Adaptive liquid-crystal phase-only passive devices

This PhD work is focused on liquid crystal based tunable phase devices with special emphasis on their design and manufacturing. In the course of the work a number of new manufacturing technologies have been implemented in the UPM clean room facilities, leading to an important improvement in the range of devices being manufactured in the laboratory. Furthermore, a number of novel phase devices have been developed, all of them including novel electrodes, and/or alignment layers. The most important manufacturing progress has been the introduction of reactive ion etching as a tool for achieving high resolution photolithography on indium-tin-oxide (ITO) coated glass and quartz substrates. Another important manufacturing result is the successful elaboration of a binding protocol of anisotropic conduction adhesives. These have been employed in high density interconnections between ITO-glass and flexible printed circuits. Regarding material characterization, the comparative study of nonstoichiometric silicon oxide (SiOx) and silica (SiO2) inorganic alignment layers, as well as the relationship between surface layer deposition, layer morphology and liquid crystal electrooptical response must be highlighted, together with the characterization of the degradation of liquid crystal devices in simulated space mission environment. A wide variety of phase devices have been developed, with special emphasis on beam steerers. One of these was developed within the framework of an ESA project, and consisted of a high density reconfigurable 1D blaze grating, with a spatial separation of the controlling microelectronics and the active, radiation exposed, area. The developed devices confirmed the assumption that liquid crystal devices with such a separation of components, are radiation hard, and can be designed to be both vibration and temperature sturdy. In parallel to the above, an evenly variable analog beam steering device was designed, manufactured and characterized, providing a narrow cone diffraction free beam steering. This steering device is characterized by a very limited number of electrodes necessary for the redirection of a light beam. As few as 4 different voltage levels were needed in order to redirect a light beam. Finally at the Wojskowa Akademia Techniczna (Military University of Technology) in Warsaw, Poland, a wedged analog tunable beam steering device was designed, manufactured and characterized. This beam steerer, like the former one, was designed to resist the harsh conditions both in space and in the context of the shuttle launch. Apart from the beam steering devices, reconfigurable vortices and modal lens devices have been manufactured and characterized. In summary, during this work a large number of liquid crystal devices and liquid crystal device manufacturing technologies have been developed. Besides their relevance in scientific publications and technical achievements, most of these new devices have demonstrated their usefulness in the actual work of the research group where this PhD has been completed. El presente trabajo de Tesis se ha centrado en el diseño, fabricación y caracterización de nuevos dispositivos de fase basados en cristal líquido. Actualmente se están desarrollando dispositivos basados en cristal líquido para aplicaciones diferentes a su uso habitual como displays. Poseen la ventaja de que los dispositivos pueden ser controlados por bajas tensiones y no necesitan elementos mecánicos para su funcionamiento. La fabricación de todos los dispositivos del presente trabajo se ha realizado en la cámara limpia del grupo. La cámara limpia ha sido diseñada por el grupo de investigación, es de dimensiones reducidas pero muy versátil. Está dividida en distintas áreas de trabajo dependiendo del tipo de proceso que se lleva a cabo. La cámara limpia está completamente cubierta de un material libre de polvo. Todas las entradas de suministro de gas y agua están selladas. El aire filtrado es constantemente bombeado dentro de la zona limpia, a fin de crear una sobrepresión evitando así la entrada de aire sin filtrar. Las personas que trabajan en esta zona siempre deben de estar protegidas con un traje especial. Se utilizan trajes especiales que constan de: mono, máscara, guantes de látex, gorro, patucos y gafas de protección UV, cuando sea necesario. Para introducir material dentro de la cámara limpia se debe limpiar con alcohol y paños especiales y posteriormente secarlos con nitrógeno a presión. La fabricación debe seguir estrictamente unos pasos determinados, que pueden cambiar dependiendo de los requerimientos de cada dispositivo. Por ello, la fabricación de dispositivos requiere la formulación de varios protocolos de fabricación. Estos protocolos deben ser estrictamente respetados a fin de obtener repetitividad en los experimentos, lo que lleva siempre asociado un proceso de fabricación fiable. Una célula de cristal líquido está compuesta (de forma general) por dos vidrios ensamblados (sándwich) y colocados a una distancia determinada. Los vidrios se han sometido a una serie de procesos para acondicionar las superficies internas. La célula se llena con cristal líquido. De forma resumida, el proceso de fabricación general es el siguiente: inicialmente, se cortan los vidrios (cuya cara interna es conductora) y se limpian. Después se imprimen las pistas sobre el vidrio formando los píxeles. Estas pistas conductoras provienen del vidrio con la capa conductora de ITO (óxido de indio y estaño). Esto se hace a través de un proceso de fotolitografía con una resina fotosensible, y un desarrollo y ataque posterior del ITO sin protección. Más tarde, las caras internas de los vidrios se acondicionan depositando una capa, que puede ser orgánica o inorgánica (un polímero o un óxido). Esta etapa es crucial para el funcionamiento del dispositivo: induce la orientación de las moléculas de cristal líquido. Una vez que las superficies están acondicionadas, se depositan espaciadores en las mismas: son pequeñas esferas o cilindros de tamaño calibrado (pocos micrómetros) para garantizar un espesor homogéneo del dispositivo. Después en uno de los sustratos se deposita un adhesivo (gasket). A continuación, los sustratos se ensamblan teniendo en cuenta que el gasket debe dejar una boca libre para que el cristal líquido se introduzca posteriormente dentro de la célula. El llenado de la célula se realiza en una cámara de vacío y después la boca se sella. Por último, la conexión de los cables a la célula y el montaje de los polarizadores se realizan fuera de la sala limpia (Figura 1). Dependiendo de la aplicación, el cristal líquido empleado y los demás componentes de la célula tendrán unas características particulares. Para el diseño de los dispositivos de este trabajo se ha realizado un estudio de superficies inorgánicas de alineamiento del cristal líquido, que será de gran importancia para la preparación de los dispositivos de fase, dependiendo de las condiciones ambientales en las que vayan a trabajar. Los materiales inorgánicos que se han estudiado han sido en este caso SiOx y SiO2. El estudio ha comprendido tanto los factores de preparación influyentes en el alineamiento, el comportamiento del cristal líquido al variar estos factores y un estudio de la morfología de las superficies obtenidas.

