This document presents the modelling and characterization of novel photonic devices based on periodic arrays of multiwalled carbon nanotubes. Multiwalled carbon nanotubes are mostly metallic in nature and interesting plasmonic effects are observed when nanotubes are grown close together, with spacing of about 400 nm. The effective electronic mass on the nanotubes changes, due to mutual coupling between them and they start displaying dielectric properties which are inherently different from the their own, forming metamaterials. We present a plasmonic high pass filtering application of carbon nanotube based metamaterials. Some promising modelling and experimental results are demonstrated showing a strong cut-off filtering effect at the plasma frequency displayed by the periodic arrays of multiwalled carbon nanotubes. The artificial negative dielectric constant displayed by the nanotube arrays was also successfully utilised for producing micron-scaled applications like optical waveguides and negative lenses for overcoming the diffraction limit. The fabrication of these optical devices using the arrays of silicon nanopillars was also considered. These arrays when fabricated at nano-scaled dimensions (of about 400 nm) present a greater degree of periodicity and require a simpler fabrication process compared to carbon nanotubes. We report the detailed computational analysis on silicon nanopillars based photonic crystals, waveguides and metamaterials which operate well within in the optical regime. However, due to the fabrication limitations, the fabricated Si nanopillars presented an inverted cone shape profile along their lengths. These inverted nanocone structures were successfully utilised for enhancing reflection from Si surfaces for applications in photovoltaic devices. Lastly we present a novel application of carbon nanotube arrays for producing micron-scale Fresnel lens arrays. Forests of carbon nanotubes were utilised as absorbing media on top of a bare silicon substrate. Optical diffraction of light across the nanotube forests produced strong focusing of light, at focal lengths of order 125 microns. Numerical simulations were in excellent agreement with the measured results.

Ce travail de thèse est composé de deux partie distinctes. La première partie est consacrée à l'étude et la modélisation des métamatériaux et tout particulièrement à leur caractérisation. Il s'agit donc de proposer des paramètres constitutifs équivalents (permittivité, perméabilité et couplage magnéto-électrique) qui permettent de remplacer les métamatériaux, composés à l'origine de réseaux périodiques de cellules discrètes, par des milieux homogènes et continus. Deux régimes sont considérés : sub-longueur d'onde et longueur d'onde. Dans le premier régime, les dimensions de la cellule de base, en particulier la période du réseau, sont très faibles devant la longueur d'onde. Le métamatériau est alors modélisé par un milieu bi-anisotrope continu. Pour le deuxième régime, les dimensions et la période sont comparables à la longueur d'onde. Les paramètres constitutifs sont alors définis au sens "local" qui prend en compte le caractère discret et périodique de la structure. Enfin, la caractérisation consiste à extraire les paramètres constitutifs à partir des coefficients de réflexion et de transmission d'une lame de métamatériau. Pour cela, nous appliquons la méthode d'inversion sous sa forme classique pour le régime "sub-longueur d'onde" et son extension aux structures périodiques pour le régime "longueur d'onde". L'une des contributions majeure de ce travail est l'application des incidences obliques (à l'interface air-métamatériau) pour extraire les paramètres constitutifs longitudinaux. La seconde partie de ce travail de thèse concerne le développement d'antennes directives à partir de sources rayonnantes à modes orthogonaux. Il est bien connu que la directivité est directement limitée par les dimensions de l'antenne ou par le nombre de sources dans un réseau. L'objectif est de mettre en œuvre une méthode alternative qui, tout au moins sur le plan conceptuel et théorique, ne relie pas la directivité à la taille de l'antenne. Nous utilisons donc la combinaison de modes rayonnants orthogonaux pour maximiser la directivité d'antennes tout en minimisant les dimensions du réseau et en limitant le couplage. Chaque source du réseau se doit rayonner un mode propre et unique qui, combiné avec une autre source, permet d'augmenter la directivité. Pour cette étude, le développement théorique des modes sphériques puis cylindriques est tout d'abord présenté afin de mettre en évidence le lien entre la directivité et les modes. En pratique, la mise en œuvre de modes sphériques a révélé des difficultés de conception, si bien que des solutions d'antennes basées sur l'anneau