# Browsing by Author "Smith, David R"

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Item Open Access Advanced Metamaterials for Beamforming and Physical Layer Processing(2023) Pande, DivyaThe design and characterization of electromagnetic metamaterial structures and their constituent subwavelength metamaterial elements are presented. The proposed structures can be employed in beamforming and physical layer processing applications. The common approach for designing such structures involves extracting the effective medium properties of the elements, a methodology inspired by early metamaterial research. The modeling and simulation method is made computationally feasible by assuming a periodic arrangement of the elements. In the case of aperiodic structures, the periodic assumption is no longer valid, and the electromagnetic behavior cannot be predicted accurately. To get a more accurate picture, the electromagnetic properties of individual elements must be evaluated to design a metamaterial structure. In this dissertation, I outline robust steps to realize electromagnetic metamaterial structures by characterizing metamaterial elements without any periodicity assumptions. The subwavelength elements are modeled as electric and magnetic dipoles, and I use dipole-based optimization techniques to design the structures. The dipolar elements are described by their electric and magnetic polarizabilities. Polarizability extraction methods to characterize the different metamaterial elements using numerical simulations are discussed in detail.

In recent years, the coupled dipole model (CDM) has been fully developed to predict the electromagnetic behavior of metamaterial given the element polarizabilities. However, the inverse problem to arrive at the desired medium given some desired behavior is a non-linear problem and can be computationally expensive to solve. Traditionally, holographic methods are used to linearize the problem in the perturbative limits limit to make it computationally tractable. The recently introduced symphotic method solves the non-linear electromagnetic inverse problem efficiently by iteratively solving two linear systems without making any assumptions. This allows one to encode multiple operations in a volume which is not possible with standard computer-generated holography design methods. Here, the two inverse design tools- holography and symphotic are investigated under the dipole framework and validated both numerically and experimentally using different metamaterial structures and corresponding elements.

Item Open Access Advances in Complex Electromagnetic Media(2009) Kundtz, NathanComplex artificial materials (metamaterials) strongly interact with light and can be used to fabricate structures which mimic a material response that has no natural equivalent. Classical tools for the design of optical or radio frequency devices are often ill-suited to utilize such media or have shortcomings in their ability to capture important physics in the device behavior. Recently it has been demonstrated that the structure of Maxwell's equations can be used to exploit this newly available freedom. By leveraging the `form-invariance' of Maxwell's equations under coordinate transforms, it is possible to develop material distributions in which light will behave as though flowing through warped coordinates. This design process is termed `transformation optics' and has inspired the creation of many novel electromagnetic structures such as the invisibility cloak.

In this dissertation the tools used in the field of transformation optics will be explored and expanded. Several new designs are discussed, each of which expands upon the ideas that have previously been employed in the field. To begin, I show that the explicit use of a transformation which extends throughout all space may be used to reduce the overall size of an optical device without changing its optical properties. A lens is chosen as a canonical device to demonstrate this behavior. For this work I provided the original idea for a compressing transformation as well as its dielectric-only implementation. I then mentored Dan Roberts as he confirmed the device properties through simulation. I further demonstrate that currents may be succesfully employed within the framework of transformation optics-resulting in novel antenna designs. For this work I suggested handling the sheet currents as the limit of a volumetric current density. I also demonstrated how an intermediate coordinate system could be used to easily handle the types of transformatios which were being explored.

For a particular functionality the choice of transformation is, in general, not unique. It is natural, then, to seek optimized transformations which reduce the complexity of the final structure. It was recently demonstrated that for some transformations a numerical scheme could be employed to find quasi-conformal transformations for which the requisite complex material distribution could be well approximated by an isotropic, inhomogeneous media. This process was previously used to demonstrate a carpet cloak-a device which masks a bump in a mirror surface. Unlike the more common transformation optical media, which exhibit strong losses at high frequencies, isotropic designs can be readily made to function at infrared or even optical frequencies.

The prospect of leveraging transformation optics in devices which operate at high frequencies, into the infrared and visible, motivates the use of quasi-conformal transformations in lens design. I demonstrate how transformation optics can be used to take a classical lens design based on spherical symmetry, such as a Luneburg lens, and warp it to suit the requirements of a planar imaging array. I report on the experimental demonstration of this lens at microwave frequencies. In the final design a lens is demonstrated in a two-dimensional field mapping waveguide to have a field of view of ~140 degrees and a bandwidth exceeding a full decade. In this work I proposed the idea of using the inverse of the quasi-conformal transform to arrive at the lens index profile. I performed all necessary simulations and wrote ray tracing code to confirm the properties of the lens. I proposed the metamaterial realization of the lens and performed the necessary retrievals for material design. I wrote code which would create the layout for an arbitrary gradient index structure in a standard computer aided drafting format. I fabricated three lenses-two of which are described in this thesis-and took all of the data shown in the thesis.

