Orientador: Fábio Simões de Vicente / Coorientador: Viviane Pilla / Banca: Alexandre Mesquista / Banca: Maria Inês Basso Bernardi / Resumo: Materiais híbridos de Sílica modificados organicamente (ORMOSIL) são muito estudados devido à variedade de alcóxidos funcionalizados com moléculas orgânicas (epóxi, vinil, amina, fenil, etc). O estudo das propriedades de materiais híbridos de sílica/orgânico preparados via sol-gel dopados com nanocristais de semicondutores do tipo núcleo/casca é de grande interesse, pois possibilita que propriedades ópticas e mecânicas sejam controladas a partir da síntese, produzindo materiais multifuncionais que apresentam características híbridas da matriz sílica/orgânico aliada aos fenômenos ópticos produzidos pelo dopante. Neste trabalho foram estudadas as propriedades ópticas de matrizes de sílica/orgânico à base de 3-glicidoxipropiltrimetoxisilano (GPTS) e tetraetilortosilicato (TEOS) preparadas pelo processo sol-gel, dopadas com nanocristais de CdSe/ZnS. As matrizes híbridas dopadas foram caracterizadas por espectroscopia óptica (absorção óptica e luminescência UV/VIS) e espectroscopia termo-óptica (lente térmica). Nanocristais de CdSe/ZnS com três tamanhos diferentes de núcleos (~2 a 5 nm) e banda de emissão centrada em 525, 560 e 615 nm (verde, laranja e vermelho, respectivamente) foram estudados quando inseridos na matriz híbrida e as caracterizações foram realizadas in-situ na matriz desde o estado inicial sol e durante a cinética de gelificação. Durante a cinética de gelificação, catalisada pela adição de NH4OH, foi observado a diminuição da intensidade de luminescência dos nanocristais com mínimo no ponto de gel da matriz e subsequente desvio para o azul com acentuado aumento da intensidade de luminescência relacionada com a consolidação da matriz. Através dos espectros de densidade óptica foi possível determinar a variação da energia de gap das amostras durante a cinética de gelificação. As medidas de lente térmica foram realizadas usando laser de excitação e prova para estudar os... / Abstract: Organically Modified Silicates (ORMOSIL) have been object of study due to the variety of the organically functionalized alkoxides (epoxy, vinyl, phenyl, etc). This allows the production of new hybrid silica materials with adjustable properties depending on the organic molecule linked to silica. The study of the properties of CdSe/ZnS core-shell nanocrystals embedded in GPTS/TEOS-derived organic/silica hybrids is of high interest due the optical and mechanical properties which can be controlled from the synthesis, producing multifunctional materials. This work aims the study of the optical properties measured in situ of CdSe/ZnS nanocrystals embedded in organic/silica hybrid colloids prepared by sol-gel technique derived from 3-glycidoxypropyltrimethoxysilane (GPTS) and tetraethylorthosilicate (TEOS). Doped organic/silica hybrid matrices were characterized by optical spectroscopy (absorption and photoluminescence) and thermo-optical spectroscopy (thermal lens). CdSe/ZnS nanocrystals with three different core sizes (~2 -5 nm) and emission band centered at 525, 560 and 615 nm (green, orange and red, respectively) were studied embedded in the hybrid matrix. The optical characterizations were performed in-situ in the matrix from the initial sol and during the gelation kinetics (transition from sol to gel). During the gelation kinetics started by NH4OH addition, an intensity decrease of the nanocrystals photoluminescence was observed during the sol to gel transition, achieving minimum photoluminescence intensity exactly at the matrix gel point with subsequent blue shift and accentuated photoluminescence intensity increase related to the matrix consolidation. Through the optical absorption spectra it was possible to determine the variation in the energy gap of the samples during the kinetics of gelation. TL transient measurements were performed using the mode-mismatched dual-beam configuration to study the effect of the CdSe/ZnS embedded in... / Mestre

