostaining for cell specific
markers on ERISM micro-cavities as detaching of the cells is not required. Furthermore, we
find that the high throughput of ERISM allows us to find significant differences between
wild-type and knock-out cell populations for a gene associated with dyslexia.
2017-06-23T00:00:00ZLiehm, PhilippThe forces biological cells apply to their environment are recognised to be critical during
processes like migration, division, wound healing, and stem cell differentiation. Methods to
measure these forces have been extremely valuable in contributing to our understanding of
cell-substrate and cell-cell interactions. However, existing force sensing techniques struggle
to measure forces cells apply perpendicular to the plane of their substrate although these
out-of-plane forces have been demonstrated to be important in many processes. In addition,
most currently used force sensing techniques require fluorescence imaging which can lead to
photo-toxic effects if high frame rates are required. Finally, many methods require detaching
of cells after the measurement which prevents measuring the same cells repeatedly or using
immunostaining, which is an important tool for linking biomechanical and biochemical
observations.
In this thesis, we introduce a novel high-throughput and low-light-intensity force sensing
technique which is inherently well suited to measure vertical forces. Elastic-Resonator-Interference-Stress-Microscopy (ERISM) measures the spatially resolved reflectance of an
elastic micro-cavity. With fully automated hyperspectral imaging and data analysis supported
by transfer-matrix modelling, this allows tracking of nanometre thickness changes
across a large area of the cavity. By combining Atomic-Force-Microscopy with a Finite-Element-Method,
we extract basic material properties of the micro-cavities to calculate
stress and force.
Using the example of different neural cells, we provide experimental evidence that ERISM
measurements can be performed over hours at high frame rates or repeatedly over weeks
for the same sample to investigate a variety of cellular processes like cell spreading, growth cone migration, and stem cell differentiation. We perform immunostaining for cell specific
markers on ERISM micro-cavities as detaching of the cells is not required. Furthermore, we
find that the high throughput of ERISM allows us to find significant differences between
wild-type and knock-out cell populations for a gene associated with dyslexia.The VIMOS Public Extragalactic Redshift Survey (VIPERS). The coevolution of galaxy morphology and colour to z ~ 1Krywult, J.Tasca, L. A. M.Pollo, A.Vergani, D.Bolzonella, M.Davidzon, I.Iovino, A.Gargiulo, A.Haines, C. P.Scodeggio, M.Guzzo, L.Zamorani, G.Garilli, B.Granett, B. R.de la Torre, S.Abbas, U.Adami, C.Bottini, D.Cappi, A.Cucciati, O.Franzetti, P.Fritz, A.Le Brun, V.Le Fèvre, O.Maccagni, D.Małek, K.Marulli, F.Polletta, M.Tojeiro, R.Zanichelli, A.Arnouts, S.Bel, J.Branchini, E.Coupon, J.De Lucia, G.Ilbert, O.McCracken, H. J.Moscardini, L.Takeuchi, T. T.https://hdl.handle.net/10023/105842022-04-08T13:30:44Z2017-02-01T00:00:00ZContext. The study of the separation of galaxy types into different classes that share the same characteristics, and of the evolution of the specific parameters used in the classification are fundamental for understanding galaxy evolution.Aims. We explore the evolution of the statistical distribution of galaxy morphological properties and colours combining high-quality imaging data from the CFHT Legacy Survey with the large number of redshifts and extended photometry from the VIPERS survey.Methods. Galaxy structural parameters were combined with absolute magnitudes, colours and redshifts in order to trace evolution in a multi-parameter space. Using a new method we analysed the combination of colours and structural parameters of early- and late-type galaxies in luminosity-redshift space.Results. We find that both the rest-frame colour distributions in the (U−B) vs. (B−V) plane and the Sérsic index distributions are well fitted by a sum of two Gaussians, with a remarkable consistency of red-spheroidal and blue-disky galaxy populations, over the explored redshift (0.5 < z < 1) and luminosity (−1.5 < B−B∗ < 1.0) ranges. The combination of the rest-frame colour and Sérsic index as a function of redshift and luminosity allows us to present the structure of both galaxy types and their evolution. We find that early-type galaxies display only a slow change in their concentrations after z = 1. Their high concentrations were already established at z ~ 1 and depend much more strongly on their luminosity than redshift. In contrast, late-type galaxies clearly become more concentrated with cosmic time with only little evolution in colour, which remains dependent mainly on their luminosity.Conclusions. The combination of rest-frame colours and Sérsic index as a function of redshift and luminosity leads to a precise statistical description of the structure of galaxies and their evolution. Additionally, the proposed method provides a robust way to split galaxies into early and late types.
