Mechanical characterization of GdBCO/Ag and YBCO single grains fabricated by top-seeded melt growth at 77 and 300 K

YBaCuO and GdBaCuO + 15 wt% Ag large, single-grain, bulk superconductors have been fabricated via the top-seeded, melt-growth (TSMG) process using a generic NdBCO seed. The mechanical behavior of both materials has been investigated by means of three-point bending (TPB) and transversal tensile tests at 77 and 300 K. The strength, fracture toughness and hardness of the samples were studied for two directions of applied load to obtain comprehensive information about the effect of microstructural anisotropy on the macroscopic and microscopic mechanical properties of these technologically important materials. Splitting (Brazilian) tests were carried out on as-melt-processed cylindrical samples following a standard oxygenation process and with the load applied parallel to the growth-facet lines characteristic of the TSMG process. In addition, the elastic modulus of each material was measured by three different techniques and related to the microstructure of each sample using optical microscopy. The results show that both the mechanical properties and the elastic modulus of both YBCO and GdBCP/Ag are improved at 77 K. However, the GdBCO/Ag samples are less anisotropic and exhibit better mechanical behavior due to the presence of silver particles in the bulk, superconducting matrix. The splitting tensile strength was determined at 77 K and both materials were found to exhibit similar behavior, independently of their differences in microstructure.

Contribution to the analysis and design of reflectarray antennas for reconfigurable beam applications at frequencies above 100 GHz using liquid crystal technology