rayonnant sont proposées pour réaliser des sources à modes orthogonaux "cylindriques". Les topologies de structures rayonnantes directives proposées associant des antennes anneaux sont validées en simulation et par la conception puis la mesure de prototypes. / The work presented in this thesis concerns two parts: characterisation methods for metamaterials and directive antennas using space eigen-modes. The first one describes the homogenisation methods of metamaterials to retrieve the constitutive parameters from scattering parameters of the metamaterial's slab. We investigated the metamaterials which present the most common properties of media: bi-isotropic metamaterials (chiral), anisotropic metamaterials (BC-SRR), uni-axial bi-anisotropic metamaterials (double omega medium) and bi-axial bi-anisotropic metamaterials (EC-SRR). The transverse and longitudinal constitutive parameters characterising the anisotropic and bi-anisotropic require to examine the media at normal and oblique incidences. In the analysis we considered to distinguish continuous media restricted to long wave limit and resonating particles lattice which are out of long wave limit. The application of continuous media approach to the resonating particle lattice with size not satisfying the long wave limit can leads to violation of causality and passivity laws. The main different between proposed two approaches is interpretation of boundary conditions (Maxwellian/non-Maxwellian) for the metamaterial slab. The inclusions of lattice we ascribe as electric and magnetic dipole moments to develop an homogeneous resonating particles lattice and to retrieve the constitutive parameters. We validated the proposed approaches and compared the retrieved constitutive parameters according to physical laws. We found that the application of Bloch admittance and equivalence to electric and magnetic dipole moments provides the physical constitutive parameters. The second part of the thesis refers to directive antenna using space eigen-modes. The directivity of the antennas is limited to the size of the antenna or number of elements in an array. In common with keeping small sizes of the array the space between radiation elements has to be minimised. The proposed directive antenna considers the radiation elements determine by the orthogonal modes, i.e. each radiator of the array corresponds to unique space eigen-mode. This allows to ensure small distance between the array elements without introduction of high mutual coupling between them. The original approach refers to superposition of spherical modes however it is not practical. Instead of spherical modes we propose superposition of cylindrical modes which are easy to be realised. For the constructive summation of cylindrical modes and maximisation of directivity in end-fire plane we determine amplitudes of the cylindrical modes excited by electric and/or magnetic vector potentials. The cylindrical modes we obtain by the annular ring antennas and miniaturised annular rings. The superposition of cylindrical modes is achieved with a stacked antenna of annular rings and a coplanar cylindrical annular rings antenna. The both antennas we realised and measured.

En accord avec l'équation de propagation des ondes élastiques dans la matière, l'expression de la vitesse du son n'interdit pas à la masse volumique et à la compressibilité effectives du milieu d'être toutes deux négatives. Comment concevoir alors un tel matériau qualifié de métamatériau acoustique ? Qu'elles sont ses principales propriétés ? Cette thèse expérimentale a pour objectif la conception et la caractérisation d'un métamatériau pour les ondes de Lamb (ondes de plaque) de fréquences comprises entre 1MHz et 10MHz.L'approche mise en place repose sur le théorème d'Helmholtz qui permet de voir la propagation d'une onde élastique d'énergie finie comme le résultat d'un processus de couplage entre deux états de vibration de la matière, l'un de cisaillement sans changement de volume (vibration de symétrie dipolaire), l'autre se faisant avec changement de volume sans cisaillement (vibration monopolaire). Ainsi, la modification des paramètres effectifs décrivant la propagation pourrait passer par un contrôle local de ces deux états élémentaires de vibration, au moyen par exemple de résonateurs locaux homogénéisables.Le modèle de matériau choisi dans le cadre de cette thèse est une hétérostructure formée par un wafer de silicium sur lequel sont gravés, en tant que cellules mécaniques élémentaires, des paire de trous et des piliers isolés de silicium. La démarche expérimentale a consisté à étudier la diffusion élastique des ondes de Lamb sur ces deux types de cellules élémentaires à l'aide d'un montage tout optique. La génération des ondes fut assurée par la focalisation le