RT acknowledges financial support from the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007- 2013)/ERC grant agreement n. 202686.
2017-02-01T00:00:00ZKrywult, J.Tasca, L. A. M.Pollo, A.Vergani, D.Bolzonella, M.Davidzon, I.Iovino, A.Gargiulo, A.Haines, C. P.Scodeggio, M.Guzzo, L.Zamorani, G.Garilli, B.Granett, B. R.de la Torre, S.Abbas, U.Adami, C.Bottini, D.Cappi, A.Cucciati, O.Franzetti, P.Fritz, A.Le Brun, V.Le Fèvre, O.Maccagni, D.Małek, K.Marulli, F.Polletta, M.Tojeiro, R.Zanichelli, A.Arnouts, S.Bel, J.Branchini, E.Coupon, J.De Lucia, G.Ilbert, O.McCracken, H. J.Moscardini, L.Takeuchi, T. T.Context. The study of the separation of galaxy types into different classes that share the same characteristics, and of the evolution of the specific parameters used in the classification are fundamental for understanding galaxy evolution.Aims. We explore the evolution of the statistical distribution of galaxy morphological properties and colours combining high-quality imaging data from the CFHT Legacy Survey with the large number of redshifts and extended photometry from the VIPERS survey.Methods. Galaxy structural parameters were combined with absolute magnitudes, colours and redshifts in order to trace evolution in a multi-parameter space. Using a new method we analysed the combination of colours and structural parameters of early- and late-type galaxies in luminosity-redshift space.Results. We find that both the rest-frame colour distributions in the (U−B) vs. (B−V) plane and the Sérsic index distributions are well fitted by a sum of two Gaussians, with a remarkable consistency of red-spheroidal and blue-disky galaxy populations, over the explored redshift (0.5 < z < 1) and luminosity (−1.5 < B−B∗ < 1.0) ranges. The combination of the rest-frame colour and Sérsic index as a function of redshift and luminosity allows us to present the structure of both galaxy types and their evolution. We find that early-type galaxies display only a slow change in their concentrations after z = 1. Their high concentrations were already established at z ~ 1 and depend much more strongly on their luminosity than redshift. In contrast, late-type galaxies clearly become more concentrated with cosmic time with only little evolution in colour, which remains dependent mainly on their luminosity.Conclusions. The combination of rest-frame colours and Sérsic index as a function of redshift and luminosity leads to a precise statistical description of the structure of galaxies and their evolution. Additionally, the proposed method provides a robust way to split galaxies into early and late types.The VIMOS Public Extragalactic Redshift Survey (VIPERS). Star formation history of passive red galaxiesSiudek, M.Małek, K.Scodeggio, M.Garilli, B.Pollo, A.Haines, C. P.Fritz, A.Bolzonella, M.de la Torre, S.Granett, B. R.Guzzo, L.Abbas, U.Adami, C.Bottini, D.Cappi, A.Cucciati, O.De Lucia, G.Davidzon, I.Franzetti, P.Iovino, A.Krywult, J.Le Brun, V.Le Fèvre, O.Maccagni, D.Marchetti, A.Marulli, F.Polletta, M.Tasca, L. A. M.Tojeiro, R.Vergani, D.Zanichelli, A.Arnouts, S.Bel, J.Branchini, E.Ilbert, O.Gargiulo, A.Moscardini, L.Takeuchi, T. T.Zamorani, G.https://hdl.handle.net/10023/105232022-04-08T13:30:47Z2017-01-01T00:00:00ZAims. We trace the evolution and the star formation history of passive red galaxies, using a subset of the VIMOS Public Extragalactic Redshift Survey (VIPERS). The detailed spectral analysis of stellar populations of intermediate-redshift passive