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M.Phil. (Optometry) / The accommodative-convergence accommodation (AC/A) ratio is a commonly used relationship in the practice of optometry. Many practitioners make use of the AC/A ratio as an aid to diagnostic and prognostic decisions. It is not perfectly clear what effects positive and negative lenses have on the AC/A ratio. A number of questions remain relating to linearity, symptomatology and equality of lens effects on the AC/A ratio. This experiment was designed and conducted in an attempt to answer some of these unanswered questions and forms the foundation of this thesis. A literature review of available knowledge related to this study is detailed. An instrument is described which was constructed to measure the necessary accommodation and vergence changes induced by various stimuli. A sample of first year students at the Rand Afrikaans University department of Optometry was selected according to various acceptance criteria. A total of 109 students were screened , of which a group of 26 students was subjected to the experimental investigation. The data were then subjected to a statistical analysis in an attempt to reveal correlations, linearity and group formations.

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Colon cancer (CC) ranks high in morbidity and mortality amongst the most frequent occurring cancers worldwide. Mortality rates are mostly caused by mis-diagnosis and the poor efficacy of treatment. The aim of this study was to enhance our insights of quantum dots, for early detection and targeted drug delivery, thereby reducing toxicity to normal cells and reducing side effects that are caused by previous colon cancer medicine. The synthesis, characterization and cytotoxicity studies of CdSe/ZnSe quantum dots (QDs), nanocrystals are reported. Toxicological properties of the Cd2+ core are reduced by capping quantum dots with ZnSe, varying chain length and functional group ligands. Fluorescence wavelength and their size is improved by varying Cd2+ source and varying nanocrystal synthesis growth temperature. CdSe/ZnSe quantum dots are characterized with FT-IR to elucidate their structure. High-resolution transmission electron microscopy (HRTEM), X-Ray Diffraction (EDX), Photoluminescence spectroscopy (PL) and Ultraviolet-visible spectroscopy (UV-Vis) are used to measure their size and composition. Ligand exchange reactions are conducted with the use of 3-Mercaptopropanoic acid (3-MPA) to facilitate bio-compatibility and stability of CdSe/ZnSe QDs. Temperature stability of various ligand capped and stabilized CdSe/ZnSe QDs are measured by using thermogravimetric analysis (TGA). Caco-2 cell line is cultured from colon cancer, and cytotoxic studies are conducted to test for cell viability of various capped 3-Mercaptopropanoic acid (3-MPA) CdSe/ZnSe quantum dots at various concentrations. Myristic acid capped CdSe/ZnSe quantum dots produce high fluorescing mono-disperse quantum dots. The capping material, synthesis temperature and Cd2+ source of CdSe/ZnSe QDs affect fluorescence wavelength and thermal stability of quantum dots. Fluorescence wavelength is improved by using CdCl2.7H2O source of Cd2+. Cytotoxicity was found to be dependent on the concentration and the capping material of quantum dots. CdSe/ZnSe quantum dots toxicity is adjusted and reduced by varying the length, size and type of the capping ligand on the surface of quantum dots.

Optical techniques were used to study the critical behaviour of the pure fluids CHF₃, CCIF₃ and Xe, and binary mixtures He-Xe and nicotine + water. We find that for all these substances, the order parameter is described by a power law in the reduced temperature t = (Tc - T)/Tc with a leading exponent β = 0.327 ± 0.002. Also, we determine the first correction to scaling exponent to be Δ = 0.43 ± 0.02 for the pure fluids and Δ = 0.50 ± 0.02 for the He-Xe system. The coexistence curve diameter in CHF₃ and CCIF₃ exhibits a deviation from rectilinear diameter, in agreement with a modern theory which interprets this behaviour as resulting from three-body effects. In contrast, no such deviation is observed in Xe where, according to that theory, it should be more pronounced than in other substances. In the polar fluid CHF₃, the order parameter, isothermal compressibility and the chemical potential along the critical isotherm were simultaneously measured in the same experiment in an effort to ensure self-consistency of the results. From the data, two amplitude ratios which are predicted to be universal are determined: Γ+⃘ /Γ-⃘= 4.8 ± 0.6 and D⃘Γ+⃘B⃘δ-₁ = 1.66 ± 0.14. In the binary liquid system nicotine + water, the diffusivity was measured both by light scattering and by interferometry. The results agree qualitatively, but differ by a factor of ≈ 2. From the light scattering data, the critical exponent of the viscosity is found to be zη = 0.044 ± 0.008. The interferometric experiments on Xe and He-Xe furnish a direct way to maesure the effects of wetting: From the data, the exponent of the surface tension is found to be n = 1.24 ± 0.06. The similarity of the order parameter and compressibility in Xe and a He-Xe mixture containing 5% He indicate that the phase transition in this He-Xe mixture is of the liquid-gas type rather than the binary liquid type. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The goal of the thesis is to make contributions to the development of two new technologies for data storage: perpendicular recording and magneto-optic recording. CoCr and rare earth-transition metal multilayers are the most suitable media for perpendicular recording and magneto-optic recording technologies, respectively. In part A of the thesis, magnetic properties of CoCr thin films produced by dc magnetron sputtering are studied for various deposition conditions. Dielectric constants and extraordinary Hall effect are also studied to provide information complementary to magnetic properties. In part B, new methods are developed for theoretical analysis of the magneto-optics of rare earth-transition metal multilayers, which can be used to optimize the readout of the recording system. Part A For dc magnetron sputtered CoCr films the perpendicular and parallel magnetic coercivities are found to be mainly controlled by the substrate temperature during film growth. Substrate temperatures between 180 and 300 C are necessary to fabricate CoCr thin films for recording media. Films produced in this manner have magnetic anisotropy constants ranging from —1.0 to +0.5 10⁶erg/cc. The magnetic anisotropy has a complicated dependence on a large number of deposition parameters and can be best controlled by the dc sputtering power and the target-to-substrate distance. Based on microstructural analysis film properties are interpreted in terms of the adatom diffusion during film growth. It is found that high adatom mobility and low deposition rate promote positive magnetic anisotropy. The dielectric constants measured by ellipsometry are found to depend on the film thickness because of the change in film morphology during film growth. The effects of asymmetric sputtering are analyzed, and the relationship between the extraordinary Hall effect and the magnetic properties is investigated. Part B The 4x4 matrix method proposed by Lin-Chung and Teitler[P. J. Lin-Chung and S. Teitler, J. Opt. Soc. Am. A 1 703(1984)] is applied to the magneto-optics of the rare earth-transition metal multilayer system. Based on a plane wave model, the above method enables one to calculate the sensitivity of the readout to the layer thicknesses as well as effects of oblique angle of incidence, anisotropy in the nonmagnetic part of the dielectric constants and misalignment of the magnetization. Finally, an improved model is presented to take into account the fact that the reading laser is a strongly focused beam instead of a plane wave. This new model is used to optimize the magneto-optic multilayer system. When the focal spot size of the reading laser beam is less than about three wavelengths, significantly different results are obtained from the focused beam and the plane wave models. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

