Band-gap engineering of bulk germanium (Ge) holds the potential for realizing a laser source, permitting full integration of monolithic circuitry on CMOS platforms. Techniques rely mainly on tensile strain and n-type doping. In this thesis, we focus on studying diffusion-based phosphorus (P) doping of Ge using spin-on dopants (SOD), and tensile strain engineering using freestanding micro-electro-mechanical systems (MEMS)-like structures. Process development of a reliable SOD recipe was conducted using furnace and rapid-thermal annealing, and successful doping up to 2.5 x 10¹⁹cm⁻³ was achieved,resulting in approximately 10 enhancement in direct-gap emission. A transition in Ge direct-gap photoluminescence (PL) behaviour is observed upon doping, from being quadratically dependent on excitation power to linear. We have also demonstrated that the limited doping concentration of P in Ge using SOD is not source limited, but more probably related to the diffusion mechanism. The other part of the project concentrated on Ge strain engineering. Previous works reported high tensile strain values based on freestanding MEMS-like structures made of Ge, yet without embedding an optical cavity (until recently). In this project, we realize this combination by fabricating Ge micro-disks as an optical cavity on top of freestanding SiO2 structures, utilizing Ge-on- Insulator wafers (GOI). 3D computer simulations were used to understand and optimize the devices, in terms of strain and optical performance. Raman spectroscopy and PL measurements confirmed simulation results showing higher tensile strain for beams with shorter lengths, with a maximum uniaxial strain of 1.3%. Splitting of light and heavy hole energy bands was observed by PL as the strain increases, agreeing with theoretical models. Direct-gap sharp-peak whispering-gallery modes (WGMs) were confined in 3 m disks with a maximum quality-factor of 200. Two loss mechanisms could be distinguished, red-shift of the absorption edge, and free-carrier absorption. In order to avoid these excitation-related losses, higher strain values combined with heavy n-type doping are required. A possible implementation using the same GOI platform is proposed for future work.

In this thesis a detailed analysis of long pulse CO₂ laser interactions with carbon fibre reinforced polymer composites (CFRPs) is presented. In particular, the fume dynamics and the fume contents are determined. Knowledge of both of these aspects of laser processing of CFRPs is important for the development of specialist fume extraction and treatment systems. The study has been conducted in a progressive manner; firstly the individual components of the composite have been characterised in respect of their thermal and optical properties, then their individual interaction with laser light quantified, and finally the whole composite has been subject to the same laser processing tests. The materials are PAN-based carbon fibres (T300, Toray Carbon Fibers America Inc.), an epoxy resin (RS-M135, PRF Composite Materials) and a 50 : 50 combination by volume as a complete CFRP from Goodfellow Cambridge Ltd. The laser processing of CFRP is known to be a challenge due to the vastly differing thermal and optical properties between the components of the composite. Despite this, it has been studied widely due to its highly desirable properties in high-technology industries and drilling with an assortment of laser sources has proved it to be a viable option. The contribution of this work is to put aspects of the laser interaction on a firmer experimental and theoretical basis in the medium irradiance regime. In particular, there is little published work on the origin and composition of fume from laser processed CFRPs and this thesis documents studies that address this knowledge gap. The fume from the carbon dioxide laser ablation of CFRPs has been found to be primarily phenol, CO₂, CO, water, methane and an aldehyde which is most likely formaldehyde. Specific regimes under which carbon fibres can be released from the surface of both the bare carbon fibre weave and the CFRP have been identified. Predominantly, this is when the fibre is cut in two places using the laser beam in a scanning mode of operation as would be used for trepanning holes, for example. A method of tracking the motion of the ejected fibres in a time resolved way using fast imaging has been shown, but also in a non-time resolved fashion by observing the incandescent streaks from them. The evidence suggests that fibres cannot be released from a single ablation site unless they are near the edge of the material, and also that any fibres released when using PAN fibres and a Gaussian beam are unlikely be thinned to the point where they are hazardous to human health. This is due to a concomitant swelling of the fibre at moderate fluences which increases its size significantly. They could however be a source of debris on the surface of the sample. By using time-resolved interferometry, the sensitivity of the plume image capture system was improved over the typical shadowgraphic method to the point that gas phase decomposition can be observed very early on in the laser interaction. This technique can be applied to finding the moment during the laser interaction at which carbon fibre or epoxy resin starts to decompose. The two materials produce very different fume compositions and the results vary significantly over a piece of CFRP as the thickness of the epoxy over-layer changes due to the woven carbon fibre structure beneath. A theoretical model has been used to analyse the data and is shown to give good agreement with the time-to-onset of thermal decomposition as a function of laser irradiance as well as closely predicting the threshold laser fluence. The time-resolved images of the plume have also been used to plot the progression of the ejecta with time so that their velocity can be measured thus informing the conditions required for efficient extraction. The fluence threshold for laser-induced damage to carbon fibre weave was measured and was found to have a threshold 1.5x higher when the fibre orientation was perpendicular to the polarization of the laser as opposed to parallel. This result was supported by calculations of the reflectivity of the fibre at the laser wavelength, which found that 1.6x more energy was absorbed by fibres that are perpendicular to the polarization of the laser. As there is a large array of fibre types, matrix compositions and laser sources, the work presented here is detailed in such a way as to allow these techniques to be applied to other systems in the future.

