This thesis reports our study of the effect of domain wall pinning by ferromagnetic (FM) metamaterials [1] in the form of periodic antidot lattices (ADL) on spin wave spectra in the reversible regime. This study was then extended to artificial quasicrystals in the form of Penrose P2 tilings (P2T). Our DC magnetization study of these metamaterials showed reproducible and temperature dependent knee anomalies in the hysteretic regime that are due to the isolated switching of the FM segments. Our dumbbell model analysis [2] of simulated magnetization maps indicates that FM switching in P2T is nonstochastic. We have also acquired the first direct, two-dimensional images of the magnetization of Permalloy films patterned into P2T using scanning electron microscopy with polarization analysis (SEMPA). Our SEMPA images demonstrate P2T behave as geometrically frustrated networks of narrow ferromagnetic film segments having near-uniform, bipolar (Ising-like) magnetization, similar to artificial spin ices (ASI). We find the unique aperiodic translational symmetry and diverse vertex coordination of multiply-connected P2T induce a more complex spin-ice behavior driven by exchange interactions in vertex domain walls, which differs markedly from the behavior of disconnected ASI governed only by dipolar interactions.

<p>Since their introduction by Mercedes Benz in the late 1990s, W-band radars operating at 76-77 GHz have found their way into more and more passenger cars. These automotive radars are typically used in adaptive cruise control, pre-collision sensing, and other driver assistance systems. While these systems are usually only about the size of two stacked cigarette packs, system size, and weight remains a concern for many automotive manufacturers.</p><p>In this dissertation, I discuss how artificially structured metamaterials can be used to improve lens-based automotive radar systems. Metamaterials allow the fabrication of smaller and lighter systems, while still meeting the frequency, high gain, and cost requirements of this application. In particular, I focus on the development of planar artificial dielectric lenses suitable for use in place of the injection-molded lenses now used in many automotive radar systems.</p><p>I begin by using analytic and numerical ray-tracing to compare the performance of planar metamaterial GRIN lenses to equivalent aspheric refractive lenses. I do this to determine whether metamaterials are best employed in GRIN or refractive automotive radar lenses. Through this study I find that planar GRIN lenses with the large refractive index ranges enabled by metamaterials have approximately optically equivalent performance to equivalent refractive lenses for fields of view approaching ±20°. I also find that the uniaxial nature of most planar metamaterials does not negatively impact planar GRIN lens performance.</p><p>I then turn my attention to implementing these planar GRIN lenses at W-band automotive radar frequencies. I begin by designing uniform sheets of W-band electrically-coupled LC resonator-based metamaterials. These metamaterial samples were fabricated by the Jokerst research group on glass and liquid crystal polymer (LCP) substrates and tested at Toyota Research Institute- North America (TRI-NA). When characterized at W-band frequencies, these metamaterials show material properties closely matching those predicted by full-wave simulations.</p><p>Due to the high losses associated with resonant metamaterials, I shift my focus to non-resonant metamaterials. I discuss the design, fabrication, and testing of non-resonant metamaterials for fabrication on multilayer LCP printed circuit boards (PCBs). I then use these non-resonant metamaterials in a W-band planar metamaterial GRIN lens. Radiation pattern measurements show that this lens functions as a strong collimating element.</p><p>Using similar lens design methods, I design a metamaterial GRIN lens from polytetrafluoroethylene-based (PTFE-based) non-resonant metamaterials. This GRIN lens is designed to match a target dielectric lens's radiation characteristics across a ±6° field of view. Measurements at automotive radar frequencies show that this lens has approximately the same radiation characteristics as the target lens across the desired field of view.</p><p>Finally, I describe the development of electrically reconfigurable metamaterials using thin-film silicon semiconductors. These silicon-based reconfigurable metamaterials were developed in close collaboration with several other researchers. My major contribution to the development of these reconfigurable metamaterials consisted of the initial metamaterial design. The Jokerst research group fabricated this initial design while TRI-NA characterized the fabricated metamaterial experimentally. Measurements showed approximately 8% variation in transmission under a 5 Volt DC bias. This variation in transmission closely matched the variation in transmission predicted by coupled electronic-electromagnetic simulation run by Yaroslav Urzhumov, one of other contributors to the development of the reconfigurable metamaterial.</p> / Dissertation

