# Browsing by Author "Smith, DR"

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Item Open Access Acoustic cloaking transformations from attainable material properties(New Journal of Physics, 2010-07-12) Urzhumov, Y; Ghezzo, F; Hunt, J; Smith, DRWe propose a general methodology and a set of practical recipes for the construction of ultra-broadband acoustic cloaks-structures that can render themselves and a concealed object undetectable by means of acoustic scattering. The acoustic cloaks presented here are designed and function analogously to electromagnetic cloaks. However, acoustic cloaks in a fluid medium do not suffer the bandwidth limitations imposed on their electromagnetic counterparts by the finite speed of light in vacuum. In the absence of specific metamaterials having arbitrary combinations of quasi-static speed of sound and mass density, we explore the flexibility of continuum transformations that produce approximate cloaking solutions. We show that an imperfect, eikonal acoustic cloak (that is, one which is not impedance matched but is valid in the geometrical optics regime) with negligible dispersion can be designed using a simple layered geometry. Since a practical cloaking device will probably be composed of combinations of solid materials rather than fluids, it is necessary to consider the full elastic properties of such media, which support shear waves in addition to the compression waves associated with the acoustic regime. We perform a systematic theoretical and numerical investigation of the role of shear waves in elastic cloaking devices. We find that for elastic metamaterials with Poisson's ratio v > 0.49, shear waves do not alter the cloaking effect. Such metamaterials can be built from nearly incompressible rubbers (with v ≈ 0.499) and fluids. We expect this finding to have applications in other acoustic devices based on the form-invariance of the scalar acoustic wave equation. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.Item Open Access Analysis of the power dependent tuning of a varactor-loaded metamaterial at microwave frequencies(Applied Physics Letters, 2010-03-26) Huang, D; Poutrina, E; Smith, DRWe present an analysis of the nonlinear, power-dependent resonance frequency shift for two metamaterial mediums consisting of arrays of varactor-loaded split ring resonators (VLSRRs). We confirm that, over a limited range of power, a VLSRR medium can be described by its second and third order nonlinear susceptibilities, making it a useful analog medium for the quantitative investigation of other nonlinear phenomena that might be achieved using inherently nonlinear materials integrated into metamaterials. Experimental measurements of the resonance frequency shift with power from fabricated VLSRR samples are found to be in excellent agreement with the analytical model. © 2010 American Institute of Physics.Item Open Access Analytic expressions for the constitutive parameters of magnetoelectric metamaterials(Physical Review E - Statistical, Nonlinear, and Soft Matter Physics, 2010-03-17) Smith, DRElectromagnetic metamaterials are artificially structured media typically composed of arrays of resonant electromagnetic circuits, the dimension and spacing of which are considerably smaller than the free-space wavelengths of operation. The constitutive parameters for metamaterials, which can be obtained using full-wave simulations in conjunction with numerical retrieval algorithms, exhibit artifacts related to the finite size of the metamaterial cell relative to the wavelength. Liu showed that the complicated, frequency-dependent forms of the constitutive parameters can be described by a set of relatively simple analytical expressions. These expressions provide useful insight and can serve as the basis for more intelligent interpolation or optimization schemes. Here, we show that the same analytical expressions can be obtained using a transfer-matrix formalism applied to a one-dimensional periodic array of thin, resonant, dielectric, or magnetic sheets. The transfer-matrix formalism breaks down, however, when both electric and magnetic responses are present in the same unit cell, as it neglects the magnetoelectric coupling between unit cells. We show that an alternative analytical approach based on the same physical model must be applied for such structures. Furthermore, in addition to the intercell coupling, electric and magnetic resonators within a unit cell may also exhibit magnetoelectric coupling. For such cells, we find an analytical expression for the effective index, which displays markedly characteristic dispersion features that depend on the strength of