http://hdl.handle.net/10023/128 2013-05-22T05:35:10Z 2013-05-22T05:35:10Z Baily, Charles Finkelstein, Noah D. http://hdl.handle.net/10023/3535 2013-05-20T15:01:04Z 2010-01-27T00:00:00Z

Abstract: Just as expert physicists vary in their personal stances on interpretation in quantum mechanics, instructors vary on whether and how to teach interpretations of quantum phenomena in introductory modern physics courses. In this paper, we document variations in instructional approaches with respect to interpretation in two similar modern physics courses recently taught at the University of Colorado, and examine associated impacts on student perspectives regarding quantum physics. We find students are more likely to prefer realist interpretations of quantum-mechanical systems when instructors are less explicit in addressing student ontologies. We also observe contextual variations in student beliefs about quantum systems, indicating that instructors who choose to address questions of ontology in quantum mechanics should do so explicitly across a range of topics.

2010-01-27T00:00:00Z Baily, Charles Finkelstein, Noah D. Just as expert physicists vary in their personal stances on interpretation in quantum mechanics, instructors vary on whether and how to teach interpretations of quantum phenomena in introductory modern physics courses. In this paper, we document variations in instructional approaches with respect to interpretation in two similar modern physics courses recently taught at the University of Colorado, and examine associated impacts on student perspectives regarding quantum physics. We find students are more likely to prefer realist interpretations of quantum-mechanical systems when instructors are less explicit in addressing student ontologies. We also observe contextual variations in student beliefs about quantum systems, indicating that instructors who choose to address questions of ontology in quantum mechanics should do so explicitly across a range of topics. Baily, Charles Finkelstein, Noah D. http://hdl.handle.net/10023/3534 2013-05-20T15:01:03Z 2010-09-16T00:00:00Z

Abstract: The perspectives of introductory classical physics students can often negatively influence how those students later interpret quantum phenomena when taking an introductory course in modern physics. A detailed exploration of student perspectives on the interpretation of quantum physics is needed, both to characterize student understanding of physics concepts, and to inform how we might teach traditional content. Our previous investigations of student perspectives on quantum physics have indicated they can be highly nuanced, and may vary both within and across contexts. In order to better understand the contextual and often seemingly contradictory stances of students on matters of interpretation, we interviewed 19 students from four introductory modern physics courses taught at the University of Colorado. We find that students have attitudes and opinions that often parallel the stances of expert physicists when arguing for their favored interpretations of quantum mechanics, allowing for more nuanced characterizations of student perspectives in terms of three key interpretive themes. We present a framework for characterizing student perspectives on quantum mechanics, and demonstrate its utility in interpreting the sometimes contradictory nature of student responses to previous surveys. We further find that students most often vacillate in their responses when what makes intuitive sense to them is not in agreement with what they consider to be a correct response, underscoring the need to distinguish between the personal and the public perspectives of introductory modern physics students.

2010-09-16T00:00:00Z Baily, Charles Finkelstein, Noah D. The perspectives of introductory classical physics students can often negatively influence how those students later interpret quantum phenomena when taking an introductory course in modern physics. A detailed exploration of student perspectives on the interpretation of quantum physics is needed, both to characterize student understanding of physics concepts, and to inform how we might teach traditional content. Our previous investigations of student perspectives on quantum physics have indicated they can be highly nuanced, and may vary both within and across contexts. In order to better understand the contextual and often seemingly contradictory stances of students on matters of interpretation, we interviewed 19 students from four introductory modern physics courses taught at the University of Colorado. We find that students have attitudes and opinions that often parallel the stances of expert physicists when arguing for their favored interpretations of quantum mechanics, allowing for more nuanced characterizations of student perspectives in terms of three key interpretive themes. We present a framework for characterizing student perspectives on quantum mechanics, and demonstrate its utility in interpreting the sometimes contradictory nature of student responses to previous surveys. We further find that students most often vacillate in their responses when what makes intuitive sense to them is not in agreement with what they consider to be a correct response, underscoring the need to distinguish between the personal and the public perspectives of introductory modern physics students. Kosmeier, Sebastian De Luca, Anna Chiara Zolotovskaya, Svetlana Di Falco, Andrea Dholakia, Kishan Mazilu, Michael http://hdl.handle.net/10023/3533 2013-05-20T11:31:06Z 2013-05-09T00:00:00Z

Abstract: The last decade has seen subwavelength focusing of the electromagnetic field in the proximity of nanoplasmonic structures with various designs. However, a shared issue is the spatial confinement of the field, which is mostly inflexible and limited to fixed locations determined by the geometry of the nanostructures, which hampers many applications. Here, we coherently address numerically and experimentally single and multiple plasmonic nanostructures chosen from a given array, resorting to the principle of optical eigenmodes. By decomposing the light field into optical eigenmodes, specifically tailored to the nanostructure, we create a subwavelength, selective and dynamic control of the incident light. The coherent control of plasmonic nanoantennas using this approach shows an almost zero crosstalk. This approach is applicable even in the presence of large transmission aberrations, such as present in holographic diffusers and multimode fibres. The method presents a paradigm shift for the addressing of plasmonic nanostructures by light.

2013-05-09T00:00:00Z Kosmeier, Sebastian De Luca, Anna Chiara Zolotovskaya, Svetlana Di Falco, Andrea Dholakia, Kishan Mazilu, Michael The last decade has seen subwavelength focusing of the electromagnetic field in the proximity of nanoplasmonic structures with various designs. However, a shared issue is the spatial confinement of the field, which is mostly inflexible and limited to fixed locations determined by the geometry of the nanostructures, which hampers many applications. Here, we coherently address numerically and experimentally single and multiple plasmonic nanostructures chosen from a given array, resorting to the principle of optical eigenmodes. By decomposing the light field into optical eigenmodes, specifically tailored to the nanostructure, we create a subwavelength, selective and dynamic control of the incident light. The coherent control of plasmonic nanoantennas using this approach shows an almost zero crosstalk. This approach is applicable even in the presence of large transmission aberrations, such as present in holographic diffusers and multimode fibres. The method presents a paradigm shift for the addressing of plasmonic nanostructures by light. Baily, Charles Dubson, Michael Pollock, Steven J. http://hdl.handle.net/10023/3530 2013-05-16T11:31:03Z 2013-01-01T00:00:00Z

Abstract: Favorable outcomes from ongoing research at the University of Colorado Boulder on student learning in junior-level electrostatics (E&M I) have led us to extend this work to upper-division electrodynamics (E&M II). We describe here our development of a set of research-based instructional materials designed to actively engage students during lecture (including clicker questions and other in-class activities); and an instrument for assessing whether our faculty-consensus learning goals are being met. We also discuss preliminary results from several recent implementations of our transformed curriculum, plans for the dissemination and further refinement of these materials, and offer some insights into student difficulties in advanced undergraduate electromagnetism.

2013-01-01T00:00:00Z Baily, Charles Dubson, Michael Pollock, Steven J. Favorable outcomes from ongoing research at the University of Colorado Boulder on student learning in junior-level electrostatics (E&M I) have led us to extend this work to upper-division electrodynamics (E&M II). We describe here our development of a set of research-based instructional materials designed to actively engage students during lecture (including clicker questions and other in-class activities); and an instrument for assessing whether our faculty-consensus learning goals are being met. We also discuss preliminary results from several recent implementations of our transformed curriculum, plans for the dissemination and further refinement of these materials, and offer some insights into student difficulties in advanced undergraduate electromagnetism. Baily, Charles Finkelstein, Noah D. http://hdl.handle.net/10023/3529 2013-05-16T11:31:01Z 2009-03-23T00:00:00Z

Abstract: Introductory undergraduate courses in classical physics stress a perspective that can be characterized as realist; from this perspective, all physical properties of a classical system can be simultaneously specified and thus determined at all future times. Such a perspective can be problematic for introductory quantum physics students, who must develop new perspectives in order to properly interpret what it means to have knowledge of quantum systems. We document this evolution in student thinking in part through pre- and post-instruction evaluations using the Colorado Learning Attitudes about Science Survey. We further characterize variations in student epistemic and ontological commitments by examining responses to two essay questions, coupled with responses to supplemental quantum attitude statements. We find that, after instruction in modern physics, many students are still exhibiting a realist perspective in contexts where a quantum-mechanical perspective is needed. We further find that this effect can be significantly influenced by instruction, where we observe variations for courses with differing learning goals. We also note that students generally do not employ either a realist or a quantum perspective in a consistent manner.

2009-03-23T00:00:00Z Baily, Charles Finkelstein, Noah D. Introductory undergraduate courses in classical physics stress a perspective that can be characterized as realist; from this perspective, all physical properties of a classical system can be simultaneously specified and thus determined at all future times. Such a perspective can be problematic for introductory quantum physics students, who must develop new perspectives in order to properly interpret what it means to have knowledge of quantum systems. We document this evolution in student thinking in part through pre- and post-instruction evaluations using the Colorado Learning Attitudes about Science Survey. We further characterize variations in student epistemic and ontological commitments by examining responses to two essay questions, coupled with responses to supplemental quantum attitude statements. We find that, after instruction in modern physics, many students are still exhibiting a realist perspective in contexts where a quantum-mechanical perspective is needed. We further find that this effect can be significantly influenced by instruction, where we observe variations for courses with differing learning goals. We also note that students generally do not employ either a realist or a quantum perspective in a consistent manner. Baily, Charles Finkelstein, Noah D. http://hdl.handle.net/10023/3527 2013-05-16T07:31:02Z 2012-02-01T00:00:00Z

Abstract: A common learning goal for modern physics instructors is for students to recognize a difference between the experimental uncertainty of classical physics and the fundamental uncertainty of quantum mechanics. Our prior work has shown that student perspectives on the physical interpretation of quantum mechanics can be characterized, and are differentially influenced by the myriad ways instructors approach interpretive themes in their introductory courses. We report how a transformed modern physics curriculum (recently implemented at the University of Colorado) has positively impacted student perspectives on quantum physics, by making questions of classical and quantum reality a central theme of the course, but also by making the beliefs of students (and not just those of scientists) an explicit topic of discussion.

2012-02-01T00:00:00Z Baily, Charles Finkelstein, Noah D. A common learning goal for modern physics instructors is for students to recognize a difference between the experimental uncertainty of classical physics and the fundamental uncertainty of quantum mechanics. Our prior work has shown that student perspectives on the physical interpretation of quantum mechanics can be characterized, and are differentially influenced by the myriad ways instructors approach interpretive themes in their introductory courses. We report how a transformed modern physics curriculum (recently implemented at the University of Colorado) has positively impacted student perspectives on quantum physics, by making questions of classical and quantum reality a central theme of the course, but also by making the beliefs of students (and not just those of scientists) an explicit topic of discussion. Baily, Charles Finkelstein, Noah D. http://hdl.handle.net/10023/3526 2013-05-15T21:31:01Z 2010-01-01T00:00:00Z

Abstract: Prior research has demonstrated how the realist perspectives of classical physics students can translate into specific beliefs about quantum phenomena when taking an introductory modern physics course. Student beliefs regarding the interpretation of quantum mechanics often vary by context, and are most often in alignment with instructional goals in topic areas where instructors are explicit in promoting a particular perspective. Moreover, students are more likely to maintain realist perspectives in topic areas where instructors are less explicit in addressing interpretive themes, thereby making such issues part of a hidden curriculum. We discuss various approaches to addressing student perspectives and interpretive themes in a modern physics course, and explore the associated impacts on student thinking.

2010-01-01T00:00:00Z Baily, Charles Finkelstein, Noah D. Prior research has demonstrated how the realist perspectives of classical physics students can translate into specific beliefs about quantum phenomena when taking an introductory modern physics course. Student beliefs regarding the interpretation of quantum mechanics often vary by context, and are most often in alignment with instructional goals in topic areas where instructors are explicit in promoting a particular perspective. Moreover, students are more likely to maintain realist perspectives in topic areas where instructors are less explicit in addressing interpretive themes, thereby making such issues part of a hidden curriculum. We discuss various approaches to addressing student perspectives and interpretive themes in a modern physics course, and explore the associated impacts on student thinking. Baily, Charles Finkelstein, Noah D. http://hdl.handle.net/10023/3525 2013-05-15T21:01:08Z 2008-01-01T00:00:00Z

Abstract: Introductory courses in classical physics are promoting in students a realist perspective, made up in part by the belief that all physical properties of a system can be simultaneously specified, and thus determined at all future times. Such a perspective can be problematic for introductory quantum physics students, who must develop new framings of epistemic and ontological resources in order to properly interpret what it means to have knowledge of quantum systems. We document this evolution in student thinking in part through pre/post instruction evaluations using the CLASS attitude survey. We further characterize variations in student epistemic and ontological commitments by examining responses to an essay question, coupled with responses to supplemental quantum attitude statements. We find that, after instruction in modern physics, many students are still exhibiting a realist perspective in contexts where a quantum perspective is needed. We also find that this effect can be significantly influenced by instruction, where we observe variations for courses with differing learning goals.

2008-01-01T00:00:00Z Baily, Charles Finkelstein, Noah D. Introductory courses in classical physics are promoting in students a realist perspective, made up in part by the belief that all physical properties of a system can be simultaneously specified, and thus determined at all future times. Such a perspective can be problematic for introductory quantum physics students, who must develop new framings of epistemic and ontological resources in order to properly interpret what it means to have knowledge of quantum systems. We document this evolution in student thinking in part through pre/post instruction evaluations using the CLASS attitude survey. We further characterize variations in student epistemic and ontological commitments by examining responses to an essay question, coupled with responses to supplemental quantum attitude statements. We find that, after instruction in modern physics, many students are still exhibiting a realist perspective in contexts where a quantum perspective is needed. We also find that this effect can be significantly influenced by instruction, where we observe variations for courses with differing learning goals. Baily, Charles Finkelstein, Noah D. http://hdl.handle.net/10023/3524 2013-05-15T21:01:07Z 2009-01-01T00:00:00Z

Abstract: Just as expert physicists vary in their personal stances on interpretation in quantum mechanics, instructors hold different views on teaching interpretations of quantum phenomena in introductory modern physics courses. There has been relatively little research in the physics education community on the variation in instructional approaches with respect to quantum interpretation, and how instructional choices impact student thinking. We compare two modern physics courses taught at the University of Colorado with similar learning environments, but where the instructors held different views on how to teach students about interpretations of quantum processes. We find significant differences in how students from these two courses responded to a survey on their beliefs about quantum mechanics; findings also suggest that instructors who choose to address student ontologies should do so across a range of topics.

