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ABSTRACT: A body-of-revolution finite-difference time-domain (BOR-FDTD) method was developed and employed to rigorously analyze axisymmetric transformation optics (TO) lenses. The novelty of the proposed BOR-FDTD technique is that analytical expressions were derived and presented to introduce obliquely incident plane waves into the total-field/scattered-field formulation, allowing for accurate simulation of BOR objects in layered media illuminated by obliquely incident waves. The accuracy of the proposed method was verified by comparing numerical results with analytical solutions. The developed code was further utilized to study the imaging properties of a cylindrical TO Luneburg lens on a substrate, demonstrating the desired focusing of light onto a flat plane.
Optics Letters 01/2013; 38(1):67-9. · 3.40 Impact Factor
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ABSTRACT: We present a detailed comparison of surface-enhanced Raman spectroscopy (SERS) signals from metallic nanoparticle arrays and their complementary hole arrays. Using an analytical model for local field enhancement, we show that the SERS enhancements of the hole arrays are closely related to their transmission spectra. This trend is experimentally confirmed and characterized by a cos(4 )θ dependence of the SERS signal on the excitation polarization angle θ. The particle arrays, on the other hand, exhibit quite different behavior because of the existence of considerable evanescent modes in the near field. Their maximal local field gains appear at wavelengths generally much larger than their localized surface plasmonic resonant wavelengths.
ACS Nano 06/2011; 5(7):5472-7. · 10.77 Impact Factor
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ABSTRACT: In this paper, we present a method to retrieve the effective electromagnetic parameters of a slab of anisotropic metamaterial from reflection and transmission coefficients (or scattering parameters). In this retrieval method, calculated or measured scattering parameters are employed for plane waves incident obliquely on a metamaterial slab at different angles. Useful analytical expressions are derived for extracting the homogeneous anisotropic medium parameters of a metamaterial. To validate the method, the effective permittivity and permeability tensor parameters for a composite split-ring resonator-wire array are retrieved and shown to be consistent with observations previously reported in the literature. This retrieval method is further incorporated into a genetic algorithm (GA) to synthesize an infrared zero-index-metamaterial with a wide field-of-view, demonstrating the utility of the new design approach. The anisotropic parameter retrieval algorithm, when combined with a robust optimizer such as GA, can provide a powerful design tool for exploiting the anisotropic properties in metamaterials to achieve specific angle dependant or independent responses.
Journal of Applied Physics. 01/2011; 109(1):013515-013515-11.
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ABSTRACT: An efficient technique is developed to fabricate optically thin metallic films with subwavelength patterns and their complements simultaneously. By comparing the spectra of the complementary films, we show that Babinet’s principle nearly holds for these structures in the optical domain. Rigorous full-wave simulations are employed to verify the experimental observations. It is further demonstrated that a discrete-dipole approximation can qualitatively describe the spectral dependence of the metallic membranes on the geometry of the constituent particles as well as the illuminating polarization.
Applied Physics Letters 11/2010; 97(19):193101-193101-3. · 3.84 Impact Factor
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ABSTRACT: Full-wave modeling of 3-D doubly periodic structures with non-orthogonal lattices is an important topic area in computational electromagnetics due to its wide range of possible applications; most notably, frequency selective surfaces (FSS) and metamaterials. The hybrid finite element boundary integral (FEBI) method has been used to analyze the "artificial puck plate" FSS using a triangular grid composed of isotropic media.
Antennas and Propagation Society International Symposium (APSURSI), 2010 IEEE; 08/2010
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ABSTRACT: In this paper, the authors present an impedance-matched low loss infrared zero index metamaterials (ZIM) with a wide field-of-view. The ZIM structure is optimized by a genetic algorithm combined with a generalized inversion method capable of retrieving the effective anisotropic constitutive parameters of the metamaterial with its angular response considered.
