In iron-doped lithium niobate and other similar crystals, exposure to light of appropriate wavelength induces small changes in the refractive index. This phenomenon is called the photorefractive effect. It allows phase holograms to be stored in these crystals. The work to be described was undertaken to obtain a better understanding of the mechanisms of the photorefractive effect and to investigate possible engineering applications. The photorefractive effect is believed to involve the spatial redistribution of photoexcited electrons among traps. This causes a space charge field to develop-which modulates the refractive index via the linear electro-optic effect. A new bulk photovoltaic effect special to ferroelectric crystals, first recognized by Glass et al., is important in the photorefractive effect in these crystals. It is shown that the finite electron transport length in this effect makes the photovoltaic current distribution spatially shifted from the light intensity pattern that causes it. Moreover, it is shown that the spatially varying photovoltaic current component which is responsible for the hologram formation decreases as the spatial frequency of the light interference pattern increases. Hologram writing by the photorefractive effect is modelled for arbitrary electron transport length. The treatment allows for the feedback effect of the space charge field and for the dark conductivity of the crystal. The model applies to uniform illumination and constant applied voltage conditions. It is shown that except in crystals where diffusion dominates the .hologram is spatially shifted from the light intensity pattern that caused it because of the finite electron transport length associated with the bulk photovoltaic effect. Experimental results which bear upon the bulk photovoltaic effect and the associated electron transport length are reported. Hologram writing with an arbitrary one-dimensional light intensity distribution is modelled allowing for the feedback effect of the space charge field at all writing times, A large scale space charge field associated with the envelope of the light is shown to affect the writing process. It is found that for any type of intensity distribution an increase in the fraction of the crystal which is illuminated improves the efficiency of the hologram writing process. Also for partially illuminated crystals the storage efficiency improves as the photoconductivity approaches the dark conductivity value from above. For a fully illuminated crystal the storage efficiency improves as the ratio of the photoconductivity to dark conductivity increases. Experimental observations of the effect of the large scale field on hologram storage are reported. Beam distortion and optically induced scattering are two problems encountered while storing holograms in lithium hipbate. We report experimental observations and theoretical models for these phenomena. It is shown that beam distortion is due to the defocusing action of the large scale refractive index change due to the envelope of light. Light scattering is suggested to be due to the lens action of the index variations due to laser speckles inside the crystal. A theoretical treatment of the spatial filtering properties of volume holograms is presented. Practical applications of volume holograms in the fields of interferometric testing and optical. communications are also discussed. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Unknown

A thesis submitted to the Faculty of Science, University of the Witwatersrand, in fulfilment of the requirements for the degree of doctor of philosophy (Physics) School of Physics. University of the Witwatersrand, Johannesburg. 24 March 2017. / This thesis is an accumulation of the work and that was carried out and published as two articles and two book chapters. Throughout the thesis, we develop and present theoretical as well as numerical model to extend the existing techniques to study the optical properties of photonic crystals, plasmonic photonic crystals and photonic quasicrystals. We start with a background review, where we cover the theoretical aspects of light–matter interaction. That is followed by a review of the physics of photonic crystals. In that chapter, we discuss the different properties of photonic crystals, plasmonic photonic crystals as well as the topic of localization. We then delve into the numerical aspects of the subject. We provide a review on the frequency domain method and the finite–differences–time–domain methods which they are both used in the work to perform different types of simulations. The frequency domain method is, then, extended to enable the numerical analysis of the optical properties in plasmonic photonic crystals. We use first order perturbation theory to study the effect of surface plasmon polaritons on the photonic band structure of plasmonic photonic crystals. We developed a simple numerical tool that extends the standard frequency domain methods to compute the photonic band structure of plasmonic photonic crystals. We then employ the two stage cut and project scheme to generate a dodecagonal two–dimensional quasiperiodic structure. The finite-differences-time–domain method is applied to simulate the propagation of electromagnetic modes in the system. We compute the transmission coefficients as well as the inverse participation ratio for a quasicrystal consisting of dielectric cylindrical rods. The analysis has shown that crystal has critical states. Furthermore, we apply the frequency domain method to quantify the localized modes in the vicinity of defects in a two–dimensional photonic crystal. We compute the intensity of those modes in the surroundings of the defects sites to identify their nature. Finally, we use the finite–differences–time–domain method to provide a second example of a quasicrystalline structure, where the states are localized. / GR2018

