This report describes an experimental study of the conduction and absorption mechanisms of Germanium in the temperature range 4.2 - 1.5 K. The results of these studies were mainly devoted to the developments of very far infrared detectors. Germanium (Ge) is a well-known semiconductor element used widely, when doped with small concentration of impurities, for detection of far infrared wavelengths up to 100~m. For doping concentrations less 16 3 than 1.0 x 10 atoms/em, the absorption of radiation in the range 100- 1000~m is very weak Because of the lack of the proper absorption mechanisms, except for some photo-hopping absorption in compensated samples around 1000llm.16 -3 In the range of doping between 1-8 x 10 cm ,there exists additional thermal activation energy not present in the lower concentrations. It was thought that this activation energy results from impurity interactions in this doping range, and hence a delocalized energy band is thus formed above the ground state level. However, the electrical conduction, the width of this band and its position, and the relevance of this band to the marked bolometric effect for 10o-lOOOWU wavelength detections are not yet clear. This thesis presents further study on this band together with its relation to the conduction and absorption mechanisms. Comparative studies were usually made for two samples of Ge differing in doping configuration, one of which does not have this additional activation energy (low concentration) . The firs two chapters give a review of the absorption and conduction mechanisms in Ge at low temperatures, and the performance relations and measurements for different types of infrared detectors. In this report, the conduction mechanism is studied for the two samples, and includes galvanometric properties, thermal properties and energy scattering processes for the carriers in the delocalized band. The absorption characteristics, 1n lOO-lOOO~ru range of the two samples were investigated. Germanium elements with absorbing surfaces are also studied using two different techniques, namely, surface ion implantation and metal film deposition. The mutual effects of the implanted surface and the bulk material are discussed and suggestions for the future of this technique are given. Finally, the design and performance of the constructed high sensitivity far infrared Ge detectors using the higher concentration sample are given. Theoretical noise limitations were reached in these detectors. Heasurements and practicaI. astronomical applications are also given.

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This thesis is concerned with a project to design and test a new Nanoparticle Spectrometer (NPS). The NPS is an instrument designed to make fast measurements of the size distribution and number concentration of aerosol samples containing particles in the size range 5–300nm. The intended application of the NPS is to take time dependant measurements of the aerosols emitted from internal combustion engines. The primary motivation for this work is ultimately the potentially detrimental effects on human health and the environment of combustion generated aerosols. In common with previous aerosol spectrometers, the Nanoparticle Spectrometer consists of a charger to give particles an electrostatic charge, a classifier, which separates the particles in an aerosol sample according to their electrical mobility (a function of size) and an array of counting devices that count the numbers of particles with different mobilities. The novelty of the NPS is the geometry of the instrument,which, it will be argued, has certain advantages. The behaviour of particles in the classifier has been modelled numerically and this model has been used to optimise the classifier geometry. Two charger designs were considered, and two analytical charger models developed and compared. The classifier model was combined with the selected charger model to create a simulation of the instrument operation, which predicts the NPS’ output signal for a given aerosol sample size distribution and number concentration. A prototype NPS was designed, built and tested experimentally. The objective of the experiments was to test the validity of the instrument model and compare the performance of the NPS to an established slow response particulate measuring instrument, the SMPS. The experiments showed good agreement between modelled and measured results, as well as close correlation between the NPS and the SMPS results across most of the instruments range. The experiments also revealed some areas in which the performance of the NPS could be improved; for instance, the modelling of diffusion in the classifier and of the fluid flow in the particle charger.

For over a decade now there has been much research into the use of superconductors in X-ray, gamma ray and other particle detectors. Detectors based on superconductor-insulator-superconductor(SIS) and superconductor-insulator-normal metal(SIN) tunnel junctions have been widely developed. To date, the predicted excellent energy resolving ability of such detectors has not been realised. Various energy loss processes have been suggested as possible causes for the failure to obtain energy resolutions close to the thermodynamic and quantum limits predicted. In my experiments, I have used both SIS and SIN tunnel junctions to investigate the transport of quasiparticles and phonons in structures similar to the proposed detector designs. I have used multiple distributed junction geometries to perform injection-detection type experiments. One junction is used to inject quasiparticles and/or phonons into the device structure, whilst the current-voltage characteristic of a second junction is monitored for a response to the injected quasiparticles/phonons. Using this type of experimental set-up, I have measured the transport of non-thermal equilibrium quasiparticles in an epitaxial niobium film. Using a simple random walk model, I have calculated an effective lifetime for quasiparticles. I have not observed the process of quasiparticle mulitiplication that has been observed by other researchers - I attribute this to differences in the microstructure of my devices and comment on the implications of this to possible quasiparticle loss mechanisms. I have investigated the energy transport in a device with a number of SIN tunnel junctions connected to a common normal metal electrode. Phonon transport via the substrate is found to be the dominant coupling process between the tunnel junctions, although the device design can result in some junctions being effectively shielded from the substrate phonons by the common electrode. Finally, the possibilities of using a superconducting heterostructure to control the rate at which quasiparticles recombine and emit phonons have been explored. Excessive recombination is believed to limit the effectiveness of large areas SIN tunnel junctions as thermometers for particle detecting bolometers.