Antennas and Propagation Society International Symposium (APSURSI), 2010 IEEE; 08/2010
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ABSTRACT: A flexible polarization-insensitive metamaterial absorber design with a wide field of view was proposed for mid-infrared applications. Both single-band and dual-band designs based on bilayer metallo-dielectric stacks were presented. According to the simulated results, the proposed metamaterial absorbers optimized using a GA (genetic algorithm) could provide absorptivity greater than 0.94 at the target frequency bands for both polarizations over a wide range of incident angles. Efforts are currently underway to fabricate and measure a prototype for confirmation of the numerically predicted absorber performance.
Antennas and Propagation Society International Symposium (APSURSI), 2010 IEEE; 08/2010
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ABSTRACT: Low loss magneto-dielectric substrates when used as substrates for patch antennas are shown to miniaturize the aperture size of the patch antennas while preserving or improving the operational bandwidth. We demonstrated that the properties of a PEC backed low loss matched impedance magneto-dielectric metamaterial slab with frequency dependent permittivity and permeability could effectively be achieved using a multi-layered thin composite structure. In this paper, we demonstrate that modified equations from [2] can be used to optimize thin composite structures to design high loss matched impedance magneto-dielectric metamaterials for absorber applications above 1 GHz. The design technique is based on using a genetic algorithm (GA) to optimize thin multi-layered metallo-dielectric metamaterial slab comprised of a periodic array of electrically small metallic frequency selective surface (FSS) screen sandwiched between two different dielectric materials, one of which is backed by a perfectly conducting ground plane. Examples will be presented to demonstrate the effectiveness of the optimized absorber structures.
Antennas and Propagation Society International Symposium (APSURSI), 2010 IEEE; 08/2010
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ABSTRACT: An improved ??smart?? interpolation approach known as model-based parameter estimation (MBPE) is applied to the wide-band interpolation of periodic method of moments (PMM) impedance matrices for normal and oblique incidence cases. Prior to interpolation, easy to calculate but hard to interpolate, phase terms are removed from the impedance matrices. An efficient spectral-domain PMM formulation is introduced for the accelerated analysis of frequency selective surface (FSS) problems with a large number of unknowns, employing a one dimensional O(N log N) FFT-based method to speed up the computation of matrix-vector products within the bi-conjugate gradient (BCG) iterative solver, which is made possible by the asymmetric multilevel block-Toeplitz structure of the impedance-matrix. The MBPE interpolation algorithm provides a faster matrix fill time than the brute force method and is comparable or even faster than the 2-D FFT-based method for a large number of unknowns. It also has the advantage that it can be applied to non-uniform gridding cases. The accuracy and efficiency of the proposed techniques for large FSS problems are demonstrated by several design examples for both the normal and oblique incidence cases. We also apply this efficient analysis tool to the design of multiband single-layer FSS filters and artificial magnetic conductors (AMC) comprised of a 2-D periodic arrangement of convoluted metallic strips in the shape of a Hilbert curve. The multiband properties of the Hilbert curve FSS filters are studied for different iteration orders (i.e., different degrees of space-filling).
IEEE Transactions on Antennas and Propagation 02/2010; · 2.15 Impact Factor
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ABSTRACT: Within the realm of metamaterial absorber design, multiband ultra-thin electromagnetic bandgap absorbers and double-sided absorber designs based on resistive frequency selective surfaces (FSSs) have not been widely investigated. In this paper, we present an efficient spectral-domain periodic method of moments (PMM) formulation for the analysis of resistive FSS screens embedded in a stratified medium. We apply a cascading procedure to generate a composite scattering matrix for the entire system consisting of multiple resistive FSS screens embedded in a stratified medium. In addition, the 2-D fast Fourier transform (FFT)-based method was applied to speed up calculation of the required impedance matrix elements. Moreover, an 0(N log N) FFT-based method with 1-D implementation together with the sparse storage technique was employed to accelerate computation of the matrix-vector product within the bi-conjugate gradient (BCG) iterative solver. This hybrid method differs from the 2-D FFT techniques reported in. Finally, this formulation will be applied to the design of multiband ultra-thin electromagnetic bandgap absorbers that operate over a wide range of incidence angles and double-sided wideband absorbers that employ resistive FSS screens.
