With the recent emergence of the field of metamaterials, the study of subwavelength propagation of plane waves and the dissipation of their energy either in the form of Joule losses in the case of electomagnetic waves or in the form of viscous dissipation in the case of acoustic waves in different interfaced media assumes great importance. with this motivation, I have worked on problems in two different areas, viz., plasmonics and surface acoustics. the first part (chapters 2 & 3) of the dissertation deals with the emerging field of plasmonics. Researchers have come up with various designs in an efort to fabricate efficient plasmonic waveguides capable of guiding plasmonic signals. However, the inherent dissipation in the form of Joule losses limits efficient usage of surface plasmon signal. a dielectric-metal-¬dielectric planar structure is one of the most practical plasmonic structures that can serve as an efficient waveguide to guide electromagnetic waves along the metal-dielectric boundary. I present here a theoretical study of propagation of surface plasmons along a symmetric dielectric-metal-dielectric structure and show how proper orientation of the optical axis of the anisotropic substrate enhances the propagation length. an equation for propagation length is derived in a wide range of frequencies. I also show how the frequency of coupled surface plasmons can be modulated by changing the thickness of the metal film. I propose a Kronig-Penny model for the plasmonic crystal, which in the long wavelength limit, may serve as a homogeneous dielectric substrate with high anisotropy which do not exist for natural optical crystals. in the second part (chapters 4 & 5) of the dissertation, I discuss an interesting effect of extraordinary absorption of acoustic energy due to resonant excitation of Rayleigh waves in a narrow water channel clad between two metal plates. Starting from the elastic properties of the metal plates, I derive a dispersion equation that gives resonant frequencies, which coincide with those observed in the experiment that was performed by Wave Phenomena Group at Polytechnic University of Valencia, Spain. Two eigenmodes with different polarizations and phase velocities are obtained from the dispersion equation. at certain critical aperture of the channel, an interesting cutoff effect, which is unusual for an acoustic wave, is observed for one of the eigenmodes with symmetric distribution of the pressure field. the theoretical prediction of the coupling and synchronization of Rayleigh waves strongly supports the experimentally measured shift of the resonant frequencies in the transmission spectra with channel aperture. the observed high level of absorption may find applications in designing metamaterial acoustic absorbers.

Les méta-surfaces permettent de contrôler efficacement es rayonnements électromagnétiques en manipulant la phase, l'amplitude et la polarisation des ondes. Pour de nombreuses applications, telles que des fonctions d’agilité, de commutations et de détection, des surfaces de haute performance sont nécessaires. L’objectif de cette thèse est d'explorer des méthodes innovantes permettant d'améliorer les facteurs de qualité des méta-surfaces dans les domaines micro-ondes et optique. La limitation des méta-surfaces est généralement liée aux pertes par rayonnement et à l'absorption non radiative. L'étude se concentre sur des solutions permettant la suppression des pertes de rayonnement en utilisant des concepts de mode sombre et de résonance de Fano. Un mécanisme d'excitation direct de mode sombre dans un résonateur individuel basé sur l’adaptation de symétrie est proposé. La possibilité d'améliorer l'efficacité de la réponse magnétique en optique est présentée. L'analyse de l'excitation par résonance de Fano dans un système de résonateurs couplés identiques est également réalisée sur la base du formalisme de la théorie des modes couplés. Deux configurations générales correspondant à l'arrangement spatial symétrique et énantiomères des résonateurs sont considérées. Dans le premier cas, le système de cavité formé par les méta-surfaces très proches fournit des caractéristiques spectrales très étroites et une sensibilité efficace élevée par rapport aux cavités Fabry-Perot conventionnelles. Dans le second cas, les caractéristiques de conception permettent de contrôler la suppression des pertes de rayonnement, permettant d'augmenter considérablement le facteur de qualité de la résonance de Fano. / Metasurfaces allows an effective control of electromagnetic radiation by manipulating phase, amplitude and polarization of electromagnetic waves. For numerous applications including tunable, switchable and sensing functionalities, high performance metasurfaces are required. The goal of this thesis is to explore innovative methods enabling to improve the quality factors of metasurfaces in microwave and optical domains. The limitation of metasurfaces is generally related to radiation losses and non-radiative absorption. The study is focused on solutions allowing the suppression of radiative losses by using concepts of dark mode and Fano resonance. A direct dark mode excitation mechanism in individual resonator based on symmetry matching is proposed. The possibility to enhance magnetic response efficiency in optical frequency range is presented. Analysis of Fano resonance excitation in a system of identical coupled resonators is also performed on the basis of coupled mode theory formalism. Two general configurations are considered corresponding to symmetric and enantiomeric space arrangement of resonators. In the first case, cavity system formed by near-field coupled metasurfaces provides sharp spectral characteristics and high efficient sensitivity compared to conventional Fabry-Perot cavities. In the second case the design features enables to control suppression of radiation losses, allowing to considerably increase Fano resonance quality factor.

