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Despite their low cost, conventional rear projection displays are bulky, and so not as attractive to consumers as other thin-panel display devices. Rear projection displays can, however, be made as thin as most flat panel displays if the image is projected via a wedge-shaped waveguide panel. Image overlapping has been found to be a major issue with conventional wedge waveguide design, but it can be removed with the insertion of a curved waveguide called the transition waveguide. This thesis reports several analysis techniques and methods for designing the transition waveguide for a wedge display panel. A simple simulation model and a parabolic prototype wedge panel were first built to demonstrate the effects of the insertion of the transition waveguide. The length and the curvature of the transition region curve were found to be the two factors that most influenced the linearity of the panel. Using various techniques of curve analysis, the curvature of the transition region was proved to be the determining factor out of the two and it was also shown that a short transition waveguide is more desirable than a long one, except when the structure of the wedge waveguide is pre-determined. A concept for generating the wedge surface profile iteratively is proposed in this thesis as a way to design the ideal transition waveguide. A prototype panel with a very short transition waveguide was built using an iterative algorithm developed by the author to prove the validity of the concept.

Scientific progress in the terahertz (0.1-10 THz) region of the electromagnetic spectrum has been impeded by a lack of coherent sources and detectors. Recent developments have seen the emergence of novel optoelectronic devices, which have generated considerable interest. One such approach is the coherent emission and detection of continuous-wave (cw) THz radiation by mixing two cw visible or near-infrared lasers in a semiconductor. The photomixer efficiency is dependent upon the carrier dynamics of the semiconductor material, in addition to the electronic properties of the antenna and electrodes. This thesis describes work undertaken to develop and characterise these components. Low-temperature-grown (LT)-GaAs is used for the photoconductor, with the necessary ultrashort (100 fs) carrier lifetimes achieved by a new approach to the post-growth annealing. This success is explained using a novel semi-quantitative model to describe the trapping of charge carriers by defects in the material. The photomixer and antenna designs are independently optimised using both finite element simulations and experimental characterisation at frequencies up to 2 THz. Detailed measurements of the polarisation state of the THz radiation allow a better understanding of the role of the photomixer and antenna at different frequencies. Antenna theory and equivalent electrical circuits are applied to assess the impedance match for optimum power transfer to the antenna, and resonant designs are shown to improve both emission and detection, particularly when coupled using a choked feed. Simulations show that the observed power enhancement at resonance is associated with improved directionality of the far-field radiation pattern. THz radiation has many potential applications, including imaging. Most established imaging systems require a femtosecond pulsed laser, but its high bulk and cost might limit commercial market penetration. In contrast, tunable cw diode lasers are small and inexpensive, and cw-imaging systems might help to generate large volume commercial THz products. For the first time, technology based on diode lasers is combined with a phase-sensitive, room-temperature homodyne detection scheme to demonstrate a system that is compact, robust, genuinely turn-key and of low cost. The unsurpassed performance of the present system, enabled by the innovations presented here, is illustrated with a variety of application examples.

The research outlined in this dissertation investigates and improves the optimum condition of the electro-optic characteristics for liquid crystal modulated photoluminescent liquid crystal displays (PLLCDs). In a comparison of TN and STN-PLLCD, the 1^st minimum condition of normally black (NB)-TN-PLLCD gives a greater integrated contrast ratio (ICR) over all collimation angles, compared to that of the 2^nd minimum NB-TN-PLLCD. The significant drop in the ICR with collimation angles for the 2^nd minimum NB-TN-PLLCD would be of no use in PLLCD applications. It was also found to be better than an optimised STN-PLLCD for every collimation angle for both 128 and 64 way multiplexed. In the case of STN-PLLCDs, both the double layer compensated STN (DSTN) and the film compensated STN (FSTN)-PLLCDs, displayed good integrated contrast ratios (ICRs) for low divergence angles (less than 10°). However, an FSTN is preferable when taking into account the cost and weight of the LC shutter. For use in a tiled display, the asymmetric response of an TN LC cell, with respect to the polar angle of incidence, results in a difference in the grey level transmission for the up and down directions, as seen by the viewer. This means that the TN effect is not suitable for use in this architecture. The symmetric response of the normally white (NW)-OCB-PLLCD was seen to be suitable for uniform image for tiled non-telecentric PLLCD displays. However, in tiled displays the collimation angle for off-normal centred beams (11.6°) should have a half-cone angle of less than 10° to achieve a 100:1 ICR.