long d'une ligne d'un faisceau laser Nd:YAG picoseconde permettant d'obtenir une source blanche acoustique. La détection fut réalisée à l'aide d'un interféromètre de Michelson doté d'un bras opto-mécanique permettant une cartographie point par point du champ de déformation de l'onde avec une sensibilité d'environ 1pm (pour une largeur de bande de 1MHz) sur une surface de 25x25mm2 avec une résolution spatiale et temporelle d'environ 1microns et 0.2nanosecondes.Cette étude aura permis de mettre en évidence la présence d'une ondelette réémise par la paire de trous ou le pilier autour de leurs fréquences de résonance et de décrire la diffusion cohérente résonante comme le résultat de l'interférence entre l'onde incidente et l'ondelette réémise. Dans le cas de la paire de trous, ces résultats ont permis d'interpréter et de comprendre le domaine de fréquences interdites d'un cristal phononique, ainsi que de décrire la dynamique de formation des modes d'une cavité phononique planaire. Dans le cas du pilier, il fut possible d'envisager la fabrication d'un système possédant les propriétés attendues pour un métamatériau acoustique.Cette thèse s'inscrit dans le projet de recherche de l'équipe Acoustique pour les Nanosciences de l'Institut des Nanosciences de Paris consacré à la structuration artificielle de la matière aux échelles micro et nanométriques pour le contrôle de la propagation des ondes élastiques. Les applications potentielles couvrent des domaines allant de la santé (imagerie haute résolution) à la défense (cape d'invisibilité) en passant par les télécommunications ou encore le bâtiment (isolation phonique).De par son caractère stratégique pour la Défense, ce travail a bénéficié du soutien de la DGA et de l'ANR sous la forme du projet ANR-ASTRID "METACTIF". Il a été effectué en collaboration avec une équipe de l'Université Lille 1, spécialisée dans la simulation numérique. Les échantillons ont été fabriqués dans le cadre d'une collaboration avec la salle blanche MIMENTO de l'Institut Femto-St de Besançon. / According with the elastic wave equation, the expression of the speed allows the motion of waves with an effective density and an effective compressibility both negative. How can we imagine and create a material of this kind, called metamaterial? What would be its properties?This experimental thesis involved to produce and to characterize a metamaterial for elastic Lamb waves (sismic waves) in the frequency range [1MHz-10MHz].On the basis of the Helmholtz theorem, the wave motion of Lamb waves is due to a coupling process between two vibrations states of the matter; one dipolar vibration (pure bending mode) coupling with one monopolar vibration (pure compressional mode). As a result, the modification of the macroscopic parameters governing the wave motion could be realised by the control of these only two vibrations states, thanks to homogeneous local resonators.Isolated silicon pillars and pairs of holes in silicon plate as potentially "good" candidate to fulfill this requirement. Structures were elaborated with deep reactive-ion etching technique (DRIE) using Bosch process in a silicon wafer.We had an experimental approach consisting in measuring the eigenfrequencies of the structures to select the one which allows fulfilling the homogenization criteria at the best and then mapping the scattered field associated to a Lamb wave interacting with the structures while vibrating onto the preselected eigenmode.To conduct this study, we used an all-optical experimental device. Generation of Lamb waves were managed to use an Nd:YAG laser focused along a line on the surface to get a white elastic source. The detection was realized with a power-operated Michelson interferometer, allowing to measure the displacement field with a resolution of around 1pm (on frequencies range of 1MHz).This study had allowed to evidence a scattering process described by the interference between the incident field and a reemitted wave emitted by the resonator. For the pairs of hole structures, these results enabled to understand the description of the bandgap of a phononic crystal in terms of bragg reflexion and to describe the dynamic of formation of phononic cavity modes. Concerning the pillars, these results enabled to make a device, using the Huygens-Fresnel principal, with the properties of a metamaterial.This PhD work follows on from the research projet of the team Acoustique pour les Nanoscience of the Nanosciences Institut of Paris (INSP).This work is jointly supported by the Agence Nationale de la Recherche and Direction Générale de l’Armement under grant ANR METACTIF. The simulations were realised in collaboration with a team at the Université Lille 1. The samples have been elaborated in MIMENTO facilities at Femto-ST institute in Besançon.