Optical metamaterials are artificial media that exhibit new properties from structuring on the nanometric scale. One of the main researches in metamaterials investigates materials with negative refractive index, which can allow the development of perfect lens and other exciting potential applications. In this thesis, we theoretically study the properties of negative-index optical fishnet metamaterials, especially the origin of their negative-valued refractive index, and also associated theoretical problems. The thesis can be divided into 4 parts. In the first part we study the light scattering at an interface between air and a semi-infinite fishnet metamaterial. With a fully-vectorial numerical method, we calculate the scattering coefficients of the interface and find that the energy transport inside the fishnet is due to a single Bloch mode, the fundamental one. Based on the single-interface scattering coefficients and the effective index of this Bloch mode we propose a new algorithm for retrieving effective optical parameters of the metamaterial. The approach emphasizes the key role played by the fundamental Bloch mode and provides retrieved parameters that are more accurate or stable than those obtained by classical methods based only on light reflection and transmission through finite-thickness metamaterial slabs. Due to the importance of the fundamental Bloch mode in the light transport in metamaterials, in the second part, based on the Bloch mode orthogonality we derive closed-form expressions for the scattering coefficients at an interface between two periodic media with slightly different geometrical parameters, which is a computationally demanding electromagnetic problem. We show that the analytical expressions are very accurate for various geometries, including dielectric waveguides and metallic metamaterials. Thus they can be useful for designing and engineering stacks of periodic structures. As shown in the first part, the fundamental Bloch mode is central to explain the negative refraction phenomenon in fishnet metamaterials. In the third part, we derive an accurate semi-analytical model for the complex propagation constant of the fishnet fundamental Bloch mode. This is achieved by analyzing light propagation and scattering inside the fishnet. The model shows that the origin of broad-band negative index of fishnets can be mainly understood as a plasmon resonance in the transversal metal-insulator-metal (MIM) channels. The plasmon resonance enhances the 'magnetic' response of fishnet and the losses associated to this resonance can be compensated by including gain in the dielectric layers of the fishnet. Furthermore, the model allows an easy and precise geometrical tailoring of fishnet metamaterials. As shown in the third part, it is the plasmon resonance in metal-insulator-metal (MIM) structures that induces the negative index of fishnet metamaterials. In the last part, we study the asymptotic behavior of 3D MIM nanoresonators, as the resonator size is shrunk below the diffraction limit. In particular we show that the quality factor increases from 10 to 100 when the resonator volume is scaled down from (λ/2n)3 to (λ/50)3. We provide a comprehensive study with a semi-analytical Fabry-Perot model. The model remains accurate over the whole size scale even in the quasi-static regime for which retardation effects are not expected. This important and counterintuitive result indicates that both localized plasmon resonances in nanoparticles and delocalized resonance in elongated plasmonic nanowires can be possibly understood as a wave-retardation based antenna problem.