Infiltration of a photonic crystal fiber with cholesteric liquid crystal and blue phase

Photonic crystal fibers (PCF) have been selectively filled with a cholesteric liquid crystal (ChLC) with special interest in the blue phase (BP) of the liquid crystal. It has been observed thermal tuning of the guided light in the visible region. A dramatically enhance appears when the phase of the liquid crystal changes from cholesteric to blue phase I (BPI). When a thermal range of the blue phase I is achieved, no changes are observed while increasing temperature from BPI through BPII and to the isotropic phase.

Electrooptic characterization of tunable cylindrical liquid crystal lenses

In this work, one-dimensional arrays of cylindrical adaptive liquid crystal lenses were manufactured and characterized; and test devices were filled with nematic liquid crystal. Comb interdigitated electrodes were designed as a mask pattern for the control electrode on the top glass substrates. A radial graded refractive index along each microsized lens was achieved by fabricating a layer of high resistance sheet deposited as a control electrode. These tunable lenses were switched by applying amplitude and frequency optimized waveforms on the control electrode. Phase profiles generated by the radial electric field distribution on each lens were measured by a convectional interferometric technique.

In situ study of Ge(100) surfaces with tertiarybutylphosphine supply in vapor phase epitaxy ambient

GaInP nucleation on Ge(100) often starts by annealing of the Ge(100) substrates under supply of phosphorus precursors. However, the influence on the Ge surface is not well understood. Here, we studied vicinal Ge(100) surfaces annealed under tertiarybutylphosphine (TBP) supply in MOVPE by in situ reflection anisotropy spectroscopy (RAS), X-ray photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED). While XPS reveals a P termination and the presence of carbon on the Ge surface, LEED patterns indicate a disordered surface probably due to by-products of the TBP pyrolysis. However, the TBP annealed Ge(100) surface exhibits a characteristic RA spectrum, which is related to the P termination. RAS allows us to in situ control phosphorus desorption dependent on temperature.

