Table of contents

Comprehensive, three-dimensional, thermal - electrical - optical self-consistent simulation of gain-guided vertical-cavity surface-emitting lasers (VCSELs) is used to study the mutual interaction in a VCSEL resonator between the carrier-concentration distribution and the laser intensity profile, called usually the spatial hole burning (SHB) effect. In the simulation, the beam-propagation method (BPM) is applied to investigate a VCSEL optical field whereas finite-element and finite-difference techniques allow modelling of heat-flux, current-spreading and carrier diffusion phenomena. BPM turns out to be very efficient in analysing laser optical fields. The SHB effect is found to influence considerably both carrier-concentration and intensity profiles in VCSEL active regions, therefore it is proved to play an important role in many physical processes (thermal, electrical and optical phenomena) occurring during operation of a VCSEL.

ZrN films of very strong (100) fibre texture on the substrates of stainless steel and tool steel were obtained by using a reactive DC magnetron sputtering technique. The residual strains along various directions in the as-deposited films were measured using an x-ray diffraction method. The results show that the variations of strains with the direction in the films deposited on both substrates do not follow that predicted for an isotropic medium. Instead, the strain variations are close to that of a single crystal. The residual stresses derived from the measured strains on the basis of the anisotropic elasticity are about 4 GPa in compression. A simple calculation indicates that the thermal stresses imposed by the substrates are relatively small. Therefore, the residual stresses in these films are mainly due to the mutual impingement of grains and the growth defects developed during the deposition process. The strong texture in the present films also results in smaller strain distributions compared with that in similar films of milder texture.

A series of metal-aluminium nitride (M-AlN) cermet materials for solar selective coatings was deposited by a novel direct current (d.c.) magnetron sputtering technology. Aluminium nitride was used as the ceramic component in the cermets, and stainless steel (SS), nickel-based alloy (NiCr), molybdenum-based alloy (TZM) and tungsten were used as the metallic components. The aluminium nitride ceramic and metallic components of the cermets were deposited by simultaneously running both an aluminium target and another metallic target in a gas mixture of argon and nitrogen. The ceramic component was deposited by d.c. reactive sputtering and the metallic component by d.c. non-reactive sputtering. The total sputtering gas pressure was 0.8-1.0 Pa and the partial pressure of reactive nitrogen gas was set at 0.020-0.025 Pa which is sufficiently high to ensure that a nearly pure AlN ceramic sublayer was deposited by d.c. reactive sputtering. Because of the excellent nitriding resistance of stainless steel and the other alloys and metal, a nearly pure metallic sublayer was deposited by d.c. sputtering at this low nitrogen partial pressure. A multilayered system, consisting of alternating metallic and AlN ceramic sublayers, was deposited by substrate rotation. This multisublayer system can be considered as a macrohomogeneous cermet layer with metal volume fraction determined by controlling the thicknesses of metallic and ceramic sublayers. Following this procedure, M-AlN cermet solar selective coatings with a double cermet layer structure were deposited. The films of these selective surfaces have the following structure: a low metal volume fraction cermet layer is placed on a high metal volume fraction cermet layer which in turn is placed on an aluminium metal infrared reflection layer. The top surface layer consists of an aluminium nitride antireflection layer. A solar absorptance of 0.92-0.96 and a normal emittance of 0.03-0.05 at room temperature have been achieved for these M-AlN cermet solar selective coatings.

Carbon nitride thin films were deposited by the reactive ionized cluster beam (RICB) technique. Fourier transform infrared (FTIR) transmission analysis shows that no marked IR peak exists for the film deposited without nitrogen, whereas a broad peak in the range 1000- and a peak due to the C3pt- 1pt- -1pt-N triple bonds are present for the films deposited with nitrogen. Both FTIR and Raman measurements obviously suggest that C1pt-N single bonds exist when nitrogen is introduced. The intensity of this band rises with increasing nitrogen pressure. The above results are also supported by x-ray photoelectron spectroscopy (XPS) analysis.

