Population transfer from the valence to conduction band in the presence of an intense laser field is explored theoretically in semiconductors and dielectrics. Experiments on intense laser driven dielectrics have revealed population transfer to the conduction band that differs from that seen in semiconductors. Our research explores two aspects of ionization in solids. (i) Current ionization theories neglect coupling between valence and conduction band and therewith the dynamic Stark shift. Our single-particle analysis identifies this as a potential reason for the different ionization behaviour. The dynamic Stark shift increases the bandgap with increasing laser intensities thus suppressing ionization to an extent where virtual population oscillation become dominant. The dynamic Stark shift plays a role dominantly in dielectrics which due to the large bandgap can be exposed to significantly higher laser intensities. (ii) In the presence of laser dressed virtual population of the conduction band, elastic collisions potentially transmute virtual into real population resulting in ionization. This process is explored in context of relaxation time approximation.

Nonlinear optical phenomena associated with high-order soliton breakup in photonic crystal fibres and squeezed state generation in three dimensional photonic crystal microcavities are investigated. In both cases, the properties of periodically patterned, high-index contrast dielectric structures are engineered to control the dispersion and local field enhancements of the electromagnetic field. Ultra-short pulse propagation in a polarization-maintaining microstructured fibre (with 1 um core diameter and 1.1 m length) is investigated experimentally and theoretically. For an 80 MHz train of 130 fs pulses with average propagating powers in the fibre up to 13.8 mW, the output spectra consist of multiple discrete solitons that shift continuously to lower energies as they propagate in the lowest transverse mode of the fibre. The number of solitons and the amount that they shift both increase with the launched power. All of the data is quantitatively consistent with solutions of the nonlinear Schrodinger equation, but only when the Raman nonlinearity is treated without approximation, and self-steepening is included. The feasibility of using a parametric down-conversion process to generate squeezed electromagnetic states in 3D photonic crystal microcavity structures is investigated for the first time. The spectrum of the squeezed light is theoretically calculated by using an open cavity quantum mechanical formalism. The cavity communicates with two main channels, which model vertical radiation losses and coupling into a single-mode waveguide respectively. The amount of squeezing is determined by the correlation functions relating the field quadratures of light coupled into the waveguide. All of the relevant model parameters are realistically estimated using 3D finite-difference time-domain (FDTD) simulations. Squeezing up to ~20% below the shot noise level is predicted for reasonable optical excitation levels. To preserve the squeezed nature of the light generated in the microcavity, a unidirectional coupling geometry from the microcavity to a ridge waveguide in a slab photonic crystal structure is studied. The structure was successfully fabricated in a silicon membrane, and experimental measurements of the efficiency for the signal coupled out of the structure are in good agreement with the result of FDTD simulations. The coupling efficiency of the cavity mode to the output channel is ~60%. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The chloride current-voltage characteristics of the membrane of sartorius fibers from Xenopus laevis were studied using a three microelectrode voltage clamp system. In fibers with normal resting potentials (-70 to -90 mV) and in fibers depolarized in 115 mM KC1 (resting potential -20 mV) the direction and degree of steady state rectification depended on extracellular pH. In alkaline solutions (pH 8.4) the current rectified outwards; with large hyperpolarizations the current recorded in normally polarized fibers was sometimes seen to diminish as the voltage was made extremely negative (the current-voltage relation exhibited a negative slope). In the depolarizing region (in depolarized fibers) the slope of the I-V relation became constant (limiting conductance) in alkaline solutions. In acid solutions (pH 5.4) the current rectified inwards with hyperpolarization and reached a limiting value with depolarization. Chloride currents decay ('inactivate') following changes of membrane potential from the resting potential (for both polarized and depolarized fibers). The kinetics of current relaxation exhibited voltage-dependent time constants depending on the size of the voltage step with a sensitivity of about -1.5 msec/mV but were independent of absolute membrane potential and external pH. Inactivation of chloride conductance was studied in two-pulse (conditioning (v₁) and test (V₂), pulses) voltage clamp experiments. In variable experiments the dependence of the initial current at the onset of was sigmoidally related to (inactivation relation). The slope of the inactivation relation was twice as steep in acid as in alkaline solutions, but was independent of the resting potential. In variable V₂ experiments, the current-voltage relation was linear over a wide voltage range and for different values of V₁, the instantaneous I-V relations converged in the outward current region; they also had zero-current potentials that became increasingly negative with respect to the holding potential as V₁ was made negative. Instantaneous chloride currents and the kinetics of current relaxation were found to depend on initial conditions when the membrane potential was changed under non-stationary conditions. The inactivation and recovery of initial current had similar timecourses as did the prolongation and recovery of the time constants. Initial currents recovered from conditioning with an exponential or sigmoid timecourse. Relaxation time constants exhibited a similar recovery pattern. The decline of initial current was initially exponentially dependent on the duration of conditioning. The time constant increased sigmoidally, or exponentially as the duration of conditioning increased. Using the data from variable conditioning step and variable test step experiments a manifold (or state space representation) was constructed that enables much of the current-voltage behavior of the chloride permeation system to be predicted. Currents recorded in voltage clamp experiments can be visualized as time-dependent flows along trajectories that are determined by the voltage. The rectification of the steady state and instantaneous current-voltage relations are related to the dispersion of the trajectories. The dependence of time constants of current transients can also be accounted for by the manifold. The results are examined in light of models for channel behavior. The instantaneous I-V characteristics exhibit some properties of channels of the electrodiffusion type. The steady state current-voltage relations are qualitatively similar to those of a model incorporating a particle within the chloride channel that either blocks or unblocks it depending on the extracellular pH. The dependence of relaxation kinetics on the size of the voltage step and on initial conditions suggest the participation of a molecule acting in a catalytic role controlling the relaxation of current transients. / Science, Faculty of / Mathematics, Department of / Unknown