[EN] Metamaterials are periodic structures whose unit cells are small compared to the wavelength at the operating frequency. Under these conditions, these artificial materials can be considered as homogeneous media whose constitutive parameters depend on the characteristics of the unit cells. The discovery of metamaterials opened a new research field that has produced many works with microwaves, optical waves and acoustic waves. In this context, the main goal of this thesis is the study of new structures based on metamaterials that allow controlling of electromagnetic energy. In particular, new solutions for localization and absorption of electromagnetic waves are proposed. The thesis has been developed in the Wave Phenomena Group of the Polytechnic University of Valencia and in collaboration with the Group of Acoustic and Electromagnetic Metamaterials at the University of Exeter. The problems studied in the first part of this thesis are energy harvesting for subsequent absorption, wireless power transfer and new systems that can be used as position sensors. To solve these problems a new type of cylindrical, multilayer and anisotropic structures known as Radial Photonic Crystals are used. The radial dependence of the constitutive parameters generates, in these structures, a behavior like a one dimensional photonic crystals. Among the results obtained with these structures, it is included the first experimental demonstration of a Radial Photonic Crystals based resonator. Absorption of electromagnetic waves by thin layers of lossy materials is the second topic of this thesis. The main target is the theoretical and experimental study of the absorption enhancement in thin layers by using two-dimensional periodic structures, also called metasurfaces. Specifically, we studied the effects of a square lattice of coaxial cavities covered by a thin layer of lossy material. As a result, an enhancement of the absorption peaks that can produce total absorption is achieved. The semi-analytical study of this structure has allowed obtaining expressions that control the position of the absorption peak and its amplitude; which have helped to develop a design methodology for total absorption systems. / [ES] Los metamateriales son estructuras periódicas cuyas celdas unidad son muy pequeñas en comparación con la longitud de onda a la frecuencia de trabajo. Bajo estas condiciones, estos materiales artificiales pueden considerarse como medios homogéneos cuyos parámetros constitutivos dependen de las características de las celdas unidad que los componen. La aparición de los metamateriales abrió un nuevo campo de investigación que ha generado multitud de trabajos en las líneas de microondas, óptica y acústica. En este contexto, el objetivo principal de esta tesis es el estudio de nuevas estructuras basadas en metamateriales que permitan el control de la energía electromagnética. En particular, plantea nuevas soluciones para problemas de localización y absorción de ondas electromagnéticas. La tesis ha sido desarrollada en el Grupo de Fenómenos Ondulatorios de la Universidad Politécnica de Valencia y en colaboración con el Grupo de Metamateriales Acústicos y Electromagnéticos de la Universidad de Exeter. Los problemas estudiados en la primera parte de esta tesis son la concentración de energía para su posterior absorción, la transferencia inalámbrica de potencia y nuevos sistemas capaces de ser empleados como sensores de posición. Para la solución de estos problemas se emplean un nuevo tipo de estructuras cilíndricas, multicapa y anisótropas conocidas como Cristales Fotónicos Radiales. La dependencia radial de los parámetros constitutivos de los materiales que componen cada una de sus capas genera, en estas estructuras, un comportamiento similar al de los cristales fotónicos unidimensionales. Entre los resultados obtenidos con estas estructuras, cabe destacar la primera demostración experimental de un resonador basado en Cristales Fotónicos Radiales. La absorción de ondas electromagnéticas por capas delgadas de materiales con pérdidas es el segundo tema tratado en esta tesis. El objetivo principal es el estudio teórico y experimental del aumento de la absorción en capas delgadas mediante el uso de estructuras periódicas bidimensionales, también llamadas metasuperficies. En concreto, se han estudiado los efectos de una red cuadrada de cavidades coaxiales