The term “metamaterials” refers to engineered structures that interact with electromagnetic fields in an unusual but controllable way that cannot be observed with natural materials. Metamaterial design at optical frequencies oftentimes makes of controllable plasmonic interactions. Light can excite collective oscillations of conduction band electrons on a metallic nanostructure. These oscillations result in localized surface plasmon modes which can provide high confinement of fields at metal-dielectric interfaces at nanoscale. Additionally scattering and absorption characteristics of plasmon modes can be controlled by geometrical features of the metallic nanostructures. This ease of controllability has lead to the development of new concepts in light manipulation and enhancement of light-material interactions. Fano resonance and plasmonic induced transparency (PIT) are among the most promising of those. The interference between different plasmon modes induced on nanostructures generates PIT/Fano resonance at optical frequencies. The unusual dispersion characteristics observed within the PIT window can be used for designing optical metamaterials to be used in various applications including bio-chemical sensing, slow light, modulation, perfect absorption, and all-optical switching. This thesis focuses on design of novel plasmonic devices to be used in these applications. The fundamental idea behind these designs is the generation of higher-order plasmon modes, which leads to PIT/Fano resonance-like output characteristics. These are then exploited together with dynamic tunability supported by graphene and field enhancement provided by nonlinear materials to prototype novel plasmonic devices. More specifically, this thesis proposes the following plasmonic device designs. I. Nano-disk Fano resonator: Open disk-like plasmonic nanostructures are preferred for bio-chemical sensing because of their higher capacity to be in contact with greater volumes of analyte. High effective refractive index required by sensing applications is achieved though the dispersion characteristics within PIT window. Higher order modes required for Fano resonance are generated through geometrical symmetry breaking by embedding a shifted and elongated cavity into a circular disk. The resulting dual band PIT can be geometrically tuned by varying the cavity's width and rotation angle. II. Tunable Terahertz Fano resonator: The possibility to dynamically tune graphene's conductivity has made it an attractive choice over conventional noble metals to generate surface plasmon modes at Terahertz frequencies. Subsequently, a polarization-independent and dynamically tunable hybrid gold-graphene structure is designed to achieve PIT/Fano resonance by allowing graphene and metallic plasmon modes to interfere. The effective group index of the resulting resonator is found to be very high (ng=1400, several times higher than all previously reported PIT devices) within the PIT window. Dynamic tunability achieved through a gate voltage applied to graphene suggests applications in switching. III. Tunable Terahertz Fano absorber: Many photonic and optical devices rely on their ability to efficiently absorb an incoming electromagnetic field. The absorption in atomically thin graphene sheet is already very high i.e., “2.3%” per layer. However, considering its atomic thickness graphene sheet remains practically transparent to Terahertz waves. The proposed absorber design makes of an asymmetrically patterned graphene layer that supports higher order plasmon modes at Terahertz frequencies. Several of these patterned layers backed by dielectric substrates are stacked on top of each other followed by reflector screen. The dynamically controllable resonances from each graphene layer and the spacing between them are fine tuned to achieve a large bandwidth of 6.9 Terahertz (from 4.7 to 11.6 Terahertz) for over 90% absorption, which is significantly higher than that of existing metallic/graphene absorbers. IV. Three state all-optical switch: The plasmonic resonances are extremely sensitive to dielectric properties of the surrounding medium. A slight change in the dielectric constant near the metal surface results in a significant change in the plasmonic resonance. This sensitivity is enhanced in the presence of a nonlinear change in the dielectric constant. To make use of this effect, Fano resonator is used in conjunction with a Kerr nonlinear material. The resulting resonator exploits multiple (higher order) surface plasmons to generate a multi-band tri-stable response in its output. This cannot be obtained using existing nonlinear plasmonic devices that make use of single mode Lorentzian resonances. Multi-band three-state optical switching that can be realized using the proposed resonator has potential applications in optical communications and computing.

