Wave scattering is the phenomenon in which a wave field interacts with physical objects. An incoming wave is scattered at the surface of the object and a scattered wave is produced. Common practical cases are acoustic, electromagnetic and elastic wave scattering. The numerical simulation of the scattering process is important, for example, in noise control, antenna design, prediction of radar cross sections and nondestructive testing. Important classes of numerical methods for accurate simulation of scattering are based on integral representations of the wave fields and theses representations require the knowledge of potentials on the surfaces of the scattering objects. The potential is typically computed by a numerical approximation of an integral equation that is defined on the surface. We first develop such numerical methods in time domain for the scalar wave equation. The efficiency of the techniques are improved by analytic quadrature and in some cases by local approximation of the potential. Most scattering simulations are done for harmonic or single frequency waves. In the electromagnetic case the corresponding integral equation method is called the method of moments. This numerical approximation is computationally very costly for high frequency waves. A simplification is suggested by physical optics, which directly gives an approximation of the potential without the solution of an integral equation. Physical optics is however only accurate for very high frequencies. In this thesis we improve the accuracy in the physical optics approximation of scalar waves by basing the computation of the potential on the theory of radiation boundary conditions. This theory describes the local coupling of derivatives in the wave field and if it is applied at the surface of the scattering object it generates an expression for the unknown potential. The full wave field is then computed as for other integral equation methods. The new numerical techniques are analyzed mathematically and their efficiency is established in a sequence of numerical experiments. The new on surface radiation conditions give, for example, substantial improvement in the estimation of the scattered waves in the acoustic case. This numerical experiment corresponds to radar cross-section estimation in the electromagnetic case.

Este trabajo resuelve el problema de gran interés para la industria aeronáutica: el cálculo de la sección recta (RCS) de blancos radar complejos en tiempo real. Para ello basta una estación de trabajo con un acelerador grafico 3-d hardware en lugar de los super-ordenadores vectoriales que requieren los métodos clásicos. El método desarrollado totalmente original e innovador, consiste en procesar gráficamente una imagen del blanco presente en la pantalla de la estación grafica. Con ello se consigue que el tiempo de cpu sea independiente del tamaño y complejidad del blanco, con lo que su principal aplicación es la estimación rápida de la RCS de blancos radar complejos. Este método, al que hemos denominado "procesado grafico", implementa las aproximaciones asintóticas de alta frecuencia que permiten obtener las principales contribuciones a la RCS: óptica física, para la reflexión en superficies; condiciones de contorno de impedancia, para la reflexión en recubrimientos absorbentes radar (RAM), método de las corrientes equivalentes para la disposición en aristas y un método grafico de iluminación global (radiosity) para las reflexiones múltiples entre superficies. Los resultados de este trabajo hacen posible, por vez primera, el diseño interactivo de formas de baja RCS "stealth" con una estación de trabajo, por lo que se ha conseguido una herramienta software para la industria aeronáutica de gran eficiencia y potencialidad en sus aplicaciones.

