Alù, Andrea
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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.Publication Enhanced Directivity From Subwavelength Infrared/Optical Nano-Antennas Loaded With Plasmonic Materials or Metamaterials(2007-11-01) Alù, Andrea; Engheta, NaderHere, we explore theoretically the concept of enhanced directivity from electrically small subwavelength radiators containing negative-parameter materials, such as plasmonic materials with negative permittivity at THz, infrared and optical frequencies. In particular, we study higher order plasmonic resonances of a subwavelength core-shell spherical nano-antenna, and we analyze the near-zone field distributions and far-field radiation patterns of such a structure when it is excited by a small dipole source, demonstrating analytically and numerically the possibility of having highly directive patterns from a nano-structure with electrically small dimensions. Radiation characteristics and intrinsic limitations on performance are analyzed in detail, and a potential application of this novel technique for super-resolution detection of the displacement of a nano-object is also pointed out.Publication Design of nanofilters for optical nanocircuits(2008-04-01) Alù, Andrea; Engheta, Nader; Young, Michael EWe theoretically and numerically study the design of optical "lumped" nanofiltering devices in the framework of our recently proposed paradigm for optical nanocircuits. In particular, we present a design of basic filtering elements, such as low-pass, pass-band, stop-band, and high-pass lumped nanofilters, for use in optical nanocircuits together with more complex designs, such as multizero or multipole nanofilters, to work at THz, infrared, and optical frequencies. Following the nanocircuit theory, we show how it is possible to design such complex frequency responses by applying simple rules, similar to those in rf circuit design, and we compare the frequency response of these optical nanofilters with classic filters in rf circuits. These findings introduce a theoretical foundation for the fabrication of nanofilters in optical lumped nanocircuit devices.Publication Plasmonic and Metamaterial Cloaking: Physical Mechanisms and Potentials(2008-09-01) Alù, Andrea; Engheta, NaderArtificial 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.Publication Cloaked Near-Field Scanning Optical Microscope Tip for Noninvasive Near-Field Imaging(2010-12-29) Alù, Andrea; Engheta, NaderNear-field imaging is a well-established technique in biomedical measurements, since closer to the detail of interest it is possible to resolve subwavelength details otherwise unresolved by regular lenses. A near-field scanning optical microscope (NSOM) tip may indeed overcome the resolution limits of far-field optics, but its proximity inherently perturbs the measurement. Here, we apply the recent concept of a ‘‘cloaked sensor’’ to an NSOM device in collection mode, showing theoretically how a proper plasmonic cover applied to an NSOM tip may drastically improve its overall measurement capabilities.Publication Sub-wavelength Focusing and Negative Refraction along Positive-Index and Negative-Index Plasmonic Nano-Transmission Lines and Nano-Layers(2005-07-03) Alù, Andrea; Engheta, NaderFollowing our recent works on the concept of plasmonic nano-inductors and nano-capacitors and related complex circuits, here we analyze the possibility of designing nano-transmission-lines (NTL) made of these basic nano-elements. We show that in the limit in which these basic circuit elements are very close to each other, they can be regarded as planar stacks of plasmonic and nonplasmonic slabs, which may be designed to act as forward (right-handed) or backward (lefthanded) NTL. Negative refraction and left-handed propagation are shown to be possible in these planar plasmonic configurations, potentially applicable in several innovative setups for subwavelength focusing, imaging and waveguiding applications.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 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 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 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.