The work presented in this thesis explores the optical properties of hierarchical structures composed of nanoscale building blocks ranging from metals to semiconductors and composites, organized through bottom-up design methods. 1) By following the dynamic generation of hot-spots in self-assembled chains of gold nanorods (NRs), we have established a direct correlation between ensemble-averaged surface-enhanced Raman scattering (SERS) and extinction properties of these nanoscale chains. Experimental results were supported by comprehensive finite-difference time-domain simulations (FDTD). The relationship established between the structure of nanorod ensembles and their optical properties provides a basis for producing dynamic, solution-based, plasmonic platforms for applications ranging from sensing to nanoelectronics. 2) We report theoretical and experimental analyses of the optical properties of side-by-side assembled gold NRs. Comprehensive FDTD simulations showed a blue shift of the surface plasmon resonance in the side-by-side assembled NR structures and a reduction of electric field intensity as the number of NRs per stack increased. These results were experimentally verified via extinction measurements and ensemble-averaged SERS spectroscopy. The experimental results and electrodynamic simulations were found to be in agreement. 3) The efficacy of hollow core photonic crystal fibers (HCPCF) as a platform for SERS spectroscopy was demonstrated. SERS measurements carried out using this platform showed the capability to monitor minute amounts of ligands on the surface of gold nanoparticles and SERS signals from HCPCF exhibited a 10-fold enhancement. Using the exchange of cetyltrimethylammonium bromide with α-methoxy-ω mercaptopolyethylene glycol on the surface of gold nanorods as an exemplary system, we showed the feasibility of using HCPCF SERS to monitor the change in surface chemistry of NRs. 4) Facile, solution-phase formation of ordered, lamellar quantum dot (QD) arrays exhibiting structural integrity and temporal stability, without the need for chemical crosslinking, was achieved. While micrometers in diameter, they are typically only two to three QD layers thick. These structures are capable of carrying a cargo of water-soluble ions, molecules, metal nanoparticles, or biomolecules. The photoluminescence of the host CdSe QDs were enhanced by the encapsulation of gold nanoparticles within the lamellae, demonstrating the ability to modulate their properties through the cargo they carry. 5) This chapter explores a bottom-up method to produce a metamaterial designed to function as an optical cloak in the visible range. A composite material consisting of an array of silver nanowires (NWs) in a dielectric host has been produced based on the theory of a non-magnetic optical cloak. The required radial array of silver NWs was achieved by electroless deposition of the metal into the channels of a porous alumina structure grown perpendicularly from the curved surface of a micrometer scale aluminum wire. The functionality of the cloak was demonstrated by partial cloaking in the visible range (540 nm).

This thesis presents the design of radio-frequency (RF) tunable active inductors (TAIs) with independent inductance (L) and quality factor (Q) tuning capability, and their application in the design of RF tunable phase shifters and directional couplers for wireless transceivers. The independent L and Q tuning is achieved using a modided gyrator-C architecture with an additional feedback element. A general framework is developed for this Q- enhancement technique making it applicable to any gyrator-C based TAI. The design of a 1.5V, grounded, 0.13um CMOS TAI is presented. The proposed circuit achieves a 0.8nH-11.7nH tuning range at 2GHz, with a peak-Q in excess of 100. Furthermore, printed and integrated versions of tunable positive/negative refractive index (PRI /NRI) phase shifters, are presented in this thesis. The printed phase shifters are comprised of a microstrip transmission-line (TL) loaded with varactors and TAIs, which, when tuned together, extends the phase tuning range and produces a low return loss. In contrast, the integrated phase shifters utilize lumped L-C sections in place of the TLs, which allows for a single MMIC implementation. Detailed experimental results are presented in the thesis. As an example, the printed design achieves a phase of -40 to +34 degrees at 2.5GHz. As another application for the TAI, a reconfigurable CMOS directional coupler is presented in this thesis. The proposed coupler allows electronic control over the coupling coefficient, and the operating frequency while insuring a low return loss and high isolation. Moreover, it allows switching between forward and backward operation. These features, combined together, would allow using the coupler as a duplexer to connect a transmitter and a receiver to a single antenna. Finally, a planar electronically steerable patch array is presented. The 4-element array uses the tunable PRI/NRI phase shifters to center its radiation about the broadside direction. This also minimizes the main beam squinting across the operating bandwidth. The feed network of the array uses impedance transformers, which allow identical interstage phase shifters. The proposed antenna array is capable of continuously steering its main beam from -27 to +22 degrees of the broadside direction with a gain of 8.4dBi at 2.4GHz.