The aim of this PhD thesis is to incorporate deformation twinning in a fullield viscoplastic crystal plasticity model based on fast Fourier transform in an effort to gain insights into its role on strain localization. This work is motivated by current experimental evidences on the important role that dislocation reactions at the twin interface play on damage initiation in materials during plastic deformation. We began first by investigating the role of slip on stress localization. To this end, we simulated the effect of macroscopic deformation path, which dictates a macroscopic stress state, as well as pre-existing microstructure in typical ferritic steel, where plastic deformation is accommodated by slip mechanism. The results show that the width of localized strain rate regions near grain boundaries is a function of the deformation path, and there is a positive correlation between local Taylor factor and local stress field, which slightly depends on deformation path. For the incorporation of mechanical twinning in twinning-induced plasticity (TWIP) steel, we implemented predominant reorientation scheme (PTR) in vpFFT, which was implemented previously in the mean field VPSC. The comparison between experimental and simulation results indicates that twin volume fraction, final texture, and stress-strain curve were satisfactorily predicted. Despite that predominant twin reorientation scheme was not suitable to capture lamellar shape of twins in the microstructure, twin domains were predicted to form and grow at or close to grain boundary regions. Finally, we surveyed current literature, which aimed at capturing the characteristic lamellar morphology of twins. Literature review shows several unsuccessful crystal plasticity simulations in capturing twin nucleation and twin lamellar shape at measocale. These inabilities can be attributed to i) twin nucleation that is controlled by local atomistic configurations and stress fluctuations at the grain boundaries, and ii) the random or stochastic nature of twin nucleation, which has been proved by EBSD observation. Based on the EBSD observations, twin nucleation depends on both microstructural (e.g, grain size, dislocation density) and loading conditions ( e.g, stress, strain). Furthermore, the propensity, frequency, and morphology of deformation twins are different among grain with the same orientation and applied boundary conditions.

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A dissertation submitted in fulfilment of the requirements for the Degree of Master of Science in Archaeology to the Faculty of Science, University of Witwatersrand, 2019 / The study examines the technology of crystal quartz pieces from Pomongwe Cave (PMG), Matopos, and south-western Zimbabwe. It focuses on the Middle and Later Stone Age (MSA and LSA) assemblages, which were excavated in the early 1960s by C. K Cooke and is housed at the Zimbabwe Museum of Human Sciences (ZMHS) in Harare. The research employs the chaîne opératoire approach to the study of crystal quartz technology, which follows all the stages of lithic production from the sourcing of the raw material through exploitation, use and discard patterns. It seeks to understand how the hunter-gatherers at Pomongwe (PMG hereafter) exploited this material and how they adapted their technologies to suit the physical and mechanical properties of crystal quartz through time at the site. On a broader scale, the study contributes to our understanding of the development of Modern Human Behaviours (MHBs). The results of the study show that the sourcing of the crystals was mainly from a secondary context throughout the MSA and LSA sequence. The presence and distribution of the material also varied throughout the MSA and LSA sequence, suggesting that the raw material was more prominent with the inhabitants during certain periods than others. The results through classification also show that not all collected crystals were exploited for lithic production, which raises questions as to why they were collected. The cores and blanks show that the crystals were mainly opened from the pyramid and exploited going towards the base of the crystal. The most notable change in the exploitation of crystal quartz throughout the MSA and LSA sequence at PMG is that of the reduction in the sizes of the blanks going towards the LSA. / TL (2020)

<p> A high temperature polymorph of α-Cu2P2O7 was found above 70°C. The lattice parameters and space groups of both phases were determined from X-ray photographs. The crystal structures of α and β-Cu2P2O7 were refined by crystallographic least squares analysis and the molecular geometry obtained was compared with that of other closely related compounds of the transition metal ion pyrophosphate series. The central oxygen atom was found to have enhanced thermal motion or disorder in agreement with I.R. spectroscopic studies of Lazarev. The bond lengths obtained for the P2O7^4- ion were discussed in reference to values predicted by Cruickshank. Evidence of a Jahn-Teller effect was found in the case of the Cu++ ion. A description is given of a relatively efficient method of measuring intensities of Bragg reflections, using a single crystal diffractometer.</p> / Thesis / Master of Science (MSc)

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