Dissertations / Theses on the topic 'METAMATERIAL ABSORBER'

Thesis advisor: Willie J. Padilla
Realization of an ideal electromagnetic absorber has long been a goal of engineers and is highly desired for frequencies above the microwave regime. On the other hand, the desire to control the blackbody radiation has long been a research topic of interest for scientists--one particular theme being the construction of a selective emitter whose thermal radiation is much narrower than that of a blackbody at the same temperature. In this talk, I will present the computational and experimental work that was used to demonstrate infrared metamaterial absorbers and selective thermal emitters. Based on these work, we further demonstrate an electrically tunable infrared metamaterial absorber in the mid-infrared wavelength range. A voltage potential applied between the metallic portion of metamaterial array and the bottom ground plane layer permits adjustment of the distance between them thus altering the electromagnetic response from the array. Our device experimentally demonstrates absorption tunability of 46.2% at two operational wavelengths. Parts of this thesis are based on unpublished and published articles by me in collaboration with others. The dissertation author is the primary researcher and author in these publications. The text of chapter two, chapter five, and chapter seven is, in part, a reprint of manuscript being prepared for publication. The text of chapter three is, in part, a reprint of material as it appears in Physical review letters 104 (20), 207403. The text of chapter four is, in part, a reprint of material as it appears in Physical Review Letters 107 (4), 45901. The text of chapter six is, in part, a reprint of material as it appears in Applied Physics Letters 96, 011906
Thesis (PhD) — Boston College, 2013
Submitted to: Boston College. Graduate School of Arts and Sciences
Discipline: Physics

Thesis advisor: Willie J. Padilla
This thesis will describe the implementation of novel imaging applications with electromagnetic metamaterials. Metamaterials have proven to be host to a multitude of interesting physical phenomena and give rich insight electromagnetic theory. This thesis will explore not only the physical theory that give them their interesting electromagnetic properties, but also the many applications of metamaterials. There is a strong need for efficient, low cost imaging solutions, specifically in the longer wavelength regime. While this technology has often been at a standstill due to the lack of natural materials that can effectively operate at these wavelengths, metamaterials have revolutionized the creation of devices to fit these needs. Their scalability has allowed them to access regimes of the electromagnetic spectrum previously unobtainable with natural materials. Along with metamaterials, mathematical techniques can be utilized to make these imaging systems streamlined and effective. Chapter 1 gives a background not only to metamaterials, but also details several parts of general electromagnetic theory that are important for the understanding of metamaterial theory. Chapter 2 discusses one of the most ubiquitous types of metamaterials, the metamaterial absorber, examining not only its physical mechanism, but also its role in metamaterial devices. Chapter 3 gives a theoretical background of imaging at longer wavelengths, specifically single pixel imaging. Chapter 3 also discusses the theory of Compressive Sensing, a mathematical construct that has allowed sampling rates that can exceed the Nyquist Limit. Chapter 4 discusses work that utilizes photoexcitation of a semiconductor to modulate THz radiation. These physical methods were used to create a dynamic THz spatial light modulator and implemented in a single pixel imaging system in the THz regime. Chapter 5 examines active metamaterial modulation through depletion of carriers in a doped semiconductor via application of a bias voltage and its implementation into a similar single pixel imaging system. Additionally, novel techniques are used to access masks generally unobtainable by traditional single pixel imagers. Chapter 6 discusses a completely novel way to encode spatial masks in frequency, rather than time, to create a completely passive millimeter wave imager. Chapter 7 details the use of telecommunication techniques in a novel way to reduce image acquisition time and further streamline the THz single pixel imager. Finally, Chapter 8 will discuss some future outlooks and draw some conclusions from the work that has been done
Thesis (PhD) — Boston College, 2015
Submitted to: Boston College. Graduate School of Arts and Sciences
Discipline: Physics

Wireless communication demands better channel capacity with a high data rate in the modern era. To fulfill these demands, the MIMO-communication systems are developed that use manifold antennas for transmitter and receiver end. MIMO is a state-of-art technology that improves the reliability of the communication systems by utilizing the diversity technique to mitigate the multi-path fading issues, where signals may come together belligerently at the receiver. Improve spectral efficiency is achieved by the total transmitted power spreading over the antennas. Thus, MIMO can increase channel capacities as well as the reliability of the communication system without sacrificing extra transmitted power or power spectrum. Several MIMO antennas have been designed in the literature to improve their characteristics in terms of impedance bandwidth; miniaturization & isolation improvement. The MIMO-communication systems with THz range are required for high data speed in Terabit/sec (Tbps). Also, it is providing very high throughput per device (from multiple Gbps to several Tera-bps) including per area efficiency (bps/km2). It is also predicted that the world monthly traffic in smartphones will be about 40 Peta-bytes in 2021, so the demand for MIMO antennas will be increased in the future. In this thesis, various microwave components for the MIMO wireless communication system has been analyzed and designed. Three major components designed and analyzed in this thesis are 1. MIMO Antennas 2. Metamaterial Absorber 3. UWB Microwave Filter MIMO Antennas: In this thesis, various MIMO antennas for UWB, SWB, and Multiband applications have been designed. Various decoupling techniques to avoid the v interference between antenna elements are designed which enhancing the diversity parameters with improved channel capacity for modern wireless applications. To mitigate the interference between bands and to improve the reliability of the signals, a notch characteristic has been introduced. SAR analysis also discusses in this thesis with the human head and confirms that proposed MIMO antennas are in the acceptable range with 1g and 10g of bio tissues given by FCC and EU for mobile and other near field applications. All the MIMO antennas with different frequency characteristics are discussed in Chapter-2 to Chapter-6. Metamaterial Absorber: To improve the isolation level in MIMO antennas as well as to minimize the Radar Cross Section (RCS) and Electromagnetic Interference (EMI), a design of multiband metamaterial absorber (MMA) for X-band applications has been suggested. This MMA provides three high absorbance bands at 8.2GHz, 9.45GHz, and 12.45GHz with 99.4%, 96.4%, and 91.25% absorbance respectively. Proposed MMA is polarization insensitive in all three bands with minimum RCS -33.2dBm2. This absorber structure has designed on FR-4 (4.4) substrate having tanδ = 0.02 with unit cell dimension 20×20×1mm3. So the proposed absorber is found appropriate for stealth aircraft, RCS and EMC reduction, isolation in MIMO antenna, imaging, and sensing in the X-band applications, discussed in Chapter-7. UWB Microwave Filter: In this research work, the design of the UWB filter with extended stopband characteristics by using a parallel-coupled line, open-ended line, multimode resonator (MMR), and defected ground structure (DGS) has been presented. This filter provides good return and insertion loss in the passband (3.1-10.6GHz) as well as stopband (10.8-18GHz). The group delay of the filter is almost constant throughout the passband. Detailed analysis of supportive coupled, feeding, and the open-ended line is vi verified with equivalent circuits. The prototype of the filter is compact as 22×20mm2 with a 109% fractional bandwidth. The proposed filter is suited for recent weather reporting Radar, Imaging, and Satellite receiver systems because simulated results have good agreement with measured results as discussed in the Chapter-8. RESEARCH OBJECTIVES: The major objectives of the research work are listed below: 1. To enhance the impedance bandwidth of the MIMO antenna and microstrip filter for various wireless applications. 2. To design and analyze the circularly polarized MIMO antenna for GPS, vehicular and 5G applications. 3. To enhance the isolation between the various elements of the MIMO antenna, to improve the various diversity parameters. 4. To enhance the specific absorption ratio (SAR) performance of the MIMO antenna for a handhold and mobile applications. 5. To design a Metasurface for stealth and isolation improvement in MIMO antenna applications.