The work presented in this thesis explores the optical properties of hierarchical structures composed of nanoscale building blocks ranging from metals to semiconductors and composites, organized through bottom-up design methods. 1) By following the dynamic generation of hot-spots in self-assembled chains of gold nanorods (NRs), we have established a direct correlation between ensemble-averaged surface-enhanced Raman scattering (SERS) and extinction properties of these nanoscale chains. Experimental results were supported by comprehensive finite-difference time-domain simulations (FDTD). The relationship established between the structure of nanorod ensembles and their optical properties provides a basis for producing dynamic, solution-based, plasmonic platforms for applications ranging from sensing to nanoelectronics. 2) We report theoretical and experimental analyses of the optical properties of side-by-side assembled gold NRs. Comprehensive FDTD simulations showed a blue shift of the surface plasmon resonance in the side-by-side assembled NR structures and a reduction of electric field intensity as the number of NRs per stack increased. These results were experimentally verified via extinction measurements and ensemble-averaged SERS spectroscopy. The experimental results and electrodynamic simulations were found to be in agreement. 3) The efficacy of hollow core photonic crystal fibers (HCPCF) as a platform for SERS spectroscopy was demonstrated. SERS measurements carried out using this platform showed the capability to monitor minute amounts of ligands on the surface of gold nanoparticles and SERS signals from HCPCF exhibited a 10-fold enhancement. Using the exchange of cetyltrimethylammonium bromide with α-methoxy-ω mercaptopolyethylene glycol on the surface of gold nanorods as an exemplary system, we showed the feasibility of using HCPCF SERS to monitor the change in surface chemistry of NRs. 4) Facile, solution-phase formation of ordered, lamellar quantum dot (QD) arrays exhibiting structural integrity and temporal stability, without the need for chemical crosslinking, was achieved. While micrometers in diameter, they are typically only two to three QD layers thick. These structures are capable of carrying a cargo of water-soluble ions, molecules, metal nanoparticles, or biomolecules. The photoluminescence of the host CdSe QDs were enhanced by the encapsulation of gold nanoparticles within the lamellae, demonstrating the ability to modulate their properties through the cargo they carry. 5) This chapter explores a bottom-up method to produce a metamaterial designed to function as an optical cloak in the visible range. A composite material consisting of an array of silver nanowires (NWs) in a dielectric host has been produced based on the theory of a non-magnetic optical cloak. The required radial array of silver NWs was achieved by electroless deposition of the metal into the channels of a porous alumina structure grown perpendicularly from the curved surface of a micrometer scale aluminum wire. The functionality of the cloak was demonstrated by partial cloaking in the visible range (540 nm).

Low-density cellular solids have demonstrated superior mechanical properties as well as multifunctional characteristics, which may provide a basis for the development of novel structured materials. In particular, cellular solids offer great design flexibility, owing to their topology, which can provide desired functionalities via targeted geometric design and proper selection of the constituent material. While stochastic configurations such as metallic foams have proven to be effective for both thermal insulation and mechanical-energy absorption, the topology of deterministic architectures is not constrained by physical processes. This allows for a variety of configurations to be tailored to simultaneously fulfill disparate tasks. An additional aspect of deterministic cellular structures is the possibility of assembling materials or structures by the spatial repetition of a unit cell. The resulting periodicity of such systems simplifies the characterization of physical properties, which can be established by analyzing the unit cell only, and will provide new opportunities in the fields of structural dynamics, where periodicity-induced impedance leads to the control of both constructive and destructive interference on propagating waves. The objective of this work is to investigate the application of the chiral cellular topology for the design of novel macrostructural, mesostructural and microstructural configurations. A truss-core airfoil, and a truss-core beam are employed as a basis to demonstrate both large-displacement capabilities within the elastic regime of the constituent material, as well as operational deflection shapes with localized dynamic deformations. Large deformation capabilities and unique operational deflection shapes are to be attributed to the unusual deformation mechanism of the chiral lattice. Mesostructural and microstructural configurations, on the other hand, are characterized by an unique mechanical behavior, complex geometry, as well as geometric design flexibility to control both static and dynamic phenomena. The propagation of elastic waves, moreover, is characterized by significant band-gap density as well as strong energy focusing dependent on frequency and wavenumber. These features suggest the chiral topology as a basis for the development of acoustic meta-materials.