El trabajo contenido en esta tesis doctoral está encuadrado en el desarrollo de antenas reconfigurables electrónicamente capaces de proporcionar prestaciones competitivas a las aplicaciones cada vez más comunes que operan a frecuencias superiores a 60 GHz. En concreto, esta tesis se centra en el estudio, diseño, e implementación de las antenas reflectarray, a las que se introduce la tecnología de cristal líquido como elemento característico con el que se consigue reconfigurabilidad de haz de forma electrónica. Desde un punto de vista muy general, se puede describir un cristal líquido como un material cuya permitividad eléctrica es variable y controlada por una excitación externa, que generalmente suele corresponderse con un campo eléctrico quasi-estático (AC). Las antenas reflectarray de cristal líquido se han escogido como objeto de estudio por varias razones. La primera de ellas tiene que ver con las ventajas que los reflectarrays, y en especial aquellos realizados en configuración planar, proporcionan con respecto a otras antenas de alta ganancia como los reflectores o los “phased-arrays”. En los reflectarrays, la alimentación a través de una fuente primaria común (característica de reflectores) y el elevado número de grados de libertad de las celdas que los componen (característica de arrays) hacen que estas antenas puedan proporcionar prestaciones eléctricas iguales o mejores que las anteriores, a un coste más reducido y con estructuras de antena más compactas. La segunda razón radica en la flexibilidad que ofrece el cristal líquido a ser confinado y polarizado en recintos de geometría variada, como consecuencia de su fluidez (propiedad de los líquidos). Por ello, la tecnología de cristal líquido permite que el propio elemento reconfigurable en las celdas de reflectarray se adapte a la configuración planar de manera que en sí mismo, el cristal líquido sea una o varias de las capas características de esta configuración. Esto simplifica de forma drástica la estructura y la fabricación de este tipo de antenas, incluso si se comparan con reflectarrays reconfigurables basados en otras tecnologías como diodos, MEMS, etc. Por tanto, su coste y desarrollo es muy reducido, lo que hace que se puedan fabricar reflectarrays reconfigurables eléctricamente grandes, a bajo coste, y en producción elevada. Un ejemplo claro de una estructura similar, y que ha tenido éxito comercial, son las pantallas de cristal líquido. La tercera razón reside en el hecho de que el cristal líquido es, hasta la fecha, de las pocas tecnologías capaces de ofrecer reconfigurabilidad del haz a frecuencias superiores a 60 GHz. De hecho, el cristal líquido permite reconfigurabilidad en un amplio margen de frecuencias, que va desde DC a frecuencias del espectro visible, incluyendo las microondas y los THz. Otras tecnologías, como los materiales ferroeléctricos, el grafeno o la tecnología CMOS “on chip” permiten también conmutar el haz en estas frecuencias. Sin embargo, la tecnología CMOS tiene un elevado coste y actualmente está limitada a frecuencias inferiores a 150 GHz, y aunque los materiales ferroeléctricos o el grafeno puedan conmutar a frecuencias más altas y en un rango más amplio, tienen serias dificultades que los hacen aún inmaduros. En el caso de los materiales ferroeléctricos, los elevados voltajes para conmutar el material los hacen poco atractivos, mientras que en el caso del grafeno, su modelado aún está en discusión, y todavía no se han arrojado resultados experimentales que validen su idoneidad. Estas tres razones hacen que los reflectarrays basados en cristal líquido sean atractivos para multitud de aplicaciones de haz reconfigurable a frecuencias superiores a 60 GHz. Aplicaciones como radar de escaneo de imágenes de alta resolución, espectroscopia molecular, radiómetros para observación atmosférica, o comunicaciones inalámbricas de alta frecuencia (WiGig) son algunas de ellas. La tesis está estructurada en tres partes. En la primera de ellas se describen las características más comunes de los cristales líquidos, centrándonos en detalle en aquellas propiedades ofrecidas por este material en fase nemática. En concreto, se estudiará la anisotropía dieléctrica (Ae) de los cristales líquidos uniaxiales, que son los que se emplean en esta tesis, definida como la diferencia entre la permitividad paralela (£//) y la perpendicular (e±): Ae = e,, - e±. También se estudiará la variación de este parámetro (Ae) con la frecuencia, y el modelado electromagnético macroscópico más general que, extraído a partir de aquella, permite describir el cristal líquido para cada tensión de polarización en celdas de geometría planar. Este modelo es de suma importancia para garantizar precisión en el desfasaje proporcionado por las diferentes celdas reconfigurables para reflectarrays que se describirán en la siguiente parte de la tesis. La segunda parte de la tesis se centra en el diseño de celdas reflectarray resonantes basadas en cristal líquido. La razón por la que se escogen estos tipos de celdas reside en el hecho de que son las únicas capaces de proporcionar rangos de fase elevados ante la reducida anisotropía dieléctrica que ofrecen los cristales líquidos. El objetivo de esta parte trata, por tanto, de obtener estructuras de celdas reflectarray que sean capaces de proporcionar buenas prestaciones eléctricas a nivel de antena, mejorando sustancialmente las prestaciones de las celdas reportadas en el estado del arte, así como de desarrollar una herramienta de diseño general para aquellas. Para ello, se estudian las prestaciones eléctricas de diferentes tipos de elementos resonantes de cristal líquido que van, desde el más sencillo, que ha limitado el estado de la técnica hasta el desarrollo de esta tesis y que está formado por un sólo resonador, a elementos que constan de varios resonadores (multi-resonantes) y que pueden ser monocapa o multicapa. En un primer paso, el procedimiento de diseño de estas estructuras hace uso de un modelo convencional de cristal líquido que ha venido siendo usado en el estado del arte para este tipo de celdas, y que considera el cristal líquido como un material homogéneo e isótropo cuya permitividad varía entre (e/7) y (e±). Sin embargo, en esta parte de la tesis se demuestra que dicho modelado no es suficiente para describir de forma genérica el comportamiento del cristal líquido en las celdas tipo reflectarray. En la tesis se proponen procedimientos más exactos para el análisis y diseño basados en un modelo más general que define el cristal líquido como un material anisótropo e inhomogeneo en tres dimensiones, y se ha implementado una técnica que permite optimizar celdas multi-resonantes de forma eficiente para conseguir elevadas prestaciones en cuanto a ancho de banda, rango de fase, pérdidas, o sensibilidad al ángulo de incidencia. Los errores cometidos en el uso del modelado convencional a nivel de celda (amplitud y fase) se han analizado para varias geometrías, usando medidas de varios prototipos de antena que usan un cristal líquido real a frecuencias superiores a 100 GHz. Las medidas se han realizado en entorno periódico mediante un banco cuasi-óptico, que ha sido diseñado especialmente para este fin. Uno de estos prototipos se ha optimizado a 100 GHz para conseguir un ancho de banda relativamente elevado (10%), pérdidas reducidas, un rango de fase mayor de 360º, baja sensibilidad al ángulo de incidencia, y baja influencia de la inhomogeneidad transversal del cristal líquido en la celda. Estas prestaciones a nivel de celda superan de forma clara aquellas conseguidas por otros elementos que se han reportado en la literatura, de manera que dicho prototipo se ha usado en la última parte de la tesis para realizar diversas antenas de barrido. Finalmente, en esta parte se presenta una estrategia de caracterización de la anisotropía macroscópica a partir de medidas de los elementos de reflectarray diseñados en banco cuasi-óptico, obteniendo resultados tanto en las frecuencias de interés en RF como en AC, y comparándolas con aquellas obtenidas mediante otros métodos. La tercera parte de la tesis consiste en el estudio, diseño, fabricación y medida de antenas reconfigurables basadas en cristal líquido en configuraciones complejas. En reflectarrays pasivos, el procedimiento de diseño de la antena se limita únicamente al ajuste en cada celda de la antena de las dimensiones de las metalizaciones que se emplean para el control de fase, mediante procesos de optimización bien conocidos. Sin embargo, en el caso de reflectarrays reconfigurables basados en cristal líquido, resulta necesario un paso adicional, que consiste en calcular de forma adecuada las tensiones de control en cada celda del reflectarray para configurar la fase requerida en cada una de ellas, así como diseñar la estructura y los circuitos de control que permitan direccionar a cada elemento su tensión correspondiente. La síntesis de tensiones es por tanto igual o más importante que el diseño de la geometría de las celdas, puesto que éstas son las que están directamente relacionadas con la fase. En el estado del arte, existen varias estrategias de síntesis de tensiones que se basan en la caracterización experimental de la curva de fase respecto al voltaje. Sin embargo, esta caracterización sólo puede hacerse a un solo ángulo de incidencia y para unas determinadas dimensiones de celda, lo que produce que las tensiones sintetizadas sean diferentes de las adecuadas, y en definitiva que se alcancen errores de fase