Crack mechanical failure in lithium niobate crystal under ion irradiation; novel simulation by extended finite elements

Swift heavy ion irradiation (ions with mass heavier than 15 and energy exceeding MeV/amu) transfer their energy mainly to the electronic system with small momentum transfer per collision. Therefore, they produce linear regions (columnar nano-tracks) around the straight ion trajectory, with marked modifications with respect to the virgin material, e.g., phase transition, amorphization, compaction, changes in physical or chemical properties. In the case of crystalline materials the most distinctive feature of swift heavy ion irradiation is the production of amorphous tracks embedded in the crystal. Lithium niobate is a relevant optical material that presents birefringence due to its anysotropic trigonal structure. The amorphous phase is certainly isotropic. In addition, its refractive index exhibits high contrast with those of the crystalline phase. This allows one to fabricate waveguides by swift ion irradiation with important technological relevance. From the mechanical point of view, the inclusion of an amorphous nano-track (with a density 15% lower than that of the crystal) leads to the generation of important stress/strain fields around the track. Eventually these fields are the origin of crack formation with fatal consequences for the integrity of the samples and the viability of the method for nano-track formation. For certain crystal cuts (X and Y), these fields are clearly anisotropic due to the crystal anisotropy. We have used finite element methods to calculate the stress/strain fields that appear around the ion-generated amorphous nano-tracks for a variety of ion energies and doses. A very remarkable feature for X cut-samples is that the maximum shear stress appears on preferential planes that form +/-45º with respect to the crystallographic planes. This leads to the generation of oriented surface cracks when the dose increases. The growth of the cracks along the anisotropic crystal has been studied by means of novel extended finite element methods, which include cracks as discontinuities. In this way we can study how the length and depth of a crack evolves as function of the ion dose. In this work we will show how the simulations compare with experiments and their application in materials modification by ion irradiation.

Crack mechanical failure in ceramic materials under ion irradiation: case of lithium niobate crystal

Swift heavy ion irradiation (ions with mass heavier than 15 and energy exceeding MeV/amu) transfer their energy mainly to the electronic system with small momentum transfer per collision. Therefore, they produce linear regions (columnar nano-tracks) around the straight ion trajectory, with marked modifications with respect to the virgin material, e.g., phase transition, amorphization, compaction, changes in physical or chemical properties. In the case of crystalline materials the most distinctive feature of swift heavy ion irradiation is the production of amorphous tracks embedded in the crystal. Lithium niobate is a relevant optical material that presents birefringence due to its anysotropic trigonal structure. The amorphous phase is certainly isotropic. In addition, its refractive index exhibits high contrast with those of the crystalline phase. This allows one to fabricate waveguides by swift ion irradiation with important technological relevance. From the mechanical point of view, the inclusion of an amorphous nano-track (with a density 15% lower than that of the crystal) leads to the generation of important stress/strain fields around the track. Eventually these fields are the origin of crack formation with fatal consequences for the integrity of the samples and the viability of the method for nano-track formation. For certain crystal cuts (X and Y), these fields are clearly anisotropic due to the crystal anisotropy. We have used finite element methods to calculate the stress/strain fields that appear around the ion- generated amorphous nano-tracks for a variety of ion energies and doses. A very remarkable feature for X cut-samples is that the maximum shear stress appears on preferential planes that form +/-45º with respect to the crystallographic planes. This leads to the generation of oriented surface cracks when the dose increases. The growth of the cracks along the anisotropic crystal has been studied by means of novel extended finite element methods, which include cracks as discontinuities. In this way we can study how the length and depth of a crack evolves as function of the ion dose. In this work we will show how the simulations compare with experiments and their application in materials modification by ion irradiation.