No description available.

This thesis focuses on the nature and role of interfacial states formed at the heterojunctions between polymer donor and polymer acceptor systems. Two classes of organic semiconductors are investigated: (1) conjugated polymers, poly (9,9-dioctylfluorene-co-bis-N,N-(4-butylphenyl)-bis-N,N-phenyl-1,4-phenylenediamine) (PFB) with poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT), and poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine) (TFB) with F8BT, and (2) helical supra-molecular assemblies of copolymers consisting of the Pt-porphyrin and perylene chromophores. First, a molecular model is developed to describe such interfacial excited electronic excitations. It is found that for stable ground-state geometries, the excited state has a strong charge-transfer character. Furthermore, when partly covalent, modelled radiative lifetimes (~10^-7s) and off-chain axis polarisation (30°) matched observed ‘exciplex’ emission. Additionally for the PFB:F8BT blend, geometries with fully ionic character are also found thus accounting for the low electroluminescence efficiency of this system. Second, the interchain and intrachain interactions are disentangled by systematically varying the interchain distance and extent of planarisation – two primary parameters that change with applied pressure. It is shown that the photophysical properties of these interfacial excited states are determined by the tradeoff between the electronic couplings and the energy mismatch between the charge-transfer and locally excited excitations upon reducing the interchain distance. In addition, increasing planarisation yields a decreased energy mismatch and a larger mixing from the locally excited excitations. Finally, a well-defined, close-packing multi-chromophoric system is investigated using a variety of optical spectroscopy techniques. The photophysical processes taking place in the system are found to strongly depend on which component of the copolymer is excited.

This thesis describes efforts to achieve an idealized architecture and to characterize the transport in polymer-based PV devices, by employing novel nanoimprint techniques. First, a novel double-imprinting process is described, which allows the fabrication of ideally structured “polymer-polymer” and “polymer-small molecule” heterojunctions, with any composition. The dimensions of both phases can be independently tailored to match the respective exciton diffusion length in either phase PV devices with extremely high densities (up to 10¹⁴/mm²) of interpenetrating nanoscale columnar features, as small as 25 nm in the active polymer blend layer, were fabricated and showed considerable improvement over the traditional blend devices. It is believed that this work advances the state of the art in polymeric organic electronic devices. Second, a non-conventional nanopatterning technique has been developed and used to fabricate well-aligned vertical ZnO nanowire arrays. This demonstrates the potential for this approach to serve as a nanostructured metal oxide scaffold for “polymer-metal oxide” hybrid PVs, as well as other nanoscaled (opto)electronic devices, due to its attractive electromechanical properties.

This thesis concentrates on the interaction between SAWs (surface acoustic wave) and low-dimensional systems studied using optical techniques. In particular SAW-driven luminescence from a lateral <i>p-n</i> junction is demonstrated. The lateral <i>p-n</i> junction is formed by molecular beam epitaxy regrowth on a patterned GaAs substrate. Silicon is used as an amphoteric dopant to create a high mobility two-dimensional electron gas on flat (100) planes and a two-dimensional hole gas on angled (311)A facets. A lateral <i>p-n</i> junction is formed at the interface between these planes. SAWs with a frequency of ~1 GHz are generated using an interdigitated transducer. When a continuous radio frequency (RF) signal is used to excite the transducer, SAW-driven light emission from the <i>p-n</i> junction is demonstrated by peaks in the current/light emission at the resonant frequency of the transducer. To investigate the nature of the luminescence further, short RF pulses are used to drive the transducer. The short pulses temporally isolate the SAW-driven light emission from any emission due unwanted pick-up of the free space electromagnetic wave. In the final section the modulation of the emission energies of a single self-assembled quantum dot by a SAW is investigated. The compression and expansion of the crystal due to the strain wave causes the energy of the dot lines to oscillate around their equilibrium values. The shape of the SAW broadened emission lines was seen to depend on the nature of the transition in the dot offering an alternative way of identifying charged and neutral species in this sample. The modulation of the dot energy levels by the SAW is used to control the time of photon emission from the system.

