A transmission-line metamaterial design of a material-embedded electric line source radiating inside a ground recess is investigated. The media embedding the recessed line source are designed such that the embedded current creates the same radiation pattern as a line source over a flat conducting ground plane. Transmission-line metamaterial unit cell designs for the embedding media obtained from the transformation electromagnetics design technique are shown. The metamaterial design of the overall embedded source configuration is numerically tested using circuit simulations. It is shown that the embedded-source design creates the same radiation characteristics as the line source above a flat ground plane at the design frequency.

An innovative implementation of the skin concept for the reduction of the radiated sound power from a vibrating structure is proposed. The skin has a periodic structure and continuously covers a vibrating beam. Thus, this skin decouples the vibrating structure from the acoustic field by modifying the wavenumber spectrum of the radiating surface. First, structural acoustics and periodic structure theories are reviewed in order to predict how bending waves propagate along a periodic beam and how this beam radiates sound. These theories are then extended to the case of multi-layered structures in order to understand the behavior of a beam loaded with a periodic skin. In order to design the beam and skin structural periods, two different methods are used: Galois sequences and an optimization process using a real-valued genetic algorithm. Simulations are run for the case of periodic beams and beams coupled with periodic smart skins in both finite and infinite configurations. Results show that periodic beam can radiate less sound than equivalent uniform structures. Results also show the potential of periodic skin for application to the structural radiation problem for frequencies higher than approximately 100Hz with an approximately 10dB of radiated sound power attenuation. / Master of Science

In this thesis, we use asymptotic methods to solve problems of wave propagation through infinite and finite (only consider those that are finite in one direction) arrays of scatterers. Both two- and three-dimensional arrays are considered. We always assume the scatterer size is much smaller than both the wavelength and array periodicity. Therefore a small parameter is involved and then the method of matched asymptotic expansions is applicable. When the array is infinite, the elastic wave scattering in doubly-periodic arrays of cavity cylinders and acoustic wave scattering in triply-periodic arrays of arbitrary shape rigid scatterers are considered. In both cases, eigenvalue problems are obtained to give perturbed dispersion approximations explicitly. With the help of the computer-algebra package Mathematica, examples of explicit approximations to the dispersion relation for perturbed waves are given. In the case of finite arrays, we consider the multiple resonant wave scattering problems for both acoustic and elastic waves. We use the methods of multiple scales and matched asymptotic expansions to obtain envelope equations for infinite arrays and then apply them to a strip of doubly or triply periodic arrays of scatterers. Numerical results are given to compare the transmission wave intensity for different shape scatterers for acoustic case. For elastic case, where the strip is an elastic medium with arrays of cavity cylinders bounded by acoustic media on both sides, we first give numerical results when there is one dilatational and one shear wave in the array and then compare the transmission coefficients when one dilatational wave is resonated in the array for normal incidence. Key words: matched asymptotic expansions, multiple scales, acoustic scattering, elastic scattering, periodic structures, dispersion relation.