mayores de 70º. De esta forma, hasta la fecha, las prestaciones a nivel de antena que se han conseguido son reducidas en cuanto a ancho de banda, rango de escaneo o nivel de lóbulos secundarios. En esta última parte de la tesis, se introduce una nueva estrategia de síntesis de tensiones que es capaz de predecir mediante simulaciones, y con alta precisión, las tensiones que deben introducirse en cada celda teniendo en cuenta su ángulo de incidencia, sus dimensiones, la frecuencia, así como la señal de polarización definida por su frecuencia y forma de onda AC. Esta estrategia se basa en modelar cada uno de los estados de permitividad del cristal líquido como un sustrato anisótropo con inhomogeneidad longitudinal (1D), o en ciertos casos, como un tensor equivalente homogéneo. La precisión de ambos modelos electromagnéticos también se discute. Con el objetivo de obtener una herramienta eficiente de cálculo de tensiones, también se ha escrito e implementado una herramienta de análisis basada en el Método de los Momentos en el Dominio Espectral (SD-MoM) para sustratos estratificados anisótropos, que se usa en cada iteración del procedimiento de síntesis para analizar cada una de las celdas de la antena. La síntesis de tensiones se ha diseñado además para reducir al máximo el efecto del rizado de amplitud en el diagrama de radiación, que es característico en los reflectarrays que están formados por celdas con pérdidas elevadas, lo que en sí, supone un avance adicional para la obtención de mejores prestaciones de antena. Para el cálculo de los diagramas de radiación empleados en el procedimiento de síntesis, se asume un análisis elemento a elemento considerando periodicidad local, y se propone el uso de un método capaz de modelar el campo incidente de forma que se elimine la limitación de la periodicidad local en la excitación. Una vez definida la estrategia adecuada de cálculo de las tensiones a aplicar al cristal líquido en cada celda, la estructura de direccionamiento de las mismas en la antena, y diseñados los circuitos de control, se diseñan, fabrican y miden dos prototipos diferentes de antena de barrido electrónico a 100 GHz usando las celdas anteriormente presentadas. El primero de estos prototipos es un reflectarray en configuración “single offset” con capacidad de escaneo en un plano (elevación o azimut). Aunque previamente se realizan diseños de antenas de barrido en 2D a varias frecuencias en el rango de milimétricas y sub-milimétricas, y se proponen ciertas estrategias de direccionamiento que permiten conseguir este objetivo, se desarrolla el prototipo con direccionamiento en una dimensión con el fin de reducir el número de controles y posibles errores de fabricación, y así también validar la herramienta de diseño. Para un tamaño medio de apertura (con un numero de filas y columnas entre 30 y 50 elementos, lo que significa un reflectarray con un número de elementos superior a 900), la configuración “single offset” proporciona rangos de escaneo elevados, y ganancias que pueden oscilar entre los 20 y 30 dBi. En concreto, el prototipo medido proporciona un haz de barrido en un rango angular de 55º, en el que el nivel de lóbulos secundarios (SLL) permanece mejor de -13 dB en un ancho de banda de un 8%. La ganancia máxima es de 19.4 dBi. Estas prestaciones superan de forma clara aquellas conseguidas por otros autores. El segundo prototipo se corresponde con una antena de doble reflector que usa el reflectarray de cristal líquido como sub-reflector para escanear el haz en un plano (elevación o azimut). El objetivo básico de esta geometría es obtener mayores ganancias que en el reflectarray “single offset” con una estructura más compacta, aunque a expensas de reducir el rango de barrido. En concreto, se obtiene una ganancia máxima de 35 dBi, y un rango de barrido de 12º. Los procedimientos de síntesis de tensiones y de diseño de las estructuras de las celdas forman, en su conjunto, una herramienta completa de diseño precisa y eficiente de antenas reflectarray reconfigurables basados en cristales líquidos. Dicha herramienta se ha validado mediante el diseño, la fabricación y la medida de los prototipos anteriormente citados a 100 GHz, que consiguen algo nunca alcanzado anteriormente en la investigación de este tipo de antenas: unas prestaciones competitivas y una predicción excelente de los resultados. El procedimiento es general, y por tanto se puede usar a cualquier frecuencia en la que el cristal líquido ofrezca anisotropía dieléctrica, incluidos los THz. Los prototipos desarrollados en esta tesis doctoral suponen también unas de las primeras antenas de barrido real a frecuencias superiores a 100 GHz. En concreto, la antena de doble reflector para escaneo de haz es la primera antena reconfigurable electrónicamente a frecuencias superiores a 60 GHz que superan los 25 dBi de ganancia, siendo a su vez la primera antena de doble reflector que contiene un reflectarray reconfigurable como sub-reflector. Finalmente, se proponen ciertas mejoras que aún deben se deben realizar para hacer que estas antenas puedan ser un producto completamente desarrollado y competitivo en el mercado. ABSTRACT The work presented in this thesis is focused on the development of electronically reconfigurable antennas that are able to provide competitive electrical performance to the increasingly common applications operating at frequencies above 60 GHz. Specifically, this thesis presents the study, design, and implementation of reflectarray antennas, which incorporate liquid crystal (LC) materials to scan or reconfigure the beam electronically. From a general point of view, a liquid crystal can be defined as a material whose dielectric permittivity is variable and can be controlled with an external excitation, which usually corresponds with a quasi-static electric field (AC). By changing the dielectric permittivity at each cell that makes up the reflectarray, the phase shift on the aperture is controlled, so that a prescribed radiation pattern can be configured. Liquid Crystal-based reflectarrays have been chosen for several reasons. The first has to do with the advantages provided by the reflectarray antenna with respect to other high gain antennas, such as reflectors or phased arrays. The RF feeding in reflectarrays is achieved by using a common primary source (as in reflectors). This arrangement and the large number of degrees of freedom provided by the cells that make up the reflectarray (as in arrays), allow these antennas to provide a similar or even better electrical performance than other low profile antennas (reflectors and arrays), but assuming a more reduced cost and compactness. The second reason is the flexibility of the liquid crystal to be confined in an arbitrary geometry due to its fluidity (property of liquids). Therefore, the liquid crystal is able to adapt to a planar geometry so that it is one or more of the typical layers of this configuration. This simplifies drastically both the structure and manufacture of this type of antenna, even when compared with reconfigurable reflectarrays based on other technologies, such as diodes MEMS, etc. Therefore, the cost of developing this type of antenna is very small, which means that electrically large reconfigurable reflectarrays could be manufactured assuming low cost and greater productions. A paradigmatic example of a similar structure is the liquid crystal panel, which has already been commercialized successfully. The third reason lies in the fact that, at present, the liquid crystal is one of the few technologies capable of providing switching capabilities at frequencies above 60 GHz. In fact, the liquid crystal allows its permittivity to be switched in a wide range of frequencies, which are from DC to the visible spectrum, including microwaves and THz. Other technologies, such as ferroelectric materials, graphene or CMOS "on chip" technology also allow the beam to be switched at these frequencies. However, CMOS technology is expensive and is currently limited to frequencies below 150 GHz, and although ferroelectric materials or graphene can switch at higher frequencies and in a wider range, they have serious difficulties that make them immature. Ferroelectric materials involve the use of very high voltages to switch the material, making them unattractive, whereas the electromagnetic modelling of the graphene is still under discussion, so that the experimental results of devices based on this latter technology have not been reported yet. These three reasons make LC-based reflectarrays attractive for many applications that involve the use of electronically reconfigurable beams at frequencies beyond 60 GHz. Applications such as high resolution imaging radars, molecular spectroscopy, radiometers for atmospheric observation, or high frequency wireless communications (WiGig) are just some of them. This thesis is divided into three parts. In the first part, the most common properties of the liquid crystal materials are described, especially those exhibited in the nematic phase. The study is focused on the dielectric anisotropy (Ac) of uniaxial liquid crystals, which is defined as the difference between the parallel (e/7) and perpendicular (e±) permittivities: Ae = e,, - e±. This parameter allows the permittivity of a LC confined in an arbitrary volume at a certain biasing voltage to be described by solving a variational problem that involves both the electrostatic and elastic energies. Thus, the frequency dependence of (Ae) is also