2009-01-01T00:00:00Z Baily, Charles Finkelstein, Noah D. Just as expert physicists vary in their personal stances on interpretation in quantum mechanics, instructors hold different views on teaching interpretations of quantum phenomena in introductory modern physics courses. There has been relatively little research in the physics education community on the variation in instructional approaches with respect to quantum interpretation, and how instructional choices impact student thinking. We compare two modern physics courses taught at the University of Colorado with similar learning environments, but where the instructors held different views on how to teach students about interpretations of quantum processes. We find significant differences in how students from these two courses responded to a survey on their beliefs about quantum mechanics; findings also suggest that instructors who choose to address student ontologies should do so across a range of topics. Krstajic, Nikola Brown, C Tom A Dholakia, Kishan Giardini, Mario Ettore http://hdl.handle.net/10023/3488 2013-05-21T13:01:00Z 2012-07-01T00:00:00Z

Abstract: We present a simple method applicable to common-path Fourier domain optical coherence tomography (OCT) in which the tissue surface is used as the reference arm. We propose using aluminium hydroxide powder as a potential tissue surface diffuser to allow wider application of this method. This technique allows one to avoid placing a reference arm reflective element, such as glass plate, on tissue, and intrinsically avoids both coherent and complex conjugate mirror artifacts associated with glass plates. Aluminium hydroxide can be sprayed onto tissue using spray nozzles commonly found in endoscopes. The sensitivity of the tissue reference arm common-path OCT image is 94 dB for a 50-mu s charge-coupled device integration time, and 97.5 dB for a 200-mu s CCD integration time. (C) 2012 Society of Photo-Optical Instrumentation Engineers (SPIE). [DOL 10.1117/1.JBO.17.7.071305]

2012-07-01T00:00:00Z Krstajic, Nikola Brown, C Tom A Dholakia, Kishan Giardini, Mario Ettore We present a simple method applicable to common-path Fourier domain optical coherence tomography (OCT) in which the tissue surface is used as the reference arm. We propose using aluminium hydroxide powder as a potential tissue surface diffuser to allow wider application of this method. This technique allows one to avoid placing a reference arm reflective element, such as glass plate, on tissue, and intrinsically avoids both coherent and complex conjugate mirror artifacts associated with glass plates. Aluminium hydroxide can be sprayed onto tissue using spray nozzles commonly found in endoscopes. The sensitivity of the tissue reference arm common-path OCT image is 94 dB for a 50-mu s charge-coupled device integration time, and 97.5 dB for a 200-mu s CCD integration time. (C) 2012 Society of Photo-Optical Instrumentation Engineers (SPIE). [DOL 10.1117/1.JBO.17.7.071305] Levell, J. W. Giardini, M. E. Samuel, I. D. W. http://hdl.handle.net/10023/3487 2013-05-12T04:05:55Z 2010-02-15T00:00:00Z

Abstract: A method employing conjugated polymer thin film blends is shown to provide a simple and convenient way of greatly enhancing the ultraviolet response of silicon photodetectors. Hybrid organic semiconductor/silicon photodetectors are demonstrated using fluorene copolymers and give a quantum efficiency of 60% at 200 nm. The quantum efficiency is greater than 34% over the entire 200-620 nm range. These devices show promise for use in high sensitivity, low cost UV-visible photodetection and imaging applications. (C) 2007 Optical Society of America

2010-02-15T00:00:00Z Levell, J. W. Giardini, M. E. Samuel, I. D. W. A method employing conjugated polymer thin film blends is shown to provide a simple and convenient way of greatly enhancing the ultraviolet response of silicon photodetectors. Hybrid organic semiconductor/silicon photodetectors are demonstrated using fluorene copolymers and give a quantum efficiency of 60% at 200 nm. The quantum efficiency is greater than 34% over the entire 200-620 nm range. These devices show promise for use in high sensitivity, low cost UV-visible photodetection and imaging applications. (C) 2007 Optical Society of America G. Kirton, P. D. Armour, A. http://hdl.handle.net/10023/3480 2013-04-17T09:31:02Z 2013-04-09T00:00:00Z

Abstract: We analyze the response of a nanomechanical resonator to an external drive when it is also coupled to a single-electron transistor (SET). The interaction between the SET electrons and the mechanical resonator depends on the amplitude of the mechanical motion leading to a strongly non-linear response to the drive which is similar to that of a Duffing oscillator. We show that the average dynamics of the resonator is well-described by a simple effective model which incorporates damping and frequency renormalization terms which are amplitude dependent. We also find that for a certain range of parameters the system displays interesting bistable dynamics in which noise arising from charge fluctuations causes the resonator to switch slowly between different dynamical states. Description: This work was supported by EPSRC [Grant number EP/I017818/1]

2013-04-09T00:00:00Z G. Kirton, P. D. Armour, A. We analyze the response of a nanomechanical resonator to an external drive when it is also coupled to a single-electron transistor (SET). The interaction between the SET electrons and the mechanical resonator depends on the amplitude of the mechanical motion leading to a strongly non-linear response to the drive which is similar to that of a Duffing oscillator. We show that the average dynamics of the resonator is well-described by a simple effective model which incorporates damping and frequency renormalization terms which are amplitude dependent. We also find that for a certain range of parameters the system displays interesting bistable dynamics in which noise arising from charge fluctuations causes the resonator to switch slowly between different dynamical states. Lagatsky, A. A. Koopmann, P. Fuhrberg, P. Huber, G. Brown, C. T. A. Sibbett, W. http://hdl.handle.net/10023/3442 2013-05-12T04:36:25Z 2012-02-01T00:00:00Z

Abstract: We report on the passive mode locking of a Tm3+:Sc2O3 laser at 2.1 mu m using a semiconductor saturable absorber mirror based on InGaAsSb quantum wells. Transform-limited 218 fs pulses are generated with an average power of 210 mW. A maximum output power of 325 mW is produced during mode locking with the corresponding pulse duration of 246 fs at a pulse repetition frequency of 124.3 MHz. A Ti:sapphire laser is used as the pump source operating at 796 nm. (C) 2012 Optical Society of America Description: This work was supported by EPSRC

2012-02-01T00:00:00Z Lagatsky, A. A. Koopmann, P. Fuhrberg, P. Huber, G. Brown, C. T. A. Sibbett, W. We report on the passive mode locking of a Tm3+:Sc2O3 laser at 2.1 mu m using a semiconductor saturable absorber mirror based on InGaAsSb quantum wells. Transform-limited 218 fs pulses are generated with an average power of 210 mW. A maximum output power of 325 mW is produced during mode locking with the corresponding pulse duration of 246 fs at a pulse repetition frequency of 124.3 MHz. A Ti:sapphire laser is used as the pump source operating at 796 nm. (C) 2012 Optical Society of America Metzger, N. K. Olle, V. F. Wonfor, A. Penty, R. V. White, I. H. Mazilu, M. Brown, C. T. A. Sibbett, W. http://hdl.handle.net/10023/3441 2013-05-12T04:36:24Z 2012-03-26T00:00:00Z

Abstract: A control algorithm is presented that addresses the stability issues inherent to the operation of monolithic mode-locked laser diodes. It enables a continuous pulse duration tuning without any onset of Q-switching instabilities. A demonstration of the algorithm performance is presented for two radically different laser diode geometries and continuous pulse duration tuning between 0.5 ps to 2.2 ps and 1.2 ps to 10.2 ps is achieved. With practical applications in mind, this algorithm also facilitates control over performance parameters such as output power and wavelength during pulse duration tuning. The developed algorithm enables the user to harness the operational flexibility from such a laser with 'push-button' simplicity. (C) 2012 Optical Society of America

2012-03-26T00:00:00Z Metzger, N. K. Olle, V. F. Wonfor, A. Penty, R. V. White, I. H. Mazilu, M. Brown, C. T. A. Sibbett, W. A control algorithm is presented that addresses the stability issues inherent to the operation of monolithic mode-locked laser diodes. It enables a continuous pulse duration tuning without any onset of Q-switching instabilities. A demonstration of the algorithm performance is presented for two radically different laser diode geometries and continuous pulse duration tuning between 0.5 ps to 2.2 ps and 1.2 ps to 10.2 ps is achieved. With practical applications in mind, this algorithm also facilitates control over performance parameters such as output power and wavelength during pulse duration tuning. The developed algorithm enables the user to harness the operational flexibility from such a laser with 'push-button' simplicity. (C) 2012 Optical Society of America Dalgarno, Paul Allan Bordello, J Morris, Rhodri St-Pierre, P Dubé, A Samuel, Ifor David William Lafontaine, Daniel Penedo-Esteiro, Juan Carlos http://hdl.handle.net/10023/3427 2013-04-18T11:31:04Z 2013-01-01T00:00:00Z

Abstract: To date, single-molecule RNA science has been developed almost exclusively around the effect of metal ions as folding promoters and stabilizers of the RNA structure. Here, we introduce a novel strategy that combines single-molecule Förster resonance energy transfer (FRET) and chemical denaturation to observe and manipulate RNA dynamics. We demonstrate that the competing interplay between metal ions and denaturant agents provides a platform to extract information that otherwise will remain hidden with current methods. Using the adenine-sensing riboswitch aptamer as a model, we provide strong evidence for a rate-limiting folding step of the aptamer domain being modulated through ligand binding, a feature that is important for regulation of the controlled gene. In the absence of ligand, the rate-determining step is dominated by the formation of long-range key tertiary contacts between peripheral stem-loop elements. In contrast, when the adenine ligand interacts with partially folded messenger RNAs, the aptamer requires specifically bound Mg2+ ions, as those observed in the crystal structure, to progress further towards the native form. Moreover, despite that the ligand-free and ligand-bound states are indistinguishable by FRET, their different stability against urea-induced denaturation allowed us to discriminate them, even when they coexist within a single FRET trajectory; a feature not accessible by existing methods.

2013-01-01T00:00:00Z Dalgarno, Paul Allan Bordello, J Morris, Rhodri St-Pierre, P Dubé, A Samuel, Ifor David William Lafontaine, Daniel Penedo-Esteiro, Juan Carlos To date, single-molecule RNA science has been developed almost exclusively around the effect of metal ions as folding promoters and stabilizers of the RNA structure. Here, we introduce a novel strategy that combines single-molecule Förster resonance energy transfer (FRET) and chemical denaturation to observe and manipulate RNA dynamics. We demonstrate that the competing interplay between metal ions and denaturant agents provides a platform to extract information that otherwise will remain hidden with current methods. Using the adenine-sensing riboswitch aptamer as a model, we provide strong evidence for a rate-limiting folding step of the aptamer domain being modulated through ligand binding, a feature that is important for regulation of the controlled gene. In the absence of ligand, the rate-determining step is dominated by the formation of long-range key tertiary contacts between peripheral stem-loop elements. In contrast, when the adenine ligand interacts with partially folded messenger RNAs, the aptamer requires specifically bound Mg2+ ions, as those observed in the crystal structure, to progress further towards the native form. Moreover, despite that the ligand-free and ligand-bound states are indistinguishable by FRET, their different stability against urea-induced denaturation allowed us to discriminate them, even when they coexist within a single FRET trajectory; a feature not accessible by existing methods. Bauer, M. Keeling, J. M. Parish, M. Lopez Rios, P. B. Littlewood, P. http://hdl.handle.net/10023/3330 2013-01-22T19:31:35Z 2013-01-07T00:00:00Z

Abstract: We calculate the photoluminescence spectrum and lifetime of a biexciton in a semiconductor using Fermi's golden rule. Our biexciton wavefunction is obtained using a Quantum Monte Carlo calculation. We consider a recombination process where one of the excitons within the biexciton annihilates. For hole masses greater than or equal to the electron mass, we find that the surviving exciton is most likely to populate the ground state. We also investigate how the confinement of excitons in a quantum dot would modify the lifetime in the limit of a large quantum dot where confinement principally affects the centre of mass wavefunction. The lifetimes we obtain are in reasonable agreement with experimental values. Our calculation can be used as a benchmark for comparison with approximate methods. Description: 6 pages, 2 figures

2013-01-07T00:00:00Z Bauer, M. Keeling, J. M. Parish, M. Lopez Rios, P. B. Littlewood, P. We calculate the photoluminescence spectrum and lifetime of a biexciton in a semiconductor using Fermi's golden rule. Our biexciton wavefunction is obtained using a Quantum Monte Carlo calculation. We consider a recombination process where one of the excitons within the biexciton annihilates. For hole masses greater than or equal to the electron mass, we find that the surviving exciton is most likely to populate the ground state. We also investigate how the confinement of excitons in a quantum dot would modify the lifetime in the limit of a large quantum dot where confinement principally affects the centre of mass wavefunction. The lifetimes we obtain are in reasonable agreement with experimental values. Our calculation can be used as a benchmark for comparison with approximate methods. Savitski, Vasili G. Metzger, Nikolaus K. Calvez, Stephane Burns, David Sibbett, W. Brown, C. T. A. http://hdl.handle.net/10023/3321 2013-05-12T04:36:25Z 2012-03-26T00:00:00Z

Abstract: We demonstrate a diode-pumped Cr:LiSAF laser with controllable and reliable fast switching between its continuous-wave and mode-locked states of operation using an optically-addressed semiconductor Bragg reflector, permitting dyed microspheres to be continuously trapped and monitored using a standard microscope imaging and on-demand two-photon-excited luminescence techniques. (C) 2012 Optical Society of America

2012-03-26T00:00:00Z Savitski, Vasili G. Metzger, Nikolaus K. Calvez, Stephane Burns, David Sibbett, W. Brown, C. T. A. We demonstrate a diode-pumped Cr:LiSAF laser with controllable and reliable fast switching between its continuous-wave and mode-locked states of operation using an optically-addressed semiconductor Bragg reflector, permitting dyed microspheres to be continuously trapped and monitored using a standard microscope imaging and on-demand two-photon-excited luminescence techniques. (C) 2012 Optical Society of America Lagatsky, A. A. Calvez, S. Gupta, J. A. Kisel, V. E. Kuleshov, N. V. Brown, C. T. A. Dawson, M. D. Sibbett, W. http://hdl.handle.net/10023/3320 2013-05-12T04:36:26Z 2011-05-09T00:00:00Z

Abstract: Efficient mode-locking in a Tm:KY(WO4)(2) laser is demonstrated by using InGaAsSb quantum-well SESAMs. Self-starting ultrashort pulse generation was realized in the 1979-2074 nm spectral region. Maximum average output power up to 411 mW was produced around 1986 nm with the corresponding pulse duration and repetition rate of 549 fs and 105 MHz respectively. Optimised pulse durations of 386 fs were produced with an average power of 235 mW at 2029 nm. (C) 2011 Optical Society of America

2011-05-09T00:00:00Z Lagatsky, A. A. Calvez, S. Gupta, J. A. Kisel, V. E. Kuleshov, N. V. Brown, C. T. A. Dawson, M. D. Sibbett, W. Efficient mode-locking in a Tm:KY(WO4)(2) laser is demonstrated by using InGaAsSb quantum-well SESAMs. Self-starting ultrashort pulse generation was realized in the 1979-2074 nm spectral region. Maximum average output power up to 411 mW was produced around 1986 nm with the corresponding pulse duration and repetition rate of 549 fs and 105 MHz respectively. Optimised pulse durations of 386 fs were produced with an average power of 235 mW at 2029 nm. (C) 2011 Optical Society of America Balagopal, Bavishna Ashok, Praveen Cheriyan Mazilu, Michael Riches, Andrew Clive Herrington, C Simon Dholakia, Kishan http://hdl.handle.net/10023/3303 2012-12-14T16:31:02Z 2012-07-09T00:00:00Z

Abstract: In the field of biomedical optics, Raman spectroscopy is a powerful tool for probing the chemical composition of biological samples. In particular, fiber Raman probes play a crucial role for in vivo and ex vivo tissue analysis. However, the high-fluorescence background typically contributed by the auto fluorescence from both a tissue sample and the fiber-probe interferes strongly with the relatively weak Raman signal. Here we demonstrate the implementation of wavelength-modulated Raman spectroscopy (WMRS) to suppress the fluorescence background while analyzing tissues using fiber Raman probes. We have observed a significant signal-to-noise ratio enhancement in the Raman bands of bone tissue, which have a relatively high fluorescence background. Implementation of WMRS in fiber-probe-based bone tissue study yielded usable Raman spectra in a relatively short acquisition time (∼30  s), notably without any special sample preparation stage. Finally, we have validated its capability to suppress fluorescence on other tissue samples such as adipose tissue derived from four different species.