Les dernières années montrent des nombreuses applications de la spectroscopie Teraheretz (THz) dans le domaine de sécurité postale, contrôle de la qualité, médecine et biologie. Après les premières expériences de l'imagerie avec un seul élément / détecteur, l'étape suivante est l'utilisation de matrices de détecteurs. Par conséquent, la nécessité de détecteurs THz sensibles, très rapides, opérant à température ambiante et intégrable facilement en matrice est devenue crucial. Les transistors à effet de champ semblaient être les candidats les plus appropriés pour la construction du première matrice pour l'imagerie THz. Ce travail présente les études des transistors à effet de champ à base de GaAs et GaN en vue de leur application comme imageurs THz. Dans la première partie, nous présentons les études de FET à base de GaAs sur une plage de fréquence très large (0,25 _2.54 THz). Nous étudions également les moyens d'accroître leur sensibilité et d'optimiser leurs puissance équivalente de bruit. Dans la deuxième partie nous étudions les transistors à base de GaN. Ce matériau , avec un grand bande interdite, pourrait être un candidat potentiel comme imageur THz travaillant à des températures élevées et / ou dans des environnements difficiles. Leur sensibilité a été étudiée en fonction de différents paramètres physiques, tels que la tension de grille, la longueur de grille, le courant de fuite de grille, la température et la direction de polarisation du rayonnement THz incidente. Nous avons étudié également l'influence de l'application de courant de drain. Les comportements observés ont été interprété / compris en utilisant des simulations numériques basées sur les modèles théoriques existants. Enfin, nous avons étudié des transistors à base de GaAs avec une structure de couplage spécifique réseaux de grille périodiques. La présence de la structure périodique permet d'améliorer le couplage entre l'onde THz incidente et le transistor. Une estimation théorique de la longueur caractéristique de détection, combinée à des calculs de l'intensité des champs THz locaux ont été utilisés pour interpréter nos résultats expérimentaux. Un bon accord avec le modèle théorique a été obtenue montrant que la détection a lieu principalement dans les zones appauvri du canal. / Last years clearly show many emerging applications of Teraheretz spectroscopy in postal and airport security, quality control, medicine and biology. After first demonstrations of imaging with a single element/detector the evident next step is use of detector matrixes. Therefore , the need for sensitive, rapid, room temperature operating, and easily integrable THz detectors became critical. Field effect transistors appeared to be the most suitable candidates for building the first matrixes focal plane arrays. This work presents the studies of different GaAs and GaN based field effect transistors in view of they application in Terahertz imagers. In the first part we present the studies of GaAs based FETs over a very wide frequency range (0.25 _2.54 THz). We study also the ways to increase their sensitivity and optimize their Noise Equivalent Power. In the second part we study the transistors based on GaN technology. This wide gap material can be a potential candidate for Terahertz imagers working at elevated temperatures or/and harsh environments. Their sensitivity has been studied as a function of various physical parameters, such as the gate voltage, gate length, the gate leakage current, temperature and direction of polarization of the incident THz radiation. We studied also the influence of a drain current. The observed behaviour was interpreted/understood using numerical simulations based on existing theoretical models. Finally, we studied GaAs based transistors with a specific coupling structure - periodic double-granting gate. The presence of periodic structure allows to improve the coupling between incident THz wave and a transistor. A theoretical estimate of the characteristic length of detection, combined with calculations of the intensity of THz local fields were used to interpret our experimental results. Good agreement with theoretical model was obtained showing that the detection takes place mainly in depleted portions of the channel.

<p>In all attempts to analyze and realize Left-Handed materials, so far, most researchers have used the same idea of extracting only some or certain behaviors of Metamaterials from a set of unit cells gathered together in a designed order. Nevertheless meeting all criteria in order to consider a media as real double-negative material has never come true.</p><p>Starting with criticizing and arguing the validity of calling any set of unit cells as a medium of propagation, the work at hand will go further demonstrating analogies between a medium which could be assigned permittivity or permeability factors and the medium consisting a set of unit cells.</p><p>After presenting the critical analysis on previous studies in the field, here it is shown that it is impossible to build Metamaterials using any number of passive unit cells. A deep insight into the concept of phase and group velocities as well as Poynting’s vector will reveal weakness of the public perception of their relation with each other. Unlike the past and current trend in analyzing these two velocities in meta-materials, they will be proven to possess the same direction.</p><p>Moreover, in this work, a solid proof over violation of energy conservation in the intersection plane between a normal material and a Left Handed material is presented which requires us to believe and accept generation of energy at this plane. This view will consequently leave meaningless all attempts to build meta-materials by passive elements.</p><p>In present work a method is proposed at which a material with positive permittivity and permeability can behave like and yield all characteristics of Metamaterials only if the foregoing parameters, while remaining positive, can vary and be governed by the magnitude of the electromagnetic field. Independence of this method from frequency broadens the range of its application and also the interest it may attract.</p>

In all attempts to analyze and realize Left-Handed materials, so far, most researchers have used the same idea of extracting only some or certain behaviors of Metamaterials from a set of unit cells gathered together in a designed order. Nevertheless meeting all criteria in order to consider a media as real double-negative material has never come true. Starting with criticizing and arguing the validity of calling any set of unit cells as a medium of propagation, the work at hand will go further demonstrating analogies between a medium which could be assigned permittivity or permeability factors and the medium consisting a set of unit cells. After presenting the critical analysis on previous studies in the field, here it is shown that it is impossible to build Metamaterials using any number of passive unit cells. A deep insight into the concept of phase and group velocities as well as Poynting’s vector will reveal weakness of the public perception of their relation with each other. Unlike the past and current trend in analyzing these two velocities in meta-materials, they will be proven to possess the same direction. Moreover, in this work, a solid proof over violation of energy conservation in the intersection plane between a normal material and a Left Handed material is presented which requires us to believe and accept generation of energy at this plane. This view will consequently leave meaningless all attempts to build meta-materials by passive elements. In present work a method is proposed at which a material with positive permittivity and permeability can behave like and yield all characteristics of Metamaterials only if the foregoing parameters, while remaining positive, can vary and be governed by the magnitude of the electromagnetic field. Independence of this method from frequency broadens the range of its application and also the interest it may attract.