described and characterised. Note that an appropriate LC modelling is quite important to ensure enough accuracy in the phase shift provided by each cell that makes up the reflectarray, and therefore to achieve a good electrical performance at the antenna level. The second part of the thesis is focused on the design of resonant reflectarray cells based on liquid crystal. The reason why resonant cells have been chosen lies in the fact that they are able to provide enough phase range using the values of the dielectric anisotropy of the liquid crystals, which are typically small. Thus, the aim of this part is to investigate several reflectarray cell architectures capable of providing good electrical performance at the antenna level, which significantly improve the electrical performance of the cells reported in the literature. Similarly, another of the objectives is to develop a general tool to design these cells. To fulfill these objectives, the electrical yields of different types of resonant reflectarray elements are investigated, beginning from the simplest, which is made up of a single resonator and limits the state of the art. To overcome the electrical limitations of the single resonant cell, several elements consisting of multiple resonators are considered, which can be single-layer or multilayer. In a first step, the design procedure of these structures makes use of a conventional electromagnetic model which has been used in the literature, which considers that the liquid crystal behaves as homogeneous and isotropic materials whose permittivity varies between (e/7) y (e±). However, in this part of the thesis it is shown that the conventional modelling is not enough to describe the physical behaviour of the liquid crystal in reflectarray cells accurately. Therefore, a more accurate analysis and design procedure based on a more general model is proposed and developed, which defines the liquid crystal as an anisotropic three-dimensional inhomogeneous material. The design procedure is able to optimize multi-resonant cells efficiently to achieve good electrical performance in terms of bandwidth, phase range, losses, or sensitivity to the angle of incidence. The errors made when the conventional modelling (amplitude and phase) is considered have been also analysed for various cell geometries, by using measured results from several antenna prototypes made up of real liquid crystals at frequencies above 100 GHz. The measurements have been performed in a periodic environment using a quasi-optical bench, which has been designed especially for this purpose. One of these prototypes has been optimized to achieve a relatively large bandwidth (10%) at 100 GHz, low losses, a phase range of more than 360º, a low sensitivity to angle of incidence, and a low influence of the transversal inhomogeneity of the liquid crystal in the cell. The electrical yields of this prototype at the cell level improve those achieved by other elements reported in the literature, so that this prototype has been used in the last part of the thesis to perform several complete antennas for beam scanning applications. Finally, in this second part of the thesis, a novel strategy to characterise the macroscopic anisotropy using reflectarray cells is presented. The results in both RF and AC frequencies are compared with those obtained by other methods. The third part of the thesis consists on the study, design, manufacture and testing of LCbased reflectarray antennas in complex configurations. Note that the design procedure of a passive reflectarray antenna just consists on finding out the dimensions of the metallisations of each cell (which are used for phase control), using well-known optimization processes. However, in the case of reconfigurable reflectarrays based on liquid crystals, an additional step must be taken into account, which consists of accurately calculating the control voltages to be applied to each cell to configure the required phase-shift distribution on the surface of the antenna. Similarly, the structure to address the voltages at each cell and the control circuitry must be also considered. Therefore, the voltage synthesis is even more important than the design of the cell geometries (dimensions), since the voltages are directly related to the phase-shift. Several voltage synthesis procedures have been proposed in the state of the art, which are based on the experimental characterization of the phase/voltage curve. However, this characterization can be only carried out at a single angle of incidence and at certain cell dimensions, so that the synthesized voltages are different from those needed, thus giving rise to phase errors of more than 70°. Thus, the electrical yields of the LCreflectarrays reported in the literature are limited in terms of bandwidth, scanning range or side lobes level. In this last part of the thesis, a new voltage synthesis procedure has been defined and developed, which allows the required voltage to be calculated at each cell using simulations that take into account the particular dimensions of the cells, their angles of incidence, the frequency, and the AC biasing signal (frequency and waveform). The strategy is based on the modelling of each one of the permittivity states of the liquid crystal as an anisotropic substrate with longitudinal inhomogeneity (1D), or in certain cases, as an equivalent homogeneous tensor. The accuracy of both electromagnetic models is also discussed. The phase errors made by using the proposed voltage synthesis are better than 7º. In order to obtain an efficient tool to analyse and design the reflectarray, an electromagnetic analysis tool based on the Method of Moments in the spectral domain (SD-MoM) has also written and developed for anisotropic stratified media, which is used at each iteration of the voltage synthesis procedure. The voltage synthesis is also designed to minimize the effect of amplitude ripple on the radiation pattern, which is typical of reflectarrays made up of cells exhibiting high losses and represents a further advance in achieving a better antenna performance. To calculate the radiation patterns used in the synthesis procedure, an element-by-element analysis is assumed, which considers the local periodicity approach. Under this consideration, the use of a novel method is proposed, which avoids the limitation that the local periodicity imposes on the excitation. Once the appropriate strategy to calculate the voltages to be applied at each cell is developed, and once it is designed and manufactured both the structure to address the voltages to the antenna and the control circuits, two complete LC-based reflectarray antennas that operate at 100 GHz have been designed, manufactured and tested using the previously presented cells. The first prototype consists of a single offset reflectarray with beam scanning capabilities on one plane (elevation and azimuth). Although several LC-reflectarray antennas that provide 2-D scanning capabilities are also designed, and certain strategies to achieve the 2-D addressing of the voltage are proposed, the manufactured prototype addresses the voltages in one dimension in order to reduce the number of controls and manufacturing errors, and thereby validating the design tool. For an average aperture size (with a number of rows and columns of between 30 and 50 elements, which means a reflectarray with more than 900 cells), the single offset configuration provides an antenna gain of between 20 and 30 dBi and a large scanning range. The prototype tested at 100 GHz exhibits an electronically scanned beam in an angular range of 55º and 8% of bandwidth, in which the side lobe level (SLL) remains better than -13 dB. The maximum gain is 19.4 dBi. The electrical performance of the antenna is clearly an improvement on those achieved by other authors in the state of the art. The second prototype corresponds to a dual reflector antenna with a liquid crystal-based reflectarray used as a sub-reflector for beam scanning in one plane (azimuth or elevation). The main objective is to obtain a higher gain than that provided by the single offset configuration, but using a more compact architecture. In this case, a maximum gain of 35 dBi is achieved, although at the expense of reducing the scanning range to 12°, which is inherent in this type of structure. As a general statement, the voltage synthesis and the design procedure of the cells, jointly make up a complete, accurate and efficient design tool of reconfigurable reflectarray antennas based on liquid crystals. The tool has been validated by testing the previously mentioned prototypes at 100 GHz, which achieve something never reached before for this type of antenna: a competitive electrical performance, and an excellent prediction of the results. The design procedure is general and therefore can be used at any frequency for which the liquid crystal exhibits dielectric anisotropy. The two prototypes designed, manufactured and tested in this thesis are also some of the first antennas that currently operate at frequencies above 100 GHz. In fact, the dual reflector antenna is the first electronically scanned dual reflector antenna at frequencies above 60 GHz (the operation frequency is 100 GHz) with a gain greater than 25 dBi, being in turn the first dual-reflector antenna with a real reconfigurable sub-reflectarray. Finally, some improvements that should be still investigated to make these antennas commercially competitive are proposed.