the coupling coefficient. We illustrate the applicability of the derived expressions by comparing to full-wave simulations on magnetoelectric unit cells. We conclude that the design of metamaterials with tailored simultaneous electric and magnetic response-such as negative index materials-will generally be complicated by potentially unwanted magnetoelectric coupling. © 2010 The American Physical Society.Item Open Access Characterization of high order modes of plasmonic antenna formed by nanoparticle/thin film hybrid structures(Proceedings of SPIE - The International Society for Optical Engineering, 2012-05-04) Chen, SY; Urzhumov, Y; Smith, DR; Lazarides, AAThe plasmonic modes of a nano-antenna formed by a nanoparticle/thin film hybrid system are investigated. Plasmonic nano-antennas are well-known for their capabilities to concentrate electromagnetic wave into extreme small region and couple the emission from active materials in proximity to the antennas into far-field region. Previously, we have shown through direct measurement of emission profile images that the nano-antennas not only enhance Raman emission but also systematically direct inelastic emission to the far-field through the dipole mode. We also showed that high order modes of the hybrid structure can be detected. Here, the higher order plasmonic modes are characterized through imaging, variable angle linearly polarized excitation, and simulation. Through spectral simulation with improved resolution, two distinct modes are found to contribute to the broad band high order mode. One has dipole-like behavior and the other has quadrupole-like behavior. The modes are characterized both through near-field distribution and farfield scattering profiles. The quadrupole-like mode can be excited by both p- and s-polarized light whereas the dipolelike mode is only excited by p-polarized light. These high order modes are not as bright as the dipole mode in the farfield spectrum but have substantial near field enhancement which can be utilized for surface-enhancing spectroscopy and sensing. In addition, characterization of high order modes may serve to clarify the interaction between nano-antenna and active materials and will lead to design rules for applications of active plasmonic structures in integrated optical circuits. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE). Photonic and Phononic Properties of Engineered Nanostructures II;82691M.Item Open Access Cross-section comparisons of cloaks designed by transformation optical and optical conformal mapping approaches(Journal of Optics, 2011-02-01) Urzhumov, YA; Kundtz, NB; Smith, DR; Pendry, JBWe review several approaches to optical invisibility designed using transformation optics (TO) and optical conformal mapping (CM) techniques. TO is a general framework for solving inverse scattering problems based on mimicking spatial coordinate transformations with distributions of material properties. There are two essential steps in the design of TO media: first, a coordinate transformation that achieves some desired functionality, resulting in a continuous spatial distribution of constitutive parameters that are generally anisotropic; and, second, the reduction of the derived continuous constitutive parameters to a metamaterial that serves as a stepwise approximation. We focus here on the first step, discussing the merits of various TO strategies proposed for the long-sought 'invisibility cloak'-a structure that renders opaque objects invisible. We also evaluate the cloaking capabilities of structures designed by the related CM approach, which makes use of conformal mapping to achieve index-only material distributions. The performance of the various cloaks is evaluated and compared using a universal measure-the total (all-angle) scattering cross section. © 2011 IOP Publishing Ltd.Item Open Access Designing three-dimensional transformation optical media using quasiconformal coordinate transformations.