This work investigates the application of homogenization techniques to the dynamic analysis of periodic solids, with emphasis on lattice structures. The presented analysis is conducted both through a Fourier-based technique and through an alternative approach involving Taylor series expansions directly performed in the spatial domain in conjunction with a finite element formulation of the lattice unit cell. The challenge of increasing the accuracy and the range of applicability of the existing homogenization methods is addressed with various techniques. Among them, a multi-cell homogenization is introduced to extend the region of good approximation of the methods to include the short wavelength limit. The continuous partial differential equations resulting from the homogenization process are also used to estimate equivalent mechanical properties of lattices with various internal configurations. In particular, a detailed investigation is conducted on the in-plane behavior of hexagonal and re-entrant honeycombs, for which both static properties and wave propagation characteristics are retrieved by applying the proposed techniques. The analysis of wave propagation in homogenized media is furthermore investigated by means of the bridging scales method to address the problem of modelling travelling waves in homogenized media with localized discontinuities. This multi-scale approach reduces the computational cost associated with a detailed finite element analysis conducted over the entire domain and yields considerable savings in CPU time. The combined use of homogenization and bridging method is suggested as a powerful tool for fast and accurate wave simulation and its potentials for NDE applications are discussed.

Electromagnetic wave scattering from a semi-infinite array of dipoles in free space is described by using asymptotic high frequency methods. An electric field integral expression is obtained and solved with asymptotic high frequency methods. An asymptotic field expression is obtained for a finite &times / infinite array of dipoles in free space. The analytical closed form expression for the array guided surface wave launching coefficient is obtained via a combination of an asymptotic high frequency analysis of a related reciprocal problem and Lorentz reciprocity integral formulation for the semi-infinite planar dipole array in which modified Kirchhoff approximation is used. The accuracy and the validity of the asymptotic analytical solutions are compared with the numerical solutions available in the literature before.

Honeycomb core sandwich panels are widely used in the aeronautic industry due to their excellent flexural stiffness to weight ratio. Generally, classical homogenized model is used to model honeycomb core sandwiches in order to have an efficient but not expensive numerical modeling. However, previous works have shown that, while the homogenized models could correctly represent the membrane waves' behavior of sandwiches in a large frequency range, they could not give satisfying simulation results for the flexural waves' behavior in the high frequency range (HF). In fact, the honeycomb core layer plays an important role in the propagation of the flexural waves, so that when the involved wavelengths become close to the characteristic lengths of honeycomb cells, the cellular microstructure starts interacting strongly with the waves and its effect should no longer be neglected, which is unfortunately not the case of the homogenized models. In the present work, we are interested in improving the theoretical and numerical analysis of HF elastic waves' propagation in honeycomb core sandwich panels by a numerical approach based on the Bloch wave theorem, which allows taking into account the periodic characteristics of the honeycomb core. In fact, by decomposing non-periodic wave solutions into their periodic Bloch wave basis modes, numerical models are defined on a basic cell and solved in a efficient way, and provide a better description and so a better understanding of the interaction between HF wave propagation phenomena and the periodic structures. Our numerical approach is developed and validated in the cases of one-dimensional periodic beam structures, of two-dimensional periodic hexagonal and rectangular beam structures and of honeycomb core sandwich plates. By solving the eigenvalue problem of the Bloch wave modes in one primitive cell of the periodic structure for all the wave vectors located in the corresponding first Brillouin zone in the phase space, the dispersion relation between the wave vector and the eigenvalue is calculated. The analysis of the dispersion relation provides important results such as: the frequency bandgaps and the anisotropic and dispersive characteristics of periodic structures, the comparison between the first Bloch wave modes to those of the classical equivalent homogenized models and the existence of the retro-propagating Bloch wave modes with a negative group velocity.