Antennas and Propagation Society International Symposium, 2009. APSURSI '09. IEEE; 07/2009
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ABSTRACT: In this paper, another efficient spatial-domain MoM solver based on the mixed potential integral equation (MPIE) is proposed for the analysis of scattering from large-scale finite periodic arrays of microstrip patches that employs sub-entire domain (SED) basis functions, which were first introduced for the free space case in W.B. Lu et al.(1991). The required spatial- domain Green's functions (GFs) are calculated efficiently by the discrete complex- imaging method (DCIM). The accuracy and efficiency of the developed techniques are demonstrated numerically for large finite periodic microstrip patch arrays having a rectangular or non-rectangular configuration.
Antennas and Propagation Society International Symposium, 2009. APSURSI '09. IEEE; 07/2009
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ABSTRACT: The hybrid finite element boundary integral (FEBI) algorithm provides an efficient analysis methodology for 3-D doubly periodic structures composed of inhomogeneous isotropic media with a wide range of applications including phased array antennas and frequency selective surfaces (FSS) [1, 2]. Recently, general bi-anisotropic media have been considered as substrates for antenna and metamaterial applications exhibiting interesting and useful properties [3]. An analysis of patch antennas residing in cavities filled with bi-anisotropic materials was presented in [3] employing the FEBI method. The spectral-domain method of moments (MoM) was applied to the analysis of periodic structures including FSS with anisotropic layered media in [4]. Scattering by periodic structures consisting of anisotropic materials was also analyzed by the FEBI approach [5]. However, very little work has been done to develop efficient techniques for evaluating scattering from 3-D doubly periodic structures composed of arbitrary inhomogeneous general bi-anisotropic media.
Antennas and Propagation Society International Symposium, 2008. AP-S 2008. IEEE; 08/2008
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ABSTRACT: 2D reconfigurable negative-index metamaterials incorporating anisotropic LCs have been presented that operate in the near-IR. The anisotropic behavior of the LCs was treated rigorously using the periodic FE-BI full-wave analysis technique. The entries in the permittivity tensor for the LCs can be tuned by changing the director orientation of the LC molecules. There are several apparent design tradeoffs between the two configurations such as number of LC layers employed, biasing requirements, and ease of fabrication.
Antennas and Propagation Society International Symposium, 2008. AP-S 2008. IEEE; 08/2008
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ABSTRACT: In this paper, an efficient spectral-domain PMM formulation is introduced for the accelerated analysis of large FSS problems. This PMM formulation incorporates the following techniques: (a) either a type of "smart" interpolation method known as model- based parameter estimation (MBPE) for efficient wide-band interpolation of the impedance matrices or an FFT-based method applied in calculating the impedance matrix elements, together with (b) an 0(N log N) FFT-based method for analyzing 3-D scattering problems, to speed up computation of the matrix-vector product within the iterative solver. The latter is possible due to the asymmetric multilevel block-Toeplitz structure [8] of the impedance-matrix that results from the uniform discretization of the FSS unit cell. Advantages of the proposed hybrid techniques are demonstrated through the presentation of several design examples.
Antennas and Propagation Society International Symposium, 2008. AP-S 2008. IEEE; 08/2008
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ABSTRACT: A near-infrared metamaterial design that is reconfigurable between almost completely transmissive and reflective states is presented. The reconfiguration is enabled by tuning the anisotropic nematic liquid crystals used as a spacer layer between two silver nanoplates in a planar doubly periodic metamaterial. The design is optimized for maximum difference in transmittance between the two states by using a genetic algorithm. For a linearly polarized illumination at normal incidence, full-wave electromagnetic analysis predicts that the optimized metamaterial film can change the transmittance between 98.7% and 0.1% at a wavelength of 1.1 microm.