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The development of high-power microwave weapons and dependence on electronics in modern weapon systems presents a high-power microwave weapons threat in future military conflicts. This study experimentally determines the absorption characteristics of simple metamaterial devices to potentially be used as protection and identification mechanisms, constructed through standard printed circuit board manufacturing processes, in the microwave region. Experimental results and analysis techniques are presented confirming absorption peaks in the anticipated microwave frequency range. The experimental results are compared to a finite-element model of these metamaterials confirming the ability to accurately model and predict absorption characteristics of similar metamaterial structures. Utilization of the absorption characteristics of these types of metamaterial structures to develop a microwave detector and/or equipment shielding is discussed. Several applications for such type of a detector are presented.

In this thesis metamaterial radar absorbers and plasmonic structures have been investigated. Following a brief overview covering metamaterial structures, and their applications in various areas of Microwave Engineering, a novel thin metamaterial wideband radar absorber, formed by two layers of resistive Hilbert curve arrays, is proposed and analysed numerically in HFSS, revealing a reduction in Monostatic Radar Cross Section (RCS) of more than 10 dB from 9.1 to 18.8 GHz (70% fractional bandwidth) for both polarizations. The structure has thickness of only 0.11λ to 0.24λ at lowest and highest frequencies respectively. The lateral dimensions are only 0.13λ to 0.3λ per unit cell at lowest and highest frequencies respectively which is several times smaller than that of recently reported circuit analogue absorbers operating in the similar frequency band. Furthermore, a wideband terahertz Hilbert curve array is proposed and analyzed both theoretically and numerically, showing an absorption bandwidth of more than one octave. This was followed by study of plasmonic cloak for subwavelength conducting objects. It was demonstrated that a plasmonic cloak designed for a conducting sphere will work for non spherical conducting objects of similar dimensions as well. Finally spoof plasmonic structures were investigated. A novel plasmonic structure based on a modified Apollonian fractal array of cylindrical coaxial apertures in an aluminium sheet was proposed and analyzed. The structure exhibits negative group velocity with less than 3.5 dB attenuation. Plasmonic structure based on Sierpinski array of apertures was also investigated and found to give quite good extraordinary transmission bandwidth.

Ce travail de thèse concerne les structures artificielles à base de métamatériaux permettant la réalisation d’absorbants parfaits. Après une brève introduction des métamatériaux, de leur fonctionnement en tant qu’absorbants et de l’état de l’art, quatre types de structures fonctionnant en bandes centimétrique ou millimétrique ont été conçus puis fabriqués à savoir (i) des réseaux de cubes BaSrTiO3 (BST) basés sur les résonances de Mie, (ii) des réseaux désordonnés composés d’anneaux métalliques mettant en jeu des effets de résonance semblables aux systèmes plasmoniques (iii) des absorbants à quatre résonateurs élémentaires sur substrat flexible et (iv) des réseaux multicouches métal-diélectrique de forme pyramidale. Pour l’ensemble, des simulations numériques, corroborées par l’expérience en guide d’onde ou en espace libre, montrent l’existence d’un moment magnétique. Celui-ci est induit par une boucle des courants de déplacement et de conduction. Pour les structures périodiques, les conditions de grande largeur de bande d’absorption ont été établies sur la base du piégeage et de la dissipation de l’énergie incidente. Pour les réseaux désordonnés, il est montré le rôle capital des couplages entre résonateurs. Des structures périodiques à base de ferroélectrique de dimensions sous longueur d’onde ont été assemblées avec succès tandis que des absorbants flexibles ont été réalisés par technique d’impression jet d’encre montrant l’amélioration d’un facteur quatre de la bande d’absorption. Des améliorations comparables ont été obtenues à l’aide de réseaux d’anneaux, dont les positions dans le plan sont désordonnées, résultant de la distribution des fréquences de résonance par effet de couplage fort entre les résonateurs
In this thesis broadband Metamaterial Perfect Absorbers (MPAs) have been investigated. Following a brief introduction of metamaterials, operating mechanisms and state of the art of MPA, four absorber types operating either at centimeter or millimeter wavelengths have been designed and fabricated namely :(i) Mie-resonance based BaSrTiO3 (BST) arrays operating at microwaves, (ii) plasmonic-type disordered ring-shaped MPA, (iii) four patches millimeter wave flexible absorbers (iv) Pyramidal metal/dielectric stacked resonator arrays. For all the structures, it was demonstrated, through numerical simulations, assessed by characterization in a waveguide configuration or in free space, that unit absorbance relies on magnetic resonances induced by a current loop combining displacement and conduction currents. For periodic arrays, the condition for a broad band operation was established via the optimization of dissipation and trapping of electromagnetic energy in the resonators. For disordered metamaterials, it was shown the major role played by the magnetic dipole-dipole interaction. From the technological side, Ferroelectrics cube arrays with subwavelength dimensions were assembled onto a metal plate while flexible multi-resonators periodic arrays were successfully fabricated by ink-jet printing showing a fourfold enhancement of the absorbance bandwidth thanks to the overlapping of resonance frequencies. Comparable improvement in the bandwidth was also pointed out with randomly position metal ring arrays due to the distribution of resonance frequencies that result from tight in-plane resonator coupling