Le but de la thèse était de simuler le spectre d'absorption de méta-matériaux pour les applications photovoltaïques. Par méta-matériaux, nous entendons une assemblée d'objets de taille nanométrique situés à distance mésoscopique. L'idée sous-jacente est qu'en modifiant la taille du nano-objet et l'arrangement géométrique, on peut ajuster le seuil d'absorption. Pour calculer ces quantités, j'ai utilisé l'état de l'art du formalisme, c'est-à-dire des méthodes ab initio.La première étape du travail a été dédiée au calcul de l'absorption d'un objet isolé (tranche de silicium, graphène, hBN). Dans le cadre de codes périodiques, on utilise une supercellule avec du vide pour isoler l'objet, et une méthode a été développée précédemment dans le groupe de Spectroscopie Théorique du LSI, pour obtenir des résultats indépendants du vide. Elle est appelée Selected-G, et a été appliquée avec succès aux surfaces de silicium. Pour une tranche isolée, une expression modifiée du potentiel coulombien dans l'espace réciproque, appelé "slab potential", doit être utilisée. Pour valider l'utilisation du potentiel de slab pour le calcul de la matrice diélectrique microscopique, j'ai simulé les spectres de perte d'énergie d'électrons pour des empilements de quelques plans de graphène, et reproduit avec succès les données expérimentales disponibles. Cela a offert la possibilité d'étudier la dispersion du plasmon d'un plan de graphène, et discuter la nature des excitations électroniques dans ce système (transitions interband ou plasmon 2D).La second étape a été consacrée à l'étude du spectre d'absorption d'une assemblée de tranches en interaction. Comme il a été mis en évidence que le formalisme de supercellule agit comme une théorie de matériau moyen avec du vide, avec l'effet erroné d'avoir des spectres dépendant de la taille de la supercellule, j'ai renversé la procédure pour extraire le spectre de la tranche en interaction, affranchi du problème du vide. La faisabilité a été démontrée sur les tranches de hBN, dont le caractère semi-conducteur à large bande interdite évite les instabilités numériques.Cela a permis de comprendre la raison pour laquelle l'absorption de la tranche en interaction de silicium apparaît à plus basse énergie que celle du matériau massif: cela vient de la présence des états de surface dans la bande interdite de la structure de bandes du massif. Néanmoins, la différence avec la tranche isolée doit être encore étudiée.La troisième partie a été dédiée à l'étude de matériaux utilisés, ou candidats, aux applications photovoltaïques comme InP et InSe. J'ai étudié dans un premier temps les structures de bandes des massifs. Pour corriger la sous-estimation de la bande interdite calculée dans l'approximation de la densité locale (LDA), j'ai calculé les corrections GW, et utilisé la fonctionnelle d'échange et corrélation de Heyd-Scuseria-Ernzerhof (HSE). Le spectre d'absorption de InP massif a été calculé en résolvant l'équation de Bethe-Salpeter, qui permet de tenir compte des effets excitoniques. Comme ce calcul est très lourd numériquement, j'ai également comparé avec le calcul beaucoup plus léger de TDDFT avec le kernel à longue portée pour introduire les effets excitoniques. Pour le massif de InSe, j'ai calculé les corrections HSE pour les valeurs propres et obtenus un bon accord avec la bande interdite expérimentale. Les spectres obtenus en TDDFT, avec le kernel à longue portée, donne de bons résultats. J'ai commencé l'étude de tranches de ces deux matériaux. Des couches épaisses de InP et InSe ont été considérées et une reconstruction de surface (2x2) a été réalisée sur InP pour obtenir une surface semi-conductrice. La structure de bande LDA et les spectres d'absorption ont été calculés. Comme des systèmes d'une telle taille sont hors de portée des calculs de corrections