red galaxies allows the build up of their stellar content to be followed over the last 8 billion years. Methods. We extracted a sample of passive red galaxies in the redshift range 0.4 Results. We find that at z ~ 1 stellar populations in low-mass passive red galaxies are younger than in high-mass passive red galaxies, similar to what is observed at the present epoch. Over the full analyzed redshift range 0.4 < z < 1.0 and stellar mass range 10 < log (Mstar/M⊙) < 12, the D4000 index increases with redshift, while HδA gets lower. This implies that the stellar populations are getting older with increasing stellar mass. Comparison to the spectra of passive red galaxies in the SDSS survey (z ~ 0.2) shows that the shape of the relations of D4000 and HδA with stellar mass has not changed significantly with redshift. Assuming a single burst formation, this implies that high-mass passive red galaxies formed their stars at zform ~ 1.7, while low-mass galaxies formed their main stellar populations more recently, at zform ~ 1. The consistency of these results, which were obtained using two independent estimators of the formation redshift (D4000 and HδA), further strengthens a scenario in which star formation proceeds from higher to lower mass systems as time passes, i.e., what has become known as the downsizing picture.
R.T. acknowledge financial support from the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement n. 202686.
2017-01-01T00:00:00ZSiudek, M.Małek, K.Scodeggio, M.Garilli, B.Pollo, A.Haines, C. P.Fritz, A.Bolzonella, M.de la Torre, S.Granett, B. R.Guzzo, L.Abbas, U.Adami, C.Bottini, D.Cappi, A.Cucciati, O.De Lucia, G.Davidzon, I.Franzetti, P.Iovino, A.Krywult, J.Le Brun, V.Le Fèvre, O.Maccagni, D.Marchetti, A.Marulli, F.Polletta, M.Tasca, L. A. M.Tojeiro, R.Vergani, D.Zanichelli, A.Arnouts, S.Bel, J.Branchini, E.Ilbert, O.Gargiulo, A.Moscardini, L.Takeuchi, T. T.Zamorani, G.Aims. We trace the evolution and the star formation history of passive red galaxies, using a subset of the VIMOS Public Extragalactic Redshift Survey (VIPERS). The detailed spectral analysis of stellar populations of intermediate-redshift passive red galaxies allows the build up of their stellar content to be followed over the last 8 billion years. Methods. We extracted a sample of passive red galaxies in the redshift range 0.4 Results. We find that at z ~ 1 stellar populations in low-mass passive red galaxies are younger than in high-mass passive red galaxies, similar to what is observed at the present epoch. Over the full analyzed redshift range 0.4 < z < 1.0 and stellar mass range 10 < log (Mstar/M⊙) < 12, the D4000 index increases with redshift, while HδA gets lower. This implies that the stellar populations are getting older with increasing stellar mass. Comparison to the spectra of passive red galaxies in the SDSS survey (z ~ 0.2) shows that the shape of the relations of D4000 and HδA with stellar mass has not changed significantly with redshift. Assuming a single burst formation, this implies that high-mass passive red galaxies formed their stars at zform ~ 1.7, while low-mass galaxies formed their main stellar populations more recently, at zform ~ 1. The consistency of these results, which were obtained using two independent estimators of the formation redshift (D4000 and HδA), further strengthens a scenario in which star formation proceeds from higher to lower mass systems as time passes, i.e., what has become known as the downsizing picture.