Des propriétés électromagnétiques (EM) intéressantes peuvent être réalisées avec des Métamatériaux (MM), qui sont des matériaux artificiels sub-longueur d’ondes. Les propriétés EM peuvent être modifiées en modifiant la géométrie des MM. Les MM sont utilisés entre autres pour la conception d’antennes miniatures, des Absorbants Radars (AR) et le contournement des Ondes Electromagnétiques (OEM). Dans cette thèse nous avons utilisé des Surfaces Sélectives en Fréquences (FSS), qui sont une famille des MM, pour la réalisation des AR fines couches large bande. Ces AR sont destinés à la réduction des interférences des OEM entre les équipements radars à l’intérieur des radomes des navires militaires pour la bande de 1-10 GHz (CEM). Les AR large bande développés dans cette thèse ont été étudiés pour différentes polarisations (TE et TM) et différentes angles d’attaque. La Signature Equivalente Radar (SER) des AR a également été étudié. Ces AR ont été fabriqués et les résultats de mesures d’absorption sont en accord avec les résultats de simulation. Afin d’améliorer les performances d’absorption en polarisation croisée, des structures d’AR en ‘échiquier’ ont été proposées. Des plus les performances de rapport d’épaisseur théorique et d’épaisseur réel des AR développés sont extrêmement intéressantes à l’état de l’art. Une étude théorique a permis de concevoir et proposer de nouvelles structures d’AR conformes couvrant une cible cylindrique métallique. Ces AR se présente sous la forme de juxtaposition de plusieurs diélectriques sectoriels. On a démontré que le rayonnement total pouvait être diminué et que les zones d’ombres causés par des objets cylindriques pouvaient également être diminuées. La validation expérimentale de ce concept constituera un résultat remarquable de la thèse
Metamaterials (MM) are artificially engineered sub wavelength materials that can provide exceptional electromagnetic properties. Their electromagnetic properties can be changed by changing their shapes. They have been used for the design of antennas, Radar Absorbers (RA), cloak devices and so on. In the aim of reducing electromagnetic interference in radomes of military vessels, in this thesis we have used Frequency Selective Surfaces, which are a family of MM, to design thin and broadband RA for the 1-10 GHz frequency band. The RA designed in this thesis have been studied for different polarizations (TE and TM) and for different incidence angles. The Radar Cross Section (RCS) of the developed RA have also been studied. These RA have been fabricated and an excellent agreement have been found between measured and simulated absorption results. In order to improve the cross-polarization absorption of our RA, a ‘chessboard’ configuration of the full structures have been proposed and studied. Furthermore, the theoretical to real thickness ratio of developed RA have been calculated and results suggest that their performances are high. Also, a theoretical study has enabled us to design conformal RA for cylindrical metallic bodies. These RA are in fact sectors of dielectrics conformed around the cylindrical target. It has been shown that the total scattering and shadow zones of cylindrical metallic bodies can be reduced. The fabrication, characterization and measurement of this concept will be a remarkable result of this thesis

Thesis (Ph.D.)--Boston University
Electromagnetic wave absorbers have been intensely investigated in the last century and found important applications particularly in radar and microwave technologies to provide anechoic test chambers, or vehicle stealth. Adding new features such as dynamic modulation, absorption frequency tunability, and nonlinearity, absorbers gain further functions as spatial light modulators, adjustable protective layers, and saturable absorbers which was a key factor in creation of ultra-fast lasers. These efforts required a rigorous search on various materials to find desired behavior. As a rather recent research field Metamaterials (MM) provide an easier path for creation of such materials by allowing engineering the interaction between electromagnetic radiation and materials. Alongside many exotic applications such as invisibility cloaking or negative refraction, MMs also made perfect, or near-unity, absorbers possible. Thanks to their ability to control electric and magnetic responses, by matching the impedance of the MMs to that of free space and simultaneously increasing the losses in the structure, perfect absorption can be achieved. This has been experimentally demonstrated in various bands of electromagnetic spectrum such as microwave, terahertz (THz), infrared, and visible. As in their earlier counterparts, adding modulation and nonlinearity to MM absorbers will broaden their contribution especially in the THz region which is nascent in terms of optical devices such as switches, modulators or detectors. With the recent developments in the THz lasers, THz nonlinear absorbers will be needed to realize ultra-fast phenomena in this region. The main focus of this thesis is incorporating conventional and novel methods to create some of the initial examples of optically controlled MM THz perfect absorbers using microfabrication tools. [TRUNCATED]

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For Terahertz (THz) imaging to be useful outside of a laboratory setting, inexpensive yet sensitive detectors such as uncooled microbolometers will be required. Metamaterials can improve THz absorption without significantly increasing the thermal mass or using exotic materials because their absorption is primarily dependent on the geometry of the materials and not their individual optical properties. Finite Element (FE) simulations revealed that an array of squares above a ground plane separated by a dielectric is efficient, yet thin. Metamaterials were fabricated and their absorption characteristics were measured using a Fourier Transform Infrared Spectrometer (FTIR) indicating that the FE simulations are accurate. Metamaterial structures tuned to a quantum cascade laser (QCL) illuminator were incorporated into a bi-material sensor, which was used for detection of THz radiation from the QCL source with good sensitivity. In the case of microbolometers, a bolometric layer needs to be embedded in the metamaterial to form a thin microbridge. Simulations indicated that if the bolometric layer was resistive enough or close enough to the ground plane, then absorption would be largely unaltered. Metamaterials with a conductive Titanium (Ti) layer embedded into the dielectric spacer were fabricated and measured with an FTIR, confirming this behavior.