sobre la que se coloca una capa delgada de un material con pérdidas. Como resultado, se consigue un aumento de la absorción que permite obtener picos de absorción total. El estudio semianalítico de esta estructura ha permitido obtener expresiones que controlan la posición del pico de absorción y su amplitud; las cuales han permitido desarrollar una metodología de diseño para sistemas de absorción total. / [CAT] Els metamateriales són estructures periòdiques en els que les cel·les unitat són molt xicotetes en comparació amb la longitud d'ona a la freqüència de treball. Tenint en consideració aquestes condicions, aquestos materials artificials poden considerar-se com a mitjans homogenis en els que els paràmetres constitutius depenen de les característiques de les cel·les unitat que els componen. A més, l'aparició dels metamateriales va obrir un nou camp d'investigació que ha generat multitud de treballs en les línies de microones, òptica i acústica. En aquest context, l'objectiu principal d'aquesta tesi és l'estudi de noves estructures basades en metamateriales que permeten el control de l'energia electromagnètica. En particular, planteja noves solucions per a problemes de localització i absorció d'ones electromagnètiques. La tesi ha sigut realitzada en el Grup de Fenòmens Ondulatoris de la Universitat Politècnica de València i en col·laboració amb el Grup de Metamateriales Acústics i Electromagnètics de la Universitat d'Exeter. Els problemes analitzats en la primera part de la tesi són la concentració d'energia per a la seua posterior absorció, la transferència inalàmbrica de potència i nous sistemes capaços de ser empleats com a sensors de posició. Per a la solució dels problemas identificats s'utilitza un nou tipus d'estructures cilíndriques, multicapa i anisòtropes conegudes com a Cristalls Fotónics Radials. La dependència radial dels paràmetres constitutius dels materials que componen cadascuna de les seues capes genera, en aquestes estructures, un comportament semblant al dels Cristalls Fotónics Unidimensionals. Entre els resultats obtinguts, cal destacar la primera demostració experimental d'un ressonador basat en Cristalls Fotónics Radials. Pel que respecta a la segon part de la tesi, l'absorció d'ones electromagnètiques per capes primes de materials amb pèrdues és tema tractat. L'objectiu principal és l'estudi teòric i experimental de l'augment de l'absorció en capes primes per mitjà de l'ús d'estructures periòdiques bidimensionals, també denominades metasuperficies. En concret, s'han examinat els efectes d'una xarxa quadrada de cavitats coaxials sobre la qual es col·loca una capa prima d'un material amb pèrdues. Com a resultat, s'aconseguix un augment de l'absorció que permet obtindre pics d'absorció total. Així mateix, l'estudi semi-analític d'aquesta estructura ha permés obtindre expressions que controlen la posició del pic d'absorció i la seua amplitud; les quals han permés desenvolupar una metodologia de disseny per a sistemes d'absorció total. / Díaz Rubio, A. (2015). Control of electromagnetic energy by metamaterials [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/54137 / TESIS

Esta tesis trata sobre el fascinante fenómeno de la transmisión extraordinaria a través de láminas metálicas nonoestructuradas periódicamente con aperturas al corte. Un efecto relacionado con la excitación de un tipo de ondas superficiales como son los plasmones de superficie. Además, en aquellas estructuras formadas por el apilamiento de dos o más láminas metálicas se consiguen nuevas funcionalidades, como magnetismo artificial que da lugar a resonancias magnéticas y por tanto la posibilidad de obtener un índice de refracción negativo.Mediante un estudio teórico y numérico se ha comprobado que este tipo de respuesta magnética efectiva se debe a la excitación de resonancias plasmónicas internas en la estructura. Obteniéndose, bajo incidencia normal, un índice de refracción efectivo negativo en la dirección de propagación en el caso de que dichas resonancias se produzcan en zonas del espectro donde se obtenga la permitividad negativa, conectando el mundo de la plasmónica con el de los metamateriales. Uno de los principales objetivos en el diseño de metamateriales es obtener un índice de refracción negativo en un gran ancho de banda. Sin embargo, este objetivo suele ser complicado de conseguier al basar los diseños en fenómenos