Engheta, Nader
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Publication Peano High Impedance Surfaces(2005-12-01) McVay, John; Hoorfar, Ahmad; Engheta, NaderFollowing our previous work on metamaterial high-impedance surfaces made of Hilbert curve inclusions, here we theoretically explore the performance of the high-impedance surfaces made of another form of space-filling curve known as the Peano curve. This metamaterial surface, formed by a 2-D periodic arrangement of Peano curve inclusions, is located above a conducting ground plane and is shown to exhibit a high surface impedance surface at certain specific frequencies. Our numerical study reveals the effect of the iteration order of the Peano curve, the surface height above the conducting ground plane and the separation distance between adjacent inclusions.Publication Cloaking a Sensor(2009-06-08) Alù, Andrea; Engheta, NaderWe propose the general concept of cloaking a sensor without affecting its capability to receive, measure, and observe an incoming signal. This may be obtained by using a plasmonic sensor, based on cloaking, made of materials available in nature at infrared and optical frequencies, or realizable as a metamaterial at lower frequencies. The result is a sensing system that may receive and transmit information, while its presence is not perceived by the surrounding, which may be of fundamental importance in a wide range of biological, optics, physics, and engineering applications.Publication Phase and Amplitude of Fractional-Order Intermediate Wave(1999-06-05) Engheta, NaderThe behavior of the amplitude and phase of the "intermediate wave", which we previously introduced as certain fractional solutions to the standard scalar Helmholtz equation, is addressed and presented. These waves effectively behave as intermediate cases between the canonical cases of the plane-wave and cylindrical wave propagation. We show that the amplitude and phase of such intermediate wave undergo interesting "evolutions" as the fractionalization parameter ν attains fractional values between zero and unity. Possible extension into the novel concept of intermediate guided-wave geometries is just speculated.Publication Physical Insight Into the “Growing” Evanescent Fields of Double-Negative Metamaterial Lenses Using Their Circuit Equivalence(2006-01-01) Alù, Andrea; Engheta, NaderPendry in his paper, “Negative refraction makes a perfect lens” (Phys. Rev. Lett., vol. 85, no. 18, pp. 3966–3969, 2000) put forward an idea for a lens made of a lossless metamaterial slab with n = -1, that may provide focusing with resolution beyond the conventional limit. In his analysis, the evanescent wave inside such a lossless double-negative (DNG) slab is “growing,” and thus it “compensates” the decaying exponential outside of it, providing the subwavelength lensing properties of this system. Here, we examine this debated issue of “growing exponential” from an equivalent circuit viewpoint by analyzing a set of distributed-circuit elements representing evanescent wave interaction with a lossless slab of DNG medium. Our analysis shows that, under certain conditions, the current in series elements and the voltage at the element nodes may attain the dominant increasing due to the suitable resonance of the lossless circuit, providing an alternative physical explanation for “growing exponential” in Pendry’s lens and similar subwavelength imaging systems.Publication Sampling and Squeezing Electromagnetic Waves through Subwavelength Ultranarrow Regions or Openings(2012-02-21) Silveirinha, Mário G.; Engheta, NaderHere, we investigate the physical mechanisms that may enable squeezing a complex electromagnetic field distribution through a narrow and/or partially obstructed region with little amplitude and phase distortions. Following our recent works, such field manipulations may be made possible by a procedure in which the incoming wave is first “sampled” “pixel by pixel” using an array of metallic waveguides, and in a second step the energy corresponding to each individual pixel is “squeezed” through a very narrow channel filled with a permittivity-near-zero material. In this work, we study in detail these processes in scenarios in which the electromagnetic wave is compressed along a single direction of space and present theoretical models that enable the analytical modeling of such phenomena. Full-wave results obtained with an electromagnetic simulator, demonstrate the possibility of compressing an incoming wave several folds through ultranarrow channels filled with silicon carbide. The “sampling and squeezing” concept may enable unparalleled control of electromagnetic waves in the nanoscale.Publication Effective Medium Approach to Electron Waves: Graphene Superlattices(2012-05-07) Silveirinha, Mário G; Engheta, NaderWe develop an effectivemedium approach to characterize the propagation of matterwaves in periodic structures, such as graphene or semiconductor superlattices. It is proven that the time evolution of the states that