Quantum molecular dynamics and ab initio studies of the crystal structure of hydrogen and deuterium

Hydrogen isotopes play a critical role both in inertial and magnetic confinemen Nuclear Fusion. Since the preferent fuel needed for this technology is a mixture of deuterium and tritium. The study of these isotopes particularly at very low temperatures carries a technological interest in other applications. The present line promotes a deep study on the structural configuration that hydrogen and deuterium adopt at cryogenic temperatures and at high pressures. Typical conditions occurring in present Inertial Fusion target designs. Our approach is aims to determine the crystal structure characteristics, phase transitions and other parameters strongly correlated to variations of temperature and pressure.

Ab initio Methodology for Calculating Elastic Constant and Sound Velocity for Hydrogen and Deuterium in Molecular Solid Phase

Hydrogen isotopes play a critical role both in inertial and magnetic confinement Nuclear Fusion. Since the preferent fuel needed for this technology is a mixture of deuterium and tritium. The study of these isotopes particularly at very low temperatures carries a technological interest in other applications. The present line promotes a deep study on the structural configuration that hydrogen and deuterium adopt at cryogenic temperatures and at high pressures. Typical conditions occurring in present Inertial Fusion target designs. Our approach is aims to determine the crystal structure characteristics, phase transitions and other parameters strongly correlated to variations of temperature and pressure. With this results is possible calculated the elastic constant and sound velocity for hydrogen and deuterium in molecular solid phase.

Point diffraction interferometer with a liquid crystal monopixel

In this work a novel point diffraction interferometer based on a variable liquid crystal wave plate (LCWP) has been implemented. The LCWP consists of a 3x3 cm2 monopixel cell with parallel alignment. The monopixel cell was manufactured such that the electrode covers the entire surface except in a centered circular area of 50 μm of diameter. This circle acts as a point perturbation which diffracts the incident wave front giving rise to a spherical reference wave. By applying a voltage to the LCWP we can change the phase of the wave front that passes through the monopixel, except at the center. Phase shifting techniques are used in order to calculate the amplitude and phase distribution of the object wave front. The system allows a digital hologram to be obtained, and by using the Fresnel diffraction integral it is possible to digitally reconstruct the different planes that constitute the three dimensional object.

Using an analytical model to design liquid crystal microlenses

We have developed new analytical expressions for designing liquid crystal (LC) microlenses. These equations are based on a novel equivalent electric circuit and can be used to create an optimum design for the LC lenses in which the lens diameter ranges from a few micrometers to several millimeters. Thus far, only experimental studies have been conducted on the LC lenses. The analytical expressions developed in this letter depend on various manufacturing parameters and can be used to design lenses with specific focal lengths and a parabolic phase profile. The required driving scheme (modal or hole-patterned) can be predicted. The LC microlenses were manufactured and electrooptically characterized: the measurements were compared using an analytical approach.

Determination of the crystal and magnetic structure of the DyCrO4-scheelite polymorph by neutron diffraction

Neutron diffraction data of DyCrO4 oxide, prepared at 4 GPa and 833 K from the ambient pressure zircon-type, reveal that crystallize with the scheelite-type structure, space group I41/a. Accompanying this structural phase transition induced by pressure the magnetic properties change dramatically from ferromagnetism in the case of zircon to antiferromagnetism for the scheelite polymorph with a T N= 19 K. The analysis of the neutron diffraction data obtained at 1.2 K has been used to determine the magnetic structure of this DyCrO4-scheelite oxide which can be described with a k = [0, 0, 0] as propagation vector, where the Dy and Cr moments are lying in the ab-plane of the scheelite structure. The ordered magnetic moments are 10 µB and 1 µB for Dy+3 and Cr+5 respectively