HSE, l'étude s'est concentrés sur des tranches beaucoup plus fine de InSe. / The purpose of the thesis was to simulate the absorption spectrum of meta-materials for photovoltaic applications. By meta-material, we mean an assembly of nanometric size objects at mesoscopic distance. The underlying idea is that by adjusting the size of the nano-object and the geometric arrangement, one could tune the absorption edge. To calculate these quantities, I used state-of-the art formalism, namely ab-initio methods.The first step of the work has been dedicated to the calculation of the absorption of an isolated object (slab of silicon, graphene, hBN). In the framework of periodic codes, one uses a supercell with vacuum to isolate the object, and a method has been developed previously in the Theoretical spectroscopy group at LSI, to provide results independent of vacuum. It is called “Selectd-G” method, and was successfully applied to silicon surfaces. For an isolated slab, a modified expression of the reciprocal space Coulomb potential, called “slab potential”, must be used. To validate the use of the slab potential on the microscopic dielectric matrix, I have simulated Electron Energy Loss spectra for slabs of few graphene layers, and successfully reproduced available experimental data. This has also offered the possibility to study the plasmon dispersion of a single graphene layer, and discuss the nature of electronic excitations in the system (intraband transitions or 2D-plasmon).The second step has been dedicated to the study of the absorption spectrum of an array of interacting slabs. Since it has been evidenced that the supercell formalism acts as an effective medium theory with vacuum, with the spurious effect of having spectra dependent on the size of the supercell, I have reversed the procedure to extract the spectrum of the interacting slab, "cured" from the vacuum problem. First, the feasibility has been demonstrated on slabs of hBN, as their semi-conducting characteristics with a the large gap prevent numerical instabilities. Then, it has allowed us to understand the reason why the absorption of the interacting slab of silicon appears at lower energy than its bulk counterpart: it is due to the presence of surface states in the gap of the bulk band structure. Nevertheless, the difference with the isolated slab must be further investigated.The third part has been dedicated to the study of materials currently used or candidates for photovoltaic applications: InP and InSe. I have first studied the band structures of bulk InP and InSe. To correct for the underestimation of the band gap in the local density approximation (LDA), I have used GW corrections and the Heyd-Scuseria-Ernzerhof (HSE) exchange-correlation functional. The absorption spectrum for bulk InP has been calculated by means of the solution of the Bethe-Salpeter equation to correctly account for the excitonic effects. As expected, the experimental macroscopic function is well reproduced. Since the calculation is numerically demanding, I have also compared the results with the much lighter calculation using TDDFT where I used the long range kernel to mimic the excitonic effects. For bulk InSe, I have calculated the HSE corrections for the eigenvalues and obtained a good agreement with the experimental band gap. The spectrum obtained within TDDFT, with the long range kernel, gives satisfying results. We have started the calculations for slabs of these two materials. Thick slabs of InP and InSe have been considered and a 2x2 reconstruction have been performed for the InP slab to recover the semi-conducting surface. The LDA band structures and absorption spectra have been calculated. Then, such large systems being out of range of HSE corrections calculations, the study has been focused on much thiner slabs in the case of InSe.