resonantes. Es por ello que en esta tesis se ha propuesto un diseño basado en el apilamiento de estructuras fishnet con diferentes grosores de dieléctrico para conseguir aumentar el ancho de banda en el cual se consigue un índice negativo. Básicamente, la obtención de tal efecto se basa en la excitación de resonancias plasmónicas a distintas frecuencias al estar formada la celda unidad por difentes grososres de dieléctrico. La hibridación que se produce entre dichas resonancias permite aumentar el ancho de banda con índice negativo. Aunque la transmisión extraordinaria esta principalmente relacionada con excitación de plasmones de superficie, los resultados mostrados en la tesis demuestran que para el caso de láminas metálicas rodeadas por dieléctricos también se consigue transmisión extraordinaria debido a la adaptación de la luz incidente a los modos soportados por los medios dieléctricos siempre y cuando el metal se encuentre estructurado periódicamente. / Ortuño Molinero, R. (2012). Extraordinary Transmission Filtering Structures based on Plasmonic Metamaterials [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/14639 / Palancia

abstract: Within the past two decades, metasurfaces, with their unique ability to tailor the wavefront, have attracted scientific attention. Along with many other research areas, RADAR cross-section (RCS)-reduction techniques have also benefited from metasurface technology. In this dissertation, a novel technique to synthesize the RCS-reduction metasurfaces is presented. This technique unifies the two most widely studied and two well-established modern RCS-reduction methods: checkerboard RCS-reduction andgradient-index RCS-reduction. It also overcomes the limitations associated with these RCS-reduction methods. It synthesizes the RCS-reduction metasurfaces, which can be juxtaposed with almost any existing metasurface, to reduce its RCS. The proposed technique is fundamentally based on scattering cancellation. Finally, an example of the RCS-reduction metasurface has been synthesized and introduced to reduce the RCS of an existing high-gain metasurface ground plane. After that, various ways of obtaining ultrabroadband RCS-reduction using the same technique are proposed, which overcome the fundamental limitation of the conventional checkerboard metasurfaces, where the reflection phase difference of (180+-37) degrees is required to achieve 10-dB RCS reduction. First, the guideline on how to select Artificial Magnetic Conductors (AMCs) is explained with an example of a blended checkerboard architecture where a 10-dB RCS reduction is observed over 83% of the bandwidth. Further, by modifying the architecture of the blended checkerboard metasurface, the 10-dB RCS reduction bandwidth increased to 91% fractional bandwidth. All the proposed architectures are validated using measured data for fabricated prototypes. Critical steps for designing the ultrabroadband RCS reduction checkerboard surface are summarized. Finally, a broadband technique to reduce the RCS of complex targets is presented. By using the proposed technique, the problem of reducing the RCS contribution from such multiple-bounces simplifies to identifying and implementing a set of orthogonal functions. Robust guidelines for avoiding grating lobes are provided using array theory. The 90 degree dihedral corner is used to verify the proposed technique. Measurements are reported for a fabricated prototype, where a 70% RCS-reduction bandwidth is observed. To generalize the method, a 45 degree dihedral corner, with a quadruple-bounce mechanism, is considered. Generalized guidelines are summarized and applied to reduce the RCS of complex targets using the proposed method. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2020

Work is focused on both theoretical knowledge of the electromagnetic field and the specific properties of metamaterials measured. First, be informed about the fundamentals of electromagnetic fields, the issue of periodic structures to the properties required metastructures. Further experimental work is designed to measure the properties of metamaterials. The measurements will be used as harmonic signals and broadband signals in the frequency range 100MHz to 10GHz. In the last part of the proposed work is carried out experimental work and are evaluated some characteristics of the antennas and measurement of the selected sample.