This thesis covers the construction and characterisation of a magneto-optical trap (MOT) for 85Rb from the very beginning. It details both the optical and mechanical aspects from laser diode assembly, tuning and stabilisation to the preparation and assembly of the vacuum system. The MOT construction forms the first goal of the project, the second was to develop a laser system capable of producing custom programmable, amplitude modulated and frequency chirped pulses (on the tens of microseconds timescale) from a continuous wave source. This involved developing software and computer control for several arbitrary signal generators linked to drivers for acousto-optic and electro-optic modulators. This chirped, pulsed laser system will be used to perform state manipulations on the 85Rb MOT cloud using two-photon Raman transitions in an adiabatic rapid passage regime. The chirped pulse system was initially tested with a rubidium vapour cell in an attempt to perform atomic interferometry that would produce spatial interference fringes along the length of the cell. However, due to the beam power requirements, the beam diameter together with the large Doppler shift at room temperature meant these fringes were not seen and so the vapour cell was replaced with the cold atom cloud in the MOT. Two-photon experiments were attempted with the MOT cloud using the chirped pulse system, however despite greatly improved laser power and detection eficiency, the signal indicating ground state population transfer via a two-photon interaction was not seen. The results indicate that the hyperfine-ground state splitting frequency has been shifted due to the proximity of the ion pump magnet to the vacuum system. Presently, efforts are being directed towards searching for the correct frequency.

The development of spatial signal processing techniques at millimetre wavelengths represents an area of science and technology that is new. At optical wavelengths, spatial signal processing techniques are well developed and are being applied to a variety of situations. In particular they are being used in pattern recognition systems with a great deal of success. At millimetre wavelengths, the kind of technology used for signal transport and processing is typically either waveguide based or quasi-optically based, or some hybrid of the two. It is the use of quasi-optical methods that opens up the possibility of applying some of the spatial signal processing techiques that up to the present time have almost exclusively been used at optical wavelengths. A generic device that opens up this dimension of spatial signal processing to millimetre wave quasi-optical systems is at the heart of the work described within this thesis. The device could be suitably called a millimetre wave quasi-optical spatial light modulator (8LM), and is identical in operation to the spatial light modulators used in many optical signal processing systems. Within this thesis both a theoretical and an experimental analysis of a specific millimetre wave quasi-optical spatial light modulator is undertaken. This thesis thus represents an attempt to open up this new area of research and development, and to establish for it, a helpful theoretical and experimental foundation. It is an area that involves a heterogeneous mix of various technologies, and it is an area that is full of potential. The development of the experimental method for measuring the beam patterns produced by millimetre wave quasi-optical spatial light modulators involved the separate development of two other components. Firstly, a sensitive, low-cost millimetre wave pyroelectric detector has been developed and characterised. And secondly, a high performance quasi-optical Faraday rotator (a polarisation rotator) has been developed and characterised. The polarisation state of a quasi-optical beam is the parameter most often exploited for signal processing applications in millimetre wave quasi-optical systems, and thus a high performance polarisation rotator has readily found many opportunities for use.

This thesis describes the process of developing a robust phase measurement technique with which to analyse semiconductor based devices intended for use in optoelectronic/all optical networks. The devices measured are prospective dispersion compensators, based either on planar photonic crystal waveguides or coupled microcavities connected by ridge waveguide. The technique was validated by measuring the phase transfer function of a Fabry Perot etalon. This demonstrated that even when detecting low optical powers (sub μW), accurate measurement of phase could quickly be carried out over a significant spectral range (~10nm). Comparison of experimental data taken from the prospective dispersion compensators with theory showed excellent agreement, which provided qualitative (cavity spacing and reflectivity) and quantitative (loss) measures of device performance. The phase measurement technique has been designed to be capable of measuring other classes of device also, including active devices such as semiconductor optical amplifiers. This suggests the phase measurement technique may be valuable in analysing the variation of dispersion as a function of applied bias, peak power or temperature for a variety of devices.