<p>Acoustic metamaterials are engineered materials that were extensively investigated over the last years mainly because they promise properties otherwise hard or impossible to find in nature. Consequently, they open the door for improved or completely new applications (e.g. acoustic superlens that can exceed the diffraction limit in imaging or acoustic absorbing panels with higher transmission loss and smaller thickness than regular absorbers). Our objective is to surpass the limited frequency</p><p>operating range imposed by the resonant mechanism that s1ome of these materials have. In addition, we want acoustic metamaterials that could be experimentally demonstrated and used to build devices with overall performances better than the previous ones reported in the literature.</p><p>Here, we start by focusing on the need of engineered metamaterials in general and acoustic metamaterials in particular. Also, the similarities between electromagnetic metamaterials and acoustic metamaterials and possible ways to realize broadband acoustic metamaterials are briefly discussed. Then, we present the experimental realization</p><p>and characterization of a two-dimensional (2D) broadband acoustic metamaterial with strongly anisotropic effective mass density. We use this metamaterial to realize a 2D broadband gradient index acoustic lens in air. Furthermore, we optimize the lens design by improving each unit cell's performance and we also realize a 2D acoustic ground cloak in air. In addition, we explore the performance of some novel applications (a 2D acoustic black hole and a three-dimensional acoustic cloak) using the currently available acoustic metamaterials. In order to overcome the limitations of our designs, we approach the active acoustic metamaterials path, which offers a broader range for the material parameters values and a better control over them. We propose two structures which contain a sensing element (microphone) and an acoustic driver (piezoelectric membrane or speaker). The material properties are controlled by tuning the response of the unit cell to the incident wave. Several samples with interesting effective mass density and bulk modulus are presented. We conclude by suggesting few natural directions that could be followed for the future research based on the theoretical and experimental results presented in this work.</p> / Dissertation

All substances above zero kelvin temperature emit fluctuating electromagnetic waves due to the random motions of charge carriers. Controlling the spectral and directional radiative properties of surfaces has wide applications in energy harvesting and thermal management. Artificial metamaterials have attracted much attention in the last decade due to their unprecedented optical and thermal properties beyond those existing in nature. This dissertation aims at tailoring radiative properties at infrared regime and enhancing the near-field radiative heat transfer by employing metamaterials. A comprehensive study is performed to investigate the extraordinary transmission, negative refraction, and tunable perfect absorption of infrared light. A polarizer is designed with an extremely high extinction ratio based on the extraordinary transmission through perforated metallic films. The extraordinary transmission of metallic gratings can be enhanced and tuned if a single layer of graphene is covered on top. Metallic metamaterials are not the unique candidate supporting exotic optical properties. Thin films of doped silicon nanowires can support negative refraction of infrared light due to the presence of hyperbolic dispersion. Long doped-silicon nanowires are found to exhibit broadband tunable perfect absorption. Besides the unique far-field properties, near-field radiative heat transfer can be mediated by metamaterials. Bringing objects with different temperatures close can enhance the radiative heat flux by orders of magnitude beyond the limit set by the Stefan-Boltzmann law. Metamaterials provide ways to make the energy transport more efficient. Very high radiative heat fluxes are shown based on carbon nanotubes, nanowires, and nanoholes using effective medium theory (EMT). The quantitative application condition of EMT is presented for metallodielectric metamaterials. Exact formulations including the scattering theory and Green’s function method are employed to investigate one- and two-dimensional gratings as well as metasurfaces when the period is not sufficiently small. New routes for enhancing near-field radiative energy transport are opened based on proposed hybridization of graphene plasmons with hyperbolic modes, hybridization of graphene plasmons with surface phonon modes, or hyperbolic graphene plasmons with open surface plasmon dispersion relation. Noncontact solid-state refrigeration is theoretically demonstrated to be feasible based on near-field thermal radiation. In addition, the investigation of near-field momentum exchange (Casimir force) between metamaterials is also conducted. Simultaneous enhancement of the near-field energy transport and suppress of the momentum exchange is theoretically achieved. A design based on repulsive Casimir force is proposed to achieve tunable stable levitation. The dissertation helps to understand the fundamental radiative energy transport and momentum exchange of metamaterials, and has significant impacts on practical applications such as design of nanoscale thermal and optical devices, local thermal management, thermal imaging beyond the diffraction limit, and thermophotovoltaic energy harvesting.