2012-07-09T00:00:00Z Balagopal, Bavishna Ashok, Praveen Cheriyan Mazilu, Michael Riches, Andrew Clive Herrington, C Simon Dholakia, Kishan In the field of biomedical optics, Raman spectroscopy is a powerful tool for probing the chemical composition of biological samples. In particular, fiber Raman probes play a crucial role for in vivo and ex vivo tissue analysis. However, the high-fluorescence background typically contributed by the auto fluorescence from both a tissue sample and the fiber-probe interferes strongly with the relatively weak Raman signal. Here we demonstrate the implementation of wavelength-modulated Raman spectroscopy (WMRS) to suppress the fluorescence background while analyzing tissues using fiber Raman probes. We have observed a significant signal-to-noise ratio enhancement in the Raman bands of bone tissue, which have a relatively high fluorescence background. Implementation of WMRS in fiber-probe-based bone tissue study yielded usable Raman spectra in a relatively short acquisition time (∼30  s), notably without any special sample preparation stage. Finally, we have validated its capability to suppress fluorescence on other tissue samples such as adipose tissue derived from four different species. Luan, Lan Auslaender, Ophir Lippman, Thomas M Hicks, Clifford William Kalisky, Beena Chu, Jiun-Haw Analytis, James G Fisher, Ian R Kirtley, John R Moler, Kathryn A http://hdl.handle.net/10023/3296 2012-12-14T13:01:03Z 2010-03-01T00:00:00Z

Abstract: We use magnetic force microscopy (MFM) to measure the local penetration depth λ in Ba(Fe0.95Co0.05)2As2 single crystals and use scanning superconducting quantum interference device susceptometry to measure its temperature variation down to 0.4 K. We observe that superfluid density ρs over the full temperature range is well described by a clean two-band fully gapped model. We demonstrate that MFM can measure the important and hard-to-determine absolute value of λ, as well as obtain its temperature dependence and spatial homogeneity. We find ρs to be uniform on the submicron scale despite the highly disordered vortex pinning.

2010-03-01T00:00:00Z Luan, Lan Auslaender, Ophir Lippman, Thomas M Hicks, Clifford William Kalisky, Beena Chu, Jiun-Haw Analytis, James G Fisher, Ian R Kirtley, John R Moler, Kathryn A We use magnetic force microscopy (MFM) to measure the local penetration depth λ in Ba(Fe0.95Co0.05)2As2 single crystals and use scanning superconducting quantum interference device susceptometry to measure its temperature variation down to 0.4 K. We observe that superfluid density ρs over the full temperature range is well described by a clean two-band fully gapped model. We demonstrate that MFM can measure the important and hard-to-determine absolute value of λ, as well as obtain its temperature dependence and spatial homogeneity. We find ρs to be uniform on the submicron scale despite the highly disordered vortex pinning. Luan, Lan Lippman, Thomas M Hicks, Clifford William Bert, Julie A Auslaender, Ophir M Chu, Jiun-Haw Analytis, James G Fisher, Ian R Moler, Kathryn A http://hdl.handle.net/10023/3295 2012-12-14T13:01:02Z 2011-02-09T00:00:00Z

Abstract: We measure the penetration depth λab(T) in Ba(Fe1-xCox)2As2 using local techniques that do not average over the sample. The superfluid density ρs(T)≡1/λab(T)2 has three main features. First, ρs(T=0) falls sharply on the underdoped side of the dome. Second, λab(T) is flat at low T at optimal doping, indicating fully gapped superconductivity, but varies more strongly in underdoped and overdoped samples, consistent with either a power law or a small second gap. Third, ρs(T) varies steeply near Tc for optimal and underdoping. These observations are consistent with an interplay between magnetic and superconducting phases.

2011-02-09T00:00:00Z Luan, Lan Lippman, Thomas M Hicks, Clifford William Bert, Julie A Auslaender, Ophir M Chu, Jiun-Haw Analytis, James G Fisher, Ian R Moler, Kathryn A We measure the penetration depth λab(T) in Ba(Fe1-xCox)2As2 using local techniques that do not average over the sample. The superfluid density ρs(T)≡1/λab(T)2 has three main features. First, ρs(T=0) falls sharply on the underdoped side of the dome. Second, λab(T) is flat at low T at optimal doping, indicating fully gapped superconductivity, but varies more strongly in underdoped and overdoped samples, consistent with either a power law or a small second gap. Third, ρs(T) varies steeply near Tc for optimal and underdoping. These observations are consistent with an interplay between magnetic and superconducting phases. Hicks, Clifford William Kirtley, John R Lippman, Thomas M Koshnick, Nicholas C Huber, Martin E Maeno, Yoshiteru Yuhasz, William M Maple, M Brian Moler, Kathryn A http://hdl.handle.net/10023/3294 2012-12-14T12:31:01Z 2010-06-01T00:00:00Z

Abstract: We present scanning superconducting quantum interference device microscopy data on the superconductors Sr2RuO4 (Tc=1.5 K) and PrOs4Sb12 (Tc=1.8 K). In both of these materials, superconductivity-related time-reversal symmetry-breaking fields have been observed by muon spin rotation; our aim was to visualize the structure of these fields. However, in neither Sr2RuO4 nor PrOs4Sb12 do we observe spontaneous superconductivity-related magnetization. In Sr2RuO4, many experimental results have been interpreted on the basis of a px±ipy superconducting order parameter. This order parameter is expected to give spontaneous magnetic induction at sample edges and order parameter domain walls. Supposing large domains, our data restrict domain wall and edge fields to no more than ∼0.1% and ∼0.2% of the expected magnitude, respectively. Alternatively, if the magnetization is of the expected order, the typical domain size is limited to ∼30 nm for random domains or ∼500 nm for periodic domains.

2010-06-01T00:00:00Z Hicks, Clifford William Kirtley, John R Lippman, Thomas M Koshnick, Nicholas C Huber, Martin E Maeno, Yoshiteru Yuhasz, William M Maple, M Brian Moler, Kathryn A We present scanning superconducting quantum interference device microscopy data on the superconductors Sr2RuO4 (Tc=1.5 K) and PrOs4Sb12 (Tc=1.8 K). In both of these materials, superconductivity-related time-reversal symmetry-breaking fields have been observed by muon spin rotation; our aim was to visualize the structure of these fields. However, in neither Sr2RuO4 nor PrOs4Sb12 do we observe spontaneous superconductivity-related magnetization. In Sr2RuO4, many experimental results have been interpreted on the basis of a px±ipy superconducting order parameter. This order parameter is expected to give spontaneous magnetic induction at sample edges and order parameter domain walls. Supposing large domains, our data restrict domain wall and edge fields to no more than ∼0.1% and ∼0.2% of the expected magnitude, respectively. Alternatively, if the magnetization is of the expected order, the typical domain size is limited to ∼30 nm for random domains or ∼500 nm for periodic domains. A. Vidotto, A. Opher, M. Jatenco-Pereira, V. I. Gombosi, T. http://hdl.handle.net/10023/3285 2013-05-12T04:35:41Z 2010-08-19T00:00:00Z

Abstract: Based on our previous work (Vidotto et al. 2009a), we investigate the effects on the wind and magnetospheric structures of weak-lined T Tauri stars due to a misalignment between the axis of rotation of the star and its magnetic dipole moment vector. In such configuration, the system loses the axisymmetry presented in the aligned case, requiring a fully 3D approach. We perform 3D numerical MHD simulations of stellar winds and study the effects caused by different model parameters. The system reaches a periodic behavior with the same rotational period of the star. We show that the magnetic field lines present an oscillatory pattern and that by increasing the misalignment angle, the wind velocity increases. Our wind models allow us to study the interaction of a magnetized wind with a magnetized extra-solar planet. Such interaction gives rise to reconnection, generating electrons that propagate along the planet's magnetic field lines and produce electron cyclotron radiation at radio wavelengths. We find that a close-in Jupiter-like planet orbiting at 0.05AU presents a radio power that is ~5 orders of magnitude larger than the one observed in Jupiter, which suggests that the stellar wind from a young star has the potential to generate strong planetary radio emission that could be detected in the near future with LOFAR. This radio power varies according to the phase of rotation of the star. We also analyze whether winds from misaligned stellar magnetospheres could cause a significant effect on planetary migration. Compared to the aligned case, we show that the time-scale tau_w for an appreciable radial motion of the planet is shorter for larger misalignment angles. While for the aligned case tau_w~100Myr, for a stellar magnetosphere tilted by 30deg, tau_w ranges from ~40 to 70Myr for a planet located at a radius of 0.05AU. (Abridged) Description: 21 pages, 16 figures, 2 tables (emulateapj.cls).

2010-08-19T00:00:00Z A. Vidotto, A. Opher, M. Jatenco-Pereira, V. I. Gombosi, T. Based on our previous work (Vidotto et al. 2009a), we investigate the effects on the wind and magnetospheric structures of weak-lined T Tauri stars due to a misalignment between the axis of rotation of the star and its magnetic dipole moment vector. In such configuration, the system loses the axisymmetry presented in the aligned case, requiring a fully 3D approach. We perform 3D numerical MHD simulations of stellar winds and study the effects caused by different model parameters. The system reaches a periodic behavior with the same rotational period of the star. We show that the magnetic field lines present an oscillatory pattern and that by increasing the misalignment angle, the wind velocity increases. Our wind models allow us to study the interaction of a magnetized wind with a magnetized extra-solar planet. Such interaction gives rise to reconnection, generating electrons that propagate along the planet's magnetic field lines and produce electron cyclotron radiation at radio wavelengths. We find that a close-in Jupiter-like planet orbiting at 0.05AU presents a radio power that is ~5 orders of magnitude larger than the one observed in Jupiter, which suggests that the stellar wind from a young star has the potential to generate strong planetary radio emission that could be detected in the near future with LOFAR. This radio power varies according to the phase of rotation of the star. We also analyze whether winds from misaligned stellar magnetospheres could cause a significant effect on planetary migration. Compared to the aligned case, we show that the time-scale tau_w for an appreciable radial motion of the planet is shorter for larger misalignment angles. While for the aligned case tau_w~100Myr, for a stellar magnetosphere tilted by 30deg, tau_w ranges from ~40 to 70Myr for a planet located at a radius of 0.05AU. (Abridged) A. Vidotto, A. Fares, R. Jardine, M. F. Donati, J. Opher, M. Moutou, C. Catala, C. I. Gombosi, T. http://hdl.handle.net/10023/3256 2013-05-12T04:35:42Z 2012-07-01T00:00:00Z

Abstract: Tau Boo is an intriguing planet-host star that is believed to undergo magnetic cycles similar to the Sun, but with a duration that is about one order of magnitude smaller than that of the solar cycle. With the use of observationally derived surface magnetic field maps, we simulate the magnetic stellar wind of Tau Boo by means of three-dimensional MHD numerical simulations. As the properties of the stellar wind depend on the particular characteristics of the stellar magnetic field, we show that the wind varies during the observed epochs of the cycle. Although the mass loss-rates we find (~2.7e-12 Msun/yr) vary less than 3 per cent during the observed epochs of the cycle, our derived angular momentum loss-rates vary from 1.1 to 2.2e32erg. The spin-down times associated to magnetic braking range between 39 and 78Gyr. We also compute the emission measure from the (quiescent) closed corona and show that it remains approximately constant through these epochs at a value of ~10^{50.6} cm^{-3}. This suggests that a magnetic cycle of Tau Boo may not be detected by X-ray observations. We further investigate the interaction between the stellar wind and the planet by estimating radio emission from the hot-Jupiter that orbits at 0.0462 au from Tau Boo. By adopting reasonable hypotheses, we show that, for a planet with a magnetic field similar to Jupiter (~14G at the pole), the radio flux is estimated to be about 0.5-1 mJy, occurring at a frequency of 34MHz. If the planet is less magnetised (field strengths roughly Earths ionospheric cutoff. According to our estimates, if the planet is more magnetised than that and provided the emission beam crosses the observer line-of-sight, detection of radio emission from Tau Boo b is only possible by ground-based instruments with a noise level of 1 mJy, operating at low frequencies. Description: 15 pages, 10 figures

2012-07-01T00:00:00Z A. Vidotto, A. Fares, R. Jardine, M. F. Donati, J. Opher, M. Moutou, C. Catala, C. I. Gombosi, T. Tau Boo is an intriguing planet-host star that is believed to undergo magnetic cycles similar to the Sun, but with a duration that is about one order of magnitude smaller than that of the solar cycle. With the use of observationally derived surface magnetic field maps, we simulate the magnetic stellar wind of Tau Boo by means of three-dimensional MHD numerical simulations. As the properties of the stellar wind depend on the particular characteristics of the stellar magnetic field, we show that the wind varies during the observed epochs of the cycle. Although the mass loss-rates we find (~2.7e-12 Msun/yr) vary less than 3 per cent during the observed epochs of the cycle, our derived angular momentum loss-rates vary from 1.1 to 2.2e32erg. The spin-down times associated to magnetic braking range between 39 and 78Gyr. We also compute the emission measure from the (quiescent) closed corona and show that it remains approximately constant through these epochs at a value of ~10^{50.6} cm^{-3}. This suggests that a magnetic cycle of Tau Boo may not be detected by X-ray observations. We further investigate the interaction between the stellar wind and the planet by estimating radio emission from the hot-Jupiter that orbits at 0.0462 au from Tau Boo. By adopting reasonable hypotheses, we show that, for a planet with a magnetic field similar to Jupiter (~14G at the pole), the radio flux is estimated to be about 0.5-1 mJy, occurring at a frequency of 34MHz. If the planet is less magnetised (field strengths roughly Earths ionospheric cutoff. According to our estimates, if the planet is more magnetised than that and provided the emission beam crosses the observer line-of-sight, detection of radio emission from Tau Boo b is only possible by ground-based instruments with a noise level of 1 mJy, operating at low frequencies. W. Hicks, Clifford S. Gibbs, Alexandra P. Mackenzie, Andrew Takatsu, Hiroshi Maeno, Yoshiteru A. Yelland, Edward http://hdl.handle.net/10023/3246 2012-12-12T13:29:49Z 2012-09-01T00:00:00Z

Abstract: We present de Haas-van Alphen and resistivity data on single crystals of the delafossite PdCoO2. At 295 K we measure an in-plane resistivity of 2.6 \mu{\Omega}-cm, making PdCoO2 the most conductive oxide known. The low-temperature in-plane resistivity has an activated rather than the usual T^5 temperature dependence, suggesting a gapping of effective scattering that is consistent with phonon drag. Below 10 K, the transport mean free path is 20 \mum, approximately 10^5 lattice spacings and an astoundingly high value for flux-grown crystals. We discuss the origin of these properties in light of our data.