Two phase transitions have been found in DEM(TCNQ)₂ at 400(3) K and at 442(6) K to 453(6) K using ESR and measurements of the microwave conductivity. These temperatures are less than the values of 415 K and 483 K previously obtained from temperature dependent Guinier measurements. Below 400(3) K two ESR lines, I and II are observed corresponding to the two stacks, B and A, in DEM(TCNQ)₂. Above 400(3) K and below 453(6) K only one line remains with the same g value as line I. The angular dependence of the g value was fitted to g[sub=‖]² = g[sub=⊥] ²cos2θ + g²sinθ and values of g[sub=⊥] = 2.003551(14) and g[sub=‖] = 2.00273015) were obtained. Between 298 K and 442(6) K the conductivity was that of a semiconductor with an exitation energy E[sub=0] = 0.385(52) e.v. The -4k[sub=⌐] phase transition is postulated at 447(9) K. The phase transition at 400(3) K is due to a transfer of spin density from stack A to stack B, and has no effect on the total spin susceptibility or on the conductivity to within the experimental error. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Microwave processing has been studied as an alternate heating technique over conventional heating for industrial applications. Some advantages include quicker and more uniform heating. Also, microwave energy offers the advantage of localized heating and smaller-sized equipment. Many ceramics, however, are difficult to heat using microwave energy by reason of the strong temperature dependence of the dielectric loss. The ability of a ceramic to absorb microwave energy, a measure of its dielectric loss, increases with temperature which makes the material more susceptible to thermal runaway. The purpose of this research is to develop a model that accurately reproduces experimental data and can be used to explore new applicator designs for continuous processing of such ceramics. A two-dimensional numerical model, created for this purpose, assumes that the ceramic is a circular cylinder that moves either longitudinally through a microwave cavity of given dimensions. By adjusting the electromagnetic field so that the absorbed power matches the measured power, the model successfully imitates experimental results and avoids thermal runaway while achieving high temperatures. / Master of Science

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No description available.