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THz radiation covers the region of the electro-magnetic (EM) spectrum between the microwaves and infra-red (IR), corresponding to frequencies from approximately 100 GHz to 10 THz. Recently, new imaging techniques, which take advantage of the special properties of THz waves, have been developed. Despite the great interest in these new techniques, limitations such as the lack of appropriate detectors and powerful sources are placing the technology in the research domain. The objective of this thesis is to characterize and analyze a set of fabricated bi-material detectors integrated with thin metamaterial films. Different experimental measurements were performed to measure the main figures of merit of the detectors and analyze them. Initially, optical microscopy was used to measure the dimensions of the sensors and stress induced curvature. Then, the thermal response of the sensors was tested and analyzed. The responsivity, the speed of operation and the minimum detected incident power were measured using a quantum cascade laser (QCL), operating at 3.8 THz. The measured experimental data agree well with the theoretical calculated values of the performance parameters.

We have studied several metamaterials structures with multiple layers by explaining them theoretically and verifying experimentally. The engineered structures we have designed work either as a perfect absorber or antireflection coating. The multilayer model as we call it Three Layer Model (TLM) has been developed, which gives the total reflection and transmission as a function of reflection and transmission of individual layers. By manipulating the amplitude and phase of the reflection and the transmission of the individual layers, we can get the required functionality of the optoelectronic devices. To get zero reflection in the both perfect absorber and the antireflection coating, the amplitude and phase conditions should be satisfied simultaneously. We have employed the numerical simulation of the structures to verify those conditions for all of the work presented here. As the theoretical retrieval method to extract the effective permittivity and effective permeability of the metamaterial contains air on the both side of the structure, we have dielectric at least on one side practically, that gives a little bit deviated result. We have modified the retrieval method to better fit with the multilayer structure by introducing air on the both side of the resonator using transfer matrix method and use it throughout all the works. We have explained the perfect absorption of the EM wave through Fabry-Perot cavity bounded by the resonator mirror and the metallic film. The metallic film acts as the close boundary whereas the resonator acts as the quasi-open boundary with very high effective permittivity, which leads to the characteristic feature of subwavelength thickness. We have shown numerically that the ultra-thin thickness makes the perfect absorber angular independent. We have also explained the phenomenon of perfect absorption through Impedance Matched Theory and Transmission Line Theory, and showed their matching with TLM. We have also developed the Meta Film Model by considering the resonator as a homogeneous thin film characterized by the effective permittivity and permeability giving rise to the same behavior as the original multilayer structure. We have shown that the resonance of the metamaterial resonator is very far from the resonance of the absorber, it behaves as the medium of high refractive index and very low loss. We have also shown that the density of states of the absorber is increased as compared to the resonator itself. We have investigated that the resonance peaks of the absorber arise from the combination of Fabry- Perot cavity modes and surface plasmon resonance modes. All the modes with increased spacer thickness are assigned with specific names describing the mode profiles. We have shown the application of perfect absorber as a refractive index sensor. It is used as a plasmonic sensor to detect the refractive index change of the chemical and biological samples. To increase the sensitivity, we have etched the dielectric spacer below the resonator, where electric field is localized and enhanced. We have found that the sensitivity (wavelength shift per refractive index change) and the Figure of Merit (FOM*) as an indicator of performance of the device both are enhanced significantly. We have employed metamaterial (MM) anti-reflection (AR) coating to avoid the shortcomings of the conventional thin film coating in three different cases of the structures. At first, we have deployed metamaterial Metal Disk Array (MDA) on the top of conventional coating material (BCB) with homogeneous substrate to enhance the transmission of EM wave. Then conventional AR coating is employed to the dispersive media (metal Hole Array) to enhance the transmission. We have shown that Impedance matched condition has been satisfied not only for homogeneous media, but for dispersive media also. At the end, we have employed the MM AR coating to the MM dispersive media (MHA). The two MM layers may interact with each other and may degrade the SPP wave of the MHA, which is essential to enhance the performance of the devices. To investigate the effect of interaction, we perform the simulation of the MDA, which shows that the resonance of the MDA is far from the antireflection resonance and hence the electric field of the SPP is significantly increased (~30%). With an improved retrieval method, the metasurface is proved to exhibit a high effective permittivity (εeff~30) and extremely low loss (tanδ~0.005). For all of the three AR structures, a classical thin film AR coating mechanism is identified through analytical derivations and numerical simulations. The properly designed εeff and μeff of the meta surface lead to the required phase and amplitude conditions for the AR coating, thereby paving the way for the improved performance of the optoelectronic devices. We have used MHA as a dispersive media to get extraordinary optical transmission (EOT). To understand the behavior of the SPP peaks, we have investigated the shifting and splitting of the spoof SPP resonance by varying the polar angle and azimuthal angle. The amplitude of extraordinary optical transmission also shows angle dependence and exhibits mirror-image or translational symmetries. Our measurements and simulations of the THz spoof SPP waves match very well with the theoretical predictions. The angle dependence results provide the important information for designing THz plasmonic devices in sensor and detector applications.