2012-09-01T00:00:00Z W. Hicks, Clifford S. Gibbs, Alexandra P. Mackenzie, Andrew Takatsu, Hiroshi Maeno, Yoshiteru A. Yelland, Edward We present de Haas-van Alphen and resistivity data on single crystals of the delafossite PdCoO2. At 295 K we measure an in-plane resistivity of 2.6 \mu{\Omega}-cm, making PdCoO2 the most conductive oxide known. The low-temperature in-plane resistivity has an activated rather than the usual T^5 temperature dependence, suggesting a gapping of effective scattering that is consistent with phonon drag. Below 10 K, the transport mean free path is 20 \mum, approximately 10^5 lattice spacings and an astoundingly high value for flux-grown crystals. We discuss the origin of these properties in light of our data. Stark, C. R. Neukirch, T. http://hdl.handle.net/10023/3154 2013-05-12T04:14:42Z 2012-01-01T00:00:00Z

Abstract: A self-consistent collisionless distribution function for the relativistic analogue of the force-free Harris sheet is presented. This distribution function is the relativistic generalization of the distribution function for the non-relativistic collisionless force-free Harris sheet recently found by Harrison and Neukirch [Phys. Rev. Lett. 102, 135003 (2009)], as it has the same dependence on the particle energy and canonical momenta. We present a detailed calculation which shows that the proposed distribution function generates the required current density profile (and thus magnetic field profile) in a frame of reference in which the electric potential vanishes identically. The connection between the parameters of the distribution function and the macroscopic parameters such as the current sheet thickness is discussed. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3677268]

2012-01-01T00:00:00Z Stark, C. R. Neukirch, T. A self-consistent collisionless distribution function for the relativistic analogue of the force-free Harris sheet is presented. This distribution function is the relativistic generalization of the distribution function for the non-relativistic collisionless force-free Harris sheet recently found by Harrison and Neukirch [Phys. Rev. Lett. 102, 135003 (2009)], as it has the same dependence on the particle energy and canonical momenta. We present a detailed calculation which shows that the proposed distribution function generates the required current density profile (and thus magnetic field profile) in a frame of reference in which the electric potential vanishes identically. The connection between the parameters of the distribution function and the macroscopic parameters such as the current sheet thickness is discussed. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3677268] Cizmar, Tomas Dholakia, Kishan http://hdl.handle.net/10023/3056 2013-05-12T04:35:43Z 2012-08-28T00:00:00Z

Abstract: There has been an immense drive in modern microscopy towards miniaturisation and fibre based technology. This has been necessitated by the need to access hostile or diffcult environments in-situ and in-vivo. Strategies to date have included the use of specialist fibres and miniaturised scanning systems accompanied by ingenious microfabricated lenses. We present a novel approach for this field by utilising disordered light within a standard multimode optical fibre for lensless microscopy and optical mode conversion. We demonstrate the modalities of bright-field and dark-field imaging and scanning fluorescence microscopy at acquisition rates allowing observation of dynamic processes such as Brownian motion of mesoscopic particles. Furthermore, we show how such control can realise a new form of mode converter and generate various types of advanced light fields such as propagation-invariant beams and optical vortices. These may be useful for future fibre based implementations of super-resolution or light sheet microscopy.

2012-08-28T00:00:00Z Cizmar, Tomas Dholakia, Kishan There has been an immense drive in modern microscopy towards miniaturisation and fibre based technology. This has been necessitated by the need to access hostile or diffcult environments in-situ and in-vivo. Strategies to date have included the use of specialist fibres and miniaturised scanning systems accompanied by ingenious microfabricated lenses. We present a novel approach for this field by utilising disordered light within a standard multimode optical fibre for lensless microscopy and optical mode conversion. We demonstrate the modalities of bright-field and dark-field imaging and scanning fluorescence microscopy at acquisition rates allowing observation of dynamic processes such as Brownian motion of mesoscopic particles. Furthermore, we show how such control can realise a new form of mode converter and generate various types of advanced light fields such as propagation-invariant beams and optical vortices. These may be useful for future fibre based implementations of super-resolution or light sheet microscopy. Wang, Yue McKeown, Neil B. Msayib, Kadhum J. Turnbull, Graham A. Samuel, Ifor D. W. http://hdl.handle.net/10023/3017 2013-05-19T00:34:08Z 2011-03-01T00:00:00Z

Abstract: This work explores the use of a polymer of intrinsic microporosity (PIM-1) as the active layer within a laser sensor to detect nitroaromatic-based explosive vapors. We show successful detection of dinitrobenzene (DNB) by monitoring the real-time photoluminescence. We also show that PIM-1 has an inherent memory, so that it accumulates the analyte during exposure. In addition, the optical gain and refractive index of the polymer were studied by amplified spontaneous emission and variable-angle ellipsometry, respectively. A second-order distributed feedback PIM-1 laser sensor was fabricated and found to show an increase in laser threshold of 2.5 times and a reduction of the laser slope efficiency by 4.4 times after a 5-min exposure to the DNB vapor. For pumping at 2 times threshold, the lasing action was stopped within 30 s indicating that PIM-1 has a very fast responsivity and as such has a potential sensing ability for ultra-low-concentration explosives.

2011-03-01T00:00:00Z Wang, Yue McKeown, Neil B. Msayib, Kadhum J. Turnbull, Graham A. Samuel, Ifor D. W. This work explores the use of a polymer of intrinsic microporosity (PIM-1) as the active layer within a laser sensor to detect nitroaromatic-based explosive vapors. We show successful detection of dinitrobenzene (DNB) by monitoring the real-time photoluminescence. We also show that PIM-1 has an inherent memory, so that it accumulates the analyte during exposure. In addition, the optical gain and refractive index of the polymer were studied by amplified spontaneous emission and variable-angle ellipsometry, respectively. A second-order distributed feedback PIM-1 laser sensor was fabricated and found to show an increase in laser threshold of 2.5 times and a reduction of the laser slope efficiency by 4.4 times after a 5-min exposure to the DNB vapor. For pumping at 2 times threshold, the lasing action was stopped within 30 s indicating that PIM-1 has a very fast responsivity and as such has a potential sensing ability for ultra-low-concentration explosives. Llama, J. Wood, K. Jardine, M. A. Vidotto, A. Helling, Ch. Fossati, L. A. Haswell, C. http://hdl.handle.net/10023/3010 2013-05-19T00:34:47Z 2011-09-01T00:00:00Z

Abstract: Near ultraviolet observations of WASP-12b have revealed an early ingress compared to the optical transit lightcurve. This has been interpreted as due to the presence of a magnetospheric bow shock which forms when the relative velocity of the planetary and stellar material is supersonic. We aim to reproduce this observed early ingress by modelling the stellar wind (or coronal plasma) in order to derive the speed and density of the material at the planetary orbital radius. From this we determine the orientation of the shock and the density of compressed plasma behind it. With this model for the density structure surrounding the planet we perform Monte Carlo radiation transfer simulations of the near UV transits of WASP-12b with and without a bow shock. We find that we can reproduce the transit lightcurves with a wide range of plasma temperatures, shock geometries and optical depths. Our results support the hypothesis that a bow shock could explain the observed early ingress. Description: 4 pages, 2 figures

2011-09-01T00:00:00Z Llama, J. Wood, K. Jardine, M. A. Vidotto, A. Helling, Ch. Fossati, L. A. Haswell, C. Near ultraviolet observations of WASP-12b have revealed an early ingress compared to the optical transit lightcurve. This has been interpreted as due to the presence of a magnetospheric bow shock which forms when the relative velocity of the planetary and stellar material is supersonic. We aim to reproduce this observed early ingress by modelling the stellar wind (or coronal plasma) in order to derive the speed and density of the material at the planetary orbital radius. From this we determine the orientation of the shock and the density of compressed plasma behind it. With this model for the density structure surrounding the planet we perform Monte Carlo radiation transfer simulations of the near UV transits of WASP-12b with and without a bow shock. We find that we can reproduce the transit lightcurves with a wide range of plasma temperatures, shock geometries and optical depths. Our results support the hypothesis that a bow shock could explain the observed early ingress. Canetta, Elisabetta Mazilu, Michael De Luca, Anna Chiara Carruthers, Antonia Erika Dholakia, Kishan Neilson, Sam Harry, Sargeant Briscoe, Tina Herrington, C Simon Riches, Andrew Clive http://hdl.handle.net/10023/3002 2013-05-12T04:06:41Z 2011-03-01T00:00:00Z

Abstract: Standard Raman spectroscopy (SRS) is a noninvasive technique that is used in the biomedical field to discriminate between normal and cancer cells. However, the presence of a strong fluorescence background detracts from the use of SRS in real-time clinical applications. Recently, we have reported a novel modulated Raman spectroscopy (MRS) technique to extract the Raman spectra from the background. In this paper, we present the first application of MRS to the identification of human urothelial cells (SV-HUC-1) and bladder cancer cells (MGH) in urine samples. These results are compared to those obtained by SRS. Classification using the principal component analysis clearly shows thatMRS allows discrimination between Raman spectra of SV-HUC-1 andMGH cells with high sensitivity (98%) and specificity (95%). MRS is also used to distinguish between SV-HUC-1 and MGH cells after exposure to urine for up to 6 h.We observe a marked change in the MRS of SV-HUC-1 and MGH cells with time in urine, indicating that the conditions of sample collection will be important for the application of this methodology to clinical urine samples.

2011-03-01T00:00:00Z Canetta, Elisabetta Mazilu, Michael De Luca, Anna Chiara Carruthers, Antonia Erika Dholakia, Kishan Neilson, Sam Harry, Sargeant Briscoe, Tina Herrington, C Simon Riches, Andrew Clive Standard Raman spectroscopy (SRS) is a noninvasive technique that is used in the biomedical field to discriminate between normal and cancer cells. However, the presence of a strong fluorescence background detracts from the use of SRS in real-time clinical applications. Recently, we have reported a novel modulated Raman spectroscopy (MRS) technique to extract the Raman spectra from the background. In this paper, we present the first application of MRS to the identification of human urothelial cells (SV-HUC-1) and bladder cancer cells (MGH) in urine samples. These results are compared to those obtained by SRS. Classification using the principal component analysis clearly shows thatMRS allows discrimination between Raman spectra of SV-HUC-1 andMGH cells with high sensitivity (98%) and specificity (95%). MRS is also used to distinguish between SV-HUC-1 and MGH cells after exposure to urine for up to 6 h.We observe a marked change in the MRS of SV-HUC-1 and MGH cells with time in urine, indicating that the conditions of sample collection will be important for the application of this methodology to clinical urine samples. Young, Adam M. H. Campbell, Elaine C. Lynch, Sarah Dunn, Malcolm H. Powis, Simon J. Suckling, John http://hdl.handle.net/10023/2984 2013-05-12T04:34:45Z 2012-06-11T00:00:00Z

Abstract: It is becoming clear that inflammation plays a significant role in a number of neurological and psychiatric conditions. Post mortem brain samples in Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, multiple sclerosis, schizophrenia and most recently autism spectrum condition, all exhibit neuroglial activation and inflammatory markers within the CSF. Many questions remain about the underlying molecular mechanisms. By adding the pro-inflammatory cytokine, TNF-alpha, to mouse brain tissue we demonstrated that the frontal lobes and temporal region, areas involved in higher functions such as memory and learning, are most susceptible to cytokine-induced inflammation via the NF-kappa B signalling pathway. We observed direct correlations between the volumetric increase and molecular expression indicating that therapeutic targets in these lobes may require different approaches when treating conditions with a central neuroinflammatory component.

2012-06-11T00:00:00Z Young, Adam M. H. Campbell, Elaine C. Lynch, Sarah Dunn, Malcolm H. Powis, Simon J. Suckling, John It is becoming clear that inflammation plays a significant role in a number of neurological and psychiatric conditions. Post mortem brain samples in Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, multiple sclerosis, schizophrenia and most recently autism spectrum condition, all exhibit neuroglial activation and inflammatory markers within the CSF. Many questions remain about the underlying molecular mechanisms. By adding the pro-inflammatory cytokine, TNF-alpha, to mouse brain tissue we demonstrated that the frontal lobes and temporal region, areas involved in higher functions such as memory and learning, are most susceptible to cytokine-induced inflammation via the NF-kappa B signalling pathway. We observed direct correlations between the volumetric increase and molecular expression indicating that therapeutic targets in these lobes may require different approaches when treating conditions with a central neuroinflammatory component. She, Zhe Di Falco, Andrea Hähner, Georg Buck, Manfred http://hdl.handle.net/10023/2933 2013-05-12T04:12:33Z 2012-02-06T00:00:00Z

Abstract: Self-assembled monolayers (SAMs) of 4'-methylbiphenyl-4-thiol (MBP0) adsorbed on polycrystalline gold substrates served as templates to control electrochemical deposition of Cu structures from acidic solution, and enabled the subsequent lift-off of the metal structures by attachment to epoxy glue. By exploiting the negative-resist behaviour of MBP0, the SAM was patterned by means of electron-beam lithography. For high deposition contrast a two-step procedure was employed involving a nucleation phase around −0.7 V versus Cu2+/Cu and a growth phase at around −0.35 V versus Cu2+/Cu. Structures with features down to 100 nm were deposited and transferred with high fidelity. By using substrates with different surface morphologies, AFM measurements revealed that the roughness of the substrate is a crucial factor but not the only one determining the roughness of the copper surface that is exposed after lift-off.

