This thesis describes the development of a monolithic dual-wavelength laser based on an InP/AlGaInP quantum dot (QD) laser structure. Each wavelength is sourced from the same active region and can be operated simultaneously or independently, with light being emitted from a common aperture. The inhomogeneity of the QD material provides a wide distribution of energies, resulting in a broad and relatively flat-topped gain spectrum, which is ideal for sourcing multiple wavelengths. Measurements of optical absorption, gain and laser threshold current densities were used to characterise the optical properties of InP/AlGaInP QDs and ascertain a suitable structure from which to fabricate the dual-wavelength source. A growth temperature of 710 °C resulted in the lowest threshold current densities, and the incorporation of tensile strain into the upper confining layers was found to reduce the temperature dependence. Optical gain measurements were used to assess how state-filling and temperature govern the gain-peak wavelength. For a fixed gain at low injection the wavelength dependence follows that of the band gap (≈ 0.17 nm/K), but at higher levels of injection it becomes relatively temperature-insensitive. A minima in wavelength sensitivity corresponded to a net gain of ≈ 28 cm-1. Edge-emitting lasers with a wavelength temperature dependence as low as 0.03 nm/K were demonstrated for temperatures up to 107 °C (380 K). An Ar-Cl2 based inductively-coupled plasma (ICP) etch process, suitable for fabricating sub-micron features, was developed to create the necessary device architecture. Using the effects of state-filling and spectrally preferential feedback, coupled-cavity ridgewaveguide lasers with unequal length sections were used to generate two wavelengths, with separations up to 61.5 ± 0.2 nm. Time resolved spectra were used to demonstrate dual-mode operation, where both wavelengths were observed to emit simultaneously. This is a promising result as it suggests that this device could potentially be used as a compact terahertz source

This thesis is concerned with the design and implementation of an Electromagnetic Acoustic Transducer (EMAT) intended for high temperature ultrasonic applications; which benefits from a novel approach using a pulsed current electromagnet (PE) and a ceramic encapsulated spaced spiral coil, both designed to operate at elevated temperatures without any active cooling. In detail all the steps taken towards the development of the PE-EMAT and its supporting electronics are presented here, together with the finite element simulations performed for the electromagnetic and ultrasonic analysis; which includes: the computation of the flux density produced by the electromagnet and its variation with lift-off (gap between core and metallic sample), the calculation and visualisation of the magnitude of the current density induced by an alternating current flowing in two different EMAT coil designs in non-magnetic and magnetic samples; as well as the validation of the propagation of the ultrasonic wave generated/detected by the PE-EMAT. The experimental results regarding the performance of the PE-EMAT are also presented. Firstly, the performance at room temperature to show the shear wave generation in paramagnatic and ferromagnetic samples, the effect of the presence of an oxide layer adhered to the sample, and the lift-off performance. Secondly, the performance at a range of different temperatures to demonstrate that the PE-EMAT has been employed successfully at temperatures up to 600 oC, without using any active cooling system, and that is capable of performing ultrasonic measurements of sample thickness and material properties, without the use of any sort of couplant, nor the necessity of sample preparation.

We present an ab initio theoretical formalism for investigating the onset of magnetic order in strongly-correlated electron systems. The formalism is based on spin density functional theory, with a self-interaction corrected local density approximation (SIC-LDA). The self-interaction correction is implemented locally, within the KKR multiple-scattering method. Thermally induced magnetic fluctuations are treated using a mean-field ‘disordered local moment’ (DLM) approach and we use a linear response technique to generate the paramagnetic spin susceptibility. We apply the formalism to the heavy rare earth metals, where the magnetic ordering tendencies are analysed in terms of the underlying electronic structure. The formation of incommensurate magnetic structures is shown to be promoted through a Fermi surface nesting mechanism. Our calculations yield an accurate, parameter free, estimate of the magnetic ordering temperature of gadolinium. Using this element as a magnetic prototype, we propose a ‘unified phase diagram’, from which the magnetic ordering tendencies of any heavy rare earth system can be found. This diagram is used to predict critical alloy concentrations, at which new magnetic phases appears. We also examine magnetic ordering in transition metal oxides and outline how our first principles linear response approach can be adapted to study compositional correlations, which we illustrate by investigating the presence of atomic short range order in gadolinium-yttrium alloys.