Modal liquid crystal microaxicon array

A novel tunable liquid crystal microaxicon array is proposed and experimentally demonstrated. The proposed structure is capable of generating tunable axicons (thousands of elements) of micrometric size, with simple control (four control voltages) and low voltage, and is totally reconfigurable. Depending on the applied voltages, control over the diameter, as well as the effective wedge angle, can be achieved. Controls over the diameter ranging from 107 to 77 μm have been demonstrated. In addition, a control over the phase profile tunability, from 12π to 24π radians, has been demonstrated. This result modifies the effective cone angle. The diameter tunability, as well the effective cone angle, results in a control over the nondiffractive Bessel beam distance. The RMS wavefront deviation from the ideal axicon is only λ∕3. The proposed device has several advantages over the existing microaxicon arrays, including being simple having a low cost. The device could contribute to developing new applications and to reducing the fabrication costs of current devices.

Impact of metal-organic vapor phase epitaxy environment on silicon bulk lifetime for III–V-on-Si multijunction solar cells

With the final goal of integrating III-V materials on silicon substrates for tandem solar cells, the influence of the Metal-Organic Vapor Phase Epitaxy (MOVPE) environment on the minority carrier properties of silicon wafers has been evaluated. These properties will essentially determine the photovoltaic performance of the bottom cell in a III-V-on-Si tandem solar cell. A comparison of the base minority carrier lifetimes obtained for different thermal processes carried out in a MOVPE reactor on Czochralski silicon wafers has been carried out. An important degradation of minority carrier lifetime during the surface preparation (i.e. H2 anneal) has been observed. Three different mechanisms have been proposed for explaining this behavior: 1) the introduction of extrinsic impurities coming from the reactor; 2) the activation of intrinsic lifetime killing impurities coming from the wafer itself; and finally, 3) the formation of crystal defects, which eventually become recombination centers. The effect of the emitter formation by phosphorus diffusion has also been evaluated. In this sense, it has been reported that lifetime can be recovered during the emitter formation either by the effect of the P on extracting impurities, or by the role of the atomic hydrogen on passivating the defects.

Selectively filled photonic liquid crystal fibers = Fibras de cristal fotónico selectivamente llenas con cristal líquido