The most well known example of a transformation optical device is the invisibility cloak. Despite the great deal of attention paid to the cloak in the literature, the most natural way in which to quantify the efficacy of the cloak-its cross-section-has never been experimentally determined. This measurement is of practical interest because the cloak provides a useful canonical example of a medium which relies on the unique properties of metamaterials-strong anisotropy, inhomogeneity and both magnetic and electric response. Thus, a cloaking cross-section measurement provides a useful way to quantify advancements in the effective medium theories which form the basis for metamaterials. I report on the first such measurements, performed on the original microwave cloaking design. The experiments were carried out in a two-dimensional TE waveguide. Explicit field maps are used to determine the Bessel decomposition of the scattered wave. It is found that the cloak indeed reduces the scattering cross-section of a concealed metal cylinder in a frequency band from 9.91 to 10.14 GHz. The maximum cross-section reduction was determined to be 24%. The total cross-section and the Bessel decomposition of the scattered wave are compared to an analytical model for the cloaking design which assumes a discrete number of loss-less, homogenized cylinders. While the qualitative features of the cloak-a reduced cross-section at the cloaking frequency-are realized, there is significant deviation from the homogenized calculation. These deviations are associated with loss and inaccuracies of the effective-medium-model for metamaterials. In this work I proposed of direct integration of the fields to perform cross-section measurements. I worked out the necessary formulas to determine the coefficients in the Bessel expansion and the resulting scattering cross-section. I mentored an undergraduate student, Dan Gaultney, who scripted the application of the cross-section analysis and took the necessary data. All of the data in this thesis, however, is based on my own implementation of the data analysis.

Item Open Access Analytic Model, Design of Waveguide-fed Metasurface Antennas and Applications to MIMO Communication Systems(2020) Yoo, InsangThis dissertation focuses on the analytic model and design of waveguide-fed meta- surface antennas using the coupled-dipole method. In particular, it is demonstrated that the coupled-dipole method can be combined with models of waveguide feeds via network theory such that the self-consistency of the model of waveguide-fed metasurfaces is maintained. Thus, the proposed approach of modeling the meta- surface antennas allows the computation of key antenna parameters for practical design and considerations. It is also demonstrated that the proposed method allows direct implementation of optimization techniques to accelerate the design process of waveguide-fed metasurfaces. Design examples with various metasurface configu- rations, including printed cavity-backed metasurface, shared aperture metasurface, cylindrical conformal metasurface, are provided to confirm the utility of the proposed approach as an efficient design tool. Given the studies on the models and physics of waveguide-fed metasurface antennas, their applications to MIMO wireless commu- nication systems are explored in later chapters. The benefits of using metasurface antennas in various propagation channels are studied, showing that MIMO systems using metasurface antennas can improve channel capacity by utilizing flexibility in pattern synthesis offered by the metasurfaces.

Item Open Access Analytical Modeling of Waveguide-fed Metasurfaces for Microwave Imaging and Beamforming(2018) Pulido Mancera, Laura MariaA waveguide-fed metasurface consists of an array of metamaterial elements excited by a guided mode. When the metamaterial elements are excited, they in turn leak out a portion of the energy traveling through the waveguide to free space. As such, a waveguide-fed metasurface acts as an antenna. These antennas possess a planar form factor that offers tremendous dexterity in forming prescribed radiation patterns; a capability that has led to revolutionary advances in antenna engineering, microwave imaging, flat optics, among others.

Yet, the common approach to model and design such metasurfaces relies on effective surface properties, a methodology that is inspired by initial metamaterial designs. This methodology is only applicable to periodic arrangements of elements, and the assumption that the neighboring elements are identical. In the scenarios where the metasurface consists of an aperiodic array, or the neighboring elements are significantly different, or the coupling to the waveguide structure changes; the aforementioned approaches cannot predict the electromagnetic response of the waveguide-fed metasurface. In this thesis, I have implemented a robust technique to model waveguide-fed metasurfaces without any assumption on the metamaterial elements' geometry or arrangement. The only assumption is that the metamaterial elements can be modeled as effective dipoles, which is usually the case given the subwavelength size of metamaterial elements.