Dans cette thèse, nous visons à concevoir des absorbants à larges bandes passantes pouvant fonctionner efficacement même en basses fréquences. Pour atteindre un tel objectif, nous profitons des propriétés des métamatériaux pour réaliser des dispositifs capables de remplacer les technologies actuelles d’absorbants destinés à des utilisations dans les chambres anéchoïques par exemple. Après avoir étudié l'état de l'art de l'absorbant à base de métamatériaux, nous avons choisi la conception pyramidale comme base de notre recherche eu égard à ses propriétés adaptées à notre application. Nous effectuons une étude paramétrique complète pour ajuster ses paramètres géométriques et les propriétés des matériaux afin d'obtenir la meilleure réponse en terme d’absorption. En outre, nous améliorons sa bande passante d'absorption relative en créant une nouvelle conception avec altitude incurvée. Ces optimisations conduisent à une augmentation de la bande passante d'absorption relative (RAB) trouvée dans les littératures récentes de 63,3% à 73,4% avec un niveau d'absorption supérieur à 80%. Dans ce travail, nous nous nous intéressons également aux contraintes de conception en basses fréquences. Pour répondre aux exigences de dimensionnement à ces fréquences nous nous orientons vers des matériaux diélectriques avec des permittivités relatives élevée afin de réduire l’encombrement de ces structures. Après avoir appliqué ces choix de matériaux, nous avons réussi à déplacer la fréquence vers les bandes UHF. Nous avons réalisé des cellules unitaires miniaturisées en appliquant une géométrie de surface minimale en tant que nouvelle méthode de miniaturisation des absorbants. De plus, pour élargir l'absorption de l'absorbant pyramidal conventionnel, nous présentons différents nouveaux prototypes. Nous citons le prototype d'une épaisseur totale de 12,7 cm, composé de 35 couches résonantes incurvées où les simulations numériques montrent une conception améliorée avec une bande d'absorption totale allant de 0,3 GHz à 30 GHz. Concernant le deuxième prototype proposé, ce dernier est composé, dans une maille élémentaire, de l’association d’absorbants opérants sur des bandes complémentaires et à base de la structure pyramidale. Après avoir introduit de nombreux facteurs d'amélioration et avoir réalisé plusieurs étapes d'optimisation, ce dernier prototype nous a permis d’atteindre une bande passante d'absorption relative de 128,69%. Tous ces prototypes sont conçus et étudiés à l'aide du simulateur numérique High-Frequency Structure Simulator (HFSS). Tous les calculs ont été effectués sur un HPC de 24 cœurs avec une mémoire système de 192 Go de RAM. Pour la fiabilité des résultats, le mode d'analyse fréquentielle en mode discret a été paramétré avec de nombreux points de fréquences pour atteindre des résultats de simulation avec un très haut niveau de précision
In this thesis, we aim to design a broadband absorber that can effectively operate at low frequencies. To achieve such an aim, we take advantage of the properties of the metamaterial to reach a stage in which the former is capable of replacing the present bulky anechoic chamber. After studying the state of the art of metamaterial absorber, we choose the pyramidal design to be the basis of our research view of its suitable properties for our application. We perform a complete parametric study to adjust its geometrical parameters and material properties to obtain the best absorption response. Besides, we enhance its relative absorptive bandwidth by making a novel curved altitude design. The latter two modifications lead to an increase in the Relative Absorptive Bandwidth (RAB) from 63.3% in the literature to 73.4% with an absorption level greater than 80%. In addition, we discuss the requirements needed to reach a low-frequency band absorber that can be summarized by the necessary high relative permittivity material dielectric substrate and the need for bigger dimensions. After applying these requirements, we succeeded to shift the frequency to UHF bands. We achieved a miniaturized unit cells by applying minimal surface geometry as a novel way in miniaturizing absorber. Moreover, to widen the broadband absorption of the conventional pyramidal absorber, we present different new absorber prototypes. We cite the prototype with a total thickness of 12.7 cm, consisting of 35 curved resonant layers where numerical simulations show an enhanced design with an absorption band from 0.3 GHz to 30 GHz. Concerning the second proposed prototype, the latter is dedicated to combining complementary bands for different pyramidal structures dimensions in one unit cell. After introducing many enhancement factors and taking into account optimization, this prototype reached a well-combined band with a relative absorptive bandwidth of 128.69%. These prototypes are tested using the numerical simulator High-Frequency Structure Simulator (HFSS). All calculations were performed on an HPC of 24 cores with a system memory of 192 GB RAM. For the reliability of the results, discrete frequency analysis mode was adjusted with numerous data points to reach simulation results with a very high level of precision

L’évolution constante des performances des sonars nécessite de nouveaux designs de revêtements absorbants pour l’acoustique sous-marine. De tels revêtements sont utilisés pour améliorer la furtivité des sous-marins, mais ils permettent également d’accroître l’efficacité des systèmes de détection embarqués. Les méta-écrans bulleux (lointains descendants des revêtements de type Alberich) représentent une solution possible pour répondre à cet enjeu. Ils sont constitués d’une distribution périodique bi-dimensionnelle de cavités d’air de taille sub-longueur d’onde emprisonnées dans une matrice viscoélastique. Lorsqu’elles sont excitées par une onde acoustique, les cavités se comportent comme des bulles d’air, et présentent une résonance basse fréquence, dite de "Minnaert". Sous certaines conditions, le méta-écran bulleux permet d’atteindre une absorption totale lorsqu’il est placé devant un réflecteur parfait. Ce travail de thèse a permis la mise au point d’un modèle phénoménologique, validé par des simulations numériques et des mesures en cuve, pour prédire les coefficients de réflexion et de transmission d’un méta-écran bulleux en fonction de ses caractéristiques géométriques et rhéologiques. Ce modèle prend en compte l’influence de la température et de la pression statique sur les performances du méta-écran, ainsi que celle de la forme des cavités
The constant evolution of sonar performance requires new designs of absorbent coatings for underwater acoustics. Such coatings are used to improve stealth of submarines but can also improve the efficiency of on-board detection systems. Bubble meta-screens (reminiscent of the so-called Alberich coatings) are a possible solution to tackle this issue. A bubble meta-screen consists of a periodic distribution of sub-wavelength air cavities trapped in a visco-elastic matrix. The cavities acoustically behave as bubbles and exhibit a low frequency resonance, known as the Minnaert resonance. Under certain conditions, the meta-screen can achieve a total absorption when placed in front of a perfect reflector. This doctoral work allowed us to build a phenomenological model, validated by numerical simulations and experiments, which can predict the reflection and transmission coefficients of the meta screen as a function of its geometric and rheological characteristics. Our model takes into account the influence of the temperature and static pressure on the performance of a meta-screen, as well as the role played by the shape of the cavities