The classification tree is an attractive method for classification as the predictions it makes are more transparent than most other classifiers. The most widely accepted approaches to tree-growth use axis-parallel splits to partition continuous attributes. Since the interpretability of a tree diminishes as it grows larger, researchers have sought ways of growing trees with oblique splits as they are better able to partition observations. The focus of this thesis is to grow oblique trees in a fast and deterministic manner and to propose ways of making them more interpretable. Finding good oblique splits is a computationally difficult task. Various authors have proposed ways of doing this by either performing stochastic searches or by solving problems that effectively produce oblique splits at each stage of tree-growth. A new approach to finding such splits is proposed that restricts attention to a small but comprehensive set of splits. Empirical evidence shows that good oblique splits are found in most cases. When observations come from a small number of classes, empirical evidence shows that oblique trees can be grown in a matter of seconds. As interpretability is the main strength of classification trees, it is important for oblique trees that are grown to be interpretable. As the proposed approach to finding oblique splits makes use of logistic regression, well-founded variable selection techniques are introduced to classification trees. This allows concise oblique splits to be found at each stage of tree-growth so that oblique trees that are more interpretable can be directly grown. In addition to this, cost-complexity pruning ideas which were developed for axis-parallel trees have been adapted to make oblique trees more interpretable. A major and practical component of this thesis is in providing the oblique.tree package in R that allows casual users to experiment with oblique trees in a way that was not possible before.

The Paris-Edinburgh cell (PEC) is a widely used opposed-anvil device for neutron scattering. Since its development, it has been used to study a number of samples loaded as solids or liquids. However, studying gases at room temperature has not yet been possible. Up until now only a few gases could be loaded as liquids, in cryogenic conditions. Thus, it was impossible to study many gases and gas mixtures and also it was difficult to use gases as pressure-transmitting media (PTM). In order to overcome these limitations, a technique that would enable loading of gases into the PEC was required. The work described in this thesis was focused on the design and use of a gas-loading system for the PEC. The challenge of designing such a system comes from the fact that the gases need to be loaded into the gasket at sufficient density in order to achieve any significant pressure during further compression in the cell. This can be achieved by using a separate pressure vessel. Because the whole PEC is too large to be placed inside the vessel, a technique of loading gas into the anvils separated from the rest of the cell had to be devised. Designing the holder for the anvils, which would make this possible, presented a major challenge as it should allow the anvils to be transferred between the vessel and the PEC, with the gasket filled with high-pressure gas. Then it needs to allow further compression of the gasket inside the PEC. The developed system consists of a pressure vessel and a locking clamp for the anvils. The pressure vessel is a closed-end thick-walled cylinder with a top cover which has an opening for a piston. The vessel is placed on the table of a hydraulic press and the piston, sealed by a high-pressure reciprocating seal, is used to transmit the force from hydraulic ram onto the anvils which are held by the clamp and placed inside the vessel. One of the anvils is fixed to the clamp and the other one is supported by spring-loaded latches - the latches engage when the anvils are pushed towards each other. Thus, when the force is applied onto the anvils to compress the gasket, latches lock the anvils in their positions stopping them from retracting and maintaining the gasket compressed after the force is released. The clamp allows the gasket to be filled with the gas and then deformed to seal the compressed gas. The locking mechanism keeps the gasket compressed, which enables the clamp to be transferred from the vessel to the PEC. After the system was built and tested, it was installed at ISIS neutron source (Oxfordshire, UK), where it has been used in several experiments. The first experiment prepared with the gas-loading system was a neutron diffraction study of nitrogen at high pressure. Nitrogen was chosen as a sample material because its high-pressure structural phase diagram is well established. Nitrogen was loaded into the gasket using the gas loader and then it was compressed in increments to 6 GPa in the PEC. β and δ phases of solid nitrogen were clearly seen in the collected neutron diffraction data. The experiment proved the usability of the gas-loading system and verified its expected performance. The second experiment utilizing the gas-loading system was to study singlecrystal and powder samples of sodium chloride (NaCl) and squaric acid (H2C4O4). For these studies argon was used as a PTM, replacing the conventionally used methanol-ethanol mixture (ME). Up until this experiment the highest pressure reported in single-crystal neutron-scattering experiments was 12 GPa. This limit was set by the solidi cation pressure of ME. With argon as the PTM, the samples were compressed to 15 GPa without any damage to the crystals. Another advantage of replacing ME with argon is improved hydrostaticity. The highest pressure that ME remain hydrostatic to is 11 GPa. Compressing beyond this point causes sheer stress acting on the sample which affects the quality of the neutron scattering data manifested in the appearance of peak-broadening in the diffraction patterns. With use of argon, the powder samples have been compressed to 18 GPa while maintaining quasi-hydrostatic pressure conditions, resulting in clean and sharp diffraction patterns without any noticeable peak-broadening.