O Estudo de Materiais Fotorrefrativos e suas Aplicações no Processamento Óptico Holográfico de Informação vêm sendo feitos por diversos Grupos de Pesquisas em vários laboratórios devido aos bons resultados obtidos com esses meios em diversas áreas. Entre outros efeitos foto-induzidos o processo que possibilita essas aplicações é o efeito fotorrefrativo, um fenômeno onde o registro holográfico no meio fotorrefrativo ocorre pela modulação do índice de refração de acordo com a distribuição espacial da intensidade da luz incidente, devido a uma redistribuição foto-induzida de cargas espaciais, e conseqüente, geração de um campo espaço-carga no meio. Neste trabalho, estuda-se a propagação e acoplamento de ondas eletromagnéticas em monocristais elétro-ópticos paraelétricos da Família das Silenitas \'BI IND. 12\'\'SI\'\'O IND. 20\' (BSO) e \'BI IND. 12\'\'TI\'\'O IND. 20\' (BTO) e nos ferroelétricos \'LI\'\'NB\'\'O IND. 3\' puros e dopados. O propósito de estudar esses materiais é caracterizá-los e, principalmente, otimizá-los para aplicação no processamento óptico e holográfico de informação. Utiliza-se técnicas de caracterização de redes holográficas fotorrefrativas e de lentes foto-induzidas para determinação de figuras de mérito e parâmetros fotorrefrativos e termo-ópticos nestes meios fotorrefrativos. Propõe-se também algumas aplicações no processamento óptico e holográfico de informação, nas áreas de pesquisa básica, tecnológicas e biomédicas, que vêm despertando grande interesse nos últimos anos, como holografia em tempo real com mapeamento de fase, interterometria speckle em tempo real e registro não-holográfico no processo de conversão incoerente-incoerente; entre outras no processamento óptico e holográfico de dados. / The Study of Photorefractive Materials and their Applications in Optical and Holographical Information Processing come being made for diverse Groups of Research in some laboratories due to the good results in many areas. Among other photo-induced effects, the process that allows these applications is the photorefractive effect, a phenomenon where the holographic recording in photorefractive medium occurs by means of the refractive index modulation due to the space distribution of the light intensity, producing a space-charge photo-induced redistribution, and consequently, the generation of a space-charge field governed by an electro-optic medium. In this work, we study the propagation and coupling of electromagnetic waves in electro-optic monocrystals of the sillemite family Bi12SiO20 (BSO) and Bi12TiO20 (BTO) and in the pure and dop-ed LiNbO3 crystals. The purpose to study these materials is to characterize them and, mainly, to optimize the conditions of their application in the optical and holographical information processing. We use photorefractive holographic gratings and photo-induced lens techniques for determination of figures of merit and photorefractive and thermo-optic parameters in these photorefractive materials. We also consider some applications in the optical and holographical information processing in the research basic, technological and biomedical areas, that come waken great interest in the last years, as phase- shifting real-time holography, real-time speckle interferometry and non-holographic recording with incoherent-incoherent conversion process; among others in the optic and holographic data processing.

No description available.