Global instability analysis and control of three-dimensional long open cavity flow

The three-dimensional wall-bounded open cavity may be considered as a simplified geometry found in industrial applications such as leading gear or slotted flats on the airplane. Understanding the three-dimensional complex flow structure that surrounds this particular geometry is therefore of major industrial interest. At the light of the remarkable former investigations in this kind of flows, enough evidences suggest that the lateral walls have a great influence on the flow features and hence on their instability modes. Nevertheless, even though there is a large body of literature on cavity flows, most of them are based on the assumption that the flow is two-dimensional and spanwise-periodic. The flow over realistic open cavity should be considered. This thesis presents an investigation of three-dimensional wall-bounded open cavity with geometric ratio 6:2:1. To this aim, three-dimensional Direct Numerical Simulation (DNS) and global linear instability have been performed. Linear instability analysis reveals that the onset of the first instability in this open cavity is around Recr 1080. The three-dimensional shear layer mode with a complex structure is shown to be the most unstable mode. I t is noteworthy that the flow pattern of this high-frequency shear layer mode is similar to the observed unstable oscillations in supercritical unstable case. DNS of the cavity flow carried out at different Reynolds number from steady state until a nonlinear saturated state is obtained. The comparison of time histories of kinetic energy presents a clearly dominant energetic mode which shifts between low-frequency and highfrequency oscillation. A complete flow patterns from subcritical cases to supercritical case has been put in evidence. The flow structure at the supercritical case Re=1100 resembles typical wake-shedding instability oscillations with a lateral motion existed in the subcritical cases. Also, This flow pattern is similar to the observations in experiments. In order to validate the linear instability analysis results, the topology of the composite flow fields reconstructed by linear superposition of a three-dimensional base flow and its leading three-dimensional global eigenmodes has been studied. The instantaneous wall streamlines of those composited flows display distinguish influence region of each eigenmode. Attention has been focused on the leading high-frequency shear layer mode; the composite flow fields have been fully recognized with respect to the downstream wave shedding. The three-dimensional shear layer mode is shown to give rise to a typical wake-shedding instability with a lateral motions occurring downstream which is in good agreement with the experiment results. Moreover, the spanwise-periodic, open cavity with the same length to depth ratio has been also studied. The most unstable linear mode is different from the real three-dimensional cavity flow, because of the existence of the side walls. Structure sensitivity of the unstable global mode is analyzed in the flow control context. The adjoint-based sensitivity analysis has been employed to localized the receptivity region, where the flow is more sensible to momentum forcing and mass injection. Because of the non-normality of the linearized Navier-Stokes equations, the direct and adjoint field has a large spatial separation. The strongest sensitivity region is locate in the upstream lip of the three-dimensional cavity. This numerical finding is in agreement with experimental observations. Finally, a prototype of passive flow control strategy is applied.