Le contrôle des vibrations à basse fréquence adapté aux structures légères est un défi scientifique ettechnologique en raison de contraintes économiques et écologiques de plus en plus strictes. De récentes études enacoustique ont portées sur l’absorption totale d’ondes basses fréquences à l’aide d’absorbeurs parfaits sublongueursd’onde. Ces métamatériaux sont obtenus en exploitant la condition de couplage critique. Unegénéralisation de cette méthode pour le domaine élastodynamique serait d’un grand intérêt pour répondre auxexigences du contrôle des vibrations de structures légères à basse fréquence.Cette thèse vise à adapter le problème d’absorption parfaite des ondes de flexion dans des systèmes 1D et 2D avecdes résonateurs locaux en utilisant la condition de couplage critique. Une étude préliminaire sur des systèmes 1D àgéométries simples sont d’abord proposée. Celle-ci propose une méthode de conception de résonateurs simplespour une absorption efficace des ondes de flexion. Une complexification du système 1D est ensuite considérée avecl’étude du couplage critique de Trou Noir Acoustique (TNA) 1D. Ceci a motivé l’interprétation de l’effet TNA à l’aidedu concept de couplage critique afin de présenter des outils clés à de futures procédures d’optimisation pour ce typede terminaisons. La condition de couplage critique est ensuite étendue aux systèmes 2D. L’absorption parfaite parle premier mode axisymétrique d’un résonateur circulaire inséré dans une plaque mince infinie est analysée. Ladiffusion multiple par une ligne de résonateurs circulaires insérés dans une plaque mince 2D infinie ou semi-infinie,appelée métaplaque, est aussi considérée dans l’optique de se rapprocher d’une application industrielle. A traverscette thèse, des modèles analytiques, des simulations numériques et des expériences sont présentés pour valider lecomportement physique des systèmes présentés
The vibration control adapted to light structures is a scientific and technological challenge due toincreasingly stringent economic and ecological standards. Meanwhile, recent studies in audible acoustics havefocused on broadband wave absorption at low frequencies by means of subwavelength perfect absorbers. Suchmetamaterials can totally absorb the energy of an incident wave. The generalisation of this method for applicationsin elastodynamics could be of great interest for the vibration control of light structures.This thesis aims at adapting the perfect absorption problem for flexural waves in 1D and 2D systems with localresonators using the critical coupling condition. A study of 1D systems with simple geometries is first proposed. Thisprovides methods to design simple resonators for an effective absorption of flexural waves. The 1D systems thenbecome more complex by studying the critical coupling of 1D Acoustic Black Holes (ABH). The ABH effect is theninterpreted using the concept of critical coupling, and key features for future optimisation procedures of ABHs arepresented. The critical coupling condition is then extended to 2D systems. The perfect absorption by the firstaxisymmetric mode of a circular resonator inserted in a thin plate is analysed. Multiple scattering by an array ofcircular resonators inserted in an infinite or semi-infinite 2D thin plate, called metaplate, is also considered to getclose to practical applications. Through this thesis, analytical models, numerical simulations and experiments areshown to validate the physical behaviour of the systems presented

碩士
國立交通大學
電子工程學系 電子研究所
104
A new structure for a nearly-perfect hyperbolic meta-material(HMM) absorber is proposed, and initial experimental verification is provided. To date, HMM PMAs are realized using tapered stacks that can provide adiabatic waveguiding over a wide spectral range. Nevertheless, the tapered nature can prevent its usage for large-area applications such as the emitters in thermophotovoltaics (TPV). The design proposed here has decent wavelength scalability and can be used from optical black holes to microwave perfect absorbers. The physics behind the HMM straight-sidewall cavity is the broadband highly confined resonance. While, in most of the cases, the broadband quasi-guided modes are weekly confined in nature, the HMM cavity can provide broadband resonances but still maintain reasonably strong oscillation strength for high absorption. This is because the photonic density of state (PDOS) is boosted dramatically by the hyperbolic dispersion of the straight-sidewall Al/SiO2 stacks.

碩士
國立交通大學
電子研究所
105
An extremely simple multiple-metal metamaterial perfect absorber (MPA) has been proposed in this project. The dimension of the proposed design, for the visible wavelength range from 400 nm to 700 nm, is only 221 nm. Even if the plasmonic excitation is absent in this proposal, our design has dimension comparable to the past effort of MPAs using plasmonics. An unity broadband absorption can be achieved with ultra-thin metallic films. In addition, the wavelength scalability is possible using our design, and the fully-planar, simple configuration facilitates large-area photonic design without the need of lithography and etching. The physics are the field penetration and the field absorption for the photons at different wavelength ranges using different metallic layers. It showed that the adjustment of the individual layer thickness was critical to attain a perfect wave impedance matching to vacuum. The titanium (Ti), nickel (Ni), and aluminum (Al) triple-metal configuration are used to demonstrate the concept experimentally, and a close match to the theoretical result is observed. The absorption band can be further widened with more stacking layers with various metals. It is believed that the proposed design is very promising in the aspects of simple processing, scalable for large-area, and broadband unity absorption. Thus, it improves the future implementation of metamaterial perfect absorbers and facilitates a wide range of relevant applications.