Le concept de l'optique de transformation qui permet de contrôler le trajet des ondes électromagnétiques à volonté en appliquant une variation spatiale judicieusement définie dans les paramètres constitutifs, est exploré pour concevoir des nouveaux types d'antennes et de dispositifs micro-ondes. Dans une première partie, basée sur la transformation de coordonnées, un dispositif d'illusion capable de modifier l'apparence d'une émission électromagnétique et de la délocaliser, est validé par le biais de simulations numériques. Un dispositif de transition de guide d'ondes, permettant d'assurer une transmission quasi totale entre deux guides d'ondes de sections transversales différentes est également conçu et validé expérimentalement par l'utilisation des résonateurs à métamatériaux. Dans une seconde partie, la transformation de l'espace basée sur l'équation de Laplace est étudiée pour concevoir des lentilles pour les applications antennaires. Une lentille de focalisation capable de restaurer les émissions en phase d'un réseau conforme d'éléments rayonnants et une lentille à dépointage qui permet de dévier la direction du faisceau rayonné d'une antenne sont conçues. Des prototypes fabriqués à partir d'un matériau isotrope tout-diélectrique grâce à la technologie d'impression en trois dimensions (3D) permettent de valider expérimentalement la fonctionnalité des lentilles sur une large bande de fréquence. / The concept of transformation optics which can control electromagnetic waves at will by applying a judiciously defined spatial variation in the constitutive parameters is explored to design novel type antennas and microwave devices. In a first part, based on coordinate transformation, an illusion device able to modify the appearance of an electromagnetic emission and then delocalize it is validated through numerical simulations. A waveguide tapering device allowing to assure quasi-total transmission between two waveguides of different cross-sections is also designed and validated experimentally through the use of metamaterial resonators. In a second part, space transformation based on Laplace's equation is studied to design lenses for antenna applications. A beam focusing lens able to restore in-phase emissions from a conformal array of radiators and a beam steering lens that allows deflecting the radiated bema of an antenna are designed. Prototypes fabricated by an all-dielectric isotropic material through three-dimensional (3D) polyjet printing technology allow to experimentally validate the functionality of the lenses on a wide frequency range.

<div>Lattices composed of bistable elements are of great significance across various fields of science and engineering due to their ability to support a class of solitary waves, called transition waves. Common with all solitary waves, transition waves carry highly concentrated energy with minimal degradation and thus have many useful engineering applications, such as extreme waveguides, bandgap transmission, vibration absorption, and energy harvesting. The rich dynamics arising from the strong nonlinearities of the constitutive bistable microstructures still have much to be unveiled for the practical implementation of the transition waves in real-world engineering structures. Especially, the quasi-particle characteristics of the transition waves can potentially address the performance limits posed by the unit cell size in linear metamaterials.</div><div><br></div><div>In this thesis, we first present an input-independent generation of transition waves in the lattices of asymmetric bistable unit cells when snap-through transitions occur at any site within the lattice. The resulting responses are invariant across the lattice except near the boundaries. These characteristics imply useful applications in broadband energy harvesting, exploiting the highly concentrated energy of the transition waves. We further observe that the inherent lattice discreteness induces dominantly monochromatic oscillatory tail following the main transition wave. This radiated energy of the tail can always be efficiently harvested through resonant transduction regardless of the input excitations. This type of bistable lattice transforms any input disturbance into an output form that can be conveniently transduced; thus, energy harvesting becomes an inherent metamaterial property of the bistable lattice.</div><div><br></div><div>To enhance the responses further for improved energy harvesting capability, we introduce engineered defects in the form of a mass impurity, inhomogeneous inter-site stiffness, and their combinations, achieving localization of energy at desired sites. Remarkably, we also observe a long-lived breather-like mode for the first time in this type of lattice. To enhance the tail motions globally across the lattice, we investigate the responses in a set of bistable lattices with the same mass and elastic densities but with different lattice spacing distances (or lattice discreteness). From the available tail energy, we observe a significant increase in the harvesting capability with the increased lattice discreteness.