Phase transitions in sintered ( were investigated by means of a differential scanning calorimeter (DSC) in combination with magnetization measurement. Below 300 K, three phase transitions were identified: charge disorder-order, paramagnetism-antiferromagnetism and antiferromagnetism-canted spin structure transitions. In contrast to the reported single-crystal sample, no field-induced metamagnetic transition occurred in the sintered sample at an applied magnetic field up to 6.5 T, and the resistivity of the sintered sample exhibited a semiconducting behaviour between 77 K and 300 K.

In a previous publication we derived the initial bunching statistics for a pulse of particles obeying Poisson statistics. Here we correct and simplify this analysis. It was predicted that, in a free electron laser (FEL) operating at short wavelengths (in the x-ray regime), the spontaneous radiation power due to shot noise could be larger than that predicted by previous theories. The results of the corrected analysis show that this is not the case.

The discharge equilibrium and the F-atom production kinetics in a phototriggered HF laser using gas mixtures containing Ne and with hydrogen or ethane have been investigated. Coupled experimental and theoretical studies have been carried out, through a 0D discharge modelling and measurements of the current, of the voltage and of the extracted laser energy, together with emission spectroscopy on F 3p excited states. A quantitative comparison of the total F-atom density produced in mixtures of with or is performed and the results are compared with the discharge pumped HF laser performance achieved with these molecules. An overall validation of the model has been obtained, both for the energy transfer to the active media and for the kinetics of F and its excited states. It is shown that the transmitted energy and electrical charge, as well as the discharge electric field, weakly depend on the type and on the concentration of the RH molecule, or , in the gas mixtures. Moreover, the F-atom production depends very slightly on the nature of RH. Therefore the fact that the laser performance achieved with hydrogen is lower than the performance achieved with ethane is not due to differences in the energy transfer and in the F-atom production kinetics in mixtures of with or .

Soda-lime glass samples have been subjected to both single and double field reversals during electric field assisted - ion exchanges in the temperature range 350-450C. Changes in refractive index profiles, chemical profiles and surface stress were observed during the exchanges and a comparison has been made with previous studies. It is shown that large reductions in surface stress seen after single field reversals, which have previously been attributed primarily to a stress relaxation mechanism, are mainly caused by the removal of a large number of ions. For example, at 395C ion removal accounts for 80-95% of the stress reduction. It is shown that the drift-diffusion model of ion transport, using concentration-independent self-diffusion coefficients, cannot consistently model this large ion removal rate. A new model is developed which is based on drift-diffusion transport with concentration-dependent self-diffusion coefficients. The effects of double field reversals are also investigated. It is suggested that the glass may undergo an irreversible change associated with ion removal.

We applied the recently developed method for determination of the power law in the degenerate four-wave mixing process in order to use a Boltzmann plot for determination of the rotational temperature of nitric oxide molecules generated in an oxyacetylene flame at atmospheric pressure. It is shown that only for a certain range of transition dipole moments can a simple power law be established and applied to temperature measurements. We checked for the influence of deviations from a simple power law due to different saturation intensities of the and lines.

An asynchronous non-iterative algorithm has been developed for the time-dependent atomic dynamics in laser-plasma simulations. This model, which includes all ionization fractions, gives a good description of the distribution of the charge states in both the ionization and the rapid recombination stages. It can be combined with the hydrocode directly to predict the behaviour of laser-plasma. We have incorporated this model with a two-dimensional Eulerian code to simulate the recombination Li-like Si laser and obtained a result that agrees with experiment.

Short-pulse laser ablation in air at 0.1 MPa leads to intense evaporation of the target material. The ablated material compresses the surrounding gas and leads to the formation of a shock wave. The incident laser radiation interacts with the partially ionized material vapour and the condensed material clusters embedded therein and affects the efficiency and quality of the ablation. Methods to increase the efficiency and quality of the ablation process require knowledge of these mechanisms. Therefore, the transmissivity of a laser-induced plasma plume was investigated in the wavelength range between 440 nm and 690 nm with a spatial resolution of about . The results show a weak dependence of the extinction coefficients over the investigated wavelength range. The spatial resolution allowed us to identify the regions behind the shock wave with the highest extinction for the visible wavelength range probed. These regions correspond to areas with high free-electron densities. To understand the mechanisms that are responsible for the heating and ionization of the vapour at the start of the excimer laser pulse, a simplified stationary model was applied. The experimental results were interpreted using Mie scattering theory on condensed material clusters, inverse bremsstrahlung absorption and absorption due to photoionization of excited material vapour atoms. The modelling shows that extinction of the laser light in a plasma with the assumed thermodynamic parameters is dominated by Mie absorption on condensed material clusters for wavelengths less than about 430 nm and is dominated by photoionization absorption and inverse bremsstrahlung above 430 nm.