are not more localized in space than the characteristic period of the structure can be described exactly through an effective Hamiltonian, and that the electronic band structure of the system can be exactly determined from the effective Hamiltonian. As an illustration of the application of the method, we characterize the mesoscopic response of graphene superlattices. It is shown that these structures may be described using simply two effective parameters: a dispersive potential, and an anisotropy tensor that characterizes the pseudospin. Our model predicts that a graphene superlattice characterized by an indefinite anisotropy tensor—such that the eigenvalues of the tensor have opposite signs—may permit the perfect tunneling of all the stationary states with a specific value of the energy when it is paired with a dual graphene superlattice with a positive definite anisotropy tensor.Publication Avoiding Metallic Walls: Use of Modal Superposition in Plasmonic Waveguides to Reduce Propagation Loss(2012-03-16) Rodríguez-Fortuño, Francisco J.; Engheta, NaderWe theoretically explore the possibility of reducing propagation loss in a metal-insulator-metal (MIM) waveguide, using mode combinations to achieve wall-avoiding field distributions along a certain propagation length. We present analytical results for several waveguides showing notable loss reduction, and we discuss the trade-offs between low loss and high confinement present in this technique.Publication Optical spectrometer at the nanoscale using optical Yagi-Uda nanoantennas(2009-05-05) Salandrino, Alessandro; Li, Jingjing; Engheta, NaderHere we present and analyze an optical spectrum analyzer at the nanometer scale that is able to distribute different frequency contents of the radiation of an optical dipole source into different directions in the space. The spectrum analyzer is composed of arrays of optical Yagi-Uda nanoantennas, forming relatively narrow radiation patterns operating at different frequencies. The optical Yagi-Uda nanoantennas composed of plasmonic core-shell nanoparticles are used as an example of building blocks for this idea in our study. Narrow radiation beams in such antenna arrays are realized by tailoring the scattering phase of the nanoparticles. The sensitivity of such an antenna array to the operating wavelength and the angular distribution of the radiation pattern, which is essential for the operation of the spectrum analyzer proposed here, is studied theoretically. The chromatic dispersion and the angular variation of the radiation pattern of such an optical spectrum analyzer are discussed in detail.Publication Electromagnetic wave propagation in the wire medium: a complex medium with long thin inclusions(2001-09-01) Moses, Charles Anthony; Engheta, NaderThe wire medium is a type of complex artificial material we conceptually envision as many identical finite-length, parallel, thin wire inclusions embedded within a host medium. It is representative of a class of novel artificial materials characterized by long thin inclusions. Unlike some conventional artificial material, the inclusions of this class are not necessarily electrically short. Here, we present our theoretical analysis for wire media and by studying certain salient features of plane-wave propagation through these media, introduce equivalent medium parameters that depend, among other parameters, on the direction of wave propagation. The approach we use separates the artificial material into its elementary planes and then uses periodic moment method techniques to individually characterize each elementary plane. Analytic formulas from periodic structure theory are then used to determine the effective wavenumber for the overall medium and the transverse impedance at the midpoint between adjacent elementary planes. Our examples show that some realizations of these media are spatially dispersive and may exhibit interesting features such as "angular windows of propagation" and other properties that are dependent on the polarization, frequency and direction of wave propagation.Publication A Positive Future for Double-Negative Metamaterials(2005-04-01) Engheta, Nader; Ziolkowski, Richard WMetamaterials (MTMs), which are formed by embedding inclusions and material components in host media to achieve composite media that may be engineered to have qualitatively new physically realizable response functions that do not occur or may not be easily available in nature, have raised a great deal of interest in recent years. In this paper, we highlight a large variety of the physical effects associated with double- and single-negative MTMs and some of their very interesting potential applications. The potential ability to engineer materials with desired electric and magnetic properties to achieve unusual physical effects offers a great deal of excitement and promise to the scientific and engineering community. While some of the applications we will discuss have already come to fruition, there are many more yet to be explored.