The work consists of optical and interferometric studies of growth, slip, and etch phenomena on diamond octahedral faces. For this study special experimental techniques have been developed. These comprise: (a) Thin film technique tor high magnification topographical studies. (b) Micro-flat for the study of rough surfaces. The study includes (1) Evaluation of inter-facial angles of growth hillocks and trigone, so familiar on diamond surfaces. This is carried out by means of multiple beam interferometry, the method competing with goniometry. A similar procedure is adopted to the etch pit. (2) Cylindrical curvature is discussed and values are obtained for both hillocks and vicinal faces. Trigons are similarly treated, but they are included in another study of a statistical nature (depth versus size). (3) A theory is developed to account for the existence of trigone. This is first presented as a speculation but is later supported by facts. (4) Slip has been substantiated to exist in diamond. This has been discovered and studied by means developed in this thesis. The slip plane has been identified as the (111) plane. An opacity has been observed (intimately connected with the slip) which has bean duly interpreted. (5) By artificially etching diamond at a much lower temperature than is usually adopted, etch pits have been observed to be spirally connected. The spirals are oriented in conformity with the crystal symmetry. The susceptibility of the shape and size of the etch pit to the etching temperature has been investigated, also the distribution of depth amongst the etch pits. (6) The temperature at which the diamond has formed is calculated, and a procedure to be adopted for the complete confirmation is outlined.

The primary objective of the work described in this thesis was to devise a red-sensitive photoresist imaging process for use in the replication of diffraction optics. In the introduction the chemistry of conventional photopolymer systems and photoresists used for holographic recording and the fabrication of diffraction gratings and diffracting optical elements is reviewed. The limitations of commercially available photoresist systems, particularly for applications requiring the use of red light are discussed. A polymer system has been investigated which could be imaged by photochemically generated free radicals, followed by a simple aqueous development procedure as required by the original specification. The polymer chosen for study was a copolymer of methyl methacrylate, methacrylic acid and 2-hydroxyethyl methacrylate. This was derivatized using methacryloyl chloride or methacrylic anhydride in order to introduce cross-linkable units to the polymer backbone. Polymers have been characterized by a number of techniques and the effect of varying composition on aqueous base solubility has been thoroughly studied. Various methods of derivatization have been employed. The ease of imaging has been found to be very sensitive to both the composition of the polymer and the extent of functionalization. High quality images have been obtained from this polymer using an organic solvent developer. Imaging experiments have been carried out on thin films of the photopolymer coated on glass using phenylazotriphenylmethane (PATM) as photoinitiator. Good images of 100 lines permillimetre (1 mm^-1) have been recorded by contact printing. Interferometry has been used to demonstrate that interference patterns having 600 and 1200 1 mm^-1 can be recorded using this polymer with PATM as initiator, exposed to an argon ion laser operating at 458nm. A number of two component photoredox initiator systems have been investigated, the light absorbing species of such systems being a dye such as methylene blue or certain cyanine dyes. The second component of these initiators may be an aryl sulphinate salt, a 1,3-diketone or some alkyl sulphides. The red light-initiated phatopolymerization of acrylamide has been demonstrated using some of these initiators and a low resolution photopolymer image has been recorded using Azure A and perinaphth-1,3-indandione as the photoinitiator system. This polymer can, in principle, produce images over a wide range of wavelengths depending on the nature of the initiator used.

Firstly, a survey is given of excitation processes occurring in gas discharges, together with an analysis of selective excitation processes in gaseous optical masers. Hollow cathode discharges have been investigated as excitation media for gas lasers. Evidence is given for the establishment of a relationship between the 632.8nm helium-neon laser discharge and transition weak to strong discharges. An optimum electron temperature for oscillation on the 632.8nm line is indicated. In a pulsed helium-iodine discharge, oscillation is reported on four new laser transitions, one an intercombination transition. Simultaneous oscillation for the first time is reported on hyperfine components of the ionised iodine lines. Hyperfine intervals of the 6p and 6s levels have been established, giving agreement between calculated and observed hyperfine components of lines. These levels are shown to be strongly perturbed. An anomalous behaviour is reported on hyperfine oscillations on the658.5nm line, which can be attributed to non-equilibrium in the populationof hyperfine sublevels of the 6s level,due to pressure/current changes. Results support the proposal that charge transfer is the selective excitation mechanism in the helium-iodine laser.

The influence of liquids in the laser beam on the emission and propagation characteristics of intense pulsed neodymium lasers is described. Results are presented on Q-switching by a thermal effect in organic solvents, on basic self-modulation and mode-locking processes in an oscillator, and on the transmission and amplification of picosecond light pulses in an inorganic liquid laser.