The work presented within this thesis details the experimental investigation of the surface waves supported on metasurfaces. Particular attention has been given to the reflection of these surface waves from planar discontinuities associated with these metasurfaces. Various experimental techniques have been developed throughout this work to characterise surface wave supporting metasurfaces. These include a new technique for measuring the dispersion of surface waves supported on metasurfaces, characterisation of the near-field associated with the surface waves, a device for launching planar phase front surface waves and finally a technique for measuring the surface wave reflection coefficient. The dispersion of surface waves on a square array of square cross-section metal pillars has been fully characterised and compared to FEM modelling. The results show that a family of surface waves may be supported by pillar or crossed slit structures rather than just holes even though there is now no lowest cut-off frequency. A family of TM surface modes have been shown to exist with dispersions which asymptote to frequencies defined by the pillar heights (slit depth) and the refractive index of the material filling the slits. Primarily this work focussed on the surface wave properties associated with a square array of square metal patches on a dielectric coated ground plane and a Sievenpiper `mushroom' metasurface. The amplitude reflection coefficient of these surface waves has been studied for three distinct systems: Firstly for surface waves incident upon the termination of a these metasurfaces to free space, secondly for surface waves incident upon the interface between a dielectric coated and uncoated metasurface and thirdly for surface waves incident on the boundary between two metaurfaces. The reflection coefficient of surface waves incident upon the termination of the metasurface to free space is found to increase significantly with the confinement of the surface mode. This confinement, and therefore the form of the reflection coefficient, is significantly different for the two metasurfaces considered due to their dispersions. This increase in the reflection coefficient is caused by both the momentum mismatch of the surface wave compared to the freely propagating modes and the different field distributions of the two modes. The reflection coefficient of surface waves incident upon the boundary between a coated and uncoated metasurface has been experimentally characterised for the metal patch array and Sievenpiper `mushroom' metasurfaces. It is shown that the addition of a thin, significantly subwavelength, dielectric overlayer onto the metasurface vastly perturbs the surface wave dispersion. The reflection coefficient of the surface waves is found to depend on the dispersion of the mode supported on the coated and uncoated metasurface and the overlayer thickness. Most noticeably the thickness of the overlayer, by comparison to the surface wave decay length, has a significant effect on scattering to free space associated with the surface wave reflection. The final system considered was designed to investigate the impedance approximation, often used to describe metasurfaces, and found it to be an incomplete description of the surface waves supported on the metasurfaces used within this study. In the impedance approximation the two surfaces considered are said to be `impedance matched` at certain frequencies. It is demonstrated that the failure of the impedance approximation to accurately describe this system is due to the behaviour of the electric field within the metasurfaces. These are not analytically described in the impedance approximation and are required for an accurate description of the surface waves supported on these metasurfaces.

Metamaterials are a class of artificial material, known to produce electromagnetic (EM) responses not found in nature due to their engineered subwavelength structure. In this thesis very thin subwavelength meshes are utilised to form layered metamaterials. The EM characteristics of the transmission and reflection response from these materials, including the polarisation converting behaviour, are explored to further understanding and develop structures to exploit and control the propagation of microwave radiation. Original experimental studies are presented across two sections; the first examines the response of stacks assembled from metallic meshes and dielectric plates; the second explores a rotated layered structure formed of square symmetric elements in a square subwavelength array that demonstrates chirality through evanescent coupling of the near fields. When metallic meshes are excited with EM radiation below the cut off frequency, only evanescently decaying fields are supported in the holes. By combining these subwavelength metallic meshes with dielectric plates in different arrangements, remarkably wide bands of high transmission and low reflection may be observed. The non-interacting resonant modes allow the response to be tuned through a suitable choice of the metallic mesh geometry and the properties of the dielectric. Further the low frequency band edge and the bandwidth are not dependent on the number of unit cells in the stack; but are dependent on the properties of the unit cell. The second section demonstrates ``evanescent handedness'' proposed as a new type of chirality. Two subwavelength square arrays of square elements are rotated with respect to one another. When the rotated arrays are positioned far from one another in the propagation direction, each acts as an effective medium layer. However when placed in close proximity the structure is shown to rotate the plane of polarisation of the incident radiation. All these mesh based structures share the property of producing an EM response that is tunable by design, allowing a structure to be tailored to a specific application.