Experiments have been conducted on the electrical breakdown of high-purity water and water mixtures. The electrical regime of interest has been carefully defined and documented to consist of electrical impulses with approximately microsecond rise time and fall time greater than 65 microseconds, on approximately 81-square-centimeter-area planar electrodes with a dielectric gap of approximately one centimeter. The results of over 25,000 shots by a Marx generator have been distilled into database form in an Excel spreadsheet and analysis performed to try to find patterns or indirect evidence into the nature of the breakdown-initiation process. An extensive review of all the experiments, which had been conducted over eight years by the Naval Surface Warfare Center and which had been designed to find the largest water-breakdown fields, was conducted with the intention of delineating the physical factors that led to breakdown. A variety of theoretical models of breakdown initiation were compared to the data, until it became clear that many of the breakdowns were dominated by impurities of various sorts. An extensive study of old and new experiments led to a more detailed understanding of the phenomenology of impurity-dominated water breakdown (such as the process of "conditioning" the electrodes and hysteresis) and the proposal of a number of new experiments to further characterize the intrinsic role of electrode materials on determining high-electric-field dielectric breakdown in water. / Ph. D.

This projects deal with dielectric materials. Characterize definition of dielectric and polarization of dielectric. Shows dielectric relaxation. Specified conception dielectric absorption and interpreted them with DC voltage charging condensator. However, this thesis be engaged in simulation of this process. Output is simulation program of dielectric absorption characteristics for two relaxation mechanism.

Thesis (PhD)--Stellenbosch University, 2002. / ENGLISH ABSTRACT: One of the most difficult problems in microwave dielectric heating is the generation and control of field and heating patterns. A technique allowing the synthesis of different, pre-determinable heating patterns by interference modulation is proposed. The proposed concept may be described by the term 'interference modulation'. Interference modulation is a technique which enables particular patterns, called features, to be obtained by signals from several sources interfering with each other. The relative phases of the signals are modulated, by which process known features may be selected. Weights are assigned to these features, which may be combined over time to form a new heating pattern. Phase changes may then be used to switch to specific, known features, with weights which will determine the contribution of each feature to the desired overall pattern. In the practical implementation described, magnetron tubes are the sources. Each of these narrow-band high-power sources was injection locked to a low power control signal. The control signals are derived from a reference source and their phase is set to select a corresponding feature. Calculation and measurement showed that reliable locking occurs with a control signal power of at least 3% of the magnetron's emitted power. Measurements of patterns were carried out with materials formed into sheets and blocks. Some were chemically prepared to reveal the overall heating pattern. The observed patters, simulations and field measurements concur, thus validating the concept and operation of the proposed topology. / AFRIKAANSE OPSOMMING: Een van die moeilikste probleme in mikrogolf diëlektriese verhitting is die opwekking en beheer van veld- en verhittingspatrone. 'n Tegniek wat voorsiening maak vir die sintese van verskillende, voorafbepaalde verhittingspatrone deur interferensie word hier voorgestel. Die voorgestelde beginsel kan beskryf word deur die term "interferensie modulasie". Interferensie modulasie is 'n tegniek wat spesifieke patrone, genoem kenmerke, moontlik maak deur seine van verskillende bronne met mekaar te laat interfereer. Bekende kenmerke kan geselekteer word deur die relatiewe fases van die seine te moduleer. Gewigte word aan hierdie kenmerke toegeken wat oor tyd gekombineer kan word om nuwe verhittingspatrone te vorm. Faseveranderings kan dan gebruik word om na 'n spesifieke, bekende kenmerk te skakel met gewigte wat die bydrae van elke kenmerk van die verlangde algehele patroon bepaal. Magnetrons word gebruik as bronne in die praktiese implimentering wat beskryf word. Elkeen van hierdie nouband, hoë drywing bronne is injeksie-gesluit met 'n lae drywing beheersein. Die beheerseine is afgekoppel van 'n verwysingsbron en hul fases is gestelom 'n ooreenstemmende kenmerk te verkry. Berekening en meting toon dat betroubare sluiting voorkom wanneer die beheersein ten minste 3% van die magnetron se uittree-drywing is. Metings van patrone is gemaak met materiale wat in lae en blokke gevorm is. Sommiges is met chemikalië voorberei sodat die algehele verhittingspatroon gesien kan word. Die waargeneemde patrone, simulasies en veldmetings stem goed ooreen en bevestig die beginsel en werking van die voorgestelde topologie.