Engheta, Nader
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Electrical and Electronics
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H. Nedwill Ramsey Professor of Electrical & Systems Engineering
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Publication Radiation Characteristics of Microstrip Dipole Antennas over a High-Impedance Metamaterial Surface made of Hilbert Inclusions(2003-06-08) McVay, John; Hoorfar, Ahmad; Engheta, NaderWe analyze numerically the radiation characteristics of center-fed microstrip short dipoles and half-wave dipoles near a high-impedance surface made of Hilbert shape flat inclusions. We study the input impedance, pattern and gain of such radiating structures. We show that the radiation resistance of a microstrip dipole increases noticeably at certain frequencies near the resonant frequency of the Hilbert surface. Moreover, antenna gain enhancement at certain frequencies is observed for all dipole sizes we have analyzed.Publication Anomolous Properties of Scattering from Cavities Partially Loaded With Double-Negative or Single-Negative Materials(2005-01-01) Bilotti, Filiberto; Alù, Andrea; Engheta, Nader; Vegni, LucioIn this paper, the theoretical justification and the numerical verification of the anomalous scattering from cavities partially filled with metamaterials are presented. A hybrid numerical formulation based on the Finite Element Method (FEM) and on the Boundary Integral (BI) for the analysis of cavity backed structures with complex loading metamaterials is first presented. The proposed approach allows the analysis of cavities filled with materials described by tensorial linear constitutive relations, which may well describe artificial metamaterials synthesized with proper inclusions in a host dielectric. It is found that cavities loaded with pairs of metamaterial layers with "resonant" features possess unusual scattering properties, and with judicious selection of constitutive parameters for these materials the transparency effect or significant enhancement in the backscattering from such cavities are obtained. This may be considered as a first step towards the analysis of the scattering and radiating features of cavity-backed patch antennas and reflect-arrays in presence of multilayered metamaterial loads.Publication Multifrequency Optical Invisibility Cloak with Layered Plasmonic Shells(2008-03-21) Alù, Andrea; Engheta, NaderHere, we theoretically suggest the possibility of employing a multilayered plasmonic shell as a cloak for reducing the total scattering cross section of a particle, simultaneously at different frequencies in the optical domain. By exploiting the frequency dispersion of plasmonic materials and their inherent negative polarizability, it is shown, theoretically and with numerical simulations, how covering a dielectric or conducting object of a certain size with this multilayered cloak may reduce its "visibility" by several orders of magnitude simultaneously at multiple frequencies.Publication Mono-Modal Waveguides Filled with a Pair of Parallel Epsilon-Negative (ENG) and Mu-Negative (MNG) Metamaterial Layers(2003-06-08) Alù, Andrea; Engheta, NaderHere we analyze guided wave propagation in a parallel-plate waveguide filled with a pair of parallel lossless slabs; one possessing negative real permittivity but positive real permeability, and the other with negative real permeability and positive real permittivity, in the range of frequency of interest. It is shown that such a waveguide can support only a single propagating mode, essentially independent of the total thickness of this structure. Furthermore, this waveguide can still possess a propagating mode even when its thickness is very small. Field distribution and dispersion relations in such a mono-modal waveguide are obtained and discussed with physical insights and intuitive description for the mathematical findings.Publication Optical nanoswitch: an engineered plasmonic nanoparticle with extreme parameters and giant anisotropy(2009-01-20) Alù, Andrea; Engheta, NaderNaturally available optical materials are known to provide a wide variety of electric responses, spanning from positive to negative permittivity values. In contrast, owing to drastically modified conduction properties at the microscopic level, at such high frequencies magnetism and conductivity are very challenging to realize. This implies that extreme (high or low) values of permittivity, although highly desirable for a wide range of optical applications, are difficult to realize in practice. Here, we suggest the design of an engineered resonant nanoparticle composed of two conjoined hemispheres, whose optical response may be changed at will from an ideal electric conductor to an ideal magnetic conductor. Near the nanoparticle internal resonant frequency, we derive a closed-form solution that describes the electromagnetic response of this nanoparticle, showing how its light interaction may become dramatically dependent on the local field polarization, passing through all possible impedance values (from zero to infinity) by a simple mechanical or polarization rotation. Considering realistic frequency dispersion and loss in optical materials, we further show that these concepts may be applied to different