Throughout this document, the simulation tool will be referred to Dipole Model. In this framework, the total response of each dipole, representing a metamaterial element, depends on the mutual interaction between elements, as well as the perturbation of the guided mode. Both effects are taken into account and, by using full-wave simulations, I have confirmed the validity of the model and the ability to predict radiation patterns that can be used for beamforming as well as for microwave imaging.

Once the capabilities of the dipole model are compared with full wave simulations of both traditional antenna designs as well as more elaborated waveguide-fed metasurfaces, I develop an analysis on the use of these metasurfaces for microwave imaging systems. These systems are used to form images of buried objects, which is crucial in security screening and synthetic aperture radar (SAR). Traditionally, the hardware needed for many imaging techniques is cumbersome, including large arrays of antennas or bulky, moving parts. However, one attractive alternative to overcome these problems is to use dynamic metasurface antennas. By quickly varying the radiation patterns generated by these antennas, enough diverse measurements can be made in order to produce high quality images in a fraction of the time.

The compact size and speed come with a trade-off: a computationally intensive optical inverse problem has to be solved, which has so far prohibited these antennas from enjoying widespread use. I address this problem by reformulating the problem to make it similar to a SAR scenario, for which fast image reconstruction algorithms already exist. By adapting an algorithm known as the Range Migration to be compatible with these metasurfaces, I can cut down on real-time computation significantly. The computer simulations performed are highly promising for the field of microwave imaging, since it is demonstrated that diffraction-limited images can be acquired in a fraction of the time, in comparison with other imaging techniques.

Item Open Access Broadband electromagnetic cloaking with smart metamaterials.(Nat Commun, 2012) Shin, Dongheok; Urzhumov, Yaroslav; Jung, Youngjean; Kang, Gumin; Baek, Seunghwa; Choi, Minjung; Park, Haesung; Kim, Kyoungsik; Smith, David RThe ability to render objects invisible with a cloak that fits all objects and sizes is a long-standing goal for optical devices. Invisibility devices demonstrated so far typically comprise a rigid structure wrapped around an object to which it is fitted. Here we demonstrate smart metamaterial cloaking, wherein the metamaterial device not only transforms electromagnetic fields to make an object invisible, but also acquires its properties automatically from its own elastic deformation. The demonstrated device is a ground-plane microwave cloak composed of an elastic metamaterial with a broad operational band (10-12 GHz) and nearly lossless electromagnetic properties. The metamaterial is uniform, or perfectly periodic, in its undeformed state and acquires the necessary gradient-index profile, mimicking a quasi-conformal transformation, naturally from a boundary load. This easy-to fabricate hybrid elasto electromagnetic metamaterial opens the door to implementations of a variety of transformation optics devices based on quasi-conformal maps.Item Open Access Coherence in Dynamic Metasurface Aperture Microwave Imaging Systems(2020) Diebold, Aaron VincentMicrowave imaging systems often utilize electrically large arrays for remote characterization of spatial and spectral content. Image reconstruction involves computational processing, the success of which depends on adequate spatial and temporal sampling at the array as dictated by the nature of the radiation and sensing strategy. Effective design of an imaging system, consisting of its hardware and algorithmic components, thus requires detailed understanding of the nature of the involved fields and their impact on the processing capabilities. One can sufficiently characterize many of the properties of such fields and systems via coherence, which quantifies their interferometric capacity in terms of statistical correlations and point spread functions. Recent work on dynamic metasurface apertures (DMAs) for microwave imaging has demonstrated the utility of these structures in active, coherent systems, supplanting traditional array architectures with lower-cost designs capable of powerful wavefront shaping. In contrast to arrays of distinct antennas, DMAs are composed of electrically large arrays of dynamically-tunable, radiating metamaterial elements to realize diverse gain patterns that can function as an encoding mechanism for coded aperture image reconstruction. With the appropriate formulation, well-established concepts from the realm of Fourier optics can be transposed from conventional array systems to DMA architectures. This dissertation furthers that task by modeling DMA imaging systems involving partial coherence and incoherence, and demonstrating new reconstruction algorithms in these contexts. Such an undertaking provides convenient opportunities for examining the origins of coherence in computational and holographic imaging systems. This insight is necessary for the development of modern approaches that seek to smoothly integrate hardware and computational elements for powerful, efficient, and innovative imaging tasks.