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

Nanomatériaux de silice peuvent être facilement fabriqués, façonné et fonctionnalisés comme plates-formes pour le greffage des nanoparticules pour des applications biomédicales et optiques. Ici, nous utilisons une méthodologie basée sur un modèle de préparer une collection variée de hélicoïdale nanoparticules d'or (PNB) superstructures ayant impartialité contrôlable et mesures structurelles en utilisant PNB que les blocs de construction, et les nanohelices de silice que le modèle. Le matériaux présentent synthétisé bien définir Agencement chiral de PNB suivant l'hélicité de nanohelices de silice, montrant des signaux plasmoniques de dichorism circulaire (CD). D'autres observations ont prouvé ce plasmon CD vient de l'arrangement chiral de PNB et cet effet est très taille, l'échelle et dépend du pH. Nous nous attendons à ce que cette stratégie d'assemblage va découvrir une meilleure vue sur les métamatériaux et de susciter la vue vers "bottom-up" des approches en nanosciences. / Silica nanomaterials can be easily fabricated, fashioned and functionalized as platforms for grafting of nanoparticles for biomedical and optical applications. Herein, we utilize a template-based methodology to prepare a diverse collection of helical gold nanoparticle (GNPs) superstructures having controllable handedness and structural metrics by using GNPs as the building blocks, and the silica nanohelices as the template. The synthesized materials exhibit well-defined chiral arrangement of GNPs following the helicity of silica nanohelices, showing plasmonic circular dichorism (CD) signals. Further observations proved this plasmon CD comes from the chiral arrangement of GNPs and this effect is highly size, scale and pH dependent. We expect that this assembly strategy will discover a better view towards metamaterials and spark the view towards “bottom-up” approaches in nanoscience.

Orientador: Prof. Dr. Carlos Eduardo Capovilla / Dissertação (mestrado) - Universidade Federal do ABC, Programa de Pós-Graduação em Engenharia Elétrica, 2016. / A tecnologia metamaterial vem sendo estudada cientificamente e suas propriedades prometem efeitos inovadores em suas aplicações. A possibilidade de manipular o comportamento eletromagnetico em um dispositivo cercado por metamateriais abre portas para a criação e otimização de novos dispositivos. Qualquer sistema que envolva dispositivos de microondas, optica e antenas, está na grade dos beneficiados por essa tecnologia. As principais caracteristicas dos metamateriais são a refração negativa e as caracteristicas intrinsecas do material com valores negativos. Assumindo esse cen'ario, nesse trabalho ser'a projetada uma célula metamaterial inspirada no conceito de lente de Lenz, que permite uma amplificação do fluxo de energia magnética na região central da estrutura. Essa célula será reconfigurada para sua implementação na estrutura de antenas planares, operando na faixa de 2,4 GHz. Tr¿es modelos de antenas são utilizadas nesse estudo, uma quasi-Yagi , uma PIFA e completando a teoria um arranjo com três PIFAs. A disposição das celulas é feita de forma periódica e estrão, assim como diagramas de radiação extraidos das simulações e testes experimentais. As conclusões são obtidas com referência nesses resultados e por meio de comparações das configurações das antenas com e sem as celulas metamateriaias em sua estrutura. / The metamaterial technology has been studied scientifically and its properties promise innovative effects in their applications. The possibility of manipulating the electromagnetic behavior on a device surrounded by metamaterials opens doors for the creation and optimization of new devices. Any system that involves microwave devices, optics and antennas is on the grid of those benefited by this technology. The main characteristics of the metamaterials are the negative refraction and the intrinsic characteristics of the material with negative values. Assuming this scenario, in this work will be projected a metamaterial cell inspired by the concept of Lenz lens, which allows an amplification of the magnetic flux in the central region of the structure. This cell will be reconfigured for its implementation in the planar antenna structure, operating in the 2.4 GHz range. Three antennas are used in this study, a quasi-Yagi, a PIFA and completing the theory a PIFA array with three elements. The arrangement of the cells is made periodically and strategically in the antenna structure, aiming for improvements in its performance. The characterization of these antennas is carried out by means of reflection and transmission parameters, as well as radiation diagrams extracted from the simulations and experimental tests. The conclusions are achieved with reference in these results and through comparisons of the antenna configurations with and without the metamaterial cells in their structure.