Alù, Andrea
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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 'Shorting Wires'(2007-10-17) Alù, Andrea; Engheta, NaderConnecting lumped circuit elements in a conventional circuit is usually accomplished by conducting wires that act as conduits for the conduction currents with negligible potential drops. More challenging, however, is to extend these concepts to optical nanocircuit elements. Here, following our recent development of optical lumped circuit elements, we show how a special class of nanowaveguides formed by a thin core with relatively large (positive or negative) permittivity surrounded by a thin concentric shell with low permittivity may provide the required analogy to 'wires' for optical nano-circuits.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 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 Evanescent Growth and Tunneling Through Stacks of Frequency-Selective Surfaces(2005-12-05) Alù, Andrea; Engheta, NaderThe presence of wave tunneling and the “growing evanescent envelope” for field distributions in suitably designed, periodically layered stacks of frequency selective surfaces (FSS) is discussed in this letter. It is known that a slab of double-negative (DNG) or left-handed metamaterial sandwiched between two vacuum half-spaces may lead to “growing” evanescent plane waves, which may be used to restore sub-wavelength information in Pendry’s “perfect” lens (Phys. Rev. Lett., vol. 85, pg. 3966, 2000). In this letter, it is shown that a completely different setup allows an analogous buildup of evanescently modulated waves. In particular, it is shown how an interface resonance phenomenon similar to the one present at the interface between metamaterials with oppositely signed constitutive parameters may be induced by a proper choice of the periodicities of the FSS stacks and the geometrical properties of these surfaces. The analysis is performed through an equivalent transmission-line approach, and some physical insights into this phenomenon are presented. Salient features, such as the complete wave tunneling through the pair of cascaded FSS, each operating at its bandgap, are presented and discussed.Publication Hertzian plasmonic nanodimer as an efficient optical nanoantenna(2008-11-01) Alù, Andrea; Engheta, NaderInspired by the geometry and shape of the classical radio-frequency radiator, the Hertzian dipole, here we analyze the design of a plasmonic optical dimer nanoantenna. We show how it may be possible to operate a pair of closely spaced spherical nanoparticles as an efficient optical nanoradiator, and how its tuning and matching properties may be tailored with great degree of freedom by designing suitable nanoloads placed at the dimer gap. In this sense, we successfully apply nanocircuit concepts to model the loading nanoparticles. High levels of optical radiation efficiency are achieved, even considering the realistic absorption of optical metals, thanks to this specific geometry and design.Publication Circuit Elements at Optical Frequencies: Nanoinductors, Nanocapacitors, and Nanoresistors(2005-08-26) Engheta, Nader; Salandrino, Alessandro; Alù, AndreaWe present the concept of circuit nanoelements in the optical domain using plasmonic and nonplasmonic nanoparticles. Three basic circuit elements, i.e., nanoinductors, nanocapacitors, and nanoresistors, are discussed in terms of small nanostructures with different material properties. Coupled nanoscale circuits and parallel and series combinations are also envisioned, which may provide road maps for the synthesis of more complex circuits in the IR and visible bands. Ideas for the optical implementation of right-handed and left-handed nanotransmission lines are also forecasted.Publication Higher-Order Resonant Power Flow Inside and Around Superdirective Plasmonic Nanoparticles(2007-10-01) Alù, Andrea; Engheta, NaderThe optical power flow around a plasmonic particle has been a topic of research interest over the years [see e.g., C. F. Bohren, Am. J. Phys. 51, 323 (1983); V. A. Bashevoy, V. A. Federov, and N. I. Zheludev, Optics Express, 13, 8372 (2005)]. Here we revisit this problem with an emphasis on higher-order resonances, and we present the theoretical results of our analysis for such power flow distribution for plasmonic nanoparticles at their multipolar resonances. Results for the second and third orders of resonances show optical power flow patterns that are significantly different from that of the first-order resonance inside and around plasmonic superdirective nanoparticles, with multi-center vortices, saddle points and saddle lines, and with an anomalous circulation of power resembling higher-order modes in a resonant cavity. A potential application of these optical flow patterns to trap or move a neighboring nanoparticle is also briefly suggested.Publication Infrared and optical invisibility cloak with plasmonic implants based on scattering cancellation(2008-08-01) Alù, Andrea; Silveirinha, Mário G; Engheta, NaderIn recent works, we have suggested that plasmonic covers may provide an interesting cloaking effect, dramatically reducing the overall visibility and scattering of a given object. While materials with the required properties may be directly available in nature at some specific infrared or optical frequencies, this is not necessarily the case for any given design frequency of interest. Here we discuss how such plasmonic covers may be specifically designed as metamaterials at terahertz, infrared, and optical frequencies using naturally available metals. Using full-wave simulations, we demonstrate that the response of a cover formed by metallic plasmonic implants may be tailored at will so that at a given frequency, it possesses the plasmonic-type properties required for cloaking applications.Publication Peculiar Radar Cross-Section Properties of Double-Negative and Single-Negative Metamaterials(2004-04-26) Alù, Andrea; Engheta, NaderHere, we give an overview of some of the results of our analysis of anomalous scattering phenomena for structures involving metamaterial layers, and we provide some physical insights and ideas for potential applications.