The electromagnetic (EM) bias is the largest source of error in the TOPEX/Poseidon and Jason-1 satellite sea surface height (SSH) estimates. Due to incomplete understanding of the physical processes which cause the bias, current operational models are based on empirical relationships between the bias wind speed and significant wave height. These models reduce RMS estimation errors of the EM bias to approximately 4 cm. To improve EM bias estimation the correlation between the bias and RMS long wave slope is studies using data from tower-based experiments in the Gulf of Mexico and Bass Straight, Australia. Models based on significant wave height and RMS slope are more accurate than models based on wave height and wind speed by at least 50% in RMS error between predicted and ground truth bias values. Nonparametric models have been proposed as a method to reduce the variability of EM bias estimates. Using tower data, nonparametric models developed from wind speed and significant wave height measurements are shown to provide some improvement over parametric models. It is also shown that the historical discrepancy between satellite and tower EM bias measurements is reduced by nonparametric modeling. A validity study of rough surface scattering models is conducted for surfaces with Gaussian and power law power spectra. Models in the study include physical optics (PO), geometrical optics, small perturbation method, and small slope approximation. Due to the prevalence of the PO approximation, particular emphasis is placed on the development of a validity criterion for the PO model. An empirical study of the PO approximation shows that the validity of the model is more accurately described by the RMS wave slope than the classic surface curvature criterion for surfaces with a Gaussian power spectrum. For surfaces with a power law PSD, the accuracy of the PO approximation is related to the significant slope (RMS surface height/wavelength of the dominant spectral peak). The validity of other models in the study are also shown to be well approximated by bounds on surface slope. An EM bias model is derived using the physical optics scattering model, hydrodynamic modulation, and non-Gaussian long wave surface statistics. Using a modulation transfer function, the hydrodynamic modulation of small wave heights is shown to be linearly related to the long wave RMS slope. The resulting EM bias model expresses the relative bias as a function of the long wave surface parameters RMS wave slope, surface skewness, and tilt modulation. Coefficients of the long wave parameters are determined by the short ocean waves, and provide insight into the physical mechanisms that cause the bias. From measured values of the ocean surface profile, estimated values of the bias are computed from the bias model. A comparison of these estimated values with in situ EM bias measurements shows a strong correlation between the estimated and measured values. Nadir and off-nadir measurements of the EM bias collected during the BYU Off-Nadir Experiment (Y-ONE) are presented. The in situ measurements are compared with bias estimates computed from an off-nadir generalization of the nadir EM bias model. From theoretical and experimental bias measurements a model of the angular dependence of the bias is developed as a function of the normalized bias at nadir.

Next generation photonics-based technologies will ultimately rely on novel materials and devices. For this purpose, phenomena at subwavelength scales are being studied to advance both fundamental knowledge and experimental capabilities. In this dissertation, concepts specific to near-field optics and experimental capabilities specific to near-field microscopy are used to investigate various aspects of the statistical properties of random electromagnetic fields in the vicinity of optically inhomogeneous media which emit or scatter radiation. The properties of such fields are being characterized within the frame of the coherence theory. While successful in describing the far-field properties of optical fields, the fundamental results of the conventional coherence theory disregard the contribution of short-range evanescent waves. Nonetheless, the specific features of random fields at subwavelength distances from interfaces of real media are influenced by the presence of evanescent waves because, in this case, both propagating and nonpropagating components contribute to the detectable properties of the radiation. In our studies, we have fully accounted for both contributions and, as a result, different surface and subsurface characteristics of inhomogeneous media could be explored. We investigated different properties of random optical near-fields which exhibit either Gaussian or non-Gaussian statistics. We have demonstrated that characteristics of optical radiation such as first- and second-order statistics of intensity and the spectral density in the vicinity of random media are all determined by both evanescent waves contribution and the statistical properties of the physical interface. For instance, we quantified the subtle differences which exist between the near- and far-field spectra of radiation and we brought the first experimental evidence that, contrary to the predictions of the conventional coherence theory, the values of coherence length in the near field depend on the distance from the interface and, moreover, they can be smaller than the wavelength of light. The results included in this dissertation demonstrate that the statistical properties of the electromagnetic fields which exist in the close proximity of inhomogeneous media can be used to extract structural information. They also suggest the possibility to adjust the coherence properties of the emitted radiation by modifying the statistical properties of the interfaces. Understanding the random interference phenomena in the near-field could also lead to new possibilities for surface and subsurface diagnostics of inhomogeneous media. In addition, controlling the statistical properties of radiation at subwavelength scales should be of paramount importance in the design of miniaturized optical sources, detectors and sensors.

No description available.