A theoretical study is made of the physics of photoexcited carriers in one of the bands of a semiconductor. The emphasis is on photoexcited hot carrier phenomena, for which the mean carrier energy deviates significantly from the thermal equilibrium value in the steady state. Very little previous theoretical work in this area has been reported. Two situations are analysed. The first is an investigation of hot photoexcited carriers in germanium and silicon at low temperatures. The carriers are excited into the band by a model black-body excitation spectrum having a mean energy in excess of the thermal energy. Full account is taken of the interaction of the carriers with impurities and phonons, and recombination is asswned to occur via a cascade mechanism. Significant carrier heating is found for trapping densities of the order 1016 cm-3 at lattice temperatures below about 300K. The steady-state carrier distribution functions are derived numerically from the Boltzmann equation in the absence of external fields. The low field transport and trapping parameters are then derived by a perturbation theory. The assumption of a linear response to applied fields is checked by an adaptation of tileHonte Carlo technique first employed by Kurosawa (1966) and Boardman et al (1968) in high field studies of semiconductors. The technique is extensively modified to suit our problem; in particular the concert of the self-scattering device is enlarged. The theory shows good agreement with the experimentally measured (Rollin and Rowell 1960) temperature variation of the Hall mobility of photoexcited holes in germanium. Agreement is also obtained with an experimental curve for the temperature dependence of the capture cross section for electrons in silicon. experimentally a 'cut-off' is found in the temperature dependence below Tile hot carrier model explains this phenomenon in terms of an anomalous temperature dependent Hall number which arises from the severe non- Maxwellian heating of the carriers. However, not all the experimental results can be explained this way and a tentative alternative mecnanism is suggested. The second situation analysed involves monochromatic photoexcitation leading to the oscillatory photoconductivity effect in many polar semiconductors. Considerable controversy has existed previously as to the origin of this effect. The distribution functions and photoconductivity are studied as a function of photon frequency and electric field strength on the basis of an analytical model and detailed Monte Carlo calculations. Good agreement is found with experiment as regards tile field dependence of the overall spectral response, confirming the assumptions of an earlier approximate analytical approach (Stocker and Kaplan 1966). For certain photon frequencies the photoexcited carriers can theoretically exhibit both total and differential negative mobility for certain ranges of applied electric field, confirming a previous approximate theory (Stocker 1967). although tile effect nas not been observed experimentally. This leads to a non-uniform field distribution in the semiconductor and the possibility of spacecharge instabilities. The possibility of steady-state negative photoconductivity is investigated with particular reference to the spatial distribution of electric field and the stability of the carrier system. The evolution and form of the instabilities and steady-states are evaluated numerically. Full account is taken of the electron and trap dynamics. It is shown that the total negative resistance state is unstable in the presence of injecting contacts. Instead either a non-uniform field distribution showing bulk positive resistance is established or there occur propagating instabilities leading to positive current oscillations.

The potential application of nitride based ceramics in structural applications is dependent on their performance at elevated temperatures. To meet these demands the microstructure of a syalon ceramic has been refined. Improved processing techniques, by the use of high purity powders and "balanced" compositions, have been shown to improve the degree of inter granular crystallisation. The key aspects are the reduction of impurity levels, particularly calcium, and the use of high nitrogen compositions such that full crystallisation is achieved. The achievement of complete crystallisation results in the elimination of subcritical crack growth during fracture and high temperature creep processes dominated by non-cavitational grain boundary diffusional processes. The mechanism for oxidation has been identified by determination of the kinetics, surface and sub-surface reactions. A temperature limit of l300 oC has been identified by the reversion of YAG to a eutectic liquid by reaction with the Si02-rich oxidation layer. In this regime oxidation rates are determined by the YAG reversion and cation out-diffusion to the oxide layer. Oxidation effects have been shown to be responsible for the onset of sub-critical crack growth at temperatures above 1300oC, where crack extension results directly from YAG reversion effects. Enhanced high temperature performance above and beyond l300oC by surface transformation and surface coating was investigated. Surface microstructures based upon BI and Si2N20 have been shown to increase oxidation resistance upto 1375oC but are difficult to form without substantial surface degradation. Si3N4 and SiC coatings deposited by chemical vapour deposition (CVD) were found to have similar effects, increasing oxidation resistance beyond 1300oC.