Significant advances have been made in the past five years in realizing photon sources based on various kinds of emitters in the 500-1000 nm spectral region. However, reliable sources emitting at wavelengths compatible with standard telecommunication optical fibre are still lacking. Self-assembled quantum dots offer a way to generate wavelength-tunable photons. They offer also relatively high radiative efficiencies, short spontaneous emission lifetimes (~1 ns) and, importantly, can easily be incorporated into compact semiconductor devices fabricated with established technologies. In this thesis, single photon sources based on InAs/GaAs quantum dots were studied and developed for applications at the technologically important wavelength around 1.3 <i>μ</i>m compatible with standard telecommunication optical fibre. A single photon source suitable for use in a quantum key distribution system was demonstrated by optically pumping an InAs/GaAs quantum dot incorporated inside a micropillar optical cavity (chapter 4). Single photon emission driven by short electrical pulses was then realised for the first time at λ ~ 1.3 <i>μ</i>m using an InAs/GaAs quantum dot embedded in a planar optical microcavity (chapter 5). Electrically driven single photon emission was also achieved for quantum dot devices based on the micropillar geometry, demonstrating the viability of such compact cavities with small optical mode volumes at both λ ~ 900 nm and 1.3 <i>μ</i>m (chapters 6 and 7). The realization of these devices surmounted the difficulty of contacting the small cross-sectional diameters (~ 2 – 2.5 <i>μ</i>m) of the micropillars. Positioning quantum dots accurately inside optical microcavities can improve the efficiency of single photon sources. Chapter 3 presents results of a study of semiconductor wafers with intentionally positioned quantum dots, demonstrating the viability of a technique for placing quantum dots reproducibly.

This thesis investigates methods of electrically tuning the photonic band gap (PBG) in chiral nematic liquid crystals (N*LC). The electrical control of the PBG is highly desirable for practical applications, however, hitherto wavelength tuning has been limited due to the destruction of the photonic band structure (PBS) by the presence of electrically-induced out-of-plane switching and electro-hydro-dynamic instabilities. Primarily, this thesis aims to find methods for stable tuning over a broadband wavelength range with a short response time upon the application of an electric field. Using electrically commanded surface switching, a stable blue-shift of the wavelength of the band-gap up to ~23 nm is demonstrated for a N*LC with no change in the quality of the PBG. This is achieved using ferroelectric liquid crystal (FLC) surface layers which rotate in the plane of the device during the application of an electric field which results in a macroscopic contraction of the helix of the N*LC. The tuning range is found to increase considerably (&gt; 100 nm) by doping the FLC compound into the bulk of the N*LC as electrically active dopants. In addition, by dispersing the FLC into the bulk it is shown that, under certain conditions, multiple band-gaps are observed. To improve the response time of the switching of the PBG, a hybrid PBS is investigated using a polymer template and an achiral liquid crystal. Due to the absence of electrical instabilities by using an achiral LC in a rigid periodic polymer template, a faster response (~43 μs) with a broad photonic band width (<i>Δλ ~</i>135 nm) and a broadband PBG shift (over 100 nm) is demonstrated. Furthermore, a PBS structure, which simultaneously reflects red-green-blue light, is fabricated using a single achiral LC.