2012-02-06T00:00:00Z She, Zhe Di Falco, Andrea Hähner, Georg Buck, Manfred Self-assembled monolayers (SAMs) of 4'-methylbiphenyl-4-thiol (MBP0) adsorbed on polycrystalline gold substrates served as templates to control electrochemical deposition of Cu structures from acidic solution, and enabled the subsequent lift-off of the metal structures by attachment to epoxy glue. By exploiting the negative-resist behaviour of MBP0, the SAM was patterned by means of electron-beam lithography. For high deposition contrast a two-step procedure was employed involving a nucleation phase around −0.7 V versus Cu2+/Cu and a growth phase at around −0.35 V versus Cu2+/Cu. Structures with features down to 100 nm were deposited and transferred with high fidelity. By using substrates with different surface morphologies, AFM measurements revealed that the roughness of the substrate is a crucial factor but not the only one determining the roughness of the copper surface that is exposed after lift-off. Ploschner, Martin Mazilu, Michael Krauss, Thomas F. Dholakia, Kishan http://hdl.handle.net/10023/2914 2013-05-12T03:32:08Z 2010-02-04T00:00:00Z

Abstract: The Maxwell stress tensor method is used to calculate the optical forces acting upon a glass nanosphere in the proximity of optically excited gold nanoantenna structures. The dependence of optical forces over a full range of excitation angles is explored: the total internal reflection excitation does not bring any particular advantage to trapping efficiency when compared to the normal incidence excitation. Our calculations show multiple trapping sites with similar trapping properties for the normal and the total internal reflection cases, respectively; furthermore, the convective heating probably dominates over any optical forces in such systems.

2010-02-04T00:00:00Z Ploschner, Martin Mazilu, Michael Krauss, Thomas F. Dholakia, Kishan The Maxwell stress tensor method is used to calculate the optical forces acting upon a glass nanosphere in the proximity of optically excited gold nanoantenna structures. The dependence of optical forces over a full range of excitation angles is explored: the total internal reflection excitation does not bring any particular advantage to trapping efficiency when compared to the normal incidence excitation. Our calculations show multiple trapping sites with similar trapping properties for the normal and the total internal reflection cases, respectively; furthermore, the convective heating probably dominates over any optical forces in such systems. Kampfrath, T. Beggs, D. M. White, T. P. Melloni, A. Krauss, T. F. Kuipers, L. http://hdl.handle.net/10023/2913 2013-05-12T03:32:07Z 2010-04-27T00:00:00Z

Abstract: We demonstrate by experiment and theory that a light pulse propagating through a Si-based photonic-crystal waveguide is adiabatically blueshifted when the refractive index of the Si is reduced on a femtosecond time scale. Thanks to the use of slow-light modes, we are able to shift a 1.3-ps pulse at telecom frequencies by 0.3 THz with an efficiency as high as 80% in a waveguide as short as 19 mu m. An analytic theory reproduces the experimental data excellently, which shows that adiabatic dynamics are possible even on the femtosecond time scale as long as the external stimulus conserves the spatial symmetry of the system.

2010-04-27T00:00:00Z Kampfrath, T. Beggs, D. M. White, T. P. Melloni, A. Krauss, T. F. Kuipers, L. We demonstrate by experiment and theory that a light pulse propagating through a Si-based photonic-crystal waveguide is adiabatically blueshifted when the refractive index of the Si is reduced on a femtosecond time scale. Thanks to the use of slow-light modes, we are able to shift a 1.3-ps pulse at telecom frequencies by 0.3 THz with an efficiency as high as 80% in a waveguide as short as 19 mu m. An analytic theory reproduces the experimental data excellently, which shows that adiabatic dynamics are possible even on the femtosecond time scale as long as the external stimulus conserves the spatial symmetry of the system. Di Falco, Andrea Ploschner, Martin Krauss, Thomas F. http://hdl.handle.net/10023/2908 2013-05-12T03:36:25Z 2010-11-04T00:00:00Z

Abstract: We report on the fabrication and characterization of plasmonic structures on flexible substrates (Metaflex) and demonstrate the optical properties of a single layer of Metaflex. The layer exhibits a plasmonic resonance in the visible region around 620 nm. We show experimental and numerical results for both nano-antennas and fishnet geometries. We anticipate the use of Metaflex as a building block for flexible metamaterials in the visible range.

2010-11-04T00:00:00Z Di Falco, Andrea Ploschner, Martin Krauss, Thomas F. We report on the fabrication and characterization of plasmonic structures on flexible substrates (Metaflex) and demonstrate the optical properties of a single layer of Metaflex. The layer exhibits a plasmonic resonance in the visible region around 620 nm. We show experimental and numerical results for both nano-antennas and fishnet geometries. We anticipate the use of Metaflex as a building block for flexible metamaterials in the visible range. Ricciardi, Armando Pisco, Marco Gallina, Ilaria Campopiano, Stefania Galdi, Vincenzo O'Faolain, Liam Krauss, Thomas F. Cusano, Andrea http://hdl.handle.net/10023/2907 2013-05-12T03:36:24Z 2010-12-01T00:00:00Z

Abstract: We report on the first experimental evidence of guided resonances (GRs) in photonic crystal slabs based on aperiodically ordered supercells. Using Ammann-Beenker (quasiperiodic, eightfold symmetric) tiling geometry, we present our study on the fabrication, experimental characterization, and full-wave numerical simulation of two representative structures (with different filling parameters) operating at near-IR wavelengths (1300-1600 nm). Our results show a fairly good agreement between measurements and numerical predictions and pave the way for the development of new strategies (based on, e.g., the lattice symmetry breaking) for GR engineering. (C) 2010 Optical Society of America

2010-12-01T00:00:00Z Ricciardi, Armando Pisco, Marco Gallina, Ilaria Campopiano, Stefania Galdi, Vincenzo O'Faolain, Liam Krauss, Thomas F. Cusano, Andrea We report on the first experimental evidence of guided resonances (GRs) in photonic crystal slabs based on aperiodically ordered supercells. Using Ammann-Beenker (quasiperiodic, eightfold symmetric) tiling geometry, we present our study on the fabrication, experimental characterization, and full-wave numerical simulation of two representative structures (with different filling parameters) operating at near-IR wavelengths (1300-1600 nm). Our results show a fairly good agreement between measurements and numerical predictions and pave the way for the development of new strategies (based on, e.g., the lattice symmetry breaking) for GR engineering. (C) 2010 Optical Society of America Portalupi, Simone L Galli, Matteo Belotti, M Andreani, Lucio Claudio Krauss, Thomas Fraser O'Faolain, Liam http://hdl.handle.net/10023/2874 2013-05-12T04:12:35Z 2011-07-12T00:00:00Z

Abstract: We report a study of photonic crystal nanocavities as a function of lattice disorder, which is implemented by deliberate radius variations of the holes forming the photonic crystal. Using cross-polarized laser light scattering, we demonstrate that lattice disorder affects the cavity mode symmetry, as is crucially evidenced by measuring scattering resonances for different sample orientations and explained by group-theoretical analysis together with calculations of the mode profile. We also quantify how the increase of lattice disorder leads to a reduction of the cavity Q factors and to a spread of both Q factors and resonance wavelengths. The trends as a function of disorder and for different radii distributions are well reproduced by theoretical calculations when both intentional (deliberate) and intrinsic (residual) disorder are taken into account. The results shed light on the effects of disorder on cavity resonances and on the interplay between intentional and intrinsic disorder, yielding reliable estimations of residual disorder (which ultimately affects technology limits) from optical measurements. Interestingly, the disorder values derived from the variation of the Q factor are lower than those derived from the wavelength spread: this suggests the occurrence of subtle interhole correlation effects that turn out to be beneficial for cavity Q factors.

2011-07-12T00:00:00Z Portalupi, Simone L Galli, Matteo Belotti, M Andreani, Lucio Claudio Krauss, Thomas Fraser O'Faolain, Liam We report a study of photonic crystal nanocavities as a function of lattice disorder, which is implemented by deliberate radius variations of the holes forming the photonic crystal. Using cross-polarized laser light scattering, we demonstrate that lattice disorder affects the cavity mode symmetry, as is crucially evidenced by measuring scattering resonances for different sample orientations and explained by group-theoretical analysis together with calculations of the mode profile. We also quantify how the increase of lattice disorder leads to a reduction of the cavity Q factors and to a spread of both Q factors and resonance wavelengths. The trends as a function of disorder and for different radii distributions are well reproduced by theoretical calculations when both intentional (deliberate) and intrinsic (residual) disorder are taken into account. The results shed light on the effects of disorder on cavity resonances and on the interplay between intentional and intrinsic disorder, yielding reliable estimations of residual disorder (which ultimately affects technology limits) from optical measurements. Interestingly, the disorder values derived from the variation of the Q factor are lower than those derived from the wavelength spread: this suggests the occurrence of subtle interhole correlation effects that turn out to be beneficial for cavity Q factors. Kotlyar, Victor V Stafeev, S S O'Faolain, Liam Soifer, V A http://hdl.handle.net/10023/2873 2013-05-12T04:12:36Z 2011-08-15T00:00:00Z

Abstract: Using a near-field scanning microscope (NT-MDT) with a 100nm aperture cantilever held 1 μm apart from a microaxicon of diameter 14 μm and period 800nm, we measure a focal spot resulting from the illumination by a linearly polarized laser light of wavelength λ ¼ 532nm, with itsFWHMbeing equal to 0:58λ, and the depth of focus being 5:6λ. The rms deviation of the focal spot intensity from the calculated value is 6%. The focus intensity is five times larger than the maximal illumination beam intensity.

2011-08-15T00:00:00Z Kotlyar, Victor V Stafeev, S S O'Faolain, Liam Soifer, V A Using a near-field scanning microscope (NT-MDT) with a 100nm aperture cantilever held 1 μm apart from a microaxicon of diameter 14 μm and period 800nm, we measure a focal spot resulting from the illumination by a linearly polarized laser light of wavelength λ ¼ 532nm, with itsFWHMbeing equal to 0:58λ, and the depth of focus being 5:6λ. The rms deviation of the focal spot intensity from the calculated value is 6%. The focus intensity is five times larger than the maximal illumination beam intensity. Ricciardi, Armando Pisco, Marco Cutolo, Antonello Cusano, Andrea O'Faolain, Liam Krauss, Thomas F. Castaldi, Giuseppe Galdi, Vincenzo http://hdl.handle.net/10023/2872 2013-05-12T04:06:49Z 2011-08-29T00:00:00Z

Abstract: We report on the experimental evidence of Fano-type guided resonances (GRs) in aperiodically-ordered photonic quasicrystal slabs. With specific reference to the Ammann-Beenker (8-fold symmetric, quasiperiodic) octagonal tiling geometry, we present our experimental results on silicon-on-insulator devices operating at near-infrared wavelengths, and compare them with the full-wave numerical predictions based on periodic approximants. Our results indicate that spatial periodicity is not strictly required for the GR excitation, and may be effectively surrogated by weaker forms of long-range aperiodic order which intrinsically provide extra degrees of freedom (e. g., higher-order rotational symmetries, richer defect states and phase-matching conditions, etc.) to be exploited in the design and performance optimization of nanostructured dielectric slabs operating in the out-of-plane configuration. The essential spectral features may be qualitatively understood in terms of phase-matching conditions derived from approximate homogenized models, and turn out to be effectively captured by full-wave modeling based on suitably-sized periodic approximants.

2011-08-29T00:00:00Z Ricciardi, Armando Pisco, Marco Cutolo, Antonello Cusano, Andrea O'Faolain, Liam Krauss, Thomas F. Castaldi, Giuseppe Galdi, Vincenzo We report on the experimental evidence of Fano-type guided resonances (GRs) in aperiodically-ordered photonic quasicrystal slabs. With specific reference to the Ammann-Beenker (8-fold symmetric, quasiperiodic) octagonal tiling geometry, we present our experimental results on silicon-on-insulator devices operating at near-infrared wavelengths, and compare them with the full-wave numerical predictions based on periodic approximants. Our results indicate that spatial periodicity is not strictly required for the GR excitation, and may be effectively surrogated by weaker forms of long-range aperiodic order which intrinsically provide extra degrees of freedom (e. g., higher-order rotational symmetries, richer defect states and phase-matching conditions, etc.) to be exploited in the design and performance optimization of nanostructured dielectric slabs operating in the out-of-plane configuration. The essential spectral features may be qualitatively understood in terms of phase-matching conditions derived from approximate homogenized models, and turn out to be effectively captured by full-wave modeling based on suitably-sized periodic approximants. Debnath, Kapil O'Faolain, Liam Gardes, F Y Thomson, D Reed, G T Krauss, Thomas Fraser http://hdl.handle.net/10023/2871 2013-05-12T04:15:33Z 2012-01-30T00:00:00Z

Abstract: We experimentally demonstrate a simple but more efficient technique to modulate and multiplex multiple WDM channels. Our design is based on a bus waveguide vertically coupled to multiple Photonic Crystal (PhC) resonator, each of which modulates an individual channel in place. The Photonic crystal resonator modulator provide very low switching energies (~fJ) while the bus waveguide can be made from a material with a low refractive index thereby allowing very efficient coupling with an optical fiber.

2012-01-30T00:00:00Z Debnath, Kapil O'Faolain, Liam Gardes, F Y Thomson, D Reed, G T Krauss, Thomas Fraser We experimentally demonstrate a simple but more efficient technique to modulate and multiplex multiple WDM channels. Our design is based on a bus waveguide vertically coupled to multiple Photonic Crystal (PhC) resonator, each of which modulates an individual channel in place. The Photonic crystal resonator modulator provide very low switching energies (~fJ) while the bus waveguide can be made from a material with a low refractive index thereby allowing very efficient coupling with an optical fiber. Krauss, Thomas F. O'Faolain, Liam Schulz, Sebastian Beggs, Daryl Matthew Morichetti, Francesco Canciamilla, Antonio Torregiani, M. Melloni, Andrea Mazoyer, Simon Lalanne, Phillippe Samarelli, Antonio Sorel, Marc De La Rue, Richard http://hdl.handle.net/10023/2870 2013-05-12T04:15:01Z 2010-02-01T00:00:00Z

Abstract: We study propagation losses in slow light photonic crystal waveguides and show that dispersion engineering can reduce the loss. We develop an improved understanding of why and how this occurs and develop an new approach to modeling these devices that provides new design insights. Description: Selected papers. Held as part of SPIE Photonics West 2010.

2010-02-01T00:00:00Z Krauss, Thomas F. O'Faolain, Liam Schulz, Sebastian Beggs, Daryl Matthew Morichetti, Francesco Canciamilla, Antonio Torregiani, M. Melloni, Andrea Mazoyer, Simon Lalanne, Phillippe Samarelli, Antonio Sorel, Marc De La Rue, Richard We study propagation losses in slow light photonic crystal waveguides and show that dispersion engineering can reduce the loss. We develop an improved understanding of why and how this occurs and develop an new approach to modeling these devices that provides new design insights. Debnath, Kapil Welna, Karl Peter Ferrera, Marcello Deasy, Kieran Lidzey, David Krauss, Thomas Fraser O'Faolain, Liam http://hdl.handle.net/10023/2869 2013-05-12T04:15:34Z 2012-01-30T00:00:00Z

Abstract: We experimentally demonstrate a new optical filter design comprising of a photonic crystal cavity and a low index bus waveguide which are monolithically integrated on a silicon-on-insulator (SOI) platform. We have fabricated oxide clad PhC cavities with a silicon nitride waveguide positioned directly above, such that there is an overlap between the evanescent tails of the two modes. We have realised an extinction ratio of 7.5dB for cavities with total Q of 50,000.