Plasmonics is the branch of photonics that is concerned with the interactions which take place between metallic structures and incident electromagnetic radiation. It is a field which has seen a recent resurgence of interest, predominantly due to the emerging fields of metamaterials and sub-wavelength optics. The original work contained within this thesis is concerned with the plasmonic resonances of metallic nanoparticles which can be excited with visible light. These structures have been placed in a variety of configurations, and the optical response of each of these configurations has been probed both experimentally, and with numerical simulations. The first chapter contains some background and describes some recent advances in the literature, set against the broad background of more general concepts which are important in plasmonics. The best starting point in describing the response of plasmonic systems is to consider individual metallic particles and this is the subject of the second chapter. Three separate modelling techniques are described and compared, and dark-field spectroscopy is used to produce experimental scattering spectra of single particles which support dipolar and higher order modes. Mie theory is used as a starting point in understanding these modes, and finite element method (FEM) modelling is used to make numerical comparisons with dark-field data. When two plasmonic particles are placed close to each other, interactions take place between them and their response is modified, sometimes considerably. This effect can be even stronger if particles are placed in large arrays. Interactions between the dipolar modes of gold particles form the basis of the third chapter. The discussion begins with pairs of particles, and the coupled dipole approximation (CDA) is introduced to describe the response. Ordered square arrays are considered and different modelling techniques are compared to experimental data. Also, random arrays have been investigated with a view to inferring the extinction spectrum of a single particle from a carefully chosen array of particles in which the inter-particle interactions are suppressed. The fourth chapter continues the theme of particles interacting in arrays, but the particles considered support quadrupolar modes (and they are silver instead of gold). The optical response is strongly modified, and an explanation is provided which overturns the accepted explanation. The final chapter of new results is somewhat different to the others in that a very different structure is considered and different parameters are extracted. Instead of far-field quantities, here, near-fields of composite structures are of interest; they can generate greatly enhanced fields in the vicinity of the structure. These enhanced fields, in turn, enhance the fluorescence and Raman emission of nearby dye molecules. A novel field integration technique is proposed which aims to mimic the experiments which were carried out using fluorescence confocal microscopy.

The possibility to manipulate electromagnetic radiation, as well as mechanical and acoustic waves has been an engaging topic since the beginning of the 20th century. Nowadays, thanks to the progress in technologies and the evolution of fabrication processes, realizing artificial materials that are able to interact with the environment in a desired fashion has become reality. The interest in micro/nanostructured metamaterials involves different field of research, ranging from optics to biology, through optoelectronics and photonics. Unfortunately, realizing experimentally these materials became highly challenging, since the size of the nanostructures are shrinking and the precision of the design became crucial for their effective operation. Disorder is, in fact, an intrinsic characteristic of fabrication processes and harnessing it by turning its unexpected effects in decisive advantages represents one of the ultimate frontiers in research. In this work we combine ab-initio FDTD simulations, fabrication process optimization and experimental results to show that, introducing disorder in metamaterials could constitute a key opportunity to enable many interesting capabilities otherwise locked. This could open up the way to novel applications in several fields, from smart network materials for solar cells and photo-electrochemical devices to all dielectric, highly-tunable structural colors.