We first consider an interacting two-dimensional electron gas in a ballistic quantum wire in an external magnetic field. Self-consistent calculations are made of the electrostatic Hall potential (EHP), the local chemical potential (LCP), and current density in a uniform ballistic quantum wire containing two-dimensional electrons in a perpendicular magnetic field B when either one or two subbands are occupied. The corresponding Hall resistances, REHP and RLCP, are also calculated. The former is nearly linear in B in spite of subband depopulation. The latter is quantised but the quantisation steps are rounded because of overlap of the forward and backward going wave functions. Secondly, self-consistent calculations are also made of wave functions and the two kinds of Hall resistances for the same system in a weak perpendicular magnetic field when several subbands are occupied. We find intermittent quenching of the Hall resistance associated with the local chemical potential as the electron density varies. The quenching is due to the overlap of opposite-going wave functions in the same subband, which is enhanced significantly by the singularity of the density of states at the subband minima aa well as by Coulomb interactions between the electrons. Finally, with a, model calculation, we demonstrate that a non-invasive measurement of intrinsic quantum Hall effect defined by the local chemical potential in a ballistic quantum wire can be achieved with the aid of a pair of voltage leads which are separated 9Y potential barriers from the wire. Biittiker's formula is used to determine the chemical potential being measured and is shown to reduce exactly to the local chemical potential in the limit of strong potential confinement in the voltage leads. Conditions for quantisation of Hall resistance and measuring local chemical potential are given.

The mixed alkali borosilicate glass system used for high-level waste immobilisation, has been studied with different divalent additive oxides such as CaO, Srf), BaO, MnO, ZnO and PbO added to the BNFL base glass (MW). A number of techniques have been employed in order to study the effect of these oxide additives. lIB MAS NMR was performed to measure the fraction of tetrahedrally coordinated boron atoms N4 and 29Si MAS NMR to measure the Qn speciation. Durability tests were carried out and compared with electrical conduction behaviour. Densities, glass transition temperatures and thermal expansion coefficients are also reported. The change in N4 with concentration has been determined for different additive oxides. The addition of BaO, CaO and srO to the base (MW) BNFL vitrification glass caused N4 to increase with alkaline earth content, and maximum values were attained at compositions depending on the M2+ ion type. In contrast, the addition of divalent oxides such as ZnO and PbO to MW reduced the N4 values, with the rate of decrease being greater for ZnO than PbO. The 29Si NMR studies have shown the presence of significant concentrations of non-bridging oxygens which can influence the aqueous corrosion behaviour of the glass. The durability tests revealed that the alkaline earth oxides BaO, srO and CaO decrease the durability. In contrast, oxides such as ZnO and PbO improve the corrosion resistance. The presence of a surface Mn7Si012 crystallisation phase was identified on corroded MnO containing glasses. The reduction in electrical conductivity with oxide addition shows a similar trend to the corrosion behaviour for chemically resistant glasses. In contrast, the opposite trend is found for less durable glasses. UV visible transmission of MnO doped glass shows a decrease in transmittance at - 380 run. The absorption band centred at - 49Onm, moves towards longer wavelength, caused by generation ofNBOs on the [Si04] tetrahedral unit.