2012-01-30T00:00:00Z Debnath, Kapil Welna, Karl Peter Ferrera, Marcello Deasy, Kieran Lidzey, David Krauss, Thomas Fraser O'Faolain, Liam We experimentally demonstrate a new optical filter design comprising of a photonic crystal cavity and a low index bus waveguide which are monolithically integrated on a silicon-on-insulator (SOI) platform. We have fabricated oxide clad PhC cavities with a silicon nitride waveguide positioned directly above, such that there is an overlap between the evanescent tails of the two modes. We have realised an extinction ratio of 7.5dB for cavities with total Q of 50,000. White, T. P. Botten, L. C. de Sterke, C. Martijn Dossou, K. B. McPhedran, R. C. http://hdl.handle.net/10023/2855 2013-05-12T04:34:32Z 2008-11-15T00:00:00Z

Abstract: We consider the coupling into a slow mode that appears near an inflection point in the band structure of a photonic crystal waveguide. Remarkably, the coupling into this slow mode, which has a group index n(g) > 1000, can be essentially perfect without any transition region. We show that this efficient coupling occurs thanks to an evanescent mode in the slow medium, which has appreciable amplitude and helps satisfy the boundary conditions but does not transport any energy. (C) 2008 Optical Society of America

2008-11-15T00:00:00Z White, T. P. Botten, L. C. de Sterke, C. Martijn Dossou, K. B. McPhedran, R. C. We consider the coupling into a slow mode that appears near an inflection point in the band structure of a photonic crystal waveguide. Remarkably, the coupling into this slow mode, which has a group index n(g) > 1000, can be essentially perfect without any transition region. We show that this efficient coupling occurs thanks to an evanescent mode in the slow medium, which has appreciable amplitude and helps satisfy the boundary conditions but does not transport any energy. (C) 2008 Optical Society of America Spurny, Marcel O'Faolain, Liam Bulla, Douglas A. P. Luther-Davies, Barry Krauss, Thomas F. http://hdl.handle.net/10023/2854 2013-05-12T04:34:27Z 2011-01-31T00:00:00Z

Abstract: We demonstrate low loss photonic crystal waveguides in chalcogenide (Ge33As12Se55) glasses. The measured losses are as low as 21dB/cm. We experimentally determine the refractive index of the thin film chalcogenide glass to be n = 2.6 and demonstrate that dispersion engineering can be performed up to a group index of n(g) = 40 in this relatively low refractive index contrast system. (C) 2011 Optical Society of America

2011-01-31T00:00:00Z Spurny, Marcel O'Faolain, Liam Bulla, Douglas A. P. Luther-Davies, Barry Krauss, Thomas F. We demonstrate low loss photonic crystal waveguides in chalcogenide (Ge33As12Se55) glasses. The measured losses are as low as 21dB/cm. We experimentally determine the refractive index of the thin film chalcogenide glass to be n = 2.6 and demonstrate that dispersion engineering can be performed up to a group index of n(g) = 40 in this relatively low refractive index contrast system. (C) 2011 Optical Society of America Li, Juntao O'Faolain, Liam Rey, Isabella H. Krauss, Thomas F. http://hdl.handle.net/10023/2853 2013-05-12T04:33:59Z 2011-02-28T00:00:00Z

Abstract: We demonstrate continuous wave four-wave mixing in silicon photonic crystal waveguides of 396 mu m length with a group index of n(g) = 30. The highest observed conversion efficiency is -24 dB for 90 mW coupled input pump power. The key question we address is whether the predicted fourth power dependence of the conversion efficiency on the slowdown factor (eta approximate to S-4) can indeed be observed in this system, and how the conversion efficiency depends on device length in the presence of propagation losses. We find that the expected dependencies hold as long as both realistic losses and the variation of mode shape with slowdown factor are taken into account. Having achieved a good agreement between a simple analytical model and the experiment, we also predict structures that can achieve the same conversion efficiency as already observed in nanowires for the same input power, yet for a device length that is 50 times shorter. (C) 2011 Optical Society of America

2011-02-28T00:00:00Z Li, Juntao O'Faolain, Liam Rey, Isabella H. Krauss, Thomas F. We demonstrate continuous wave four-wave mixing in silicon photonic crystal waveguides of 396 mu m length with a group index of n(g) = 30. The highest observed conversion efficiency is -24 dB for 90 mW coupled input pump power. The key question we address is whether the predicted fourth power dependence of the conversion efficiency on the slowdown factor (eta approximate to S-4) can indeed be observed in this system, and how the conversion efficiency depends on device length in the presence of propagation losses. We find that the expected dependencies hold as long as both realistic losses and the variation of mode shape with slowdown factor are taken into account. Having achieved a good agreement between a simple analytical model and the experiment, we also predict structures that can achieve the same conversion efficiency as already observed in nanowires for the same input power, yet for a device length that is 50 times shorter. (C) 2011 Optical Society of America Chalcraft, A. R. A. Lam, S. Jones, B. D. Szymanski, D. Oulton, R. Thijssen, A. C. T. Skolnick, M. S. Whittaker, D. M. Krauss, T. F. Fox, A. M. http://hdl.handle.net/10023/2852 2013-05-12T04:34:26Z 2011-03-14T00:00:00Z

Abstract: We investigate the energy splitting, quality factor and polarization of the fundamental modes of coupled L3 photonic crystal cavities. Four different geometries are evaluated theoretically, before experimentally investigating coupling in a direction at 30 degrees to the line of the cavities. In this geometry, a smooth variation of the energy splitting with the cavity separation is predicted and observed, together with significant differences between the polarizations of the bonding and anti-bonding states. The controlled splitting of the coupled states is potentially useful for applications that require simultaneous resonant enhancement of two transitions. (C) 2011 Optical Society of America

2011-03-14T00:00:00Z Chalcraft, A. R. A. Lam, S. Jones, B. D. Szymanski, D. Oulton, R. Thijssen, A. C. T. Skolnick, M. S. Whittaker, D. M. Krauss, T. F. Fox, A. M. We investigate the energy splitting, quality factor and polarization of the fundamental modes of coupled L3 photonic crystal cavities. Four different geometries are evaluated theoretically, before experimentally investigating coupling in a direction at 30 degrees to the line of the cavities. In this geometry, a smooth variation of the energy splitting with the cavity separation is predicted and observed, together with significant differences between the polarizations of the bonding and anti-bonding states. The controlled splitting of the coupled states is potentially useful for applications that require simultaneous resonant enhancement of two transitions. (C) 2011 Optical Society of America Spasenovic, Marko White, Thomas P. Ha, Sangwoo Sukhorukov, Andrey A. Kampfrath, Tobias Kivshar, Yuri S. de Sterke, C. Martijn Krauss, Thomas F. Kuipers, L. (Kobus) http://hdl.handle.net/10023/2851 2013-05-12T04:34:24Z 2011-04-01T00:00:00Z

Abstract: We experimentally study the fields close to an interface between two photonic crystal waveguides that have different dispersion properties. After the transition from a waveguide in which the group velocity of light is v(g) similar to c/10 to a waveguide in which it is v(g) similar to c/100, we observe a gradual increase in the field intensity and the lateral spreading of the mode. We attribute this evolution to the existence of a weakly evanescent mode that exponentially decays away from the interface. We compare this to the situation where the transition between the waveguides only leads to a minor change in group velocity and show that, in that case, the evolution is absent. Furthermore, we apply novel numerical mode extraction techniques to confirm experimental results. (C) 2011 Optical Society of America

2011-04-01T00:00:00Z Spasenovic, Marko White, Thomas P. Ha, Sangwoo Sukhorukov, Andrey A. Kampfrath, Tobias Kivshar, Yuri S. de Sterke, C. Martijn Krauss, Thomas F. Kuipers, L. (Kobus) We experimentally study the fields close to an interface between two photonic crystal waveguides that have different dispersion properties. After the transition from a waveguide in which the group velocity of light is v(g) similar to c/10 to a waveguide in which it is v(g) similar to c/100, we observe a gradual increase in the field intensity and the lateral spreading of the mode. We attribute this evolution to the existence of a weakly evanescent mode that exponentially decays away from the interface. We compare this to the situation where the transition between the waveguides only leads to a minor change in group velocity and show that, in that case, the evolution is absent. Furthermore, we apply novel numerical mode extraction techniques to confirm experimental results. (C) 2011 Optical Society of America Ha, Sangwoo Spasenovic, Marko Sukhorukov, Andrey A. White, Thomas P. de Sterke, C. Martijn Kuipers, L. Kobus Krauss, Thomas F. Kivshar, Yuri S. http://hdl.handle.net/10023/2850 2013-05-12T04:34:24Z 2011-04-01T00:00:00Z

Abstract: We develop a systematic approach for simultaneous extraction of the dispersion relations and profiles of multiple modes in periodic waveguides though a special global optimization procedure applied to near-field electric field measurements in the waveguide plane. We apply this method to perform in-depth analysis of experimental data on wave propagation close to an interface between waveguide sections with different dispersion characteristics, and we successfully identify several modes contributing to the experimentally measured fields. We find clear evidence that when the group velocity is reduced across the interface, evanescent modes that facilitate the excitation of propagating slow-light waves appear, confirming previous theoretical predictions. (C) 2011 Optical Society of America

2011-04-01T00:00:00Z Ha, Sangwoo Spasenovic, Marko Sukhorukov, Andrey A. White, Thomas P. de Sterke, C. Martijn Kuipers, L. Kobus Krauss, Thomas F. Kivshar, Yuri S. We develop a systematic approach for simultaneous extraction of the dispersion relations and profiles of multiple modes in periodic waveguides though a special global optimization procedure applied to near-field electric field measurements in the waveguide plane. We apply this method to perform in-depth analysis of experimental data on wave propagation close to an interface between waveguide sections with different dispersion characteristics, and we successfully identify several modes contributing to the experimentally measured fields. We find clear evidence that when the group velocity is reduced across the interface, evanescent modes that facilitate the excitation of propagating slow-light waves appear, confirming previous theoretical predictions. (C) 2011 Optical Society of America Corcoran, Bill Pelusi, Mark D. Monat, Christelle Li, Juntao O'Faolain, Liam Krauss, Thomas F. Eggleton, Benjamin J. http://hdl.handle.net/10023/2849 2013-05-12T04:34:23Z 2011-05-01T00:00:00Z

Abstract: We demonstrate all-optical demultiplexing of a high-bandwidth, time-division multiplexed 160 Gbit/s signal to 10 Gbit/s channels, exploiting slow light enhanced four-wave mixing in a dispersion engineered, 96 mu m long planar photonic crystal waveguide. We report error-free (bit error rate < 10(-9)) operation of all 16 demultiplexed channels, with a power penalty of 2.2-2.4 dB, highlighting the potential of these structures as a platform for ultracompact all-optical nonlinear processes. (C) 2011 Optical Society of America

2011-05-01T00:00:00Z Corcoran, Bill Pelusi, Mark D. Monat, Christelle Li, Juntao O'Faolain, Liam Krauss, Thomas F. Eggleton, Benjamin J. We demonstrate all-optical demultiplexing of a high-bandwidth, time-division multiplexed 160 Gbit/s signal to 10 Gbit/s channels, exploiting slow light enhanced four-wave mixing in a dispersion engineered, 96 mu m long planar photonic crystal waveguide. We report error-free (bit error rate < 10(-9)) operation of all 16 demultiplexed channels, with a power penalty of 2.2-2.4 dB, highlighting the potential of these structures as a platform for ultracompact all-optical nonlinear processes. (C) 2011 Optical Society of America Rey, Isabella H. Lefevre, Yannick Schulz, Sebastian A. Vermeulen, Nathalie Krauss, Thomas F. http://hdl.handle.net/10023/2848 2013-05-12T04:34:22Z 2011-07-20T00:00:00Z

Abstract: The prospect for low pump-power Raman amplification in silicon waveguides has recently been boosted by theoretical studies discussing the enhancement of nonlinear phenomena in slow-light structures. In principle, the slowing down of either the pump or the signal beam is equivalent in terms of Raman gain, but in the presence of losses, we show that they play different roles in determining the net signal gain. We also investigate the impact of the mode profile in realistic slow-light waveguides on the total gain, an effect that is usually neglected in the context of stimulated Raman scattering. By taking representative losses and mode shapes into account, we provide a realistic estimation of the achievable performance of slow-light photonic crystal waveguides.