(Phys Rev Lett, 2010-11-05) Landy, NI; Kundtz, N; Smith, DRWe introduce an approach to the design of three-dimensional transformation optical (TO) media based on a generalized quasiconformal mapping approach. The generalized quasiconformal TO (QCTO) approach enables the design of media that can, in principle, be broadband and low loss, while controlling the propagation of waves with arbitrary angles of incidence and polarization. We illustrate the method in the design of a three-dimensional carpet ground plane cloak and of a flattened Luneburg lens. Ray-trace studies provide a confirmation of the performance of the QCTO media, while also revealing the limited performance of index-only versions of these devices.Item Open Access Electronically reconfigurable metal-on-silicon metamaterial(Physical Review B - Condensed Matter and Materials Physics, 2012-08-08) Urzhumov, Y; Lee, JS; Tyler, T; Dhar, S; Nguyen, V; Jokerst, NM; Schmalenberg, P; Smith, DRReconfigurable metamaterial-based apertures can play a unique role in both imaging and in beam-forming applications, where current technology relies mostly on the fabrication and integration of large detector or antenna arrays. Here, we report the experimental demonstration of a voltage-controlled, silicon-based electromagnetic metamaterial operating in the W-band (75-110 GHz). In this composite semiconductor metamaterial, patterned gold metamaterial elements serve both to manage electromagnetic wave propagation while simultaneously acting as electrical Schottky contacts that control the local conductivity of the semiconductor substrate. The active device layers consist of a patterned metal on a 2-μm-thick n-doped silicon layer, adhesively bonded to a transparent Pyrex wafer. The transmittance of the composite metamaterial can be modulated over a given frequency band as a function of bias voltage. We demonstrate a quantitative understanding of the composite device through the application of numerical approaches that simultaneously treat the semiconductor junction physics as well as wave propagation. © 2012 American Physical Society.Item Open Access Experimental determination of the quadratic nonlinear magnetic susceptibility of a varactor-loaded split ring resonator metamaterial(Applied Physics Letters, 2010-07-05) Larouche, S; Rose, A; Poutrina, E; Huang, D; Smith, DRThis letter presents a quantitative measurement of the second harmonic generated by a slab of varactor loaded split ring resonator metamaterial and the retrieval of the effective quadratic nonlinear magnetic susceptibility χ(2)musing an approach based on transfer matrices. The retrieved value of χ(2)m is in excellent agreement with that predicted by an analytical effective medium theory model. © 2010 American Institute of Physics.Item Open Access Going beyond Axisymmetry: 2.5D Vector Electromagnetics(2012-10) Urzhumov, YA; Landy, N; Ciraci, C; Smith, DRLinear wave propagation through inhomogeneous structures of size R≫λ (Fig.1) is a computationally challenging problem, in particular when using finite element methods, due to the steep increase of the number of degrees of freedom as a function of R/λ. Fortunately, when the geometry of the problem possesses symmetries, one may choose an appropriate basis in which the stiffness matrix of the discretized problem is block-diagonal. A particular scenario is the case of a cylindrically-symmetric geometry, where an appropriate basis is the set of cylindrical waves with all possible azimuthal numbers (m). Each of the excited cylindrical harmonics propagate through the structure independently of all other harmonics, and therefore the fields associated with that harmonic can be found by solving an essentially two-dimensional PDE problem in the ρ-z (half)-plane. The cylindrical waves have a prescribed dependence on the azimuthal angle variable (φ), hence the name – 2.5D electromagnetics. This novel approach is applied to the problem of cloaking and wave scattering off a spherical nanoparticle on metallic and/or dielectric substrates.Item Open Access Isotropic-medium three-dimensional cloaks for acoustic and electromagnetic waves(Journal of Applied Physics, 2012-03-01) Urzhumov, Y; Landy, N; Smith, DRWe propose a generalization of the two-dimensional eikonal-limit cloak derived from a conformal transformation to three dimensions. The proposed cloak is a spherical shell composed of only isotropic media; it operates in the transmission mode and requires no mirror or ground plane. Unlike the well-known omnidirectional spherical cloaks, it may reduce visibility of an arbitrary object only for a very limited range of observation angles. In the short-wavelength limit, this cloaking structure restores not only the trajectories of incident rays, but also their phase, which is a necessary ingredient to complete invisibility. Both scalar-wave (acoustic) and transverse vector-wave (electromagnetic) versions are presented. © 2012 American Institute of Physics.Item Open Access Magnetic levitation of metamaterial bodies enhanced with magnetostatic surface resonances(Physical Review B - Condensed Matter and Materials Physics, 2012-02-27) Urzhumov, Y; Chen, W; Bingham, C; Padilla, W; Smith, DRWe propose that macroscopic objects built from negative-permeability metamaterials may experience resonantly enhanced magnetic force in low-frequency magnetic