Resonant injection of sub-picosecond optical pulses were explored as an ultra-fast analogue to injection locking, to influence the phase of the semiconductor laser on short time-scales. This allowed for efficiently altering the absolute phase of the lasing modes (without exciting new modes to lase) and produced mode interference beating; both demonstrated in vertical cavity surface emitting lasers (VCSELs). The ultra-fast optical sampling was extended to resolve polarization degrees of freedom, showing the nonlinear coupling between transverse modes through the charge carriers. Phase-sensitive double-pulse injection, commonly used for coherent control, was applied to VCSELs to introduce a field component into a specifically selected subset of the transverse lasing modes. The self-organization between transverse modes in VCSELs was investigated with resonant pulse injection and optical sampling. At different bias currents, mode-locking was observed to produce both a train of 2 ps pulses with an 11 ps repetition period, and self-organization of the laser mode resonances to tones within the Fibonacci sequence (with a 19 GHz fundamental tone). It was demonstrated that the nonlinear optical sampling technique could be used to measure the coherence length and type of fluctuations within the VCSEL. While the pulsed mode-locking showed the expected shot-noise fluctuations from spontaneous emission, the Fibonacci-type mode-locking was more stable and showed popcorn noise. The emergence of the Fibonacci-type mode-locking was explained by a spatio-temporal theory of the symmetry-breaking interaction between nearly degenerate modes and the carrier density. Edge-emitting semiconductor laser structures, with and without an optical grating, were also investigated. Resonant and non-resonant optical pulses were used to free laser light from a trapped defect state within an optical grating, and create carrier-heating relaxation oscillations. These effects were reproduced with a spatio-temporal model. Double pulse injection was used to measure the separate effects of group-velocity dispersion and gain curvature on pulse propagation within a Fabry-Pérot laser.

The use of an A1N interlayer to allow the growth of crack-free AlGaN on a GaN template is investigated. The impact of using an A1N interlayer on the active region of a device is examined by investigating a series of GaN/AlGaN quantum well structures. It is observed by WBDF that the a-type dislocations generated at the A1N interlayer form ‘staircase’ structures in the quantum well stack where all dislocation segments are in edge orientation A model is proposed where misfit dislocation segments at each well interface are formed by climb, in contrast to the dislocation glide at the lowest interface that might be expected in other systems. Both HAADF imaging and conventional TEM are used to characterise a series of Al-GaN/GaN quantum cascade laser (QCL) structures. These devices have great potential in the field of fibre optic communication. Changes in the layer spacing near V-defects, and the effects that changing substrate from sapphire to bulk GaN have on the mechanisms for relaxation in this system are identified and discussed.

It is believed that the Silicon Germanium (SiGe) materials system offers numerous benefits over GaAs/AlGaAs for operation within the THz gap - absorption coefficients are lower due to the non-polar nature of the SiGe lattice and the potential for integration with Si chips exists. Furthermore, operating within the valence band allows surface normal emission to be observed and vertical cavity surface emitting lasers to be fabricated using transitions between light (LH) and heavy (HH) hole subbands and poly-Si/silicon dioxide Bragg reflectors. This dissertation reports upon recent advances made in FIR Quantum Cascade Emitters (QCEs) based within the SiGe materials system. Initial measurements were designed purely to demonstrate the ability of the vertical intersubband transition to absorb radiation. Such structures were also observed to emit and spectroscopy was performed allowing the origin of such emission to be verified as being due to intersubband transitions. QCEs were then designed and processed, allowing the observation of the first surface-normal emission from a QCE in the absence of a grating to be observed. Further designs demonstrated the primary dependence upon the strain within the quantum wells of the energy of the LH1-HH1 transition. The scalability of the active regions has also been demonstrated - the strain symmeterised growth allowing hundreds of layers to be grown at a uniformly high standard. A shift from vertical (intrawell) to diagonal (interwell) transitions using photon assisted tunnelling lead to the theoretical observation of population inversion within the system. One of the key requirements for lasing, the existence of population inversion demonstrates both the potential and feasibility for a QCL to be fabricated in SiGe.

Three-dimensional displays provide a sensation of physical depth and have demonstrated their functional effectiveness in several task or leisure-oriented applications. However, their relatively poor performance and high cost compared to present two-dimensional displays largely reduce users' interest to go for this further dimension in image presentation. The main theme of this research is to upgrade the quality of 3D video image to the state-of-the-art 2D image standard while keeping the cost low by a better embodiment of existing 3D architectures. The multi-projector autostereoscopic display has been chosen for further development due to the flexibility of field-of-view and screen size to be improved with available technologies. The problem of dark areas in the viewing space has firstly been solved by a novel projection lens array with the lenses' exit pupils abutting. Further improvements have been made by introducing planarisation techniques and unconventional imaging optics. The proposed configuration is comprised of a waveguide screen with a one-dimensional liquid crystal based switchable element array and a projection optical system featuring a circularly symmetric field lens, a diffuser and collimated image sources. The device has a thickness equal to the height of only one line and ensures a field-of-view up to 60<SUP>o</SUP>. The prototype of the optical system has been tested for image quality, which shows that good quality can be achieved by simple and low cost optics. A test on the cross-view effect has also demonstrated that a perfect autostereoscopic 3D image can be rendered with careful alignment. On the other hand, the analysis and test of liquid crystal grating cells have shown that this device will be adequate as the switchable element embedded in a waveguide screen to perform line-multiplexing and light deflection for 3D applications, but modification is required if efficiency and contrast ratio are to be improved.