Accommodating different degrees of coherence in a microwave imaging system can substantially relax demands on hardware components including phase stability and synchronization, or on algorithmic procedures such as calibration. In addition, incoherent operation can yield improved images free of coherent diffraction artifacts and speckle. Finally, an understanding of coherence can unlock fundamentally distinct applications, such as passive imaging and imaging with ambient illumination, that can benefit from the flexibility of a DMA system but have yet to be demonstrated under such an architecture. To this end, I formulate a unified framework for analyzing and processing array imaging systems in the Fourier domain, and demonstrate a method for transforming a DMA-based system to an equivalent array representation under active, coherent operation. I then investigate the role of spatial coherence in a two-dimensional holographic imaging system, and experimentally demonstrate some results using a collection of DMAs. I conduct a similar investigation in the context of single-pixel ghost imaging, which allows coherent and incoherent imaging directly from intensity measurements, thereby relaxing hardware phase requirements. I then formulate a model for partially coherent fields in a DMA imaging system, and provide several reconstruction strategies and example simulations. I finally restrict this general case to passive, spectral imaging of spatially and temporally incoherent sources, and experimentally demonstrate a compressive imaging strategy in this context.

Item Open Access Compressive holography.(2012) Lim, Se HoonCompressive holography estimates images from incomplete data by using sparsity priors. Compressive holography combines digital holography and compressive sensing. Digital holography consists of computational image estimation from data captured by an electronic focal plane array. Compressive sensing enables accurate data reconstruction by prior knowledge on desired signal. Computational and optical co-design optimally supports compressive holography in the joint computational and optical domain. This dissertation explores two examples of compressive holography : estimation of 3D tomographic images from 2D data and estimation of images from under sampled apertures. Compressive holography achieves single shot holographic tomography using decompressive inference. In general, 3D image reconstruction suffers from underdetermined measurements with a 2D detector. Specifically, single shot holographic tomography shows the uniqueness problem in the axial direction because the inversion is ill-posed. Compressive sensing alleviates the ill-posed problem by enforcing some sparsity constraints. Holographic tomography is applied for video-rate microscopic imaging and diffuse object imaging. In diffuse object imaging, sparsity priors are not valid in coherent image basis due to speckle. So incoherent image estimation is designed to hold the sparsity in incoherent image basis by support of multiple speckle realizations. High pixel count holography achieves high resolution and wide field-of-view imaging. Coherent aperture synthesis can be one method to increase the aperture size of a detector. Scanning-based synthetic aperture confronts a multivariable global optimization problem due to time-space measurement errors. A hierarchical estimation strategy divides the global problem into multiple local problems with support of computational and optical co-design. Compressive sparse aperture holography can be another method. Compressive sparse sampling collects most of significant field information with a small fill factor because object scattered fields are locally redundant. Incoherent image estimation is adopted for the expanded modulation transfer function and compressive reconstruction.Item Open Access Control of radiative processes using tunable plasmonic nanopatch antennas.(Nano Lett, 2014-08-13) Rose, Alec; Hoang, Thang B; McGuire, Felicia; Mock, Jack J; Ciracì, Cristian; Smith, David R; Mikkelsen, Maiken HThe radiative processes associated with fluorophores and other radiating systems can be profoundly modified by their interaction with nanoplasmonic structures. Extreme electromagnetic environments can be created in plasmonic nanostructures or nanocavities, such as within the nanoscale gap region between two plasmonic nanoparticles, where the illuminating optical fields and the density of radiating modes are dramatically enhanced relative to vacuum. Unraveling the various mechanisms present in such coupled systems, and their impact on spontaneous emission and other radiative phenomena, however, requires a suitably reliable and precise means of tuning the plasmon resonance of the nanostructure while simultaneously preserving the electromagnetic characteristics of the enhancement region. Here, we achieve this control using a plasmonic platform consisting of colloidally synthesized nanocubes electromagnetically coupled to a metallic film. Each nanocube resembles a nanoscale patch antenna (or nanopatch) whose plasmon resonance can be changed independent of its local field enhancement. By varying the size of the nanopatch, we tune the plasmonic resonance by ∼ 200 nm, encompassing the excitation, absorption, and emission spectra corresponding to Cy5 fluorophores embedded within the gap region between nanopatch and film. By sweeping the plasmon resonance but keeping the field enhancements roughly fixed, we demonstrate fluorescence enhancements exceeding a factor of 30,000 with detector-limited enhancements of the spontaneous emission rate by a factor of 74. The experiments are supported by finite-element simulations that reveal design rules for optimized fluorescence enhancement or large Purcell factors.Item Open Access Control of Surface Plasmon Substrates and Analysis of Near field Structure(2011) Chen, Shiuan-YehThe electromagnetic properties of various plasmonic nanostructures are investigated. These nanostructures, which include random clusters, controlled clusters and particle-film hybrids are applied to surface-enhanced Raman scattering (SERS). A variety of techniques are utilized to fabricate, characterize, and model these SERS-active structures, including nanoparticle functionalization, thin film deposition, extinction spectroscopy, elastic scattering spectroscopy, Raman scattering spectroscopy, single-assembly scattering spectroscopy, transmission electron microscopy, generalized Mie theory, and finite element method.