Defending against cybercrime: advances in the detection of malicious servers and the analysis of client-side vulnerabilities

Esta tesis se centra en el análisis de dos aspectos complementarios de la ciberdelincuencia (es decir, el crimen perpetrado a través de la red para ganar dinero). Estos dos aspectos son las máquinas infectadas utilizadas para obtener beneficios económicos de la delincuencia a través de diferentes acciones (como por ejemplo, clickfraud, DDoS, correo no deseado) y la infraestructura de servidores utilizados para gestionar estas máquinas (por ejemplo, C & C, servidores explotadores, servidores de monetización, redirectores). En la primera parte se investiga la exposición a las amenazas de los ordenadores victimas. Para realizar este análisis hemos utilizado los metadatos contenidos en WINE-BR conjunto de datos de Symantec. Este conjunto de datos contiene metadatos de instalación de ficheros ejecutables (por ejemplo, hash del fichero, su editor, fecha de instalación, nombre del fichero, la versión del fichero) proveniente de 8,4 millones de usuarios de Windows. Hemos asociado estos metadatos con las vulnerabilidades en el National Vulnerability Database (NVD) y en el Opens Sourced Vulnerability Database (OSVDB) con el fin de realizar un seguimiento de la decadencia de la vulnerabilidad en el tiempo y observar la rapidez de los usuarios a remiendar sus sistemas y, por tanto, su exposición a posibles ataques. Hemos identificado 3 factores que pueden influir en la actividad de parches de ordenadores victimas: código compartido, el tipo de usuario, exploits. Presentamos 2 nuevos ataques contra el código compartido y un análisis de cómo el conocimiento usuarios y la disponibilidad de exploit influyen en la actividad de aplicación de parches. Para las 80 vulnerabilidades en nuestra base de datos que afectan código compartido entre dos aplicaciones, el tiempo entre el parche libera en las diferentes aplicaciones es hasta 118 das (con una mediana de 11 das) En la segunda parte se proponen nuevas técnicas de sondeo activos para detectar y analizar las infraestructuras de servidores maliciosos. Aprovechamos técnicas de sondaje activo, para detectar servidores maliciosos en el internet. Empezamos con el análisis y la detección de operaciones de servidores explotadores. Como una operación identificamos los servidores que son controlados por las mismas personas y, posiblemente, participan en la misma campaña de infección. Hemos analizado un total de 500 servidores explotadores durante un período de 1 año, donde 2/3 de las operaciones tenían un único servidor y 1/2 por varios servidores. Hemos desarrollado la técnica para detectar servidores explotadores a diferentes tipologías de servidores, (por ejemplo, C & C, servidores de monetización, redirectores) y hemos logrado escala de Internet de sondeo para las distintas categorías de servidores maliciosos. Estas nuevas técnicas se han incorporado en una nueva herramienta llamada CyberProbe. Para detectar estos servidores hemos desarrollado una novedosa técnica llamada Adversarial Fingerprint Generation, que es una metodología para generar un modelo único de solicitud-respuesta para identificar la familia de servidores (es decir, el tipo y la operación que el servidor apartenece). A partir de una fichero de malware y un servidor activo de una determinada familia, CyberProbe puede generar un fingerprint válido para detectar todos los servidores vivos de esa familia. Hemos realizado 11 exploraciones en todo el Internet detectando 151 servidores maliciosos, de estos 151 servidores 75% son desconocidos a bases de datos publicas de servidores maliciosos. Otra cuestión que se plantea mientras se hace la detección de servidores maliciosos es que algunos de estos servidores podrán estar ocultos detrás de un proxy inverso silente. Para identificar la prevalencia de esta configuración de red y mejorar el capacidades de CyberProbe hemos desarrollado RevProbe una nueva herramienta a través del aprovechamiento de leakages en la configuración de la Web proxies inversa puede detectar proxies inversos. RevProbe identifica que el 16% de direcciones IP maliciosas activas analizadas corresponden a proxies inversos, que el 92% de ellos son silenciosos en comparación con 55% para los proxies inversos benignos, y que son utilizado principalmente para equilibrio de carga a través de múltiples servidores. ABSTRACT In this dissertation we investigate two fundamental aspects of cybercrime: the infection of machines used to monetize the crime and the malicious server infrastructures that are used to manage the infected machines. In the first part of this dissertation, we analyze how fast software vendors apply patches to secure client applications, identifying shared code as an important factor in patch deployment. Shared code is code present in multiple programs. When a vulnerability affects shared code the usual linear vulnerability life cycle is not anymore effective to describe how the patch deployment takes place. In this work we show which are the consequences of shared code vulnerabilities and we demonstrate two novel attacks that can be used to exploit this condition. In the second part of this dissertation we analyze malicious server infrastructures, our contributions are: a technique to cluster exploit server operations, a tool named CyberProbe to perform large scale detection of different malicious servers categories, and RevProbe a tool that detects silent reverse proxies. We start by identifying exploit server operations, that are, exploit servers managed by the same people. We investigate a total of 500 exploit servers over a period of more 13 months. We have collected malware from these servers and all the metadata related to the communication with the servers. Thanks to this metadata we have extracted different features to group together servers managed by the same entity (i.e., exploit server operation), we have discovered that 2/3 of the operations have a single server while 1/3 have multiple servers. Next, we present CyberProbe a tool that detects different malicious server types through a novel technique called adversarial fingerprint generation (AFG). The idea behind CyberProbe’s AFG is to run some piece of malware and observe its network communication towards malicious servers. Then it replays this communication to the malicious server and outputs a fingerprint (i.e. a port selection function, a probe generation function and a signature generation function). Once the fingerprint is generated CyberProbe scans the Internet with the fingerprint and finds all the servers of a given family. We have performed a total of 11 Internet wide scans finding 151 new servers starting with 15 seed servers. This gives to CyberProbe a 10 times amplification factor. Moreover we have compared CyberProbe with existing blacklists on the internet finding that only 40% of the server detected by CyberProbe were listed. To enhance the capabilities of CyberProbe we have developed RevProbe, a reverse proxy detection tool that can be integrated with CyberProbe to allow precise detection of silent reverse proxies used to hide malicious servers. RevProbe leverages leakage based detection techniques to detect if a malicious server is hidden behind a silent reverse proxy and the infrastructure of servers behind it. At the core of RevProbe is the analysis of differences in the traffic by interacting with a remote server.

The active digestion of uniparental chloroplast DNA in a single zygote of Chlamydomonas reinhardtii is revealed by using the optical tweezer

The non-Mendelian inheritance of organelle genes is a phenomenon common to almost all eukaryotes, and in the isogamous alga Chlamydomonas reinhardtii, chloroplast (cp) genes are transmitted from the mating type positive (mt+) parent. In this study, the preferential disappearance of the fluorescent cp nucleoids of the mating type negative (mt−) parent was observed in living young zygotes. To study the change in cpDNA molecules during the preferential disappearance, the cpDNA of mt+ or mt− origin was labeled separately with bacterial aadA gene sequences. Then, a single zygote with or without cp nucleoids was isolated under direct observation by using optical tweezers and investigated by nested PCR analysis of the aadA sequences. This demonstrated that cpDNA molecules are digested completely during the preferential disappearance of mt− cp nucleoids within 10 min, whereas mt+ cpDNA and mitochondrial DNA are protected from the digestion. These results indicate that the non-Mendelian transmission pattern of organelle genes is determined immediately after zygote formation.

Efficient construction of a large nonimmune phage antibody library: The production of high-affinity human single-chain antibodies to protein antigens

A large library of phage-displayed human single-chain Fv antibodies (scFv), containing 6.7 × 109 members, was generated by improving the steps of library construction. Fourteen different protein antigens were used to affinity select antibodies from this library. A panel of specific antibodies was isolated with each antigen, and each panel contained an average of 8.7 different scFv. Measurements of antibody–antigen interactions revealed several affinities below 1 nM, comparable to affinities observed during the secondary murine immune response. In particular, four different scFv recognizing the ErbB2 protein had affinities ranging from 220 pM to 4 nM. Antibodies derived from the library proved to be useful reagents for immunoassays. For example, antibodies generated to the Chlamydia trachomatis elementary bodies stained Chlamydia-infected cells, but not uninfected cells. These results demonstrate that phage antibody libraries are ideally suited for the rapid production of panels of high-affinity mAbs to a wide variety of protein antigens. Such libraries should prove especially useful for generating reagents to study the function of gene products identified by genome projects.

