Microresonators for organic semiconductor and fluidic lasers
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This thesis describes a number of studies of microstructured optical resonators, designed with the aim of enhancing the performance of organic semiconductor lasers and exploring potential applications. The methodology involves the micro-engineering of the photonic environment in order to modify the pathways of the emitted light and control the feedback mechanism. The research focuses on designing new organic microstructures using established semi-analytical and numerical methods, developing fabrication techniques using electron-beam lithography, and optically characterising the resulting structures. Control of the feedback mechanism in conjugated polymer lasers is first investigated by studying Distributed Feedback or photonic crystal resonators based on a square feedback lattice. This study identified the diffraction to free space radiation as a major source of loss in current microstructured resonator designs. By cancelling the coupling to free space through the use of different feedback symmetries and diffraction orders, a threshold reduction by almost an order of magnitude is demonstrated. The introduction of mid-gap defect photonic states in an otherwise uniformly periodic structure was studied in Distributed Bragg Reflector (DBR) resonators. This enabled GaN diode pumped polymer lasers to be demonstrated, indicating that the transition from complex excitation sources to more compact systems is possible. Devices for potential applications in the field of optical communications are also explored by demonstrating a polymer DBR laser based on silicon. In this way, the potential for integrating conjugated polymers with silicon photonics is confirmed. Photonic crystal fibres, which have a periodic microstructure in the transverse direction, are explored as an alternative means for controlling the optical properties of organic lasers. Fluidic fibre organic lasers were demonstrated as efficient sources with good spectral purity. In these devices, mechanisms to tune the emission wavelength were explored and the origin of the frequency selection mechanism was investigated.
Thesis, PhD Doctor of Philosophy
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