Thesis (MScEng)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: Radar Cross Section (RCS) is an important parameter in radar engineering. Often, electrically large structures are of interest in RCS analysis due to the high operating frequencies of radar systems. Simulation of these problems can be more e cient than measurement due to the cost associated with measurement. The Method of Moments/Physical Optics (MoM/PO) hybrid method combines the advantages of the MoM and PO, making it suited to solving electrically large problems that may contain some small complex detail. The requirement for high meshing resolution when analysing some electromagnetic problems, however, signi cantly increases memory requirements. As a result, the hybrid MoM/PO becomes computationally expensive for electrically large problems. In this work, a linear phase term is introduced into the RWG basis function formulation of the MoM/PO hybrid. The addition of the linear phase term allows the use of large triangular mesh elements in the PO region, resulting in the analysis of electrically large problems. The bene t of this formulation is that it allows a reduction in computational cost whilst maintaining the accuracy of the hybrid MoM/PO. This improved hybrid is tested on various planar test cases and results show that it attains the same level of accuracy as the original MoM/PO hybrid. / AFRIKAANSE OPSOMMING: Radardeursnit is 'n belangrike parameter in radaringenieurswese. As gevolg van die hoë frekwensies wat deur baie radarstelsels gebruik word, is elektriesgroot probleme dikwels van belang in die berekening van die radardeursnit van teikens. Die modellering en berekening van die radardeursnit van teikens kan meer kostedoeltre end as metings wees, as gevolg van die beduidende koste van radardeursnitmetings. Die hibriede Moment-Metode/Fisiese-Optika tegniek kombineer die voordele van die twee tegnieke, wat dit geskik maak vir elektries-groot probleme met klein, komplekse detail. Indien die gewone benadering egter gevolg word om 'n hoë resolusie faset-model te gebruik, bly dit berekeningsintensief met groot rekenaar geheuevereistes vir elektries-groot probleme. In hierdie studie word 'n lineêre fase term ingesluit in die formulering van die Rao-Wilton-Glisson (RWG) basisfunksies vorm van die hibriede Moment-Metode/Fisiese-Optika tegniek. Die toevoeging van die lineêre fase term maak dit moontlik om groot driehoekfasette in die Fisiese-Optika gebied te gebruik, wat beteken dat elektries-groot probleme makliker opgelos kan word. Die voordeel van hierdie nuwe formulering is dat die berekeningslas en -tyd verminder word terwyl die akkuraatheid van die oorspronklike hibriede Moment-Metode/Fisiese-Optika tegniek behou word. Hierdie verbeterde hibriede tegniek word getoets aan die hand van verskeie platvlak toetsgevalle en die resultate dui daarop dat die akkuraatheid vergelykbaar is met die van die oorspronklike hibriede Moment-Metode/Fisiese-Optika tegniek.

In high frequency computational electromagnetics, rigorous numerical methods be come unrealistic tools due to computational demand increasing with the frequency. Instead approximations to the solutions of the Maxwell equations can be employed to evaluate th electromagnetic fields. In this thesis, we present the implementations of three high frequency approximat methods. The first two, namely the Geometrical Theory of Diffraction (GTD) and th Physical Optics (PO), are commonly used approximations. The third is a new invention that will be referred to as the Surface Current Extraction-Extrapolation (SCEE). Specifically, the GTD solver is a flexible and modular software package which use Non-Uniform Rational B-spline (NURBS) surfaces to model complex geometries. The PO solver is based on a triangular description of the surfaces and includes fas shadowing by ray tracing as well as contribution from edges to the scattered fields. GTD ray tracing was combined with the PO solver by a well thought-out software architecture Both implementations are now part of the GEMS software suite, the General ElectroMag netic Solvers, which incorporates state-of-the-art numerical methods. During validations both GTD and PO techniques turned out not to be accurate enough to meet the indus trial standards, thus creating the need for a new fast approximate method providing bette control of the approximations. In the SCEE approach, we construct high frequency approximate surface currents ex trapolated from rigourous Method of Moments (MoM) models at lower frequency. T do so, the low frequency currents are projected onto special basis vectors defined on th surface relative to the direction of the incident magnetic field. In such configuration, w observe that each component displays systematic spatial patterns evolving over frequenc in close correlation with the incident magnetic field, thus allowing us to formulate a fre quency model for each component. This new approach is fast, provides good control of th error and represents a platform for future development of high frequency approximations. As an application, we have used these tools to analyse the radar detectability of a new marine distress signaling device. The device, called "Rescue-Wing", works as an inflatabl radar reflector designed to provide a strong radar echo useful for detection and positionin during rescue operations of persons missing at sea. / QC 20101004