Flexibility of the Kir6.2 inward rectifier K+ channel pore

Interactions of sulfhydryl reagents with introduced cysteines in the pore-forming (Kir6.2) subunits of the KATP channel were examined. 2-Aminoethyl methanethiosulfonate (MTSEA+) failed to modify Cd2+-insensitive control-Kir6.2 channels, but rapidly and irreversibly modified Kir6.2[L164C] (L164C) channels. Although a single Cd2+ ion is coordinated by L164C, four MTSEA+ “hits” can occur, each sequentially reducing the single-channel current. A dimeric fusion of control-Kir6.2 and L164C subunits generates Cd2+-insensitive channels, confirming that at least three cysteines are required for coordination, but MTSEA+ modification of the dimer occurs in two hits. L164C channels were not modified by bromotrimethyl ammoniumbimane (qBBr+), even though qBBr+ caused voltage-dependent block (as opposed to modification) that was comparable to that of MTSEA+ or 3-(triethylammonium)propyl methanethiosulfonate (MTSPTrEA+), implying that qBBr+ can also enter the inner cavity but does not modify L164C residues. The Kir channel pore structure was modeled by homology with the KcsA crystal structure. A stable conformation optimally places the four L164C side chains for coordination of a single Cd2+ ion. Modification of these cysteines by up to four MTSEA+ (or three MTSPTrEA+, or two qBBr+) does not require widening of the cavity to accommodate the derivatives within it. However, like the KcsA crystal structure, the energy-minimized model shows a narrowing at the inner entrance, and in the Kir6.2 model this narrowing excludes all ions. To allow entry of ions as large as MTSPTrEA+ or qBBr+, the entrance must widen to >8 Å, but this widening is readily accomplished by minimal M2 helix motion and side-chain rearrangement.

Destruction of a single chlorophyll is correlated with the photoinhibition of photosystem II with a transiently inactive donor side.

Pigments destroyed during photoinhibition of water-splitting photosystem II core complexes from the green alga Chlamydomonas reinhardtii were studied. Under conditions of a transiently inactivated donor side, illumination leads to an irreversible inhibition of the electron transfer at the donor side that is paralleled by the destruction of chlorophylls a absorbing maximally around 674 and 682 nm. The observed stochiometry of 1 +/- 0.1 destroyed chlorophyll per inhibited photosystem II suggests that chlorophyll destruction could be the primary photodamage causing the inhibition of photosystem II under these conditions.

Photoconductivity and Electro-Optical Properties of Wide Band Gap Metal Oxides

The PhD activity described in this Thesis was focused on the study of metal-oxide wide-bandgap materials, aiming at fabricating new optoelectronic devices such as solar-blind UV photodetectors, high power electronics, and gas sensors. Photocurrent spectroscopy and DC photocurrent time evolution were used to investigate the performance of prototypes under different atmospheres, temperatures and excitation wavelengths (or dark conditions). Cathodoluminescence, absorption spectroscopy, XRD and SEM were used to assess structural, morphologic, electrical and optical properties of materials. This thesis is divided into two main sections, each describing the work done on a different metal-oxide semiconductor. 1) MOVPE-grown Ga2O3 thin films for UV solar-blind photodetectors and high power devices The semiconducting oxides, among them Ga2O3, have been employed for several decades as transparent conducting oxide (TCO) electrodes for fabrication of solar cells, displays, electronic, and opto-electronic devices. The interest was mainly confined to such applications, as these materials tend to grow intrinsically n-type, and attempts to get an effective p-type doping has consistently failed. The key requirements of TCO electrodes are indeed high electrical conductivity and good transparency, while crystallographic perfection is a minor issue. Furthermore, for a long period no high-quality substrates and epi-layers were available, which in turn impeded the development of a truly full-oxide electronics. Recently, Ga2O3 has attracted renewed interest, as large single crystals and high-quality homo- and hetero-epitaxial layers became available, which paved the way to novel application areas. Our research group spent the last two years in developing a low temperature (500-700°C) MOVPE growth procedure to obtain thin films of Ga2O3 on different substrates (Dept. of Physics and IMEM-CNR at UNIPR). We obtained a significant result growing on oriented sapphire epitaxial films of high crystalline, undoped, pure phase -Ga2O3 (hexagonal). The crystallographic properties of this phase were investigated by XRD, in order to clarify the lattice parameters of the hexagonal cell. First design and development of solar blind UV photodetectors based on -phase was carried out and the optoelectronic performance is evaluated by means of photocurrent spectroscopy. The UV-response is adequately fast and reliable to render this unusual phase a subject of great interest for future applications. The availability of a hexagonal phase of Ga2O3 stable up to 700°C, belonging to the same space group of gallium nitride, with high crystallinity and tunable electrical properties, is intriguing in view of the development of nitride-based devices, by taking advantage of the more favorable symmetry and epitaxial relationships with respect to the monoclinic β-phase. In addition, annealing at temperatures higher than 700°C demonstrate that the hexagonal phase converts totally in the monoclinic one. 2) ZnO nano-tetrapods: charge transport mechanisms and time-response in optoelectronic devices and sensors Size and morphology of ZnO at the nanometer scale play a key role in tailoring its physical and chemical properties. Thanks to the possibility of growing zinc oxide in a variety of different nanostructures, there is a great variety of applications, among which gas sensors, light emitting diodes, transparent conducting oxides, solar cells. Even if the operation of ZnO nanostructure-based devices has been recently demonstrated, the mechanisms of charge transport in these assembly is still under debate. The candidate performed an accurate investigation by photocurrent spectroscopy and DC-photocurrent time evolution of electrical response of both single-tetrapod and tetrapod-assembly devices. During the research done for this thesis, a thermal activation energy enables the performance of samples at high temperatures (above about 300°C). The energy barrier is related to the leg-to-leg interconnection in the assembly of nanotetrapods. Percolation mechanisms are responsible for both the very slow photo-response (minutes to hours or days) and the significant persistent photocurrent. Below the bandgap energy, electronic states were investigated but their contribution to the photocurrent are two-three order of magnitude lower than the band edge. Such devices are suitable for employ in photodetectors as well as in gas sensors, provided that the mechanism by which the photo-current is generated and gas adsorption on the surface modify the conductivity of the material are known.