Orientador: Luiz Carlos Kretly / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Elétrica e de Computação / Made available in DSpace on 2018-08-24T13:36:57Z (GMT). No. of bitstreams: 1 Silva_MauricioWeberBenjoda_D.pdf: 10953654 bytes, checksum: 6b4d1b6000f187a807b5cec8ba653713 (MD5) Previous issue date: 2014 / Resumo: Neste trabalho, as diferentes propriedades de superfícies seletivas em frequência, FSS - Frequency Selective Surfaces, são analisadas. As FSS são estruturas planares com células periódicas e podem ser classificadas como uma classe de metamateriais. Para tanto, o mecanismo de trabalho dessas estruturas foi extensivamente estudado, e um método próprio, baseado no modelo de circuito equivalente em conjunto com simulações de onda completa foi proposto. A ferramenta desenvolvida é útil para uma análise preliminar rápida de FSS, a qual foi utilizada para criar uma base de dados de elementos conhecidos na literatura. Diferente dos modelos de análise clássicos, a modelagem analítica proposta, que é uma das principais contribuições do trabalho, usa um simples algoritmo para aproximar a resposta de superfícies seletivas em frequência com geometrias arbitrárias, para incidências normal e oblíqua e para substratos com quaisquer espessuras. Nesse sentido, após a simulação eletromagnética da estrutura, é possível computar a resposta de uma FSS com diferentes parâmetros sem o consumo de tempo das simulações de onda completa. O modelo usa as características peculiares de superfícies de alta impedância, HIS - High Impedance Surface, que dentro de determina faixa comporta-se como condutor magnético perfeito, PMC - Perfect Magnetic Conductor, enquanto no restante da banda tem comportamento de um condutor elétrico perfeito, PEC - Perfect Electric Conductor, para sintetizar absorvedores finos e planares de micro-ondas. As estruturas, compostas de superfície seletiva em frequência resistivas sobre um substrato dielétrico aterrado, são projetadas visando aplicação em diferentes faixas de frequência de absorção e diferentes larguras de banda. Na faixa de 5,5 GHz, objetivou-se satisfazer as especificações dos sistemas WIMAX, WLAN, com os padrões IEEE 802.11a, bem como sistemas de radar, enquanto sinais de outras faixas podem trafegar com atenuação mínima ou nula. Para a faixa mais elevada, projetou-se uma estrutura que oferece absorção sobre a faixa de frequências de 10 GHz a 18 GHz, que pode ser empregada visando aplicações na banda-X e banda-Ku. O método de modelagem para a FSS e para os absorvedores propostos foi validado fisicamente através de montagens experimentais e instrumentação, especialmente desenvolvidas para estas estruturas. Os protótipos dos absorvedores fabricados são extremamente finos e foram medidos por meio de setups de medida em campo aberto e em câmara anecóica. As estruturas projetadas mostraram excelente desempenho para as faixas medidas, mantendo refletividade tipicamente abaixo de -10 dB ao longo de toda a banda. A metodologia desenvolvida nesta pesquisa pode ser ampliada para diferentes faixas de frequências, larguras de banda e aplicações / Abstract: In this work, the different properties of frequency selective surfaces - FSS are analyzed. Frequency selective surfaces are planar structures with periodic cells and can be classified as a kind of metamaterials. To this end, the working mechanism of these structures has been extensively studied, and a proper method based on the equivalent circuit model in conjunction with full-wave simulations was proposed. The developed tool is useful for a fast preliminary analysis of FSS, which was used to create a database of known elements presented in the literature. Unlike of classical analysis model, the proposed analytical modeling, which is one of the main thesis contributions, uses a simple algorithm for approximate the response of frequency selective surfaces with arbitrary shape, for normal and oblique incidence and for substrates with all thicknesses. In this sense, after the electromagnetic simulation of the structure, it is possible to compute the response of an FSS with different parameters without the time consuming full-wave simulations. The model uses the unique characteristics of High-Impedance Surfaces - HIS, which for certain frequency range, behaves as Perfect Magnetic Conductor - PMC, while outside this band behaves as a Perfect Electric Conductor - PEC, for synthesizing thin planar microwave absorbers. The structures, comprising resistive frequency selective surfaces over a grounded dielectric substrate, are designed aiming different absorption frequency bands and different bandwidths. In the 5.5 GHz frequency range, the aim was to satisfy the specifications of WiMAX, WLAN systems, in view of the IEEE 802.11a standards, as well as radar systems, while signals from other bands can travel across with zero or minimal attenuation. To the highest range, the designed structure provides absorption over 10 GHz to 18 GHz frequency range, and can be applied to the X- and Ku- band. The modeling method for the FSS and the proposed absorbers was physically validated through experimental setups and instrumentation, especially developed for these structures. The prototype of the fabricated absorbers are extremely thin and were characterized by using free space and anechoic chamber measurement setups. The designed structures showed excellent performance for measurements ranges, with reflectivity typically below -10 dB over the entire band. The methodology developed in this research can be extended to different frequency bands, bandwidth and applications / Doutorado / Eletrônica, Microeletrônica e Optoeletrônica / Doutor em Engenharia Elétrica