</div><div><br></div><div>Next, the effect of functional grading on the onsite and inter-site stiffnesses are investigated to augment the control of the transition waves in the bistable lattices. The unidirectionality still remains in the direction of decreasing stiffness, while a boomerang-like wave reversal occurs in the direction of increasing stiffness. Both the compression and rarefaction transition waves are allowed to propagate, enabling continuous transmission of the transition waves without complex resetting mechanisms, thus expanding the bistable lattices' functionality for practical applications.</div><div><br></div><div>The observed input-independent dynamics of the one-dimensional bistable lattices can be extended to higher-dimensional metastructures by allowing macrostructural flexibility. Metabeams composed of spring-joined bistable elements are subjected to in-plane sinusoidal input at the microstructural level, and the out-of-plane responses at the macrosctructural level are measured. As long as transition waves are triggered within the metabeam, the most dominant output frequency occurs near the natural frequency of the macroscopic structure regardless of the input excitations initiating the transition waves, yielding energy transfer between uncorrelated frequencies.</div><div><br></div><div>Finally, high-fidelity in-house numerical solvers are developed for the massively parallelized computation of the problems involving generic bistable architectures, addressing the problem size limit. The improved numerical solution accuracy and computational performance, compared to those of commercial solvers, provide great potential to discover new dynamics by drastically expanding the accessible analysis regimes.</div><div><br></div><div>The experiments, simulations, and theoretical contributions in this thesis illustrate the possibilities afforded by strongly nonlinear phenomena to tailor the dynamics of materials systems. Importantly, the presented results show mechanisms to affect global dynamic properties unconstrained by the unit cell size, thereby offering new routes to extreme dynamics beyond current metamaterial architectures.</div>

Lightweight mechanical metamaterials with exception specific stiffness and strength are useful in many applications, such as transportation, aerospace, architectures and etc. These materials show great potential in mechanical tasks where weight of the material is restrained due to economy or energy reasons. To achieve both the lightweight and the high specific mechanical properties, the metamaterials are often in form of periodic cellular structures with well-designed unit cells. The strategies in designing and improving such cellular structures become the key in the studies of such mechanical metamaterials. In this work, we use both experimental and numerical approaches while probing three types of mechanical metamaterials: i) composite bending dominated hollow lattices (HLs); ii) triply periodic minimal surfaces (TPMSs) and extended TPMSs (eTPMSs); iii) corrugated TPMSs. We have demonstrated a few strategies in designing and improving the specific stiffness or strength via these examples of mechanical metamaterials. Using carbon/ceramic composite in the bending dominated HLs, we prove that using the composite layered material against the single layer ceramic is effective in improving the specific mechanical performances of the mechanical metamaterials. Next, while studying the nature of TPMS, we discover that under isotropic deformation TPMSs are stretch dominated with no stress concentrations within the shell structure. They also have an optimal specific bulk modulus approaching the H-S upper bound. Furthermore, we establish a strategy to smoothly connect the zero-mean-curvature surfaces in TPMSs with the extension of zero-Gaussion-curvature surfaces, forming new ‘eTPMSs”. These new shellular structures trade off its isotropy and have improved specific Young’s modulus along their stiffest orientation compared to their TPMS base structures. Lastly, we introduce corrugated sub-structures to existing TPMSs to improve their mechanical properties, such as Young’s modulus, yield strength and failure strength in compression. It is found that the corrugated sub-structure can effectively suppress the local bending behavior and redirect crack propagation while such structures were under uniaxial compression.