Optical and ultraviolet observations of a strong flare in the young, single K2 dwarf LQ Hya

We present high-resolution optical echelle spectra and IUE observations during a strong flare on 1993 December 22 in the very active, young, rapidly rotating, single K2 dwarf LQ Hya. The initial impulsive phase of the flare, which started sometime between 2:42 ut and 4:07 ut, was characterized by strong optical continuum enhancement and blueshifted emission lines with broad wings. The optical chromospheric lines reached their maximum intensity at ≈ 5:31 ut, by which time the blueshift vanished and the optical continuum enhancement had sharply decreased. Thereafter, the line emission slowly decreased and the lines redshift in a gradual phase that lasted at least two more hours. The Mg II lines behaved similarly. Quiescent C IV flux levels were not recovered until 21 h later, though a data gap and a possible second flare make the interpretation uncertain. In addition to the typically flare-enhanced emission lines (e.g., H α and H β), we observe He I D_3 going into emission, plus excess emission (after subtraction of the quiescent spectrum) in other He I and several strong neutral metal lines (e.g., Mg I b). Flare enhancement of the far-ultraviolet continuum generally agrees with an Si I recombination model. We estimate the total flare energy, and discuss the broad components, asymmetries and Doppler shifts seen in some of the emission lines.

Optical probing of spin fluctuations of a single paramagnetic Mn atom in a semiconductor quantum dot

We analyzed the photoluminescence intermittency generated by a single paramagnetic spin localized in an individual semiconductor quantum dot. The statistics of the photons emitted by the quantum dot reflect the quantum fluctuations of the localized spin interacting with the injected carriers. Photon correlation measurements, which are reported here, reveal unique signatures of these fluctuations. A phenomenological model is proposed to quantitatively describe these observations, allowing a measurement of the spin dynamics of an individual magnetic atom at zero magnetic field. These results demonstrate the existence of an efficient spin-relaxation channel arising from a spin exchange with individual carriers surrounding the quantum dot. A theoretical description of a spin-flip mechanism involving spin exchange with surrounding carriers gives relaxation times in good agreement with the measured dynamics.

Tight-Binding Approximations in 1D and 2D Coupled-Cavity Photonic Crystal Structures

Light confinement and controlling an optical field has numerous applications in the field of telecommunications for optical signals processing. When the wavelength of the electromagnetic field is on the order of the period of a photonic microstructure, the field undergoes reflection, refraction, and coherent scattering. This produces photonic bandgaps, forbidden frequency regions or spectral stop bands where light cannot exist. Dielectric perturbations that break the perfect periodicity of these structures produce what is analogous to an impurity state in the bandgap of a semiconductor. The defect modes that exist at discrete frequencies within the photonic bandgap are spatially localized about the cavity-defects in the photonic crystal. In this thesis the properties of two tight-binding approximations (TBAs) are investigated in one-dimensional and two-dimensional coupled-cavity photonic crystal structures We require an efficient and simple approach that ensures the continuity of the electromagnetic field across dielectric interfaces in complex structures. In this thesis we develop \textrm{E} -- and \textrm{D} --TBAs to calculate the modes in finite 1D and 2D two-defect coupled-cavity photonic crystal structures. In the \textrm{E} -- and \textrm{D} --TBAs we expand the coupled-cavity \overrightarrow{E} --modes in terms of the individual \overrightarrow{E} -- and \overrightarrow{D} --modes, respectively. We investigate the dependence of the defect modes, their frequencies and quality factors on the relative placement of the defects in the photonic crystal structures. We then elucidate the differences between the two TBA formulations, and describe the conditions under which these formulations may be more robust when encountering a dielectric perturbation. Our 1D analysis showed that the 1D modes were sensitive to the structure geometry. The antisymmetric \textrm{D} mode amplitudes show that the \textrm{D} --TBA did not capture the correct (tangential \overrightarrow{E} --field) boundary conditions. However, the \textrm{D} --TBA did not yield significantly poorer results compared to the \textrm{E} --TBA. Our 2D analysis reveals that the \textrm{E} -- and \textrm{D} --TBAs produced nearly identical mode profiles for every structure. Plots of the relative difference between the \textrm{E} and \textrm{D} mode amplitudes show that the \textrm{D} --TBA did capture the correct (normal \overrightarrow{E} --field) boundary conditions. We found that the 2D TBA CC mode calculations were 125-150 times faster than an FDTD calculation for the same two-defect PCS. Notwithstanding this efficiency, the appropriateness of either TBA was found to depend on the geometry of the structure and the mode(s), i.e. whether or not the mode has a large normal or tangential component.

Production of superpositions of coherent states in traveling optical fields with inefficient photon detection

We develop an all-optical scheme to generate superpositions of macroscopically distinguishable coherent states in traveling optical fields. It nondeterministically distills coherent-state superpositions (CSS's) with large amplitudes out of CSS's with small amplitudes using inefficient photon detection. The small CSS's required to produce CSS's with larger amplitudes are extremely well approximated by squeezed single photons. We discuss some remarkable features of this scheme: it effectively purifies mixed initial states emitted from inefficient single-photon sources and boosts negativity of Wigner functions of quantum states.

Simple microcavity for single-photon generation

A new design of an optical resonator for generation of single-photon pulses is proposed. The resonator is made of a cylindrical or spherical piece of a polymer squeezed between two flat dielectric mirrors. The mode characteristics of this resonator are calculated numerically. The numerical analysis is backed by a physical explanation. The decay time and the mode volume of the fundamental mode are sufficient for achieving more than 96% probability of generating a single-photon in a single-mode. The corresponding requirement for the reflectivity of the mirrors (similar to 99.9%) and the losses in the polymer ( 100 dB/m) are quite modest. The resonator is suitable for single-photon generation based on optical pumping of a single quantum system such as an organic molecule, a diamond nanocrystal, or a semiconductor quantum dot if they are imbedded in the polymer. (C) 2005 Optical Society of America.