2011-07-20T00:00:00Z Rey, Isabella H. Lefevre, Yannick Schulz, Sebastian A. Vermeulen, Nathalie Krauss, Thomas F. The prospect for low pump-power Raman amplification in silicon waveguides has recently been boosted by theoretical studies discussing the enhancement of nonlinear phenomena in slow-light structures. In principle, the slowing down of either the pump or the signal beam is equivalent in terms of Raman gain, but in the presence of losses, we show that they play different roles in determining the net signal gain. We also investigate the impact of the mode profile in realistic slow-light waveguides on the total gain, an effect that is usually neglected in the context of stimulated Raman scattering. By taking representative losses and mode shapes into account, we provide a realistic estimation of the achievable performance of slow-light photonic crystal waveguides. Monat, Christelle Spurny, Marcel Grillet, Christian O'Faolain, Liam Krauss, Thomas F. Eggleton, Benjamin J. Bulla, Douglas Madden, Steve Luther-Davies, Barry http://hdl.handle.net/10023/2847 2013-05-12T04:34:21Z 2011-08-01T00:00:00Z

Abstract: We demonstrate third-harmonic generation (THG) in a dispersion-engineered slow-light photonic crystal waveguide fabricated in AMTIR-1 chalcogenide glass. Owing to the relatively low loss and low dispersion in the slow-light (c/30) regime, combined with the high nonlinear figure of merit of the material (similar to 2), we obtain a relatively large conversion efficiency (1.4 x 10(-8)/W-2), which is 30x higher than in comparable silicon waveguides, and observe a uniform visible light pattern along the waveguide. These results widen the number of applications underpinned by THG in slow-light platforms, such as the direct observation of the spatial evolution of the propagating mode. (C) 2011 Optical Society of America

2011-08-01T00:00:00Z Monat, Christelle Spurny, Marcel Grillet, Christian O'Faolain, Liam Krauss, Thomas F. Eggleton, Benjamin J. Bulla, Douglas Madden, Steve Luther-Davies, Barry We demonstrate third-harmonic generation (THG) in a dispersion-engineered slow-light photonic crystal waveguide fabricated in AMTIR-1 chalcogenide glass. Owing to the relatively low loss and low dispersion in the slow-light (c/30) regime, combined with the high nonlinear figure of merit of the material (similar to 2), we obtain a relatively large conversion efficiency (1.4 x 10(-8)/W-2), which is 30x higher than in comparable silicon waveguides, and observe a uniform visible light pattern along the waveguide. These results widen the number of applications underpinned by THG in slow-light platforms, such as the direct observation of the spatial evolution of the propagating mode. (C) 2011 Optical Society of America Xiong, C. Monat, Christelle Clark, Alex S. Grillet, Christian Marshall, Graham D. Steel, M. J. Li, Juntao O'Faolain, Liam Krauss, Thomas F. Rarity, John G. Eggleton, Benjamin J. http://hdl.handle.net/10023/2811 2013-05-12T04:33:56Z 2011-09-01T00:00:00Z

Abstract: We report the generation of correlated photon pairs in the telecom C-band at room temperature from a dispersion-engineered silicon photonic crystal waveguide. The spontaneous four-wave mixing process producing the photon pairs is enhanced by slow-light propagation enabling an active device length of less than 100 mu m. With a coincidence to accidental ratio of 12.8 at a pair generation rate of 0.006 per pulse, this ultracompact photon pair source paves the way toward scalable quantum information processing realized on-chip. (C) 2011 Optical Society of America

2011-09-01T00:00:00Z Xiong, C. Monat, Christelle Clark, Alex S. Grillet, Christian Marshall, Graham D. Steel, M. J. Li, Juntao O'Faolain, Liam Krauss, Thomas F. Rarity, John G. Eggleton, Benjamin J. We report the generation of correlated photon pairs in the telecom C-band at room temperature from a dispersion-engineered silicon photonic crystal waveguide. The spontaneous four-wave mixing process producing the photon pairs is enhanced by slow-light propagation enabling an active device length of less than 100 mu m. With a coincidence to accidental ratio of 12.8 at a pair generation rate of 0.006 per pulse, this ultracompact photon pair source paves the way toward scalable quantum information processing realized on-chip. (C) 2011 Optical Society of America Husko, C. Vo, T. D. Corcoran, B. Li, J. Krauss, T. F. Eggleton, B. J. http://hdl.handle.net/10023/2810 2013-05-12T04:33:55Z 2011-10-10T00:00:00Z

Abstract: We demonstrate an ultracompact, chip-based, all-optical exclusive-OR (XOR) logic gate via slow-light enhanced four-wave mixing (FWM) in a silicon photonic crystal waveguide (PhCWG). We achieve error-free operation (<10(-9)) for 40 Gbit/s differential phase-shift keying (DPSK) signals with a 2.8 dB power penalty. Slowing the light to v(g) = c/32 enables a FWM conversion efficiency, eta, of -30 dB for a 396 mu m device. The nonlinear FWM process is enhanced by 20 dB compared to a relatively fast mode of v(g) = c/5. The XOR operation requires approximate to 41 mW, corresponding to a switching energy of 1 pJ/bit. We compare the slow-light PhCWG device performance with experimentally demonstrated XOR DPSK logic gates in other platforms and discuss scaling the device operation to higher bit-rates. The ultracompact structure suggests the potential for device integration. (C) 2011 Optical Society of America

2011-10-10T00:00:00Z Husko, C. Vo, T. D. Corcoran, B. Li, J. Krauss, T. F. Eggleton, B. J. We demonstrate an ultracompact, chip-based, all-optical exclusive-OR (XOR) logic gate via slow-light enhanced four-wave mixing (FWM) in a silicon photonic crystal waveguide (PhCWG). We achieve error-free operation (<10(-9)) for 40 Gbit/s differential phase-shift keying (DPSK) signals with a 2.8 dB power penalty. Slowing the light to v(g) = c/32 enables a FWM conversion efficiency, eta, of -30 dB for a 396 mu m device. The nonlinear FWM process is enhanced by 20 dB compared to a relatively fast mode of v(g) = c/5. The XOR operation requires approximate to 41 mW, corresponding to a switching energy of 1 pJ/bit. We compare the slow-light PhCWG device performance with experimentally demonstrated XOR DPSK logic gates in other platforms and discuss scaling the device operation to higher bit-rates. The ultracompact structure suggests the potential for device integration. (C) 2011 Optical Society of America Beggs, Daryl M. Krauss, Thomas F. Kuipers, L. Kampfrath, Tobias http://hdl.handle.net/10023/2809 2013-05-12T04:33:55Z 2012-01-18T00:00:00Z

Abstract: We demonstrate, by theory and experiment, the ultrafast tilting of the dispersion curve of a photonic-crystal waveguide following the absorption of a femtosecond pump pulse. By shaping the pump-beam cross section with a nanometric shadow mask, different waveguide eigenmodes acquire different spatial overlap with the perturbing pump, leading to a local flattening of the dispersion by up to 11%. We find that such partial mode perturbation can be used to adiabatically compress the spectrum of a light pulse traveling through the waveguide.

2012-01-18T00:00:00Z Beggs, Daryl M. Krauss, Thomas F. Kuipers, L. Kampfrath, Tobias We demonstrate, by theory and experiment, the ultrafast tilting of the dispersion curve of a photonic-crystal waveguide following the absorption of a femtosecond pump pulse. By shaping the pump-beam cross section with a nanometric shadow mask, different waveguide eigenmodes acquire different spatial overlap with the perturbing pump, leading to a local flattening of the dispersion by up to 11%. We find that such partial mode perturbation can be used to adiabatically compress the spectrum of a light pulse traveling through the waveguide. Beggs, Daryl M. Rey, Isabella H. Kampfrath, Tobias Rotenberg, Nir Kuipers, L. Krauss, Thomas F. http://hdl.handle.net/10023/2808 2013-05-12T04:33:54Z 2012-05-21T00:00:00Z

Abstract: We introduce the concept of an indirect photonic transition and demonstrate its use in a dynamic delay line to alter the group velocity of an optical pulse. Operating on an ultrafast time scale, we show continuously tunable delays of up to 20 ps, using a slow light photonic crystal waveguide only 300 mu m in length. Our approach is flexible, in that individual pulses in a pulse stream can be controlled independently, which we demonstrate by operating on pulses separated by just 30 ps. The two-step indirect transition is demonstrated here with a 30% conversion efficiency.

2012-05-21T00:00:00Z Beggs, Daryl M. Rey, Isabella H. Kampfrath, Tobias Rotenberg, Nir Kuipers, L. Krauss, Thomas F. We introduce the concept of an indirect photonic transition and demonstrate its use in a dynamic delay line to alter the group velocity of an optical pulse. Operating on an ultrafast time scale, we show continuously tunable delays of up to 20 ps, using a slow light photonic crystal waveguide only 300 mu m in length. Our approach is flexible, in that individual pulses in a pulse stream can be controlled independently, which we demonstrate by operating on pulses separated by just 30 ps. The two-step indirect transition is demonstrated here with a 30% conversion efficiency. Nissen, Felix Schmidt, Sebastian Biondi, Matteo Blatter, Gianni Tureci, Hakan E. Keeling, Jonathan Mark James http://hdl.handle.net/10023/2596 2013-05-13T10:01:01Z 2012-06-08T00:00:00Z

Abstract: We study the coherence and fluorescence properties of the coherently pumped and dissipative Jaynes-Cummings-Hubbard model describing polaritons in a coupled-cavity array. At weak hopping we find strong signatures of photon blockade similar to single-cavity systems. At strong hopping the state of the photons in the array depends on its size. While the photon blockade persists in a dimer consisting of two coupled cavities, a coherent state forms on an extended lattice, which can be described in terms of a semi-classical model.

2012-06-08T00:00:00Z Nissen, Felix Schmidt, Sebastian Biondi, Matteo Blatter, Gianni Tureci, Hakan E. Keeling, Jonathan Mark James We study the coherence and fluorescence properties of the coherently pumped and dissipative Jaynes-Cummings-Hubbard model describing polaritons in a coupled-cavity array. At weak hopping we find strong signatures of photon blockade similar to single-cavity systems. At strong hopping the state of the photons in the array depends on its size. While the photon blockade persists in a dimer consisting of two coupled cavities, a coherent state forms on an extended lattice, which can be described in terms of a semi-classical model. Reardon, Christopher Di Falco, Andrea Welna, Karl Krauss, Thomas http://hdl.handle.net/10023/2494 2013-05-12T03:31:50Z 2009-03-02T00:00:00Z

Abstract: We demonstrate beam deflection and multiple channel communication in free space optical communications using microprisms integrated directly onto an array of vertical cavity surface emitting lasers (VCSELs). The design and fabrication of such a transmitter is presented, and shown to achieve beam deflection of up to 10 in a planar configuration. A location discovery application, for use within a distributed network, is put forward and analysed. (c) 2009 Optical Society of America

2009-03-02T00:00:00Z Reardon, Christopher Di Falco, Andrea Welna, Karl Krauss, Thomas We demonstrate beam deflection and multiple channel communication in free space optical communications using microprisms integrated directly onto an array of vertical cavity surface emitting lasers (VCSELs). The design and fabrication of such a transmitter is presented, and shown to achieve beam deflection of up to 10 in a planar configuration. A location discovery application, for use within a distributed network, is put forward and analysed. (c) 2009 Optical Society of America O'Faolain, Liam Schulz, S. A. Beggs, D. M. White, T. P. Spasenovic, M. Kuipers, L. Morichetti, F. Melloni, A. Mazoyer, S. Hugonin, J. P. Lalanne, P. Krauss, T. F. http://hdl.handle.net/10023/2482 2013-05-19T04:31:12Z 2010-12-20T00:00:00Z

Abstract: Slow light devices such as photonic crystal waveguides (PhCW) and coupled resonator optical waveguides (CROW) have much promise for optical signal processing applications and a number of successful demonstrations underpinning this promise have already been made. Most of these applications are limited by propagation losses, especially for higher group indices. These losses are caused by technological imperfections ("extrinsic loss") that cause scattering of light from the waveguide mode. The relationship between this loss and the group velocity is complex and until now has not been fully understood. Here, we present a comprehensive explanation of the extrinsic loss mechanisms in PhC waveguides and address some misconceptions surrounding loss and slow light that have arisen in recent years. We develop a theoretical model that accurately describes the loss spectra of PhC waveguides. One of the key insights of the model is that the entire hole contributes coherently to the scattering process, in contrast to previous models that added up the scattering from short sections incoherently. As a result, we have already realised waveguides with significantly lower losses than comparable photonic crystal waveguides as well as achieving propagation losses, in units of loss per unit time (dB/ns) that are even lower than those of state-of-the-art coupled resonator optical waveguides based on silicon photonic wires. The model will enable more advanced designs with further loss reduction within existing technological constraints. (C) 2010 Optical Society of America

2010-12-20T00:00:00Z O'Faolain, Liam Schulz, S. A. Beggs, D. M. White, T. P. Spasenovic, M. Kuipers, L. Morichetti, F. Melloni, A. Mazoyer, S. Hugonin, J. P. Lalanne, P. Krauss, T. F. Slow light devices such as photonic crystal waveguides (PhCW) and coupled resonator optical waveguides (CROW) have much promise for optical signal processing applications and a number of successful demonstrations underpinning this promise have already been made. Most of these applications are limited by propagation losses, especially for higher group indices. These losses are caused by technological imperfections ("extrinsic loss") that cause scattering of light from the waveguide mode. The relationship between this loss and the group velocity is complex and until now has not been fully understood. Here, we present a comprehensive explanation of the extrinsic loss mechanisms in PhC waveguides and address some misconceptions surrounding loss and slow light that have arisen in recent years. We develop a theoretical model that accurately describes the loss spectra of PhC waveguides. One of the key insights of the model is that the entire hole contributes coherently to the scattering process, in contrast to previous models that added up the scattering from short sections incoherently. As a result, we have already realised waveguides with significantly lower losses than comparable photonic crystal waveguides as well as achieving propagation losses, in units of loss per unit time (dB/ns) that are even lower than those of state-of-the-art coupled resonator optical waveguides based on silicon photonic wires. The model will enable more advanced designs with further loss reduction within existing technological constraints. (C) 2010 Optical Society of America Kerry, Philip S. Ayllon, Juan Taylor, Margaret A. Hass, Claudia Lewis, Andrew Garcia-Sastre, Adolfo Randall, Richard E. Hale, Benjamin G. Russell, Rupert J. http://hdl.handle.net/10023/2177 2013-05-12T04:09:34Z 2011-03-28T00:00:00Z

Abstract: Influenza A virus NS1 protein is a multifunctional virulence factor consisting of an RNA binding domain (RBD), a short linker, an effector domain (ED), and a C-terminal 'tail'. Although poorly understood, NS1 multimerization may autoregulate its actions. While RBD dimerization seems functionally conserved, two possible apo ED dimers have been proposed (helix-helix and strand-strand). Here, we analyze all available RBD, ED, and full-length NS1 structures, including four novel crystal structures obtained using EDs from divergent human and avian viruses, as well as two forms of a monomeric ED mutant. The data reveal the helix-helix interface as the only strictly conserved ED homodimeric contact. Furthermore, a mutant NS1 unable to form the helix-helix dimer is compromised in its ability to bind dsRNA efficiently, implying that ED multimerization influences RBD activity. Our bioinformatical work also suggests that the helix-helix interface is variable and transient, thereby allowing two ED monomers to twist relative to one another and possibly separate. In this regard, we found a mAb that recognizes NS1 via a residue completely buried within the ED helix-helix interface, and which may help highlight potential different conformational populations of NS1 (putatively termed 'helix-closed' and 'helix-open') in virus-infected cells. 'Helix-closed' conformations appear to enhance dsRNA binding, and 'helix-open' conformations allow otherwise inaccessible interactions with host factors. Our data support a new model of NS1 regulation in which the RBD remains dimeric throughout infection, while the ED switches between several quaternary states in order to expand its functional space. Such a concept may be applicable to other small multifunctional proteins. Description: Work in St. Andrews was supported by the Medical Research Council, UK (RER and RJR), and the Scottish Funding Council (RJR).