Optics Letters 04/2008; 33(6):545-7. · 3.40 Impact Factor
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ABSTRACT: The interaction of electromagnetic waves with an arbitrarily shaped three-dimensional (3-D) homogeneous chiral object located above a lossy half space is investigated using the method of moments (MoM) via the coupled mixed potential integral equations (MPIEs). Based on the surface equivalence principle, the equivalent surface electric and magnetic currents are used to replace the homogeneous chiral target in the presence of the half space. Two coupled MPIEs are developed for the unknown equivalent surface electric and magnetic currents by utilizing the continuity condition of the tangential total electric and magnetic field components on the chiral body's surface. The well-known Galerkin procedure with Rao-Wilton-Glisson (RWG) basis functions is applied to solve this problem. The spatial domain half-space Green's functions are obtained from the corresponding spectral domain Green's functions via the discrete complex image method (DCIM) combined with the generalized-pencil of function (GPOF) technique. The reciprocity theorem is employed to calculate the far-zone scattered field. Numerical results are presented for characterizing electromagnetic scattering by a 3-D arbitrarily shaped homogenous chiral object located above a lossy half space so as to demonstrate the accuracy and efficiency of the proposed technique.
IEEE Transactions on Antennas and Propagation 01/2008; · 2.15 Impact Factor
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ABSTRACT: In this paper, a simple four-layer configuration has been developed from the physical point of view for modeling a densely forested environment in the VHF/UHF band. In the model, the first layer, i.e. the uppermost layer, is air, where the incident and scattered fields are considered, while the canopies of the trees are modeled as a homogeneous medium in the second layer. The third layer is again air where the conducting and non-metallic objects are located. The fourth layer is a semi-infinite half space. Based on this, a MoM model has been developed for analysis of electromagnetic scattering from complex targets located in the third layer of the four-layer configuration.
Antennas and Propagation Society International Symposium, 2007 IEEE; 07/2007
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ABSTRACT: We consider the scattering of an arbitrarily shaped 3D chiral object above half space, a topic of interest to applications in which reduction in the scattering from targets has a significant impact. At microwave frequency, chiral materials can be used to minimize scattering, and as substrates for printed antennas to lower the resonance frequency. To the best of our knowledge, scattering from chiral targets located above half space has not been adequately addressed in the literature. The paper aims to develop a formulation based on the mixed potential integral equations (MPIEs) for electromagnetic scattering of an arbitrarily shaped 3D homogeneous chiral object located above half space. This is an extension of the scattering from homogeneous dielectric targets located above half space (Michalski, K.A. and Zheng, D., 1990). The difference is that the field produced by the equivalent surface currents in an unbounded homogeneous chiral medium is the sum of two circularly polarized waves with different sense of rotation and different phase velocities. The time dependence of exp(jωt) is assumed and suppressed.
Antennas and Propagation Society International Symposium, 2004. IEEE; 07/2004
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ABSTRACT: The prediction of electromagnetic scattering from rough surface and subsurface targets has been a problem of long-term interest, as the targets above or below the rough surface are affected by the surrounding medium. The fact that a combined object and rough surface scattering mode can be applied to studies of landmine detection, foliage and surface remote sensing, or target detection and signal processing problems provides further motivation for model development. The contribution from the rough surface can significantly modify the scattering from the target, as compared to that from a flat surface. Some approximate analytical solutions can only be found in the case of small roughness limit The development of numerical models is therefore of interest since these models avoid unnecessary approximations and thus can clarify important scattering effects where analytical methods failed. However, to the authors' knowledge, the electromagnetic scattering by partially buried cylinders at the dielectric rough surface interface between two media was not investigated. This problem will be discussed here, by means of the method of moments with point-matching and pulse basis functions.
Antennas and Propagation Society International Symposium, 2003. IEEE; 07/2003
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ABSTRACT: The efficient evaluation of scattering cross section (SCS) of targets located above a half space is investigated using the reciprocity theorem and the spatial dyadic Green's function (SDGF) obtained via the discrete complex image method with generalised pencil function. The numerical results showed that the SDGF is accurate not only in the near and intermediate field region, but also the SCS in the far field.
Electronics Letters 05/2003; · 0.96 Impact Factor