Thesis advisor: Willie J. Padilla / Progress in the field of metamaterials has started coming to a point where the field may finally begin to emerge as a viable solution to many electromagnetic challenges facing the community. No where is that more true then at terahertz frequencies where there lies an immense opportunity for growth. The development of mature technologies within this region of the electromagnetic spectrum would provide a valuable resource to become available for a multitude of applications. In order to achieve this, the necessary first steps of identifying viable materials and paths to integrate these with metamaterials will need to be completed. In this dissertation, we examine several different paths to achieve dynamic metamaterial electromagnetic response at terahertz frequencies, and demonstrate several paths to package these devices into imaging systems. In Chapter 1, we introduce the basic theory and design principles of metamaterials. We also describe the experimental techniques involved in the study of terahertz metamaterials. Chapter 2 presents a computational and experimental study investigating the integration of high electron mobility transistors with metamaterials allowing for high speed modulation of incident terahertz radiation. In Chapters 3 and 4, we investigate several different paths to create tunable terahertz metamaterial absorbers. Chapter 3 presents an investigation where we encapsulate a metametarial absorber unit cell with liquid crystals. We study both computationally and experimentally the tuning mechanism of the absorber as the liquid crystal refractive index is controlled as a function of the applied electric field strength and modulation frequency. In Chapter 4, we form a doped semiconducting metamaterial spatial light modulator with multi-color super-pixels composed of arrays of electronically controlled terahertz metamaterial absorbers. We computationally and experimentally study the independent tunability of each pixel in the spatial array and demonstrate high speed modulation. Chapter 5 introduces a multiplex imaging approach by using a terahertz spatial light modulator to enable terahertz imaging with a single pixel detector. We demonstrate the capability for high speed image acquisition, currently only limited by the commerical software used to reconfigure the spatial masks. We also configure the system to capture high fidelity images of varying complexity. In Chapter 6, we show how a metamaterial absorber can be implemented into a detector focal plane array for high sensitivity, low mutual coupling, and broad angle performance. Finally, we summarize in Chapter 7 the achievments of the research presented and highlight the direction of future work. / Thesis (PhD) — Boston College, 2013. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

During this PhD study, metamaterials incorporating active devices such as varactors and non-Foster circuits, were researched and designed. Starting from the research on tuneable metamaterials, an electronically controlled leaky-wave (LW) antenna based on composite right/left handed (CRLH) transmission line (TL) structure was proposed which could perform a broadband beam-fixing function with the frequency range from 1 to 4 GHz. In addition, scanning from forward to backward at a fixed frequency can be achieved by manipulating the biasing voltage applied to the varactors. Most of this study has been devoted to the non-Foster active metamaterials. First, the characterization of active magnetic metamaterials with non-Foster loads was presented. Based on the equivalent circuit model, stability of an actively-loaded loop array was examined through different analysis techniques, further to give the design specifications to achieve the broadband non-dispersive negative-Re(μ) (MNG) or μ-near-zero (MNZ) magnetic properties. Moreover, the wave propagation in the actively-loaded medium was investigated. By relating the dispersion characteristics and the effective medium properties, we henceforth proposed the design of zero-loss and broadband metamaterials. This thesis also has covered the study of active high impedance surfaces (HIS) with non-Foster loads. As a two-dimensional metamaterial structure, HIS have been widely used in the microwave and antenna engineering. However it can be easily seen that the performance of a general passive HIS is always limited by the narrow bandwidth, thus making a broadband HIS desirable. In this work, an analytical solution to achieving a stable broadband HIS structure is given by incorporating appropriate negative impedance converter (NIC) circuits. Simulation results have verified the design approach. Finally, the design of NIC circuits was presented as the key part of the realization of active metamaterials. Two schemes have been adopted to realize the design of NICs, one is the operational amplifier (op-amp) based NIC, and another is based on discrete transistors. Both types of NICs were introduced and studied in this thesis.