2011-03-28T00:00:00Z Kerry, Philip S. Ayllon, Juan Taylor, Margaret A. Hass, Claudia Lewis, Andrew Garcia-Sastre, Adolfo Randall, Richard E. Hale, Benjamin G. Russell, Rupert J. Influenza A virus NS1 protein is a multifunctional virulence factor consisting of an RNA binding domain (RBD), a short linker, an effector domain (ED), and a C-terminal 'tail'. Although poorly understood, NS1 multimerization may autoregulate its actions. While RBD dimerization seems functionally conserved, two possible apo ED dimers have been proposed (helix-helix and strand-strand). Here, we analyze all available RBD, ED, and full-length NS1 structures, including four novel crystal structures obtained using EDs from divergent human and avian viruses, as well as two forms of a monomeric ED mutant. The data reveal the helix-helix interface as the only strictly conserved ED homodimeric contact. Furthermore, a mutant NS1 unable to form the helix-helix dimer is compromised in its ability to bind dsRNA efficiently, implying that ED multimerization influences RBD activity. Our bioinformatical work also suggests that the helix-helix interface is variable and transient, thereby allowing two ED monomers to twist relative to one another and possibly separate. In this regard, we found a mAb that recognizes NS1 via a residue completely buried within the ED helix-helix interface, and which may help highlight potential different conformational populations of NS1 (putatively termed 'helix-closed' and 'helix-open') in virus-infected cells. 'Helix-closed' conformations appear to enhance dsRNA binding, and 'helix-open' conformations allow otherwise inaccessible interactions with host factors. Our data support a new model of NS1 regulation in which the RBD remains dimeric throughout infection, while the ED switches between several quaternary states in order to expand its functional space. Such a concept may be applicable to other small multifunctional proteins. Bhaseen, M. J. Mayoh, J. Simons, B. D. Keeling, J. http://hdl.handle.net/10023/2172 2013-05-12T04:09:36Z 2012-01-17T00:00:00Z

Abstract: Motivated by experiments observing self-organization of cold atoms in optical cavities we investigate the collective dynamics of the associated non-equilibrium Dicke model. The model displays a rich semiclassical phase diagram of long time attractors including distinct superradiant fixed points, bistable and multistable coexistence phases and regimes of persistent oscillations. We explore the intrinsic timescales for reaching these asymptotic states and discuss the implications for finite duration experiments. On the basis of a semiclassical analysis of the effective Dicke model we find that sweep measurements over 200ms may be required in order to access the asymptotic regime. We briefly comment on the corrections that may arise due to quantum fluctuations and states outside of the effective two-level Dicke model description.

2012-01-17T00:00:00Z Bhaseen, M. J. Mayoh, J. Simons, B. D. Keeling, J. Motivated by experiments observing self-organization of cold atoms in optical cavities we investigate the collective dynamics of the associated non-equilibrium Dicke model. The model displays a rich semiclassical phase diagram of long time attractors including distinct superradiant fixed points, bistable and multistable coexistence phases and regimes of persistent oscillations. We explore the intrinsic timescales for reaching these asymptotic states and discuss the implications for finite duration experiments. On the basis of a semiclassical analysis of the effective Dicke model we find that sweep measurements over 200ms may be required in order to access the asymptotic regime. We briefly comment on the corrections that may arise due to quantum fluctuations and states outside of the effective two-level Dicke model description. Hutton, Richard D. Craggs, Timothy D. White, Malcolm F Penedo, J. Carlos http://hdl.handle.net/10023/2089 2013-05-12T03:03:51Z 2010-03-01T00:00:00Z

Abstract: XPF is a structure-specific endonuclease that preferentially cleaves 3' DNA flaps during a variety of repair processes. The crystal structure of a crenarchaeal XPF protein bound to a DNA duplex yielded insights into how XPF might recognise branched DNA structures, and recent kinetic data have demonstrated that the sliding clamp PCNA acts as an essential cofactor, possibly by allowing XPF to distort the DNA structure into a proper conformation for efficient cleavage to occur. Here, we investigate the solution structure of the 3'-flap substrate bound to XPF in the presence and absence of PCNA using intramolecular Forster resonance energy transfer (FRET). We demonstrate that recognition of the flap substrate by XPF involves major conformational changes of the DNA, including a 90 degrees kink of the DNA duplex and organization of the single-stranded flap. In the presence of PCNA, there is a further substantial reorganization of the flap substrate bound to XPF, providing a structural basis for the observation that PCNA has an essential catalytic role in this system. The wider implications of these observations for the plethora of PCNA-dependent enzymes are discussed.

2010-03-01T00:00:00Z Hutton, Richard D. Craggs, Timothy D. White, Malcolm F Penedo, J. Carlos XPF is a structure-specific endonuclease that preferentially cleaves 3' DNA flaps during a variety of repair processes. The crystal structure of a crenarchaeal XPF protein bound to a DNA duplex yielded insights into how XPF might recognise branched DNA structures, and recent kinetic data have demonstrated that the sliding clamp PCNA acts as an essential cofactor, possibly by allowing XPF to distort the DNA structure into a proper conformation for efficient cleavage to occur. Here, we investigate the solution structure of the 3'-flap substrate bound to XPF in the presence and absence of PCNA using intramolecular Forster resonance energy transfer (FRET). We demonstrate that recognition of the flap substrate by XPF involves major conformational changes of the DNA, including a 90 degrees kink of the DNA duplex and organization of the single-stranded flap. In the presence of PCNA, there is a further substantial reorganization of the flap substrate bound to XPF, providing a structural basis for the observation that PCNA has an essential catalytic role in this system. The wider implications of these observations for the plethora of PCNA-dependent enzymes are discussed. Torres, Maria Leilani Antkowiak, Maciej Cizmarova, Hana Ferrier, David Ellard Keith Dholakia, Kishan Gunn-Moore, Frank J http://hdl.handle.net/10023/2007 2013-05-12T04:04:48Z 2011-06-01T00:00:00Z

Abstract: We demonstrate a system for the combined optical injection and trapping of developing embryos. A Ti:sapphire femtosecond laser in tandem with a spatial light modulator, is used to perform fast and accurate beam-steering and multiplexing. We show successful intracellular delivery of a range of impermeable molecules into individual blastomeres of the annelid Pomatoceros lamarckii embryo by optoinjection, even when the embryo is still enclosed in a chorion. We also demonstrate the ability of the femtosecond laser optoinjection to deliver materials into inner layers of cells in a well-developed embryo. By switching to the continuous wave mode of the Ti:sapphire laser, the same system can be employed to optically trap and orient the 60 μm sized P. lamarckii embryo whilst maintaining its viability. Hence, a complete all-optical manipulation platform is demonstrated paving the way towards single-cell genetic modification and cell lineage mapping in emerging developmental biology model species.

2011-06-01T00:00:00Z Torres, Maria Leilani Antkowiak, Maciej Cizmarova, Hana Ferrier, David Ellard Keith Dholakia, Kishan Gunn-Moore, Frank J We demonstrate a system for the combined optical injection and trapping of developing embryos. A Ti:sapphire femtosecond laser in tandem with a spatial light modulator, is used to perform fast and accurate beam-steering and multiplexing. We show successful intracellular delivery of a range of impermeable molecules into individual blastomeres of the annelid Pomatoceros lamarckii embryo by optoinjection, even when the embryo is still enclosed in a chorion. We also demonstrate the ability of the femtosecond laser optoinjection to deliver materials into inner layers of cells in a well-developed embryo. By switching to the continuous wave mode of the Ti:sapphire laser, the same system can be employed to optically trap and orient the 60 μm sized P. lamarckii embryo whilst maintaining its viability. Hence, a complete all-optical manipulation platform is demonstrated paving the way towards single-cell genetic modification and cell lineage mapping in emerging developmental biology model species. Mazilu, M. Baumgartl, J. Kosmeier, S. Dholakia, K. http://hdl.handle.net/10023/1805 2013-05-12T04:03:53Z 2011-01-17T00:00:00Z

Abstract: We report a mathematically rigorous technique which facilitates the optimization of various optical properties of electromagnetic fields in free space and including scattering interactions. The technique exploits the linearity of electromagnetic fields along with the quadratic nature of the intensity to define specific Optical Eigenmodes (OEi) that are pertinent to the interaction considered. Key applications include the optimization of the size of a focused spot, the transmission through sub-wavelength apertures, and of the optical force acting on microparticles. We verify experimentally the OEi approach by minimising the size of a focused optical field using a superposition of Bessel beams. (C) 2011 Optical Society of America

2011-01-17T00:00:00Z Mazilu, M. Baumgartl, J. Kosmeier, S. Dholakia, K. We report a mathematically rigorous technique which facilitates the optimization of various optical properties of electromagnetic fields in free space and including scattering interactions. The technique exploits the linearity of electromagnetic fields along with the quadratic nature of the intensity to define specific Optical Eigenmodes (OEi) that are pertinent to the interaction considered. Key applications include the optimization of the size of a focused spot, the transmission through sub-wavelength apertures, and of the optical force acting on microparticles. We verify experimentally the OEi approach by minimising the size of a focused optical field using a superposition of Bessel beams. (C) 2011 Optical Society of America Pollacco, D L Skillen, I Cameron, Andrew Collier Christian, D J Hellier, C Irwin, J Lister, T A Street, R A West, R G Anderson, D Clarkson, W I Deeg, H Enoch, B Evans, A Fitzsimmons, A Haswell, C A Hodgkin, S Horne, Keith Douglas Kane, S R Keenan, F P Maxted, P F L Norton, A J Osborne, J Parley, N R Ryans, R S I Smalley, B Wheatley, P J Wilson, D M http://hdl.handle.net/10023/1617 2013-05-19T00:31:51Z 2006-10-01T00:00:00Z

Abstract: The SuperWASP cameras are wide- field imaging systems at the Observatorio del Roque de los Muchachos on the island of La Palma in the Canary Islands, and at the Sutherland Station of the South African Astronomical Observatory. Each instrument has a field of view of some 482 deg(2) with an angular scale of 13".7 pixel(-1), and is capable of delivering photometry with accuracy better than 1% for objects having. V similar to 7.0 -11.5. Lower quality data for objects brighter than are stored in the project archive. The systems, while designed to monitor fields with high cadence, are capable of surveying the entire visible sky every 40 minutes. Depending on the observational strategy, the data rate can be up to 100 Gbytes per night. We have produced a robust, largely automatic reduction pipeline and advanced archive, which are used to serve the data products to the consortium members. The main science aim of these systems is to search for bright transiting exoplanet systems suitable for spectroscopic follow- up observations. The first 6 month season of SuperWASP-North observations produced light curves of similar to 6.7 million objects with 12.9 billion data points.

2006-10-01T00:00:00Z Pollacco, D L Skillen, I Cameron, Andrew Collier Christian, D J Hellier, C Irwin, J Lister, T A Street, R A West, R G Anderson, D Clarkson, W I Deeg, H Enoch, B Evans, A Fitzsimmons, A Haswell, C A Hodgkin, S Horne, Keith Douglas Kane, S R Keenan, F P Maxted, P F L Norton, A J Osborne, J Parley, N R Ryans, R S I Smalley, B Wheatley, P J Wilson, D M The SuperWASP cameras are wide- field imaging systems at the Observatorio del Roque de los Muchachos on the island of La Palma in the Canary Islands, and at the Sutherland Station of the South African Astronomical Observatory. Each instrument has a field of view of some 482 deg(2) with an angular scale of 13".7 pixel(-1), and is capable of delivering photometry with accuracy better than 1% for objects having. V similar to 7.0 -11.5. Lower quality data for objects brighter than are stored in the project archive. The systems, while designed to monitor fields with high cadence, are capable of surveying the entire visible sky every 40 minutes. Depending on the observational strategy, the data rate can be up to 100 Gbytes per night. We have produced a robust, largely automatic reduction pipeline and advanced archive, which are used to serve the data products to the consortium members. The main science aim of these systems is to search for bright transiting exoplanet systems suitable for spectroscopic follow- up observations. The first 6 month season of SuperWASP-North observations produced light curves of similar to 6.7 million objects with 12.9 billion data points. Hunter, Robert Iain Robertson, Duncan Alexander Goy, P Smith, Graham Murray http://hdl.handle.net/10023/1595 2013-05-12T01:35:03Z 2007-05-01T00:00:00Z

Abstract: Faraday rotators using permanently magnetized ferrite materials are used to make quasi-optical isolators and circulators at millimeter wave and sub-millimeter wave frequencies that have far higher performance than their waveguide equivalents. This paper demonstrates state-of-the-art performance for four-port quasi-optical circulators with 60-dB isolation, 0.2-dB insertion loss, and better than 80-dB return loss for devices centered at 94 GHz. A method is presented for the accurate characterization of the complex permeability and permittivity of permanently magnetized ferrites via a series of frequency and polarization dependent transmission and reflection measurements. The dielectric and magnetic parameters for the sample are determined by fitting theoretical curves to the measured data. These fitted parameters are then used in a model for a complete quasi-optical Faraday rotator, including matching layers, allowing the accurate design and fabrication of these devices for any specific operational frequency band in the millimeter wave and sub-millimeter wave regime. Examples are given showing typical results and demonstrating how temperature cycling can significantly improve the temperature stability of these devices, while allowing fine tuning of the center frequency. We also indicate the performance possible at higher frequencies to above 1 THz and outline performance of truly planar isolators where lossy polarizer material is built into the Faraday rotator matching structure.

2007-05-01T00:00:00Z Hunter, Robert Iain Robertson, Duncan Alexander Goy, P Smith, Graham Murray Faraday rotators using permanently magnetized ferrite materials are used to make quasi-optical isolators and circulators at millimeter wave and sub-millimeter wave frequencies that have far higher performance than their waveguide equivalents. This paper demonstrates state-of-the-art performance for four-port quasi-optical circulators with 60-dB isolation, 0.2-dB insertion loss, and better than 80-dB return loss for devices centered at 94 GHz. A method is presented for the accurate characterization of the complex permeability and permittivity of permanently magnetized ferrites via a series of frequency and polarization dependent transmission and reflection measurements. The dielectric and magnetic parameters for the sample are determined by fitting theoretical curves to the measured data. These fitted parameters are then used in a model for a complete quasi-optical Faraday rotator, including matching layers, allowing the accurate design and fabrication of these devices for any specific operational frequency band in the millimeter wave and sub-millimeter wave regime. Examples are given showing typical results and demonstrating how temperature cycling can significantly improve the temperature stability of these devices, while allowing fine tuning of the center frequency. We also indicate the performance possible at higher frequencies to above 1 THz and outline performance of truly planar isolators where lossy polarizer material is built into the Faraday rotator matching structure. Wu, L Mazilu, M Karle, T Krauss, Thomas Fraser http://hdl.handle.net/10023/1593 2013-05-12T00:32:47Z 2002-07-01T00:00:00Z

Abstract: We utilize the anomalous dispersion of planar photonic crystals near the dielectric band edge to control the wavelength-dependent propagation of light. We typically observe an angular swing of up to 10degrees as the input wavelength is changed from 1290 urn to 1310 mn, which signifies an angular dispersion of 0.5 degree/nm. Such a strong angular dispersion is of the order required for wavelength-division multiplexing systems. This is the first demonstration of the "superprism" effect in a planar configuration with a small lattice.

2002-07-01T00:00:00Z Wu, L Mazilu, M Karle, T Krauss, Thomas Fraser We utilize the anomalous dispersion of planar photonic crystals near the dielectric band edge to control the wavelength-dependent propagation of light. We typically observe an angular swing of up to 10degrees as the input wavelength is changed from 1290 urn to 1310 mn, which signifies an angular dispersion of 0.5 degree/nm. Such a strong angular dispersion is of the order required for wavelength-division multiplexing systems. This is the first demonstration of the "superprism" effect in a planar configuration with a small lattice.