Initially, the generalized Mie theory is applied to calculate the near-field of the small random clusters to explain their SERS signal distribution. The nonlinear trend of SERS intensity versus size of clusters is demonstrated in experiments and near-field simulations.

Subsequently, controlled nanoparticle clusters are fabricated for quantitative SERS. A 50 nm gold nanoparticle and 20nm gold nanoparticles are tethered to form several hot spots between them. The SERS signal from this assembly is compared with SERS signals from single particles and the relative intensities are found to be consistent with intensity ratios predicted by near-field calculation.

Finally, the nanoparticle/film hybrid structure is studied. The scattering properties and SERS activity are observed from gold nanoparticles on different substrates. The gold nanoparticle on gold film demonstrates high field enhancement. Raman blinking is observed and implies a single molecule signal. Furthermore, the doughnut shape of Raman images indicates that this hybrid structure serves as nano-antenna and modifies the direction of molecular emission.

In additional to the primary gap dipole utilized for SERS, high order modes supported by the nanoparticle/film hybrid also are investigated. In experiments, the HO mode show less symmetry compared to the gap dipole mode. The simulation indicates that the HO modes observed may be comprised of two gap modes. One is quadrupole-like and the other is dipole-like in terms of near-field profile. The analytical treatment of the coupled dipole is performed to mimic the imaging of the quadrupole radiation.