This thesis reports research conducted on point defects in single crystal diamond utilising the complementary techniques of electron paramagnetic resonance (EPR), optical absorption and photoluminescence (PL) spectroscopy. Intentional silicon-doping of chemical vapour deposition (CVD) diamond allowed the production of samples grown on differently oriented substrates and containing distinct silicon isotopic abundances. The EPR spectrum of the neutral charge state of the silicon split-vacancy centre in diamond, (Si-V)0, has been characterised in the literature. Evidence for the assignment of the 1.31 eV zero phonon line (ZPL) seen in absorption and PL to the 3A2g -> 3A1u transition arising at (Si-V)0 is presented. Reversible charge transfer between the negatively charged centre, (Si-V)− (ZPL at 1.68 eV), and (Si-V)0 enabled the determination of calibration factors relating defect concentrations to their respective ZPL intensities. Preferential alignment of trigonal centres, such as (Si-V), in CVD material grown on {110}-substrates has been observed. The formation of (Si-V) centres during CVD synthesis and via irradiation and annealing of silicon-doped diamond was studied. Variable temperature EPR spectroscopy under illumination was used to investigate the optical spin polarisation (SP) of the (Si-V)0 3A2g ground state. Two different mechanisms for the SP are considered; selective intersystem crossing of (Si-V)0, and photoionisation of (Si-V)−. The properties of (Si-V)0 are compared to those of the extensively studied negatively charged nitrogen vacancy centre, (N-V)−. The effective spin-lattice relaxation and spin polarisation rates for both centres during continuous illumination are explored using pulsed EPR methods. A new defect, labelledWAR3, with spin S = 1 2 was observed in silicon-doped diamond and characterised using multifrequency EPR. Analysis of the data revealed that WAR3 is the neutral charge state of a silicon divacancy complex decorated by a hydrogen atom, (Si-V2:H)0. The experimentally derived 29Si and 1H hyperfine parameters are in good agreement with the values calculated using the spindensity- functional technique, confirming this model and ruling out a non-planar structure.

This thesis presents the theoretical framework required to apply spectrally resolved x-ray Thomson scattering (XRTS) as a diagnostic method for warm dense matter. In particular, the theory is generalised to allow for the description of systems with multiple ion species where all mutual correlations are taken into account within the new approach. Supplemented with the theory presented, XRTS is now a promising diagnostics for high-energy-density matter containing different chemical elements or mixtures of different materials. The signal measured at XRTS contains the unshifted Rayleigh peak and frequency-shifted features. The first is related to elastic scattering from electrons co-moving with the ions whilst the second occurs due to scattering from free electrons and excitation/ionisation events. The focus of this thesis lies on the elastic scattering feature which requires the ion structure and the electron density around the ion as input for the theoretical modelling. The ion structure is obtained from quantum simulations (DFT-MD) and classical hypernetted-chain (HNC) equations. The analysis of the DTF-MD simulation data reveals that partial ionisation yields strong modifications of the ion-ion interactions. Similar effects are found for the form of the electron screening cloud around an ion. On the basis of the newly developed theory and structural models, multicomponent effects on the XRTS signal are studied. It is shown that the Rayleigh feature is very sensitive to the ratio of the elements in the scattering volume and their mutual correlations. These results indicate that XRTS is well-suited to probe the properties of complex materials and the process of mixing in the WDM regime. The advanced theories are finally applied to experimental spectra. The procedure allows for both extracting the basic plasma parameters and assessing the quality of the theoretical models applied. Comparisons with several experiments demonstrated that the non-collective regime (large scattering angle) is reasonably well understood whereas the collective regime (small scattering angle/long wavelength limit) still holds challenges. The collective regime is problematic as here strong correlations and screening are highly relevant and, thus, a yet unknown description for fully coupled quantum systems needs to be applied.

A generalization of the non-local coherent potential approximation (NL-CPA) for the treatment of short range ordering (SRO) effects in complex unit cell solids with different forms of disorder is proposed. The method is discussed with respect to fully first-principles Korringa, Kohn and Rostoker multiple scattering theory, and a tight-binding model hamiltonian. Examples from the context of applications to metallic alloys, diluted magnetic semiconductors, and half-metals in the presence of antisite disorder are discussed.