A theoretical study of plasma density control in a RF resonant field has been carried out. The equation expressing the RF resonant field is derived from the dispersion relation for electrostatic electron waves propagating at an angle relative to the magnetic field. Calculations were executed by varying the magnetic flux density at 0.133 Pa and demonstrate the RF resonant field in the condition under which parameters satisfy the equation. The RF resonant field allows an electric field to be induced in bulk plasma, which generates ions in the bulk as a result of electron impact ionization. The low magnetic flux density, 5 mT, facilitates control of the plasma density in the bulk by allowing adjustment of the extent of the ionization even at low pressures such as 0.133 Pa.

New methods make it possible to form considerable flux densities of Rydberg atoms of alkali metals. It is now possible to study the transport processes in regions where the density of Rydberg atoms is large. Examples of such studies have been given by Svensson and coworkers. In the present study, ions and Rydberg states are formed by desorption at 1300-1800 K from an Ir surface covered by a thin graphite layer. Due to the very large cross sections for collision processes involving Rydberg species, the Rydberg state-rich plasma between the Ir emitter and a cold grid electrode is not collision free, even at a pressure of mbar. Electron or emission takes place from the grid at a rate controlled by the flux of and . The transition to penetration of and through the cloud of excited species between the emitter and grid is observed directly by molecular beam and ion sampling to detectors in a separate chamber. There is a space-charge limited behaviour for the positive current through the plasma as well as, in some modes, a clear positive saturation current, which shows that little gas phase ionization takes place. A current larger than expected from the saturation current as well as maxima in the voltage dependences are observed at high Rydberg densities. These effects are probably caused by space charge compensation due to a dielectric phase of condensed excited species, which means, for example, that the effective distance between the emitter and grid is decreased, as observed. The temperature variation of the space charge limited behaviour gives an activation energy of , while the saturation current gives an activation energy of . This agrees well with the electronic excitations at 0.90 eV and at 0.84 eV, which should be important steps in a thermal excitation process from the ground state of Cs.

The low-frequency behaviour of the isotropic elastic modulus and mechanical loss tangent of modified lead titanate ferro-piezoelectric ceramics have been studied by the three-point bending technique. Measurements of these parameters as a function of the temperature were performed in the frequency range of 0.2 to 20 Hz. The results show anomalies in both parameters at the transition temperature and below this temperature. The anomalies below the transition temperature have wide maxima in mechanical loss whose temperatures increase when the measuring frequency increases. Values for activation energies were obtained that suggest the existence of mechanical relaxation mechanisms where point defects and ferroelectric domains are involved. The main species of point defects should be oxygen vacancies. Comparisons between experimental results and existing theoretical models are made.

We investigated ferroelastic domain switching due to stress in single crystals grown by a slow evaporation method. The ferroelastic domain switching stress due to the saturation effect is about 0.9 MPa. The temperature dependence of the stress-strain hysteresis was also studied. From this result, it was determined that the single crystal undergoes a phase transition near K. In other words, the phase transition due to an external mechanical stress applied to crystals causes a transformation from the ferroelastic to the paraelastic phase. From these results, it was confirmed that crystals have ferroelastic characteristics below .

This paper presents an experimental study of the coordination number of binary packings by the use of the liquid bridge technique and provides detailed information about the distributed coordination numbers corresponding to different types of contacts between small and large components and their dependence on particle size distribution. The results indicate that increasing the volume fraction of small component increases the small-to-small and large-to-small contacts and decreases the small-to-large and large-to-large contacts; and this trend is more apparent for a packing with a large size difference. For the packings under gravity, the overall mean coordination number is essentially a constant and independent of particle size distribution.