Alù, Andrea
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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 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 Effects of size and frequency dispersion in plasmonic cloaking(2008-10-01) Alù, Andrea; Engheta, NaderThe plasmonic venue to realize invisibility and cloaking [A. Alù 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.Publication Transmission-line analysis of ε-near-zero–filled narrow channels(2008-07-01) Alù, Andrea; Engheta, Nader; Silveirinha, Mário GFollowing our recent interest in metamaterial-based devices supporting resonant tunneling, energy squeezing, and supercoupling through narrow waveguide channels and bends, here we analyze the fundamental physical mechanisms behind this phenomenon using a transmission-line model. These theoretical findings extend our theory, allowing us to take fully into account frequency dispersion and losses and revealing the substantial differences between this unique tunneling phenomenon and higher-frequency Fabry-Perot resonances. Moreover, they represent the foundations for other possibilities to realize tunneling through arbitrary waveguide bends, both in E and H planes of polarization, waveguide connections, and sharp abruptions and to obtain analogous effects with geometries arguably simpler to realize.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 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 Pairing an Epsilon-Negative Slab With a Mu-Negative Slab: Resonance, Tunneling and Transparency(2003-10-01) Alù, Andrea; Engheta, NaderHere, we analyze the transverse-magnetic (TM) wave interaction with a pair of slabs, one being an epsilon-negative (ENG) layer in which the real part of permittivity is assumed to be negative while its permeability has positive real part, and the other being a mu-negative (MNG) layer that has the real part of its permeability negative but its permittivity has positive real part. Although wave interaction with each slab by itself has predictable features, we show that the juxtaposition and pairing of such ENG and MNG slabs may, under certain conditions, lead to some unusual features, such as resonance, complete tunneling, zero reflection and transparency. The field distributions inside and outside such paired slabs are analyzed, and the Poynting vector distributions in such structures are studied. Using equivalent transmission-line models, we obtain the conditions for the resonance, complete tunneling and transparency, and we justify and explain the field behavior in these resonant paired structures. Salient features of the tunneling conditions, such as the roles of material parameters, slab thicknesses, dissipation, and angle of incidence are discussed. The analogy and correspondence between the ENG-MNG pair and the pair of a slab of conventional material juxtaposed with a “double-negative” medium is also discussed. Finally, a conceptual idea for a potential application of such a “matched” lossless ENG-MNG pair in “ideal” image displacement and image reconstruction is proposed.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 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 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.