fields. Resonant enhancement of the time-averaged force originates from magnetostatic surface resonances (MSRs), which are analogous to the electrostatic resonances of negative-permittivity particles, well known as surface plasmon resonances in optics. We generalize the classical problem of the MSR of a homogeneous object to include anisotropic metamaterials and consider the most extreme case of anisotropy, where the permeability is negative in one direction but positive in the others. It is shown that deeply subwavelength objects made of such indefinite (hyperbolic) media exhibit a pronounced magnetic dipole resonance that couples strongly to uniform or weakly inhomogeneous magnetic field and provides strong enhancement of the magnetic force, enabling applications such as enhanced magnetic levitation. © 2012 American Physical Society.Item Open Access Magnetic superlens-enhanced inductive coupling for wireless power transfer(Journal of Applied Physics, 2012-03-15) Huang, D; Urzhumov, Y; Smith, DR; Hoo Teo, K; Zhang, JWe investigate numerically the use of a negative-permeability perfect lens for enhancing wireless power transfer between two current carrying coils. The negative permeability slab serves to focus the flux generated in the source coil to the receiver coil, thereby increasing the mutual inductive coupling between the coils. The numerical model is compared with an analytical theory that treats the coils as point dipoles separated by an infinite planar layer of magnetic material Urzhumov, Phys. Rev. B 19, 8312 (2011). In the limit of vanishingly small radius of the coils, and large width of the metamaterial slab, the numerical simulations are in excellent agreement with the analytical model. Both the idealized analytical and realistic numerical models predict similar trends with respect to metamaterial loss and anisotropy. Applying the numerical models, we further analyze the impact of finite coil size and finite width of the slab. We find that, even for these less idealized geometries, the presence of the magnetic slab greatly enhances the coupling between the two coils, including cases where significant loss is present in the slab. We therefore conclude that the integration of a metamaterial slab into a wireless power transfer system holds promise for increasing the overall system performance. © 2012 American Institute of Physics.Item Open Access Metamaterial-enhanced coupling between magnetic dipoles for efficient wireless power transfer(Physical Review B - Condensed Matter and Materials Physics, 2011-05-18) Urzhumov, Y; Smith, DRNonradiative coupling between conductive coils is a candidate mechanism for wireless energy transfer applications. In this paper we propose a power relay system based on a near-field metamaterial superlens and present a thorough theoretical analysis of this system. We use time-harmonic circuit formalism to describe all interactions between two coils attached to external circuits and a slab of anisotropic medium with homogeneous permittivity and permeability. The fields of the coils are found in the point-dipole approximation using Sommerfeld integrals which are reduced to standard special functions in the long-wavelength limit. We show that, even with a realistic magnetic loss tangent of order 0.1, the power transfer efficiency with the slab can be an order of magnitude greater than free-space efficiency when the load resistance exceeds a certain threshold value. We also find that the volume occupied by the metamaterial between the coils can be greatly compressed by employing magnetic permeability with a large anisotropy ratio. © 2011 American Physical Society.Item Open Access Nanophotonics: Optical time reversal with graphene(2013-07) Urzhumov, YA; Ciraci, C; Smith, DRWould you ever guess that a microscopic flake of graphite could reverse the diffraction of light? An experiment that demonstrates just such an effect highlights the exciting optical applications of graphene — an atomic layer of carbon with a two-dimensional honeycomb lattice.Item Open Access Nonlinear long-range plasmonic waveguides(Physical Review A - Atomic, Molecular, and Optical Physics, 2010-09-13) Degiron, A; Smith, DRWe report on plasmonic waveguides made of a thin metal stripe surrounded on one or both sides by a Kerr nonlinear medium. Using an iterative numerical method, we investigate the stationary long-range plasmons that exist for self-focusing and self-defocusing Kerr-type nonlinearities. The solutions are similar to the well-known case of infinitely wide nonlinear waveguides-they are strongly power-dependent and can experience symmetry-breaking bifurcations under appropriate conditions. © 2010 The American Physical Society.Item Open Access Nonlinear oscillator metamaterial model: numerical and experimental verification.