M.Sc. (Physics) / Please refer to full text to view abstract

Lidar has the ability to supplant or compliment many current measurement technologies in ocean optics. Lidar measures Inherent Optical Properties over long distances without impacting the orientation and assemblages of particles it measures, unlike many systems today which require pumps and flow cells. As an active sensing technology, it has the benefit of being independent of time of day and weather. Techniques to interpret oceanographic lidar lags behind atmospheric lidar inversion techniques to measure optical properties due to the complexity and variability of the ocean. Unlike in the atmosphere, two unknowns in the lidar equation backscattering at 180o (𝛽𝜋) and attenuation (c) do not necessarily covary. A lidar system developed at the Harbor Branch Oceanographic Institute is used as a test bed to validate a Monte-Carlo model to investigate the inversion of optical properties from lidar signals. Controlled tank experiments and field measurements are used to generate lidar waveforms and provide optical situations to model. The Metron EODES backscatter model is used to model waveforms. A chlorophyll based forward optical model provides a set of 1500 unique optical situations which are modeled to test inversion techniques and lidar geometries. Due to issues with the lidar system and model the goal of validating the model as well as a more mature inversion experiment were not completed. However, the results are valuable to show the complexity and promise of lidar systems. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2020. / FAU Electronic Theses and Dissertations Collection

El texto completo de este trabajo no está disponible en el Repositorio Académico UPC por restricciones de la casa editorial donde ha sido publicado. / The electronic structure, band structure and optical properties of compounds GaAs(1−x)Mx (M = Fe,Cu), for x=0.25, 0.75, and 1 are discussed via Full-potential linearized augmented plane wave calculations. An increase in absorption in the visible spectrum is observed for all compounds, which, however, is associated with an increasingly metallic character as the metal concentration increases, to the point that compound GaFe is an indirect degenerate semiconductor. The addition of Fe and Cu to the compound decreases its stability, as demonstrated by the formation enthalpies, which become progressively more positive as the content in As is decreased. The calculations were performed using both the PBE and PBEsol exchange correlation potentials and the TB-mBJ method was used to verify the band structure obtained. / Consejo Nacional de Ciencia, Tecnología e Innovación Tecnológica / Revisón por pares

Nowadays, studying materials for renewable energy applications are highly de- manded. Hybrid halide perovskites have proven to be promising materials for such technology since their first application in solar cells in 2008, with a power conversion efficiency of 2.7%. Since then, hybrid halide perovskites have proven their superior properties for light-absorbing devices. In this scope, studying the optical properties is ultimately essential. This work investigates the tempera- ture dependence of the optical spectra for formamidinium lead iodide/bromide perovskites (FAPb[IxBr1-x]3 (0 ≤ x ≤ 1) using spectroscopic ellipsometry mea- surements, empirical optical modeling, density functional theory, and molecular dynamics. Five FAPb[IxBr1-x]3 perovskite samples were fabricated by a hybrid processing technique. External Quantum Efficiency measurements reported an energy bandgap range between 1.58 eV and 1.77 eV for the resulted samples. Next, multi-angle spectroscopic ellipsometry measurements were applied with a temperature-controlled stage, allowing the variance of temperature from 25 ◦C to 75 ◦C. The results show a blue shift in the optical spectra at elevated tempera- tures. We then conducted a temperature-dependent empirical model that predicts the optical spectra for the sample of study at higher temperatures using input data of the spectra at room temperature. The model reports low mean squared errors which are less than ≈ 2 around the bandgap, and further development can be applied for better utilization. First-principles investigations were conducted on four FAPb[IxBr1-x]3 per- ovskite unit cells. Structural optimization was applied with assuming fixed angles of the lattice. Atomic configuration was chosen to achieve minimal ground state energies. Ab initio molecular dynamics simulations were applied to each opti- mized structures at target temperatures of 300 K and 350 K using Berendsen thermostat. The simulation time was 4ps with 1fs time step, and the electronic energy bandgap was calculated at each step using PBE functional. The simula- tions reported a rotational motion for the FA molecule that showed to be faster at 350 K, along with higher mean energy bandgap compared to the reported value at 300 K. The optical spectra were extracted using a snapshot from the resulted structures. Similar to the spectroscopic ellipsometry measurements, a temperature induced blue shift was reported. Overall, this work detects and predicts the temperature-dependent optical spectra and confirms the role of the atomic thermal motion. With further devel- opment, higher accuracy can be achieved along with broadening the materials of study for photovoltaic and optoelectronic applications.