Engheta, Nader
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Publication Single-Negative, Double-Negative, and Low-index Metamaterials and their Electromagnetic Applications(2007-02-01) Alù, Andrea; Engheta, Nader; Erentok, A.; Ziolkowski, Richard WMetamaterials that are engineered media characterized by electromagnetic constitutive parameters with anomalous values may show counterintuitive properties in their interactions with electromagnetic waves. Here, we review some of the properties and potential applications we have recently presented in the technical literature: properties and applications in which plasmonic materials and metamaterials may be utilized to overcome some conventional physical limits. Resonances arising in electrically small regions of interface where these materials are paired with common materials are shown to be potentially attractive for this purpose in some electromagnetic problems, for instance, in guiding and radiating structures. The anomalous refractive properties at such "complementary" interfaces and the negative values of polarizability attainable in such materials are also shown to offer potentials for several applications.Publication Guided Propogation Along Quadrupolar Chains of Plasmonic Nanoparticles(2009-06-12) Alù, Andrea; Engheta, NaderThe employment of periodic arrays of plasmonic nanoparticles has been proposed by several groups for enhanced transmission or absorption and for realizing optical nanowaveguides. Generally, due to their small transverse dimensions, such linear arrays have been operated near their dipolar resonance. However, it has been recently shown that nanoscale plasmonic particles may also support higher-order resonances, which provide some advantages in different applications. Here we derive a full-wave analytical closed-form dispersion equation for the guided and leaky modes supported by linear chains of nanoparticles near a quadrupolar resonance. We show that, despite the vanishing bandwidth of the individual quadrupolar resonance in each of the nanoparticles composing the chain, the overall bandwidth of quadrupolar chain guidance is relatively large due to strong coupling, even considering realistic losses and frequency dispersion of optical materials. Applications for low-damping optical nanotransmission lines and leaky-wave nanoantennas are suggested.Publication On the Near-Zone Inverse Doppler Effect(1980-07-01) Engheta, Nader; Mickelson, Alan R.; Papas, Charles H.Attention is invited to the recently discovered inverse Doppler effect which occurs in the near-zone field of an antenna emitting a continuous wave. On approaching the antenna, the received signal is blue-shifted in the far zone and then red-shifted in the near zone; and on receding from the antenna, the received signal is blue-shifted in the near zone and then red-shifted in the far zone. Calculations are presented for the case where the antenna is a simple dipole. It is shown that this effect gives not only the vector velocity of the moving receiver but also its range, i.e., its distance from the antenna.Publication Adaptive Algorithms for 2–Channel Polarization Sensing under Various Polarization Statistics with Non-Uniform Distributions(2006-08-01) Yemelyanov, Konstantin M; Pugh, Edward N; Lin, Shih-Schön; Engheta, NaderThe polarization of light carries much useful information about the environment. Biological studies have shown that some animal species use polarization information for navigation and other purposes. It has been previously shown that a bio-inspired Polarization Difference Imaging technique can facilitate detection and feature extraction of targets in scattering media. It has also been established by S. Tyo1 that "Polarization Sum" and "Polarization Difference" are the optimum pair of linear combinations of images taken through two orthogonally oriented linear polarizers of a scene having a uniform distribution of polarization directions. However, in many real environments the scene has a non-uniform distribution of polarization directions. Using principal component analysis of the polarization statistics of the scene, here we develop a method to determine the two optimum information channels with unequal weighting coefficients that can be formed as linear combinations of the images of a scene taken through a pair of linear polarizers not constrained to the horizontal and vertical directions of the scene We determine the optimal orientations of linear polarization filters that enhance separation of a target from the background, where the target is defined as an area with distinct polarization characteristics as compared to the background. Experimental results confirm that in most situations adaptive polarization difference imaging outperforms "conventional" polarization difference imaging with fixed channels.Publication Experimental Verification of n = 0 Structures for Visible Light(2013-01-02) Vesseur, Ernst Jan R; Coenen, Toon; Caglayan, Humeyra; Engheta, Nader; Polman, AlbertWe fabricate and characterize a metal-dielectric nanostructure with an effective refractive index n=0 in the visible spectral range. Light is excited in the material at deep subwavelength resolution by a 30-keV electron beam. From the measured spatially and angle-resolved emission patterns, a vanishing phase advance, corresponding to an effective ϵ=0 and n=0, is directly observed at the cutoff frequency. The wavelength at which this condition is observed can be tuned over the entire visible or near-infrared spectral range by varying the waveguide width. This