(Opt Express, 2011-04-25) Poutrina, E; Huang, D; Urzhumov, Y; Smith, DRWe verify numerically and experimentally the accuracy of an analytical model used to derive the effective nonlinear susceptibilities of a varactor-loaded split ring resonator (VLSRR) magnetic medium. For the numerical validation, a nonlinear oscillator model for the effective magnetization of the metamaterial is applied in conjunction with Maxwell equations and the two sets of equations solved numerically in the time-domain. The computed second harmonic generation (SHG) from a slab of a nonlinear material is then compared with the analytical model. The computed SHG is in excellent agreement with that predicted by the analytical model, both in terms of magnitude and spectral characteristics. Moreover, experimental measurements of the power transmitted through a fabricated VLSRR metamaterial at several power levels are also in agreement with the model, illustrating that the effective medium techniques associated with metamaterials can accurately be transitioned to nonlinear systems.Item Open Access Probing the ultimate limits of plasmonic enhancement.(Science, 2012-08-31) Ciracì, C; Hill, RT; Mock, JJ; Urzhumov, Y; Fernández-Domínguez, AI; Maier, SA; Pendry, JB; Chilkoti, A; Smith, DRMetals support surface plasmons at optical wavelengths and have the ability to localize light to subwavelength regions. The field enhancements that occur in these regions set the ultimate limitations on a wide range of nonlinear and quantum optical phenomena. We found that the dominant limiting factor is not the resistive loss of the metal, but rather the intrinsic nonlocality of its dielectric response. A semiclassical model of the electronic response of a metal places strict bounds on the ultimate field enhancement. To demonstrate the accuracy of this model, we studied optical scattering from gold nanoparticles spaced a few angstroms from a gold film. The bounds derived from the models and experiments impose limitations on all nanophotonic systems.Item Open Access Relaxation approach for the generation of inhomogeneous distributions of uniformly sized particles(Applied Physics Letters, 2010-07-12) Hunt, J; Kundtz, N; Landy, N; Smith, DRFor many applications in gradient index devices and photonic crystals, it is necessary to be able to design semicrystalline distributions of particles where the lattice constant of the distribution is an arbitrary function of position. We propose a method to generate such distributions which is physically motivated by a system of interacting particles, and apply it to the design and implementation of a microwave gradient index lens. While the demonstration was preformed at microwave wavelengths, this technique would also be particularly useful for designing devices for operation at IR and visible wavelengths where the fabrication of distributions of uniformly sized holes or columns is very easy. © 2010 American Institute of Physics.Item Open Access Structurally Rigid Elastic Composites for Acoustic Imaging Countermeasures(2013-06-07) Urzhumov, YA; Starr, AF; Smith, DRWe explore the possibilities coming from transformation acoustics and beyond for creating rigid elastic composite shells capable of suppressing the total scattering cross-section of acoustically large objects. The reported design methodology is based on generalized shape and topology optimization, and the outcomes are suitable for rapid prototyping techniques.Item Open Access Towards macroscopic optical invisibility devices: geometrical optics of complex materials(2012-01-18) Urzhumov, YA; Smith, DRRecently, a path towards macroscopic, transparent optical cloaking devices that may conceal objects spanning millions of wavelengths has been proposed [1]. Such devices are designed using transformation optics (TO) [2,3]. In this paper, we offer further analysis and improvements to the concept using the method of geometrical optics extended to complex photonic media with an arbitrary dispersion relation. A technique for solving the highly nonlinear partial differential equation of the eikonal using the finite element method is presented. Aberra-tions caused by the non-quadratic part of the dispersion relation are demonstrated quantitatively in a numerical experiment. An analytical argument based on the scalability of the eikonal phase is presented, which points to-wards a solution that removes this type of aberration in each order of the k-perturbation theory, thus restoring the perfect cloaking solution.