[pt] Antenas refletoras para cobertura omnidirecional vêm sendo utilizadas em diversos estudos de micro-ondas e ondas milimétricas. A principal motivação para trabalhar nestas bandas, entre outras aplicações, é o desenvolvimento de sistemas de comunicação sem fio de banda larga. O presente trabalho utiliza uma lente dielétrica na abertura do alimentador da antena refletora para reduzir a largura do feixe e, simultaneamente, evitar a presença de lóbulos laterais na região de cobertura. Uma apropriada modelagem da lente reduz o tamanho da antena sem degradar as características de radiação. O trabalho é dividido em duas partes: a modelagem de lentes dielétricas; e a modelagem de reflertores. A modelagem de lentes dielétricas circularmente simétricas utilizando os princípios da Óptica Geométrica para controlar a largura de feixe do diagrama de radiação transmitido pela lente. O modelo é feito a partir do deslocamento do foco da geratriz da lente, o novo foco virtual é o ponto geométrico onde convergem os raios emergentes da lente. A rotação da geratriz da superfície refletora faz que este ponto se torne um anel cáustico virtual. As lentes modeladas têm como alimentador uma corneta coaxial que fornece um diagrama de radiação circularmente simétrico. O diagrama de radiação transmitido pela lente é calculado usando as aproximações da ótica geométrica e ótica física em campo distante e próximo. Os resultados são comparados com os resultados simulados em um software especializado de simulação eletromagnética. A modelagem de refletores baseada nas propriedades da Ótica Geométrica. O refletor é uma superfície de revolução obtida através da rotação de uma geratriz em torno ao eixo de simetria. Neste trabalho, a geratriz é descrita por uma sucessão de seções de cônicas concatenadas, esta metodologia utiliza o diagrama de radiação transmitido da lente para obter uma distribuição de campo no plano vertical previamente especificada, que nosso caso é uma distribuição constante. O diagrama de radiação em campo distante radiado pelo refletor é calculado usando as aproximações da física ótica, estes resultados são 6 comparados com os resultados simulados em um software especializado em simulação eletromagnética. / [en] Reflector antennas for omnidirectional coverage have been considered in several studies of microwave and millimeter wave. The main motivation to work in these bands, among other applications, is the development of wireless broadband communication systems. This work uses a dielectric lens in the aperture of the reflector antenna feeder to reduce the beamwidth and simultaneously avoid the presence of side lobes in the coverage area. An appropriate lens modeling reduces the antenna size without degrading the radiation characteristics. The work is divided into two parts: the modeling of lens and modeling of reflector.The modeling of lens using the principles of the Geometrical Optics to control the radiation pattern transmitted by the lens. The modeling depends on the lens focus displacement, which is the geometric point of convergence of the rays emerging from the lens. The rotation of the lens generatrix causes this point to become a virtual caustic ring. The lens has a coaxial feed horn that provides a circularly symmetrical radiation pattern. The radiation pattern transmitted by the lens is calculated using the approximation of Geometric Optics and Physical Optics in the near and far field; these results are compared with the simulated results in a specialized electromagnetic simulation software. The reflector modeling is based on the properties of Geometric Optics. The reflector is a revolution surface obtained by rotating a generatrix around the symmetry axis. In this work, the generatrix is described by a sequence of concatenated conical sections. This method uses the transmitted radiation pattern of the lens to obtain a previously specified field distribution in the vertical plane, which is a constant distribution in the present case. The far field radiation pattern of the reflector is calculated using the Physical Optics approximation. These results are compared with the simulated results in a specialized electromagnetic simulation software.

While exact methods, such as DDA or T-matrix, can be applied to particles withsizes comparable to the wavelength, computational demands mean that they are size limited. For particles much larger than the wavelength, the Geometric Optics approximation can be employed, but in doing so wave effects, such as interference and diffraction, are ignored. In between these two size extremes there exists a need for computational techniques which are capable of handling the wide array of ice crystal shapes and sizes that are observed in cirrus clouds. The Beam Tracing model developed within this project meets these criteria. It combines aspects of geometric optics and physical optics. Beam propagation is handled by Snell's law and the law of reflection. A beam is divided into reflected and transmitted components each time a crystal facet is illuminated. If the incident beam illuminates multiple facets it is split, with a new beam being formed for each illuminated facet. The phase-dependent electric field amplitude of the beams is known from their ampli- tude (Jones) matrices. These are modified by transmission and reflection matrices, whose elements are Fresnel amplitude coefficients, each time a beam intersects a crystal facet. Phase tracing is carried out for each beam by considering the path that its 'centre ray' would have taken. The local near-field is then mapped, via a surface integral formulation of a vector Kirchhoff diffraction approximation, to the far-field. Once in the far-field the four elements of the amplitude matrix are trans- formed into the sixteen elements of the scattering matrix via known relations. The model is discussed in depth, with details given on its implementation. The physical basis of the model is given through a discussion of Ray Tracing and how this leads to the notion of Beam Tracing. The beam splitting algorithm is described for convex particles followed by the necessary adaptations for concave and/or ab- sorbing particles. Once geometric aspects have been established details are given as to how physical properties of beams are traced including: amplitude, phase and power. How diffraction is implemented in the model is given along with a review of existing diffraction implementations. Comparisons are given, first against a modified Ray Tracing code to validate the geometric optics aspects of the model. Then, specific examples are given for the cases of transparent, pristine, smooth hexagonal columns of four different sizes and orientations; a highly absorbing, pristine, smooth hexagonal column and a highly absorbing, indented, smooth hexagonal column. Analysis of two-dimensional and one-dimensional intensity distributions and degree of linear polarisation results are given for each case and compared with results acquired through use of the Amster- dam Discrete-Dipole Approximation (ADDA) code; with good agreement observed. To the author's best knowledge, the Beam Tracer developed here is unique in its ability to handle concave particles; particles with complex structures and the man- ner in which beams are divided into sub-beams of quasi-constant intensity when propagating in an absorbing medium. One of the model's potential applications is to create a database of known particle scattering patterns, for use in aiding particle classification from images taken by the Small Ice Detector (SID) in-situ probe. An example of creating such a database for hexagonal columns is given.