Item Open Access Controlling the second harmonic in a phase-matched negative-index metamaterial.(Phys Rev Lett, 2011-08-05) Rose, Alec; Huang, Da; Smith, David RNonlinear metamaterials have been predicted to support new and exciting domains in the manipulation of light, including novel phase-matching schemes for wave mixing. Most notable is the so-called nonlinear-optical mirror, in which a nonlinear negative-index medium emits the generated frequency towards the source of the pump. In this Letter, we experimentally demonstrate the nonlinear-optical mirror effect in a bulk negative-index nonlinear metamaterial, along with two other novel phase-matching configurations, utilizing periodic poling to switch between the three phase-matching domains.Item Open Access Coordinated analysis of delayed sprites with high-speed images and remote electromagnetic fields(2010) Li, JingboOne of the most dramatic discoveries in solar-terrestrial physics in the past two decades is the sprite, a high altitude optical glow produced by a lightning discharge. Previous sprite studies including both theoretical modeling and remote measurements of optical emissions and associated radio emissions have revealed many important features. However, in-situ measurements, which are critical for understanding the microphysics in sprites and constraining the existing models, are almost impossible because of the sprites' small time scale (a few ms) and large spatial scale (tens of km). In this work, we infer the lightning-driven ambient electric fields by combining remote measured electromagnetic fields with numerical simulations. To accomplish this, we first extract the lightning source current from remotely measured magnetic fields with a deconvolution technique. Then we apply this current source to an existing 2-D Finite Difference Time Domain (FDTD) model to compute the electric fields at sprite altitudes. These inferred electric fields make up for the deficiency of lacking in-situ measurements. A data set collected at two observation sites in 2005 combines simultaneous measurements of sprite optical emissions and sprite-producing lightning radiated electromagnetic fields. Sprite images from a high speed camera and the measured wideband magnetic fields removed the limitations imposed by the small sprite temporal scale and allow us to precisely determine the sprite initiation time and the time delay from its parent lightning discharge. For 83 sprites analyzed, close to 50% of them are delayed for more than 10 ms after the lightning discharges and empirically defined as long-delayed sprites. Compared with short-delayed sprites, which are driven by the lightning return stroke, all these long-delayed sprites are associated with intense continuing current and large total charge moment changes. Besides that, sferic bursts and slow intensifications are frequently detected before those long-delayed sprites. These observations suggest a different initiation mechanism of long-delayed sprites. To reveal that, we inferred the lightning-driven electric fields at the sprite initiation time and altitude. Our results show that although long-delayed sprites are mainly driven by the continuing current instead of the lightning return stroke, the electric fields required to produce those long-delayed sprites are essentially the same as fields to produce short-delayed sprites. Thus the initiation mechanism of long delayed sprite is consistent with the conventional breakdown model. Our results also revealed that the slow (5{20ms) intensifications in continuing current can significantly increase high altitude electric fields and play a major role in initiating delayed sprite. Sferic bursts, which were suggested as a direct cause of long-delayed sprites in previous studies, are linked to slow intensifications but not causal. Previous studies from remote measured low frequency radio emissions indicate that substantial electric current flows inside the sprite body. This charge motion, with unknown location and amount, is related to the detailed internal microphysics of sprite development that is in turn connected to the impact sprites have on the mesosphere. In our data, the recorded high speed images show the entire development history of sprite streamers. By assuming streamers propagate along the direction of local electric fields, we estimate the amount of electric charge in sprites. Our results show that individual bright core contains significant negative space charge between 0.01 to 0.03 C. Numerical simulations also indicate that this sprite core region is at least partial or perhaps the dominant source of the positive charge in the downward positive polarity streamers. Thus the average amount of charge in each downward streamer is at least 2 - 4 103 C. The connection between these charge regions is consistent with previous observations. The reported amount and location of the electric charge provide the initial condition and key data to constrain the existing streamer models. After initiation, sprite streamers propagate in the inhomogeneous medium from a strong field region to a weak field region. The propagation properties reflect the physics in sprite development. For the first time we measured the downward streamer propagation behaviors over the full sprite altitude extent. We found that downward streamers accelerate to a maximum velocity of 1 - 3 x 107 m/s and then immediately decelerate at an almost constant rate close to 10 10 m/s2. The deceleration processes dominant downward streamer propagation in both time and distance. Lightning driven electric fields have been inferred at streamer tip locations during their propagation. We found that most of the deceleration process occurs at a electric field less than 0.1 Ek. The results also show the dependence of sprite termination altitude on the ambient electric field. A minimum ambient electric field about 0.05 Ek is consistently observed for streamers in different sprites or at different locations in a single sprite. These streamer propagation properties as well as their connections to the ambient electric fields can be applied to further constrain the streamer models.Item Open Access Design, Analysis, And Characterization Of Metamaterial Quasi-Optical Components For Millimeter-Wave Automotive Radar(2013) Nguyen, Vinh NgocSince 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.

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.

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.

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.

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.

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.

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.

Item Open Access Designing and building microwave metamaterials(2009) Liu, RuopengItem Open Access Development of Analog Nonlinear Materials Using Varactor Loaded Split-ring Resonator Metamaterials(2013) Huang, DaAs research in electromagnetics has expanded, it has given rise to the examination of metamaterials, which possess nontrivial electromagnetic material properties such as engineered permittivity and permeability. Aside from their application in the microwave industry, metamaterials have been associated with novel phenomena since their invention, including sub-wavelength focusing in negative refractive index slabs, evanescent wave amplification in negative index media, and invisibility cloaking and its demonstration at microwave frequency with controlled material properties in space.

Effective medium theory plays a key role in the development and application of metamaterials, simplifying the electromagnetic analysis of complex engineered metamaterial composites. Any metamaterial composite can be treated as a homogeneous or inhomogeneous medium, while every unit structure in the composite is represented by its permittivity and permeability tensor. Hence, studying an electromagnetic wave's interaction with complex composites is equivalent to studying the interaction between the wave and an artificial material.

This dissertation first examines the application of a magnetic metamaterial lens in wireless power transfer (WPT) technology, which is proposed to enhance the mutual coupling between two magnetic dipoles in the system. I examine and investigate the boundary effect in the finite sized magnetic metamaterial lens using a numerical simulator. I propose to implement an anisotropic and indefinite lens in a WPT system to simplify the lens design and relax the lens dimension requirements. The numerical results agree with the analytical model proposed by Smith et al. in 2011, where lenses are assumed to be infinitely large.