博士
國立交通大學
電子研究所
107
Recently, metamaterial perfect absorber (MPA) has been widely investigated. At first, by using metamaterials, we can resolve the issue of narrow absorption band for photonic devices using anti-reflection coating structure and fulfill an ideal broadband perfect absorber. However, the researches of wavelength selectivity perfect absorber have gradually increased owing to its versatile applications. In this thesis, we integrate the planar perfect absorber with an aperiodic filter composed of two alternating dielectric thin films. The genetic algorithm is used to determine the geometry of this proposed structure. By adjusting each layer thickness and total thickness of the aperiodic dielectric filter, we can achieve four kinds of functional absorbers, including low-pass, high-pass, band-reject and band-pass absorbers. We also conduct the experiment of band-reject absorber and measure the reflectance and transmittance by UV-VIS-NIR spectrometer and therefore the absorbance can be calculated. The band-reject property is shown in the measurement result and it can sustain until the incident angle reaches 60o. In the second part, we implement the similar concept on thermophotovoltaics. There are many studies about thermophovoltaics in recent years since thermophotovoltaics can reach higher conversion efficiency than conventional solar cells. The key factor is the design and fabrication of the thermal emitter. An ideal thermal emitter should have a narrow emission band whose emission peak wavelength is slightly smaller than the wavelength corresponding to the bandgap of photodiode, and long wavelength emittance suppression in order to prevent redundant thermal emission. According to these principles, we proposed a planar thermal emitter. By adjusting each layer thickness of the aperiodic stacking, the emission peak wavelength can be shifted. The preliminary experiment is conducted to fabricate a thermal emitter whose emission peak wavelength is 1500nm. The effect of materials used and surface roughness for emission spectrum is also investigated. Finally, there are some extra work on PHP Laravel framework and Phonegap. The ultimate goal is to build a simple website which can store the experiment data or create an app which can monitor the condition during experiment. First, we will use Laravel framework to build a simple website for data CRUD operation. Then, Phonegap is used to convert the simple website into an app running on the cell phone.

博士
國立交通大學
電子研究所
105
Hyperbolic metamaterial (HMM) absorber is widely investigated recently. However, the proposed tapered structure needs repeated lithography and etching steps, which are very time-consuming. As a result, several methods are proposed to deal with this problem. Firstly, straight-sidewall structure is utilized whose performance mainly depends on pattern design. Random pattern is preferable to realize broadband and high absorption. Nevertheless, simulation on random pattern is time-consuming. Consequently, experiment genetic algorithm is proposed to conduct the optimization process directly in this study. Secondly, planar field-penetration structure can also be a potential solution method. Multiple metal-dielectric stacking is utilized to accomplish the high absorption and broadband absorption. The proposed structure is fabricated without any lithography and etching steps. Time and cost needed for fabrication is thus decrease. Thirdly, constructing a perfect absorption layer through electrophoretic deposited (EPD) carbon nanotubes (CNTs) are also demonstrated in this study. The combination of the CNT layer and a resonant cavity is proposed in this study. Experimental result confirms that the excessively large thickness issue, associated with past vertically aligned CNT ideal blackbodies, can be resolved while the broadband absorption is still maintained.

碩士
逢甲大學
航太與系統工程學系
102
Metamaterials refer to special artificial materials which do not exist in nature, and such materials have unique features in sound, electromagnetic and light waves. The mechanism is regulated by a periodic pattern of the material to generate electric and magnetic resonances so as to enhance the filtering or absorbing ability. In this study, the carbonyl iron powder is mixed with epoxy resin to fabricate different weight percentage of absorber samples, of which the complex permittivity and permeability are measured. The performance of metamaterial absorbers is evaluated and optimally designed by ANSYS HFSS to obtain the periodic patterns of metamaterials which are fabricated by wet etching. From the results of optimal design, the thickness of lower and upper carbonyl iron layers is 0.6 mm and 1.6 mm, respectively, and the thickness of metamaterial with circular-ring patterns is 0.2 mm. The experimental results shows that the absorbing bandwidth of –10 dB is 8.2 GHz for an incident angle of 21º. The reflection loss reaches –22.24 dB at 8.92 GHz and –17.21 dB at 16.52 GHz. Keywords: metamaterial, carbonyl iron, reflection loss

碩士
逢甲大學
航太與系統工程所
100
Metamaterials are materials with special physical quantities which can not be found in natural materials. Metamaterials are compound structures or materials which are artificially made through periodical arrangement of the material units to obtain unusual negative permittivity and negative permeability of the metamaterials. In this study, three different periodical structures, i.e., four-legged, circular ring, square loop structures combined with carbonyl iron-epoxy layers are used to design the microwave absorbing metamaterials. ANSYS HFSS is used to simulate and evaluate the reflection loss performance of each design for different thickness of carbonyl iron-epoxy layers. With the built-in optimal technique in ANSYS HFSS, a design goal of -10 dB reflection loss (i.e., 90% incident energy is absorbed) is aimed in the frequency range of 2 ~ 18 GHz. Three different structures are designed to meet the light-weight, thin in thinness and broadband requirements. The -10 dB absorption bandwidth for the four-legged metamaterials is from 6.88 GHz to 17.44 GHz with a thickness of 2.49 mm while the bandwidth for the circular ring is 6.72 GHz to 17.36 GHz with 2.45 mm thickness. The bandwidth for the square loop metamaterials is from 6.40 GHz to 16.80 GHz with a thickness of 2.63 mm.