The molecular design, synthesis, spectroscopic and photophysical characterization of a series of cyanostilbene-based compounds are studied in this thesis. The thermal, electrochemical and aggregation induced emission (AIE) properties of these cyanostilbene-based compounds, as well as their application in organic lighting-emitting diodes, live cell imaging, chemical vapor sensor were investigated. Chapter 1 gives a brief introduction on the aggregation-caused quenching (ACQ) behavior of the conventional organic luminogens and the discovery and proposed mechanism of AIE phenomenon. Furthermore, some examples and the applications of these AIE compounds will be discussed. In Chapter 2, triphenylamine- and carbazole-containing cyanostilbene-based derivatives are presented. From the examination of the emission profile, they are all AIE-active through comparison of the photoluminescence intensity in dissolved and in aggregated states. Additionally, the calculation of the enhancement ratio (I/I0 – 1) of each fluorophore was performed in order to quantify its AIE effect. One of our cyanostilbene-based luminogens has achieved an enhancement ratio with a value of 1128. This cyanostilbene-based luminogens has also shown good performance in OLED investigation. In addition to the OLEDs application, the selected cyanostilbene-based luminogens with solid-state emission, cell-permeability and reversible switch-on/off capability have illustrated the positive result in live-cell imaging and chemical vapor sensing. Conjugated polymer with high molecular weight is the superior option by overcoming the weaknesses of low-molecular-weight luminogens with excellent thin-film form ability and comparatively simple and inexpensive fabrication processes. The design and synthesis of the cyanostilbene-based polymeric chromophores are described in Chapter 3. The polymerization of the AIE-active diacetylene ligands by connection of trans-[Pt(PBu3)2] unit at both ends has successfully retained their AIE behavior. In contrast, the ACQ problem has occurred on the polymers with organic spacers and the AIE-active ligands. From the DFT calculation on the Pt polymers and the blue shift of emission spectra in high water content suggested that the AIE phenomenon of Pt polymers is probably originated from the elimination of the non-radiative intramolecular charge transfer (ICT) process. Owing to the high demand in red-emitting materials in the applications of electroluminescent devices, fluorescent sensing and bio-imaging, effort has been made to design a system with the new chromophores with donor (D) – acceptor (A) system and thus to synthesize phenothiazine (D)-containing cyanostilbene (A)-based derivatives which are depicted in Chapter 4. Consistent with the conventional AIE-active luminogens with a successively climb of photoluminescence intensities in response to the increase of water proportion, phenothiazine-containing cyanostilbene-based derivatives has exhibited a V-shape fashion of emission intensity. It suggests that the emission of chromophores started to be quenched due to the increase of solvent polarity, overriding that of the molecular aggregation when a “small” volume of water is being introduced. While aggregate formation was dominant from the addition of a “large” amount of poor solvent, less polar local environment was created which suppressed the non-radiative transition to the ICT state and intensified the emission efficiency. Phenothiazine (D) – cyanostilbene (A) system has created a series of red-emitting chromophores with great tunability for the sake of achieving the desired emission color and better emission efficiency. To functionalize these AIE-active cyanostilbene-based chromophores, pyridine group was attached to the compounds to take the advantage of its metal-chelating capability, which is discussed in Chapter 5. The AIE features of cyanostilbene-based compounds can be preserved after the introduction of the pyridyl unit. Even it possessed a weak photoluminescence in its dilute solution which suggest that the high electron delocalization within the molecule has rigidified the structure to some extent, it is transformed to a highly emissive state with a high proportion of water. The exclusive variation of emission behavior with obvious bathochromic shift and boost of emission spectrum in the presence of cadmium-(II) ion has demonstrated its potential metal ion sensing ability. Chapter 6 and 7 present the concluding remarks and the experimental data of the compounds of Chapter 2 to 5, respectively.

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.