By manipulating the microwave properties of a magnetic metamaterial, the nonlinear properties come into the scope of this research. I chose split-ring resonators (SRR) loaded with varactors to develop nonlinear metamaterials. Analogous to linear metamaterials, I developed a nonlinear effective medium model to characterize nonlinear processes in microwave nonlinear metamaterials. I proposed both experimental and numerical methods here for the first time to quantify nonlinear metamaterials' effective properties. I experimentally studied three nonlinear processes: power-dependent frequency tuning, second harmonic generation, and three-wave mixing. Analytical results based on the effective medium model agree with the experimental results under the low power excitation assumption and non-depleted pump approximation. To overcome the low power assumption in the effective medium model for nonlinear metamaterials, I introduced general circuit oscillation models for varactor/diode-loaded microwave metamaterial structures, which provides a qualitative prediction of microwave nonlinear metamaterials' responses at relatively high power levels when the effective medium model no longer fits.

In addition to 1D nonlinear processes, this dissertation also introduces the first 2D microwave nonlinear field mapping apparatus, which is capable of simultaneously capturing both the magnitude and phase of generated harmonic signals from nonlinear metamaterial mediums. I designed a C-band varactor loaded SRR that is matched to the frequency and space limitation of the 2D mapper. The nonlinear field generation and scattering properties from both a single nonlinear element and a nonlinear metamaterial medium composite are experimentally captured in this 2D mapper, and the results qualitatively agree with numerical results based on the effective medium model.

Item Open Access Dynamic Metasurface Apertures for Computational Imaging(2018) Sleasman, TimothyMicrowave imaging platforms conventionally take the form of antenna arrays or synthetic apertures. Inspired by methods in the optical regime, computational microwave imaging has recently taken hold as an alternative approach that uses spatially-diverse waveforms to multiplex scene information. In this dissertation, we use dynamic metasurface apertures to demonstrate improved hardware characteristics and capabilities in computational microwave imaging systems. In particular, we demonstrate waveguide-fed and cavity-backed dynamic metasurface apertures. A waveguide-fed dynamic metasurface aperture consists of a waveguide device loaded with numerous independently tunable metamaterial elements, each of which couples energy from the guided mode into a reconfigurable radiation pattern. We explicate design considerations for a waveguide-fed dynamic metasurface aperture, optimize its usage, and utilize it in computational imaging. In addition, we leverage the dynamic aperture's agility to demonstrate through-wall imaging and beamforming for synthetic aperture radar. Significant attention is also devoted to imaging with a single frequency, an approach which can ease the complexity and improve the performance of the required RF components.

Expanding on the waveguide-fed instantiation, we investigate cavity-backed dynamic apertures. These apertures employ disordered cavity modes to feed a multitude of radiating elements. We investigate this approach with two structures: a volumetric cavity, where we tune the boundary condition, and a planar PCB-based cavity, where the radiating elements are tuned. Capable of generating diverse radiation patterns, we use these structures to assess the utility of dynamic tuning in computational imaging systems. Many of the architectures studied in this dissertation chart a path toward a low-cost dynamic aperture with a favorable form factor, a platform which provides immense control over its emitted fields for a variety of microwave applications.

Item Open Access Electromagnetic Metamaterials for Antenna Applications(2010) Sajuyigbe, AdesojiThis dissertation examines the use of artificial structured materials -- known as metamaterials -- in two antenna applications in which conventional dielectric materials are otherwise used. In the first application, the use of metamaterials to improve the impedance matching of planar phased array antennas over a broad range of scan angles is explored. A phased array antenna is composed of an array of antenna elements and enables long-distance signal propagation by directional radiation. The direction of signal propagation is defined as the scan angle. The power transmission ratio of a phased array is the ratio of the radiated power to the input power, and depends on the scan angle. The variation in the power transmission ratio is due to the different mutual coupling contributions between antenna elements at different scan angles. An optimized stack of dielectric layers, known as a wide-angle impedance matching layer (WAIM), is used to optimize the power transmission ratio profile over a broad range of scan angles. In this work, the use of metamaterials to design anisotropic WAIMs with access to a larger range of constitutive parameters -- including magnetic permeability -- to offer an improved power transmission ratio at a broad range of scan angles is investigated.

In the second antenna application, a strategy to create maximally transmissive and minimally reflective electromagnetic radome materials using embedded metamaterial inclusions is introduced. A radome is a covering used to protect an antenna from weather elements or provide structural function such as the prevention of aerodynamic drag. A radome should be made from a fully transparent and non-refractive material so that radiated fields from and to the enclosed antenna are not disrupted. The aim of this research was to demonstrate that embedded metamaterial inclusions can be used to isotropically adjust the dielectric properties of a composite material to a desired value. This strategy may lead to the creation of a structural material with electromagnetic properties close to air, thus reducing the detrimental scattering effects often associated with conventional radome materials.