El presente trabajo de Tesis se ha centrado en el diseño, fabricación y caracterización de dispositivos basados en fibras de cristal fotónico infiltrado selectivamente con cristales líquidos, polímeros y una mezcla de ambos. Todos los dispositivos son sintonizables, y su área de aplicación se centra en comunicaciones ópticas y sensores. La manipulación y fusionado de fibras fotónicas, el llenado selectivo de determinadas cavidades y la alineación recíproca de fibras mantenedoras de polarización son tareas muy específicas y delicadas para las que se requieren protocolos muy estrictos. Previo a la fabricación de dispositivos ha sido necesaria por tanto una tarea de sistematización y creación de protocolos de fabricación. Una vez establecidos se ha procedido a la fabricación y caracterización de dispositivos. Los dispositivos fabricados se enumeran a continuación para posteriormente detallar una a una las singularidades de cada uno. • Interferómetros intermodales hechos a partir de una porción de fibra fotónica soldada entre dos fibras estándar, bien monomodo o PANDA (mantenedora de polarización). Estos interferómetros han sido sumergidos o bien llenados selectivamente con cristales líquidos para así sintonizar la señal interferométrica guiada a través de la fibra. • Infiltración de fibras fotónicas con cristales líquidos colestéricos con especial énfasis en la fase azul (blue phase) de estos materiales. Las moléculas de cristal líquido se autoalinean en volumen por lo que la infiltración de fibras fotónicas con estos cristales líquidos es muy interesante, pues es conocida la dificultad de alinear apropiadamente cristales líquidos dentro de cavidades micrométricas de las fibras fotónicas. • Grabación de redes holográficas de forma selectiva en las cavidades de una fibra fotónica. Estas redes holográficas, llamadas POLICRYPS (POlymer-LIquid CRYstal-Polymer Slices), son redes fabricadas a base de franjas de polímero y cristal líquido alineado perpendicularmente a dichas franjas. Las franjas son a su vez perpendiculares al eje de la fibra como lo puede ser una red de Bragg convencional. El cristal líquido, al estar alineado perpendicularmente a dichos franjas y paralelo al eje de la fibra, se puede conmutar aplicando un campo eléctrico externo, modificando así el índice efectivo de la red. Se puede fabricar por lo tanto una red de Bragg sintonizable en fibra, muy útil en comunicaciones ópticas. • Llenado selectivo de fibras fotónicas con polidimetilsiloxano (PDMS), un polímero de tipo silicona. Si se realiza un llenado selectivo asimétrico se puede inducir birrefringencia en la fibra. El índice de refracción del PDMS tiene una fuerte dependencia térmica, por lo que se puede sintonizar la birrefringencia de la fibra. • Estudio teórico de llenado selectivo de fibras fotónicas con PDMS dopado con nanopartículas de plata de 5, 40 y 80 nm. Estas nanopartículas poseen un pico de absorción en torno a los 450 nm debido a resonancias superficiales localizadas de plasmones (LSPR). La resonancia del plasmon tiene una fuerte dependencia con el índice de refracción del material colindante, y al ser éste PDMS, la variación de índice de refracción se ve amplificada, obteniendo una absorción sintonizable. Se ha propuesto la fabricación de polarizadores sintonizables usando esta técnica. Como ya se ha dicho, previamente a la fabricación ha sido necesaria la protocolización de diversos procedimientos de fabricación de alta complejidad, así como protocolizar el proceso de toma de medidas para optimizar los resultados. Los procedimientos que han requerido la formulación de protocolos específicos han sido los siguientes: • Llenado selectivo de cavidades en una fibra fotónica. Dichas fibras tienen generalmente un diámetro externo de 125 μm, y sus cavidades son de entre 5 y 10 μm de diámetro. Se han desarrollado tres técnicas diferentes para el llenado/bloqueado selectivo, pudiéndose combinar varios protocolos para la optimización del proceso. Las técnicas son las siguientes: o Llenado y bloqueado con un prepolímero. Dicho prepolímero, también llamado adhesivo óptico, está inicialmente en estado líquido y posee una cierta viscosidad. Las cavidades de la fibra fotónica que se desea llenar o bloquear poseen un diámetro diferente al resto, por lo que en el proceso de llenado aparecen dos frentes de llenado dependientes de su diámetro. A mayor diámetro, mayor velocidad de llenado. Polimerizando cuando existe dicha diferencia en los frentes se puede cortar por medio, obteniendo así una fibra parcialmente bloqueada. o Colapsamiento de las cavidades de menor diámetro mediante aplicación de calor. El calor producido por un arco voltaico de una soldadora de fibra estándar fusiona el material exterior de la fibra produciendo el colapsamiento de las cavidades de menor diámetro. En esta técnica también es necesaria una diferencia de diámetros en las cavidades de la fibra. o Bloqueo una a una de las cavidades de la fibra fotónica con adhesivo óptico. Este procedimiento es muy laborioso y requiere mucha precisión. Con este sistema se pueden bloquear las cavidades deseadas de una fibra sin importar su diámetro. • Alineación de una fuente de luz linealmente polarizada con una fibra mantenedora de polarización ya sea PANDA o fotónica. Así mismo también se han alineado entre sí fibras mantenedoras de polarización, para que sus ejes rápidos se fusionen paralelos y así el estado de polarización de la luz guiada se mantenga. • Sistematización de toma de medidas para caracterizar los interferómetros modales. Éstos son altamente sensibles a