abstract: The objective of this dissertation is to study the use of metamaterials as narrow-band and broadband selective absorbers for opto-thermal and solar thermal energy conversion. Narrow-band selective absorbers have applications such as plasmonic sensing and cancer treatment, while one of the main applications of selective metamaterials with broadband absorption is efficiently converting solar energy into heat as solar absorbers. This dissertation first discusses the use of gold nanowires as narrow-band selective metamaterial absorbers. An investigation into plasmonic localized heating indicated that film-coupled gold nanoparticles exhibit tunable selective absorption based on the size of the nanoparticles. By using anodized aluminum oxide templates, aluminum nanodisc narrow-band absorbers were fabricated. A metrology instrument to measure the reflectance and transmittance of micro-scale samples was also developed and used to measure the reflectance of the aluminum nanodisc absorbers (220 µm diameter area). Tuning of the resonance wavelengths of these absorbers can be achieved through changing their geometry. Broadband absorption can be achieved by using a combination of geometries for these metamaterials which would facilitate their use as solar absorbers. Recently, solar energy harvesting has become a topic of considerable research investigation due to it being an environmentally conscious alternative to fossil fuels. The next section discusses the steady-state temperature measurement of a lab-scale multilayer solar absorber, named metafilm. A lab-scale experimental setup is developed to characterize the solar thermal performance of selective solar absorbers. Under a concentration factor of 20.3 suns, a steady-state temperature of ~500 degrees Celsius was achieved for the metafilm compared to 375 degrees Celsius for a commercial black absorber under the same conditions. Thermal durability testing showed that the metafilm could withstand up to 700 degrees Celsius in vacuum conditions and up to 400 degrees Celsius in atmospheric conditions with little degradation of its optical and radiative properties. Moreover, cost analysis of the metafilm found it to cost significantly less ($2.22 per square meter) than commercial solar coatings ($5.41-100 per square meter). Finally, this dissertation concludes with recommendations for further studies like using these selective metamaterials and metafilms as absorbers and emitters and using the aluminum nanodiscs on glass as selective filters for photovoltaic cells to enhance solar thermophotovoltaic energy conversion.
Dissertation/Thesis
Doctoral Dissertation Mechanical Engineering 2018

The use of a patterned resistive sheet as an infrared-selective absorber, including the effects of a mechanical support dielectric layer is discussed. Also, modified dielectric coated Salisbury Screen can improve both the wavelength selectivity and the speed of thermal response for microbolometers. These patterned resistive sheets and Modified dielectric coated Salisbury Screen are a modified form of classical Salisbury Screens that utilize a resistive absorber layer placed a quarter-wavelength in front of a mirror. These structures can show a narrower detection bandwidth when compared to conventional microbolometers. For a Modified dielectric coated Salisbury Screen for multi-spectral system, wavelength selectivity can be varied by changing the distance to the mirror, and for patterned resistive sheet, wavelength selectivity can be varied by changing the lithographically drawn parameters of the array. Hence, different pixels in a focal plane array can be designed to produce a “multi-color” infrared imaging system. Also, the thermal mass of microbolometer is reduced using patterned resistive structure.
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博士
國立清華大學
材料科學工程學系
104
The researches on THz gap (0.1-30 THz) have attracted much attention in recent years because the THz gap is a transition regime between microwave and far-infrared (IR), i.e., the watershed between the electronic and optical responses so that matters in THz gap in nature possess weak electronic and optical responses, and such weak responses hinder generation and detection of THz signals. Therefore, we eager to apply metamaterials to THz devices based on the properties of metamaterials that is well developed in microwave, IR and even visible region such as their scalability and strong interaction with waves; based on these properties, we expect to expand the applications in the THz gap. In this dissertation, we delve into three different applications at the THz gap. The first research topic is to construct a three-dimensional (3D) negative index medium (NIM) through rotating a split ring resonator. Such 3D NIM, unlike traditional NIM integrating two independent magnetic and electric resonators together, achieves electric and magnetic responses simultaneously and also negative index via a monolithic structure, that is, a metallic hemispherical shell; the shell could simplify the fabrication process of 3D NIM. Furthermore, this monolithic shell is independent of polarization and insensitive to incident angles due to its high symmetry that are favorable in practical applications. Next, in the second project, we modulate the frequencies of magnetic and electric resonances so that permeability and permittivity of the metamaterial intersect with each other to match the impedance of free space, thus leading to suppression of reflectance. On the other hand, via the resonance nature of metamaterials, the imaginary part of index is enhanced when resonance occurred, so transmittance is reduced. While reflectance and transmittance of a material are simultaneously approaching to zero, a perfect absorber could be achieved. Hence, in this topic, we employed stochastic design process to generate a double-sided metamaterial perfect absorber that is composed of a dielectric layer sandwiched by two identical metallic patterns and could absorb the electromagnetic wave from two sides, solving the drawback of traditional metamaterial absorbers, i.e., single operating direction. Noteworthily, such perfect absorber owns a sub-wavelength thickness, thus miniaturizing the devices and providing a promising future compared to conventional absorbers. Afterward, based on the previous stochastic design process, we consider increasing the efficiency of the design process of metamaterials with desired goals. Consequently, in the third project, instead of trial and error process, we utilize computer-aided genetic algorithms (GAs) to efficiently optimize the existing metamaterials and come out the best metamaterial patterns. A bandpass filter, an important unit on future THz communications, is chosen as our target to execute GA and then approach behaviors of ultra-broad fractional bandwidth 82.8% and band-edge transition of 58.3 dB/octave. Besides, the 38-μm-thick and optimized bandpass filter, which is much thinner compared to conventional THz filter. Such miniaturized ultra-broadband and sharp-transition filters profit the development of a THz optical system. To summarize, in this dissertation, we devote ourselves into three different THz devices including 1. Polarization independent and high incident-angle tolerable 3D negative index media, 2. Stochastically designed double-sided perfect absorbers and finally 3. Ultra-broad bandwidth and sharp transition metamaterial THz bandpass filters via genetic algorithm. Such devices validate the exotic properties of metamaterials and would have a huge impact on the field of the THz gap.