Chapter 1 introduces the concept of metamaterials and discusses the use of subwavelength metallic structures to artificially engineer constitutive parameters such as permeability of permittivity. In Chapter 2, the analytical formulations that enable the characterization of the transmission performance of a planar phased array covered with anisotropic impedance matching layers are developed. Chapter 3 discusses the design rules that must govern the design parameters of anisotropic WAIMs realizable using metamaterials, and also presents examples of anisotropic impedance matching layers that provide a maximum power transmission ratio for most scan angles. In addition, numerical and experimental results on a metamaterial placed over a phased array are presented. In Chapter 4, the feasibility of using metamaterials to realize a minimally transparent and fully transmissive radome material is numerically investigated. In Chapter 5, experimental results that corroborate earlier numerical simulation results are analyzed.

Item Open Access Enhancing imaging systems using transformation optics.(Opt Express, 2010-09-27) Smith, David R; Urzhumov, Yaroslav; Kundtz, Nathan B; Landy, Nathan IWe apply the transformation optical technique to modify or improve conventional refractive and gradient index optical imaging devices. In particular, when it is known that a detector will terminate the paths of rays over some surface, more freedom is available in the transformation approach, since the wave behavior over a large portion of the domain becomes unimportant. For the analyzed configurations, quasi-conformal and conformal coordinate transformations can be used, leading to simplified constitutive parameter distributions that, in some cases, can be realized with isotropic index; index-only media can be low-loss and have broad bandwidth. We apply a coordinate transformation to flatten a Maxwell fish-eye lens, forming a near-perfect relay lens; and also flatten the focal surface associated with a conventional refractive lens, such that the system exhibits an ultra-wide field-of-view with reduced aberration.Item Open Access Far-field analysis of axially symmetric three-dimensional directional cloaks.(Opt Express, 2013-04-22) Ciracì, Cristian; Urzhumov, Yaroslav; Smith, David RAxisymmetric radiating and scattering structures whose rotational invariance is broken by non-axisymmetric excitations present an important class of problems in electromagnetics. For such problems, a cylindrical wave decomposition formalism can be used to efficiently obtain numerical solutions to the full-wave frequency-domain problem. Often, the far-field, or Fraunhofer region is of particular interest in scattering cross-section and radiation pattern calculations; yet, it is usually impractical to compute full-wave solutions for this region. Here, we propose a generalization of the Stratton-Chu far-field integral adapted for 2.5D formalism. The integration over a closed, axially symmetric surface is analytically reduced to a line integral on a meridional plane. We benchmark this computational technique by comparing it with analytical Mie solutions for a plasmonic nanoparticle, and apply it to the design of a three-dimensional polarization-insensitive cloak.Item Open Access Flow stabilization with active hydrodynamic cloaks(Physical Review E - Statistical, Nonlinear, and Soft Matter Physics, 2012-11-21) Urzhumov, Yaroslav A; Smith, David RWe demonstrate that fluid flow cloaking solutions, based on active hydrodynamic metamaterials, exist for two-dimensional flows past a cylinder in a wide range of Reynolds numbers (Re's), up to approximately 200. Within the framework of the classical Brinkman equation for homogenized porous flow, we demonstrate using two different methods that such cloaked flows can be dynamically stable for Re's in the range of 5-119. The first highly efficient method is based on a linearization of the Brinkman-Navier-Stokes equation and finding the eigenfrequencies of the least stable eigenperturbations; the second method is a direct numerical integration in the time domain. We show that, by suppressing the von Kármán vortex street in the weakly turbulent wake, porous flow cloaks can raise the critical Reynolds number up to about 120 or five times greater than for a bare uncloaked cylinder. © 2012 American Physical Society.Item Open Access Fluid flow control with transformation media.(Phys Rev Lett, 2011-08-12) Urzhumov, Yaroslav A; Smith, David RWe introduce a new concept for the manipulation of fluid flow around three-dimensional bodies. Inspired by transformation optics, the concept is based on a mathematical idea of coordinate transformations and physically implemented with anisotropic porous media permeable to the flow of fluids. In two situations-for an impermeable object placed either in a free-flowing fluid or in a fluid-filled porous medium-we show that the object can be coated with an inhomogeneous, anisotropic permeable medium, such as to preserve the flow that would have existed in the absence of the object. The proposed fluid flow cloak eliminates downstream wake and compensates viscous drag, hinting at the possibility of novel propulsion techniques.

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