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A novel microstrip patch antenna topology is presented for achieving a dual-band response with arbitrarily closely spaced resonances. This topology is based on a coupled transmission line structure in order to take advantage of the separation in propagation constants for parallel (even-mode) and anti-parallel (odd-mode) current modes. Applying a metamaterials inspired design approach, periodic reactive load­ings are used to design the underlying transmission line to have specific propagation constants necessary to realize a desired separation between two resonant frequencies. Using a single probe feed for a finite coupled line segment, both even-and odd-mode resonances can be excited to radiate efficiently at their respective design frequencies. The efficiency of the odd-mode radiation is enhanced by separating the two lines, while strong coupling is maintained by inserting a series of narrowly-separated thin loops between them. Several example resonant antenna designs, in the 2.45 GHz band, are presented. The directivities of these microstrip patch antennas are enhanced by optimizing the physical length of the resonant structure. For a resonant antenna obtained by cas­cading several unit cells of reactively loaded microstrip segments, dispersion analysis is employed for the unit-cell design. Maximum directivity is achieved by choosing the overall physical length to be slightly less than a half wavelength in free space at the design frequency. This gain optimization is applied to three coupled-line antennas, as well as a single resonance patch. Excellent agreement is observed between simulated and measured responses across all designs. The potential of loading the coupled line structure with active components is also explored. Varactor diodes are placed on coupled-line structures in two configurations. In one configuration, both resonant frequencies are affected. In the other configura­tion, only the odd-mode characteristics are reconfigured. In this way, the resonant frequency of either one or both modes can be adjusted by applying a DC bias voltage to the varactor diode loading elements. Two antennas, one employing each of these topologies, were designed and fabricated. Control of the resonant frequency over the predicted range through applying a bias voltage is observed with the fabricated prototypes.

Phased microphone arrays are valuable tools for aeroacoustic measurements that can measure the directivity of multiple acoustic sources. However, when deployed in closed test-section wind tunnels, the acoustics suffer due to intense pressure fluctuations contained in the wall-bound turbulent boundary layer. Furthermore, phased microphone arrays require many sensors distributed over a large aperture to ensure good spatial resolution over a wide frequency range. Microphone arrays of such large count are not always feasible due to constraints in space and cost. This thesis describes an alternative approach for measuring single broadband acoustic sources that uses an acoustic metasurface. The metasurface is comprised of a meandering channel of quarter-wave cavities and an array of equally spaced half-wave open through-cavities. A series of tests were conducted in Virginia Tech's Anechoic Wall-Jet Tunnel where combinations of a wall-bound turbulent jet-flow and a single broadband acoustic source were used to excite the metasurface and produce acoustic surface waves. Measurements of the acoustic surface waves were performed using two methods: a pair of traversing microphones scanning the pressure field along the length of the metasurface 0.25 mm beneath its bottom face, and an array of unequally spaced microphones embedded inside the metasurface. Spectral analysis on the measurements revealed that the inclusion of multiple through-cavities leads to constructive reinforcement of select acoustic surface waves as a function of the acoustic source location. In the case of the embedded microphones, acoustic beamforming was applied in order to extract spatial information. This reinforcement was observed during measurements made with both flow and acoustic excitation, up to Wall-Jet Tunnel nozzle exit speeds of 40 m/s beyond which it was no longer seen. A series of quiescent measurements made with a range of speaker locations constituted a calibration for the metasurface which was used to locate an unknown broadband acoustic source within an The Root-Mean-Square (RMS) error of 1.06 degrees. / Master of Science / Phased microphone arrays are valuable tools for aeroacoustic measurements that can measure the directivity of multiple acoustic sources within a sound field. When used in conjunction with signal processing techniques, such as delay-and-sum beamforming, a researcher or engineer can obtain an intuitive view of the sound field and distinguish between multiple sources over a wide frequency range. However, these microphone arrays often utilize dozens of microphones which raises the array's complexity and cost. Furthermore, when a phased microphone array is mounted flush to the wall of a wind tunnel test section, it is submerged under a turbulent boundary layer which imposes intense pressure fluctuations on the microphones making it difficult to identify acoustic sources. Boundary layers form at the interface between a fluid and solid interface. This thesis describes experimentation performed in the Virginia Tech Anechoic Wall-Jet Tunnel on a new type of pressure sensing microphone array that leverage acoustic metamaterial technology. The acoustic metamaterial shields the microphones from the flow, lessening the influence of the turbulent boundary layer on the measurement. The focus in this thesis is on the novel array's ability to locate a single broadband acoustic source using as few as six microphones. The metasurface was installed in the Wall-Jet Tunnel test plate such that an array of evenly spaced through-cavities are flush to the surface. The through-cavities communicate the pressure field on top of the test surface to a meandering channel of interconnected closed cavities below. Near the resonant depth frequencies of the closed cavities, acoustic surface waves form which are evanescent pressure waves that are bound to the surface or structure that support them. The interference between the acoustic surface waves generated at each through-cavity leads to reinforced acoustic surface waves which are sensitive to the direction of a broadband source. In all, an acoustic metamaterial was tested under a variety of conditions such as: Wall-Jet Tunnel flow speed, speaker location, and the number of through-cavities open. The performance of the novel array and future plans are discussed.