Aujourd'hui, la sécurité des cyclistes sur les routes est une préoccupation majeure. Les statistiques d'accidents montrent une augmentation de 12% des décès de cyclistes depuis 2010. L'objectif du projet national français CYCLOPE est d'améliorer la sécurité des cyclistes grâce à des dispositifs électroniques innovants. Dans le cadre de ce projet, ces travaux de thèse se concentrent sur l'amélioration de la détectabilité du cycliste par un véhicule équipé d'un radar anticollision. L'objectif est d'augmenter la réflexion du système vélo-cycliste pour que le niveau d'énergie rétrodiffusé soit suffisant pour provoquer une alarme. Pour atteindre cet objectif, deux approches sont étudiées dans cette thèse. La première approche vise à concevoir un réflecteur rétrodirectif inspiré des techniques utilisées pour les antennes réseaux réflecteurs. Ce réflecteur est destiné à être placé sur le vélo, comme un catadioptre. Le réflecteur utilise deux panneaux imprimés formant un dièdre aplati, dans un souci de compacité. L'utilisation des motifs imprimés permet de réaliser une loi de phase sur les panneaux et, par ce biais, de restaurer le comportement rétrodirectif mis à mal par l'applatissement. Une étude théorique du mode de fonctionnement de cette structure complexe est proposée, permettant de dégager ces potentialités mais également ces limitations. Finalement, une conception complète est menée pour un prototype à 24 GHz. Sa fabrication et sa caractérisation montrent sa capacité à améliorer la surface radar d'un cycliste. Dans la deuxième approche, le dispositif réfléchissant est destiné à équiper le cycliste lui-même, comme un gilet fluorescent. Le torse humain est modalisé sous la forme d'un cylindre multi-couches diélectriques avec pertes. En revêtant ce cylindre d'une couche de nature réfléchissante, il est possible d'améliorer sa rétrodiffusion. Le travail se concentre donc sur la recherche du profil de revêtement optimal. Pour ce faire, un modèle numérique basé sur la solution mathématique exacte du cylindre infini multi-couches est proposé. En utilisant ce modèle, on constate que la couche requise doit être réalisée par des matériaux à permittivité négative, synthétisables grâce à des métamatériaux. L 'étude est ensuite étendue au cas d'un cylindre fini, plus représentatif de la réalité. Des comparaisons sont menées avec une simulation électromagnétique rigoureuse / A cyclist safety nowadays on roads is a major concern. Accident statistics show an increase of 12 % of deaths of cyclists since 2010. In the French national project CYCLOPE, the aim is improve the safety of cyclists by developing innovative electronic devices. In the context of this project, the work in this thesis is concerned with enhancing the detectability of the cyclist by a vehicle equipped with an anti-collision radar. The objective hence is to enhance the reflection by the bicycle-cyclist system so that the backscattered energy level would be sufficient to cause an alarm. In order to attain this objective, two approaches are investigated in the thesis. In the first, the design of a retrodirective reflector inspired by reflectarrays is studied. This device is going to be attached to the bicycle to operate as a retroreflector. The reflector is formed of two panels attached and flattened till a certain degree. Equipping each of the two panels with printed phasing cells permits for the construction of a phase law that restores the retrodirective behavior of the corner dihedral after it being flattened. A theoretical study performed on the retrodirective mechanism permits to uncover the potentials as well as limits of such reflector. Finally, a prototype operating at 24 GHz is realized. Its performance validates its capacity to enhance the RCS of a bicycle-cyclist. In the second approach, the possibility of enhancing the backscattering by the cyclist body using a reflective jacket is discussed. The human torso can be modeled as a multi-layered lossy dielectric cylinder. By coating this cylinder with a layer of reflective nature, it is possible to enhance its backscattering. The work hence concentrates on finding the corresponding optimum coat profile. Therefore, a numerical model based on the exact mathematical solution of the multilayered infinite cylinder is proposed. Using this model, it is found that the required coat should be made of negative permittivity materials, synthesizable using metamaterials. The study then is concentrated on the case of a finite cylinder which more accurately represents the real case scenario. Finally, Electromagnetic full-wave simulations are performed to compare the infinite-based numerical model to the finite-based simulation results.