No description available.

This thesis focuses on experiments involving coupled cavity THz QCLs. These take the form of either monolithic devices with etched gaps to form two cavities on the same device, or separate devices aligned under vacuum and at cryogenic temperatures using a specially built piezo-electric micropositioner system. After an introductory chapter briefly reviewing the current state of QCL research, processing techniques and other standard experimental techniques, the thesis begins with the presentation of some experiments dealing with the effects of optical feedback between the two cavities. This leads on to a theoretical chapter in which a mathematical model of a double section QCL is presented, along with some explanation of some unexpected results seen experimentally. The final chapter returns to experimental work. Findings from the previous chapters are used to attempt to develop methods for the measurement of gain and loss in QCLs using coupled cavities, something for which there is currently no reliable method at THz frequencies and which involves considerable experimental difficulties. The thesis concludes with a discussion of possible future work including an extension of the mathematical model, and other possibilities to improve the gain measurement method.

Mid-infrared quantum cascade lasers (QCLs) have been extensively developed since their realisation in 1994, with a spectral range covered from 3.4<I>μ</I>m (88THz) to 24<I>μ</I>m (12.5THz). This is a direct result of advances in molecular beam epitaxy and band-structure engineering. QCLs are fabricated from multi-quantum well semiconductor heterostructures in which an appropriate engineering of the thickness and composition of the semiconductor layers adjusts the intersubband transition energies, offering considerable design flexibility of the band profile. By application of a suitable electric field and stacking together successive active regions, each injected electron cascades through the device, generating a number of photons. QCLs have shown considerable advances in performance with high powers and room temperature operation demonstrated. Extension of this quantum cascade scheme to the far-infrared, or terahertz (THz) range, is now being investigated, where the lack of sources remains acute. Specifically, operation is sought at energies smaller than the characteristic LO phonon energy of the semiconductor material, where currently no lasing has been shown (<36meV, 9 THz). The dynamics of this spectral range, however, are considerably different to those in the mid-infrared. LO phonon emission is effectively forbidden for subband spacings less than the phonon energy but increases in electron-electron scattering are expected to dominate. Although THz electroluminescence has been shown from cascade structures, systematic investigations into key parameters have not been reported. This dissertation reports a comprehensive study of THz electroluminescence from n-type A1GaAs/GaAs quantum cascade emitters as a basis for understanding the radiative and scattering mechanisms that occur in this spectral range, forming the foundations of the development of a THz semiconductor laser. The electroluminescence was correlated thoroughly with band structure calculations, along with the structural and electrical properties of the samples. The many features observed in the far-infrared were characterised, with the intersubband peaks investigated extensively to confirm their origin.

No description available.

The dissertation is an account of the author's research into the analysis, design, and implementations of knowledge-based (expert) systems for applications in the field of scanning electron microscopy. The specific tasks of interest have been fault diagnosis and instrument control. This research represents the first utilisation of knowledge-based techniques within electron microscopy. The dissertation begins by providing background material on the scanning electron microscope and surveying the expert systems literature with regard to critical areas of system design. The target applications and the associated research objectives are next summarised. A set of novel algorithms for improving the microscope by enhanced automatic control of fundamental instrument parameters, namely tungsten filament saturation and alignment, are presented, and the area of automatic focus algorithms is reinvestigated. Research into the design and implementation of an expert system for microscope fault diagnosis, including mechanisms for interfacing the expert system to the Internet, is discussed. This is followed by a detailed presentation of the development of a knowledge-based approach to instrument control. The next segment of this dissertation concentrates upon theoretical issues arising from the preceding work. A proposal is made to classify network-aware expert systems, and the impact of the Internet and the World Wide Web upon such systems is discussed within this context. An analysis of the work is performed to indicate how the practical portions of the research may be generalised to other tasks within scanning electron microscopy and outwith to other scientific instruments.

Fibre Bragg gratings (FBG) are periodic changes of the refractive index in the core of optical fibres and are written to filter specific wavelengths from a broadband spectrum to be re-routed to another area of an optical network. The filtered wavelengths, or channels, act as a means for carrying differentiated information down the optical fibre. To maximise the amount of information that can be transmitted through the optical fibre, channels may be as close as 100 GHz, or 0.2 nm at l = 1550 nm. However, temperature fluctuations experienced by FBGs cause the mean filtered wavelength to shift as much as 1.5 nm. Either an increase in channel spacing is required, which will reduce the network capacity, or the temperature-dependent shift in Bragg wavelength must be eliminated. Compressing the grating as the temperature increases can reduce the shift in Bragg wavelength. This can be done by mounting the FBG on a platform with the appropriate negative thermal expansion. This research presents a device to reduce the temperature-dependent shift. The dimensional changes of the device were studied both theoretically and experimentally with predictions giving good agreement with experimental results. It is also shown that the tolerances are improved six-fold over current devices. It gives the smallest temperature-dependent shift in Bragg wavelength over the required temperature range that has been reported. These experiments show that the temperature-dependent shift in Bragg wavelength is non-linear and to further reduce the shift requires a platform with a non-linear coefficient of thermal expansion. No such device currently exists. It is shown that the structure presented can be modified to give the required non-liner strain. A theoretical analysis describes the effects of temperature on the dimensional changes and shows it can obtain the required non-linearity with the required overall thermal expansion with suitable modifications. Experimental results show the structure non-linearly compensates for the temperature-dependent shift in Bragg wavelength. However, further work is required to achieve manageable tolerances to completely thermally compensate a FBG.