diversas variables por lo que el proceso de medida es complejo. Se deben aislar variables de forma estrictamente controlada. Aunque todos los dispositivos tienen en común el llenado selectivo de cavidades en una fibra fotónica cada dispositivo tiene sus peculiaridades, que van a ser explicadas a continuación. ABSTRACT The present Thesis has been centered in the design, fabrication and characterization of devices based on photonic crystal fibers selectively filled with liquid crystals, polymers and a mixture of both. All devices are tunable and their work field is optical communications and sensing The handling and splicing of photonic crystal fibers, the selective filling of their holes and the aligning of polarization maintaining fibers are very specific and delicate tasks for which very strict protocols are required. Before the fabrication of devices has therefore been necessary task systematization and creation of manufacturing protocols. Once established we have proceeded to the fabrication and characterization of devices. The fabricated devices are listed below and their peculiarities are detailed one by one: • Intermodal interferometers made with a portion of photonic crystal fiber spliced between two optical communication fiber pigtails, either single mode or PANDA (polarization-maintaining) fiber. These interferometers have been submerged or selectively filled with liquid crystals to tune the interferometric guided signal. • Infiltration of photonic fibers with cholesteric liquid crystals with special emphasis on their blue phase (blue phase). The liquid crystal molecules are self-aligning in volume so the infiltration of photonic fibers with these liquid crystals is very interesting. It is notoriously difficult to properly align liquid crystals within micron cavities such as photonic fibers. • Selectively recording of holographic gratings in the holes of photonic crystal fibers. These holographic gratings, called POLICRYPS (POlymer-LIquid CRYstal-Polymes Slices), are based on walls made of polymer and liquid crystal aligned perpendicular to them. These walls are perpendicular to the axis of the fiber as it can be a conventional Bragg grating. The liquid crystal is aligned perpendicular to the walls and parallel to the fiber axis, and can be switched by applying an external electric field and thus change the effective index of the grating. It is thus possible to manufacture a tunable Bragg grating fiber, useful in optical communications. •Asymmetrically selective filling of photonic crystal fibers with a silicone polymer like called polydimethylsiloxane (PDMS) to induce birefringence in the fiber. The refractive index of PDMS has temperature dependence, so that the birefringence of the fiber can be tuned. • Theoretical study of photonic crystal fibers selectively filled with PDMS doped with silver nanoparticles of 5, 40 and 80 nm. These nanoparticles have an absorption peak around 450 nm due to localized surface plasmon resonances (LSPR). Plasmon resonance has a strong dependence on the refractive index of the adjacent material, and as this is PDMS, the refractive index variation is amplified, obtaining a tunable absorption. Fabrication of tunable polarizers using this technique has been proposed. Before starting the fabrication, it has been necessary to optimize several very delicate procedures and different protocols have been designed. The most delicate procedures are as follows: • Selective filling of holes in a photonic crystal fiber. These fibers generally have an outer diameter of 125 μm, and their holes have a diameter around between 5 and 10 μm. It has been developed three different techniques for filling / selective blocking, and they can be combined for process optimization. The techniques are: o Filling and blocked with a prepolymer. This prepolymer also called optical adhesive is initially in liquid state and has a certain viscosity. The holes of the photonic crystal fiber that are desired to be filled or blocked should have a different diameter, so that in the filling process appear two different fronts depending on the hole diameter. The holes with larger diameter are filled faster. Then the adhesive is polymerized when there is such a difference on the front. A partially blocked fiber is obtained cutting between fronts. o Collapsing of holes of smaller diameter by application of heat. The heat produced by an arc of a standard fusion splicer fuses the outer fiber material producing the collapsing of the cavities of smaller diameter. In this technique also you need a difference of diameters in the fiber holes. o Blocking one by one the holes of photonic crystal fiber with optical adhesive. This procedure is very laborious and requires great precision. This system can block unwanted cavities regardless fiber diameter. • Aligning a linearly polarized light source with a polarization-maintaining fiber (either a PANDA fiber as a photonic crystal fiber). It is needed also an aligning between polarization-maintaining fibers, so that their fast axes parallel merge and that is state of polarization of light guided is maintained. • Systematization of taking measurements to characterize the modal interferometers. These are highly sensitive to several variables so the measurement process is very complicated. Variables must be fixed in a very controlled manner. Although all devices have the common characteristic of being selectively filled PCFs with some kind of material, each one has his own peculiarities, which are explained below.

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.