geometries, with possibility for future experimental feasibility. We forecast various applications of this geometry as an optical nanoswitch, a novel nanocircuit element and as a building block for novel optical metamaterials.Publication Effects of size and frequency dispersion in plasmonic cloaking(2008-10-01) Alù, Andrea; Engheta, NaderThe plasmonic venue to realize invisibility and cloaking [A. Alù and N. Engheta, Phys. Rev. E 72, 016623 (2005)] is analyzed here in terms of its limitations and its frequency dispersion relative to the cloak size. Intrinsic limits due to causality and comparison with transformation-based cloaking techniques are discussed and analyzed. An interestingly simple low-dispersion cloak is also suggested for background materials with larger refractive index. These results may shed light on this scattering cancellation phenomenon, suggesting potential applications in scattering reduction and noninvasive probing.Publication Transmission-line analysis of ε-near-zero–filled narrow channels(2008-07-01) Alù, Andrea; Engheta, Nader; Silveirinha, Mário GFollowing our recent interest in metamaterial-based devices supporting resonant tunneling, energy squeezing, and supercoupling through narrow waveguide channels and bends, here we analyze the fundamental physical mechanisms behind this phenomenon using a transmission-line model. These theoretical findings extend our theory, allowing us to take fully into account frequency dispersion and losses and revealing the substantial differences between this unique tunneling phenomenon and higher-frequency Fabry-Perot resonances. Moreover, they represent the foundations for other possibilities to realize tunneling through arbitrary waveguide bends, both in E and H planes of polarization, waveguide connections, and sharp abruptions and to obtain analogous effects with geometries arguably simpler to realize.Publication Subwavelength Planar Leaky-Wave Components with Metamaterial Bilayers(2007-03-01) Alù, Andrea; Bilotti, Filiberto; Engheta, Nader; Vegni, LucioThe potential use of metamaterial planar bilayers for synthesizing compact subwavelength leaky-wave radiators in the microwave regime is analyzed in detail. In particular, the possibility of pairing "complementary" metamaterials in order to reduce the dimensions of microwave components is explored for the leaky-wave operation of an open waveguide consisting of a grounded pair of planar layers. In connection with our similar findings in other setups employing such complementary pairings, here we show how the compact resonance at the interface between "negative" and "positive" materials may also be properly exploited in this context. Choosing materials with low constitutive parameters, moreover, shows to be effective for enhancing the directivity of these components. We explore in detail the notable guidance and radiation properties of the anomalous natural modes supported by these bilayered structures, giving some physical insights into the anomalous phenomenon and considering the possible limitations in some realistic setups.Publication Dynamical theory of artificial optical magnetism produced by rings of plasmonic nanoparticles(2008-08-01) Alù, Andrea; Engheta, NaderWe present a detailed analytical theory for the plasmonic nanoring configuration first proposed by Alù et al. [Opt. Express 14, 1557 (2006)], which is shown to provide negative magnetic permeability and negative index of refraction at infrared and optical frequencies. We show analytically how the nanoring configuration may provide superior performance when compared to some other solutions for optical negative-index materials, offering a more "pure" magnetic response at these high frequencies, which is necessary for lowering the effects of radiation losses and absorption. Sensitivity to losses and the bandwidth of operation of this magnetic inclusion are also investigated in detail and compared with other available setups.Publication Mode Excitation by a Line Source in a Parallel-Plate Waveguide Filled with a Pair of Parallel Double-Negative and Double-Positive Slabs(2003-06-22) Alù, Andrea; Engheta, NaderIn this paper, we investigate theoretically the excitation of modes by a line source, as well as their modal structure and dispersion, in a parallel-plate waveguide that is filled with a pair of parallel slabs; one being a lossless "double-negative (DNG)" material and the other being a lossless conventional "double-positive (DPS)" medium. Previously, we have shown that such "conjugate" pairing of DNG and DPS materials may lead to reduction of size in one-dimensional cavity resonators and waveguides. The analysis presented here describes the proper modes in such a 1-D parallel plate waveguide, and reveals the possibility of no cut-off thickness for this class of parallel-plate waveguides, which implies that a proper TE or TM mode may always be excited in such a waveguide independent of the overall thickness of the waveguide. The excitation of guided modes by a line source and their power-flow peculiarities are discussed, and the analogy to the concept of "open 1-D cavity" in such a waveguide is mentioned. Some of the interesting features and physical insights regarding the power flow in these guided modes will be presented.