With the increasing demand on wireless and portable devices, the radio frequency front end blocks are required to feature properties such as wideband, high frequency, multiple operating frequencies, low cost and compact size. However, the current radio frequency system blocks are designed by combining several individual frequency band blocks into one functional block, which increase the cost and size of devices. To address these issues, it is important to develop novel approaches to further advance the current design methodologies in both space and spectrum domains. In recent years, the concept of artificial materials has been proposed and studied intensively in RF/Microwave, Terahertz, and optical frequency range. It is a combination of conventional materials such as air, wood, metal and plastic. It can achieve the material properties that have not been found in nature. Therefore, the artificial material (i.e. meta-materials) provides design freedoms to control both the spectrum performance and geometrical structures of radio frequency front end blocks and other high frequency systems. In this dissertation, several artificial materials are proposed and designed by different methods, and their applications to different high frequency components and circuits are studied. First, quasi-conformal mapping (QCM) method is applied to design plasmonic wave-adapters and couplers working at the optical frequency range. Second, inverse QCM method is proposed to implement flattened Luneburg lens antennas and parabolic antennas in the microwave range. Third, a dual-band compact directional coupler is realized by applying artificial transmission lines. In addition, a fully symmetrical coupler with artificial lumped element structure is also implemented. Finally, a tunable on-chip inductor, compact CMOS transmission lines, and metamaterial-based interconnects are proposed using artificial metal structures. All the proposed designs are simulated in full-wave 3D electromagnetic solvers, and the measurement results agree well with the simulation results. These artificial material-based novel design methodologies pave the way toward next generation high frequency circuit, component, and system design.

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Parabolic solar trough receivers are used to collect heat via the mean of a heat transfer fluid. This component is one among a myriad of the Concentrated Solar Power (CSP) devices. Parabolic troughs reach high temperatures around 400 ºC. improving the Parabolic Solar Trough's receiver with an IR selective coating will increase the heat transfer absorbed by the heat transfer fluid and reduce the radiative heat loss. Thus, optimizing the receiver will ameliorate the efficiency of the electrical production for a CSP. The parabolic solar receiver existing in industry currently are made of stainless steel with no specific coating for IR solar rays spectrum selection. Therefore, the heat transferred through the absorber is limited to certain light spectrum. Furthermore, numerous receivers proposed are made from materials that contaminates their optical properties when oxidized such as aluminum [1]. The heat transfer and optical analysis of the PTC are essential to optimize and understand its performance under high temperatures and reduce the heat loss. In this paper, our focus is on presenting a super-lattice IR selective coating to minimize the radiative heat loss. Making use of the power of metamaterials to confection optical properties that are inexistent in nature, the coating will serve to maximize the tube's reflectance above 70% in the IR. Not only does the selective coating enhance the optical properties of the receiver, but also it ensures performance stability for high temperatures.