The dependence of certain properties of nitrogen lasers on various parameters have been examined and the design of two original nitrogen lasers is presented. The output characteristics of these lasers have been studied as a function of gas pressure, gas flow rate and direction, applied potential, repetition rate, electrode geometry, drive circuitry including the effects of preionization, and the effect of optical feedback by the addition of an optical cavity. The operation of these two lasers in a master oscillator/power amplifier configuration is described and the optimization of this system by varying the drive circuit parameters has been achieved. In the optimized state this configuration produced as much energy per pulse as did the amplifier when operating separately as an oscillator. However, the oscillator/amplifier system produces an output with some additional useful characteristics, such as lower beam divergence and a high degree of polarization. The design of a dye laser which uses a grating at grazing incidence and three mirrors is described. This laser utilizes the polarized output from the oscillator/amplifier system to generate simultaneously two independently tunable wavelengths. A novel pumping arrangement is used which ensures that there is no mode competition between the two wavelengths, that they can have any polarization ratio, that the linewidth of each wavelength can be independently varied and allows the two wavelengths to be generated in different dyes.

A brief survey is given of the progress made in the study and development of Q-switched lasers in recent years, and a review of the theory relevant to the work described, is outihed. A gain-switched giant pulse ruby laser system, employing two ruby rods of differing lengths in a single resonant cavity coupled with a rotating prism,has been developed. A systematic study of the output characteristics of this system under various conditions has been carried out. High resolution spectroscopy, including nanosecond time-resolution, of the emission of this laser, and also of a Pockels cell switched laser, has rendered possible the direct observation of the axial and off-axial mode structure, and has shown the existence of an intensity dependent frequency shift towards higher frequency in the giant pulses. When the two ruby rods in the gain-switched device are differentially cooled, simultaneous two frequency output pulses with powers in the tens of megawatt region are obtained, the wavelength separation of the two frequencies being tunable from 1 to 5.5Å. The suitability of this device for certain experiments in non-linear optics, such as the generation of the sum and difference frequencies in crystals, and nonlinear resonance mixing in plasmas, is considered. The two frequency output of the laser system has been mixed in ADP, to generate the sum frequency, and the conversion efficiency of the latter process compared with that of second harmonic generation.

A general method, of approach to resonant non-linear optical phenomena involving travelling waves has been developed. Maxwell's equations are solved for the electric field in a spherical Fabry-Perot type optical resonator, which encloses uniaxial arisotropic media. The specific case of propagation perpendicular to the optic axis is considered but the theory can be extended to cover the general case including double refraction. In the presence of more than one optical field of this form, when the medium enclosed in the resonator is non-linear, by expanding the polarisation in terms of the electric field in the normal way coupled mode equations are obtained for amplitudes of the eigen modes concerned. This general formalism is then used to examine resonant second harmonic generation in the small conversion approximation from a fundamental beam in the lowest order 0-0 mode. Analytical solutions are obtained in three limiting cases, weak focussing, strong focussing and the focus removed infinitely from the non-linear medium. The general case is solved numerically. From the results the values of the variable parameters can he obtained which give the maximum output in any given mode. Graphs are presented giving the output variation in 0-0 and 0-2 modes with focus position, phase matching, focussing and spot size. From there it can be seen that the optimum focussing for the 0-0 mode occurs at 1/20 = 5.65 (1 crystal length, zo one half the confocal parameter). Secondly degenerate parametric amplification between two lowest order inodes is examined under the approximation that the pump beam is undepleted. An analytic solution is obtained for the single pass amplification in the weak focussing limit and preliminary results of numerical computations for the general case are given. From these results a value for the optimum threshold condition is calculated.

Methods for the production and detection of shock waves are reviewed with particular emphasis on the production of shock waves by exploding wires and by the absorption of energy from a pulsed laser system. An exploding wire system is described for the generation of shockwaves in perspex plates. Observation is made using the stress induced bi-refrigence, and 'time integrated' photographs of shock waves are produced. At lower energies the exploding wire produces a mixture of wire vapour and molten droplets. The disintegration of the droplets is shown and described in terms of the mechanism shown by Lane and Edwards⁽¹⁰⁴⁾ to operate forwater droplets. A modified Kerr cell shutter with an unsymmetric electrode geometry is described with reference to possible use as a fast light shutter for measuring shock wave velocities. A development of this shutter has one of the electrodes in the Kerr cell consisting of a stack of razor blades. Light which has been diffracted from an electrostrictively induced phase grating near the blade edges, has been detected. A number of mechanisms are described which take effect within the modified cell. A system for the production and detection of shock waves by a Q-switched ruby laser is described. Shock waves are produced in liquids both by electrical breakdown and by evaporation of the surface material from a metal target. Experimental results show the production of shock waves with velocities up to 10⁴ m/s in water. The ablation wave produced when the surface of a metal wire is evaporated by a laser has been photographed. This shock wave accelerates as it moves towards the centre of the wire. Finally, the process of stimulated Brillouin scattering is described as the pulsed laser is focused into a liquid.