Copyright (c) 2009 All rights reserved. http://works.bepress.com/nader_engheta Recent documents in Nader Engheta en-us Sun, 31 May 2009 09:37:57 PDT 3600 http://works.bepress.com/nader_engheta/101 http://works.bepress.com/nader_engheta/101 Wed, 17 Dec 2008 11:47:11 PST The plasmonic venue to realize invisibility and cloaking [A. Alů 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. Andrea Alů http://works.bepress.com/nader_engheta/100 http://works.bepress.com/nader_engheta/100 Wed, 17 Dec 2008 11:47:05 PST Inspired 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. Andrea Alů http://works.bepress.com/nader_engheta/99 http://works.bepress.com/nader_engheta/99 Tue, 09 Dec 2008 13:08:42 PST The anomalous interaction of light with plasmonic materials has fascinated scientists and non-scientists for centuries. The recent interest in research on anomalous resonant phenomena involving plasmonic nanoparticles may be associated with the relevant advancements in nanotechnology of the last decade, which allow realization of artificial materials with a tailored anomalous electromagnetic response, with large design flexibility. We have recently proposed several novel applications that involve plasmonic nanoparticles, employed as nanoantennas, nanocircuit elements or nanowaveguides. In particular, in the framework of our recent paradigm for scaling the circuit concepts to optical frequencies [1], we have suggested that the combination of plasmonic and non-plasmonic nanoparticles may give rise to a novel type of circuitry at frequencies (IR and optical) at which metals lose their highly conductive properties. In this context, we have proposed the synthesis and design of nanotransmission-lines [2-3] and backward-wave nanomaterials [4], the optical equivalent of connecting "shorting" wires [5], combinations of parallel and series nanocircuit elements [6-7] and basic collections of nanofilters [8]. Andrea Alů http://works.bepress.com/nader_engheta/98 http://works.bepress.com/nader_engheta/98 Fri, 21 Nov 2008 09:59:03 PST Artificial materials, metamaterials and plasmonic media have recently received tremendous attention from the scientific communities, media and general public, following novel ideas and suggestions for their potential use in a variety of applications such as cloaking. Here we briefly review and highlight some of the available solutions for invisibility and cloaking that employ metamaterials and plasmonic materials at various frequencies. We briefly overview some of the different cloaking mechanisms recently proposed in the literature, such as plasmonic cloaking based on scattering cancellation, coordinate-transformation cloaking and anomalous localized resonances for cloaking, in particular providing some details for scattering-cancellation-based plasmonic cloaking. We mention the main analogies and differences among these various approaches, and we discuss some possible ideas for realizations and applications of these results, with particular attention to the physical phenomena involved. Andrea Alů http://works.bepress.com/nader_engheta/97 http://works.bepress.com/nader_engheta/97 Mon, 27 Oct 2008 11:44:52 PDT In 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. Mario G. Silveirinha http://works.bepress.com/nader_engheta/96 http://works.bepress.com/nader_engheta/96 Mon, 27 Oct 2008 11:44:46 PDT We present a detailed analytical theory for the plasmonic nanoring configuration first proposed by Alů 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. Andrea Alů http://works.bepress.com/nader_engheta/95 http://works.bepress.com/nader_engheta/95 Thu, 18 Sep 2008 08:44:54 PDT Here we explore the radiation features of optical nanoantennas, analyzing the concepts of optical input impedance, optical radiation resistance, impedance matching, and loading of plasmonic nanodipoles. We discuss how the concept of antenna impedance may be applied to optical frequencies and how its quantity may be properly defined and evaluated. We exploit these concepts in the optimization of nanoantenna loading by optical nanocircuit elements, extending classic concepts of radio-frequency antenna theory to the visible regime for the proper design and matching of plasmonic nanoantennas. Andrea Alů http://works.bepress.com/nader_engheta/94 http://works.bepress.com/nader_engheta/94 Thu, 04 Sep 2008 11:55:02 PDT Connecting 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. Andrea Alů http://works.bepress.com/nader_engheta/93 http://works.bepress.com/nader_engheta/93 Thu, 04 Sep 2008 11:54:56 PDT Here, 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. Andrea Alů http://works.bepress.com/nader_engheta/92 http://works.bepress.com/nader_engheta/92 Thu, 04 Sep 2008 11:54:51 PDT Following our recent idea of using plasmonic and nonplasmonic nanoparticles as nanoinductors and nanocapacitors in the infrared and optical domains [N. Engheta et al., Phys. Rev. Letts. 95, 095504 (2005)], in this work we analyze in detail some complex circuit configurations involving series and parallel combinations of these lumped nanocircuit elements at optical frequencies. Using numerical simulations, it is demonstrated that, after a proper design, the behavior of these nanoelements may closely mimic that of their lower-frequency [i.e., radio frequency (rf) and microwave] counterparts, even in relatively complex configurations. In addition, we analyze here in detail the concepts of nanoinsulators and nanoconnectors in the optical domain, demonstrating how these components may be crucial in minimizing the coupling between adjacent optical nanocircuit elements and in properly connecting different branches of the nanocircuit. The unit nanomodules for lumped nanoelements are introduced as building blocks for more complex nanocircuits at optical frequencies. Numerical simulations of some complex circuit scenarios considering the frequency response of these nanocircuits are presented and discussed in detail, showing how practical applications of such optical nanocircuit concepts may indeed be feasible within the current limits of nanotechnology. Mário G. Silveirinha