<p>Climate change and the energy crisis are substantial challenges facing the human species, and they are projected to threaten life on our planet. For millions of years, the sun has been the main source of energy for life on Earth; this inspires ongoing research efforts focusing on a “sunlight to fuel” energy solution. Photosynthesis is nature’s tool to derive energy from the sun. Hence, scientists focus on the biochemistry of this phenomenon to employ photosynthesis in a man-made device. Such a device is able to convert solar energy to chemical energy through a light-driven cycle of the chemical reactions which produce hydrogen gas, later used as fuel. This process, often called “artificial photosynthesis,” needs efficient catalysts which can be incorporated into a molecular assembly and other microscopic structures or immobilized on an electrode surface. </p><p>Additionally, evolution, in the course of billions of years, chose manganese as an abundant and effective metal to facilitate the process of photosynthesis. These manganese atoms formed a cluster and an optimized ligand field to maximize efficiency. The photochemistry and photo-physics process behind photosynthesis is yet to be fully understood and implemented in a man-made apparatus with comparable efficiency and durability. </p><p>Photosynthesis requires a source of electrons. Water is an abundant molecule on earth that can provide the electrons needed for the photosynthesis. Although water is ubiquitous, it is one of the most stable molecules; hence, splitting it demands a well-designed system with strong oxidizing capability. Because a single atom of oxygen is highly reactive, there should be at least four oxidation states in the system to remove four electrons and release molecular oxygen: O2. The O-O bond formation is one of the most important steps in photosynthesis to fully understand. Lacking a thorough knowledge of this step prevents design and fabrication of robust and active water oxidizing catalysts. To fully understand O-O formation, one should perform a comprehensive study of each of the intermediates of the system. In other words, we need an understanding of the structure and electronic configuration of the system (natural or artificial) from the moment that a water molecule attaches to the catalyst (usually a metal core, central in the complex), until the moment that oxygen released as an O2 molecule. </p><p>There are multiple possible mechanisms to explain O-O formation. Two mechanisms that were extensively studied in this thesis are water nucleophilic attack and radical coupling. The prevailing view about oxygen formation in the catalysts that we study here explains the O-O bond formation by nucleophilic attack of a water molecule to a highly oxidized ruthenium (RuV=O) species. In this hypothesis, all polypyridine ligands that are coordinated to ruthenium remain neutral during the water oxidation process, while the formation of RuV=O (the key intermediate) would require a relatively high free energy (about 1.8 to 2 eV); use of computational (numerical) calculations determine this to be thermodynamically inaccessible. Furthermore, the failure of spectroscopic techniques to confirm the presence of RuV=O calls the validity of this model into question.)</p><p>Alternatively, radical coupling hypothesis considers another pathway to oxygen bond formation. Here, one of the nitrogen atoms coordinated to ruthenium in polypyridine plays a crucial role. We hypothesize that after formation of RuIV=O (which is spectroscopically observed), one nitrogen decoordinates from the metallic core (ruthenium) and oxidizes to form Ru-ON species. This N-oxide ligand can be further oxidized to form a ligand cation radical. It has been shown that [ligand-NO]+• can have almost no energy barrier for O-O bond formation via spin alignment. The study of the role of N-oxide is one of the main focuses of this work. Since this hypothesis does not require RuV=O nor water nucleophilic attack, it explains the process of water oxidation and opens further avenues for the design of future catalysts.</p><p>To confirm our hypothesis, I employed several spectroscopic methods and computational calculations. This new pathway predicts new intermediates exclusive to this model. Our objective is to prove their presence by in situ spectroscopy and test the possibility of formation of each intermediate computationally, to see if their formation is thermodynamically feasible. </p><div><br></div>

The potential for high spatio-temporal resolution microwave measurements has urged the adoption of the signals of opportunity (SoOp) passive radar technique for use in remote sensing. Recent trends in particular target highly complex remote sensing problems such as root-zone soil moisture and snow water equivalent. This dissertation explores the continued open-sourcing of the SoOp coherent bistatic scattering model (SCoBi) and its use in soil moisture sensing applications. Starting from ground-based applications, the feasibility of root-zone soil moisture remote sensing is assessed using available SoOp resources below L-band. A modularized, spaceborne model is then developed to simulate land-surface scattering and delay-Doppler maps over the available spectrum of SoOp resources. The simulation tools are intended to provide insights for future spaceborne modeling pursuits.