Alù, Andrea
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Publication Theory and potentials of multi-layered plasmonic covers for multi-frequency cloaking(2008-11-27) Alù, Andrea; Engheta, NaderWe have recently suggested that suitably designed plasmonic layers may cloak a given object simultaneously at multiple frequencies (Alù and Engheta 2008 Phys. Rev. Lett. 100 113901). Here, we extend our theory and fully analyze this possibility, highlighting the potentials of this plasmonic cloaking technique and its fundamental limitations dictated by the passivity and causality of the materials involved. The cloaking mechanism relies on the scattering cancellation properties of plasmonic materials. By exploiting their inherent frequency dispersion, it is possible to reduce the 'visibility' of a given object by several orders of magnitude simultaneously at multiple frequencies, such that any of the particular layers composing the cloak is responsible for noticeable reduction of scattering at each frequency of operation.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 Metamaterial Covers Over a Small Aperture(2006-06-01) Alù, Andrea; Engheta, Nader; Bilotti, Filiberto; Vegni, LucioRecently, there has been an increased interest in the problem of wave transmission through sub-wavelength apertures, following successful experimental demonstration by several groups for enhancing optical power transmission through nano-scale holes in metallic screens due to properly designed periodic corrugation. Oliner, Jackson, and their co-workers explained and justified this phenomenon as the result of the excitation of the leaky waves supported by the corrugated screen. Here we discuss in detail the mechanism and analysis for another setup we have recently proposed, in which metamaterial layers with special parameters may be utilized as covers over a single sub-wavelength aperture in a perfectly electric conducting (PEC) flat screen in order to increase the wave transmission through this aperture, and we provide a detailed physical insights and analytical explanation for this aperture setup that may lead to similar, potentially even more pronounced effects when the proper metamaterial layers are used in the entrance and the exit face of the hole in the flat PEC screen with no corrugation. Some numerical results confirming this theory are presented and discussed. We also investigate the sensitivity of the transmission enhancement to the geometrical and electromagnetic parameters of this structure.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 Cloaking mechanism with antiphase plasmonic satellites(2008-11-12) Alù, Andrea; Silveirinha, Mário G; Engheta, NaderIn this work we theoretically demonstrate the possibility of cloaking a given object by surrounding it with a finite collection of suitably dimensioned, discrete "antiphase" plasmonic scatterers. It is shown that the total scattering from the object may approximately be canceled by the currents induced on a finite number of the plasmonic "satellite" scatterers, effectively making the whole system invisible to an external observer. Unlike other approaches, the proposed solution allows one to cloak a given object in a noninvasive manner since the antiphase satellites, being finite in number, do not need to fully cover and be in direct contact with the cloaked object.Publication Anomalous Mode Coupling in Guided-Wave Structures Containing Metamaterials with Negative Permittivity and Permeability(2002-08-26) Alù, Andrea; Engheta, NaderThe peculiar characteristics of the mode coupling between standard dielectric waveguides and waveguides formed by a material with negative index of refraction are presented in this talk. Some of our theoretical results, describing the anti-directional (antiparallel) nature of such coupling, are shown for the case of the two-parallel-slab geometry.Publication Robustness in Design and Background Variations in Metamaterial/Plasmonic Cloaking(2008-05-17) Alù, Andrea; Engheta, NaderHere we discuss our recent numerical results concerning the robustness of the scattering cancellation effect produced by a plasmonic cloak with near-zero permittivity and correspondingly negative polarizability. Being based on an integral effect and on an intrinsically non-resonant phenomenon, we show how variations of the geometrical parameters of the design and changes in the background do not sensibly affect the invisibility properties of the plasmonic/metamaterial cloak. Design examples are presented and discussed in order to highlight this robustness and to provide some insights into this cloaking phenomenon, different from other cloaking techniques recently presented in the literature.Publication Coupling of optical lumped nanocircuit elements and effects of substrates(2007-10-17) Alù, Andrea; Engheta, Nader; Salandrino, AlessandroWe present here an analytical quasi-static circuit model for the coupling among small nanoparticles excited by an optical electric field in the framework of the optical lumped nanocircuit theory [N. Engheta, A. Salandrino, and A. Alù, Phys. Rev. Lett. 95, 095504 (2005)]. We derive how coupling effects may affect the corresponding nanocircuit model by adding lumped controlled sources that depend on the optical voltages applied on the coupled particles as coupled lumped elements. With the same technique, we may model the presence of a substrate located underneath the nanocircuit elements, relating its presence to the coupling with a properly modeled image nanoparticle. These results are of importance in the understanding and the design of complex optical nanocircuits at infrared and optical frequencies.Publication Nanoinsulators and nanoconnectors for optical nanocircuits(2008-03-21) Alù, Andrea; Silveirinha, Mário G; Engheta, Nader; Li, JingjingFollowing 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.Publication Input Impedance, Nanocircuit Loading, and Radiation Tuning of Optical Nanoantennas(2008-07-25) Alù, Andrea; Engheta, NaderHere 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.