n=0 plasmonic nanostructure may serve as a new building block in nanoscale optical integrated circuits and to control spontaneous emission as experimentally demonstrated by the strongly enhanced radiative optical density of states over the entire n=0 structure.Publication Epsilon-Near-Zero (ENZ) Metamaterials and Electromagnetic Sources: Tailoring the Radiation Phase Pattern(2007-04-15) Alù, Andrea; Silveirinha, Mário G; Salandrino, Alessandro; Engheta, NaderIn this work, we investigate the response of epsilon-near-zero (ENZ) metamaterials and plasmonic materials to electromagnetic source excitation. The use of these media for tailoring the phase of radiation pattern of arbitrary sources is proposed and analyzed numerically and analytically for some canonical geometries. In particular, the possibility of employing planar layers, cylindrical shells or other more complex shapes made of such materials in order to isolate two regions of space and to tailor the phase pattern in one region, fairly independent of the excitation shape present in the other, is demonstrated with theoretical arguments and some numerical examples. Physical insights into the phenomenon are also presented and discussed together with potential applications of the phenomenon.Publication Polarizabilities and effective parameters for collections of spherical nanoparticles formed by pairs of concentric double-negative, single-negative, and/or double-positive metamaterial layers(2005-05-01) Alù, Andrea; Engheta, NaderUnusual scattering effects from tiny spherical particles may be obtained when concentric shells are designed by pairing together "complementary" double-negative, single-negative, and/or standard double-positive materials. By embedding these highly polarizable scatterers in a host medium one can achieve a bulk medium with interesting effective parameters. Some physical insights and justifications for the anomalous polarizability of these concentric spherical nanoparticles and the effective parameters of the bulk composite medium are discussed.Publication Electrostatic "Fractional" Image Methods for Perfectly Conducting Wedges and Cones(1996-12-01) Engheta, NaderIn our earlier work, we introduced a definition for the electric charge "fractional-order" multipoles using the concept of fractional derivatives and integrals [l]. Here, we utilize that definition to introduce a detailed image theory for the two-dimensional (2-D) electrostatic potential distributions in front of a perfectly conducting wedge with arbitrary wedge angles, and for the three-dimensional potential in front of a perfectly conducting cone with arbitrary cone angles. We show that the potentials in the presence of these structures can be described equivalently as the electrostatic potentials of sets of equivalent "image" charge distributions that effectively behave as "fractional-order" multipoles; hence, the name "fractional" image methods. The fractional orders of these so-called fractional images depend on the wedge angle (for the wedge problem) and on the cone angle (for the cone problem). Special cases where these fractional images behave like the discrete images are discussed, and physical justification and insights into these results are given.Publication The Fast Multipole Method (FMM) for Electromagnetic Scattering Problems(1992-06-01) Engheta, Nader; Murphy, William D.; Rokhlin, Vladimir; Vassiliou, Marius S.The fast multipole method (FMM) was developed by Rokhlin to solve acoustic scattering problems very efficiently. We have modified and adapted it to the second-kind-integral-equation formulation of electromagnetic scattering problems in two dimensions. The present implementation treats the exterior Dirichlet (TM) problem for two-dimensional closed conducting objects of arbitrary geometry. The FMM reduces the operation count for solving the second-kind integral equation (SKIE) from O(n3) for Gaussian elimination to O(n4/3) per conjugated-gradient iteration, where n is the number of sample points on the boundary of the scatterer. We also present a simple technique for accelerating convergence of the iterative method: "complexifying" k, the wavenumber. This has the effect of bounding the condition number of the discrete system; consequently, the operation count of the entire FMM (all iterations) becomes O(n4/3). We present computational results for moderate values of ka, where a is the characteristic size of the scatterer.Publication On Fractional Paradigm and Intermediate Zones in Electromagnetism: II. Cylindrical and Spherical Observations(1999-10-20) Engheta, NaderExtending our previous work for the planar case [1], in this Letter we present fractionalization of the kernels of integral transforms that link the field quantities over two coaxial cylindrical surfaces of observation for the two-dimensional (2-D) monochromatic wave propagation, and over two concentric spherical surfaces of observation for the three-dimensional (3-D) wave propagation. With the proper radial normalizations, we show that the fractionalized kernels, with fractionalization parameter ν that here could attain complex values between zero and unity, can effectively be regarded as the kernels of the integral transforms that provide the radially normalized field quantities over the coaxial cylindrical surfaces (for 2-D case) and over the concentric spherical surfaces (for 3-D case) between the two original surfaces. Like in the planar case [1], here the fractionalized kernels can supply another way of interpreting the fields in the intermediate zones.