arXiv subjects

Nader Engheta

Publications and source records attributed to Nader Engheta.

At least 91 records · Page 5Linked to original sources

arXiv2011

Fourier Optics on Graphene

Using numerical simulations, here we demonstrate that a single sheet of graphene with properly designed inhomogeneous, nonuniform conductivity distributions can act as a convex lens for focusing and collimating the transverse-magnetic (TM) surface Plasmon polariton (SPP) surface wave propagating along the graphene. Consequently, we show that the graphene can act as a platform capable of obtaining spatial Fourier transform of infra-red (IR) SPP signals. This may lead to rebirth of the field of Fourier Optics on a one-atom-thick structure.

physics.optics↗

arXiv2011

Electromagnetic tunneling of obliquely-incident waves through a single-negative slab paired with a double-positive uniaxial slab

We show that, under appropriate oblique-incidence and polarization conditions, the inherent opaqueness of a homogeneous, isotropic single-negative slab may be perfectly compensated (in the ideal lossless case) by a homogenous, anisotropic (uniaxial) double-positive slab, so that complete tunneling (with total transmission and zero phase delay) occurs. We present an analytical and numerical study aimed at deriving the basic design rules, elucidating the underlying physical mechanisms, and exploring the role of the various involved parameters.

physics.optics↗

arXiv2011

Analytical study of spherical cloak/anti-cloak interactions

The intriguing concept of "anti-cloaking" has been recently introduced within the framework of transformation optics (TO), first as a "countermeasure" to invisibility-cloaking (i.e., to restore the scattering response of a cloaked target), and more recently in connection with "sensor invisibility" (i.e., to strongly reduce the scattering response while maintaining the field-sensing capabilities). In this paper, we extend our previous studies, which were limited to a two-dimensional cylindrical scenario, to the three-dimensional spherical case. More specifically, via a generalized (coordinate-mapped) Mie-series approach, we derive a general analytical full-wave solution pertaining to plane-wave-excited configurations featuring a spherical object surrounded by a TO-based invisibility cloak coupled via a vacuum layer to an anti-cloak, and explore the various interactions of interest. With a number of selected examples, we illustrate the cloaking and field-restoring capabilities of various configurations, highlighting similarities and differences with respect to the cylindrical case, with special emphasis on sensor-cloaking scenarios and ideas for approximate implementations that require the use of double-positive media only.

physics.optics↗

arXiv2011

Electromagnetic tunneling through a single-negative slab paired with a double-positive bi-layer

We show that resonant tunneling of electromagnetic fields can occur through a three-layer structure composed of a single-negative (i.e., either negative-permittivity or negative-permeability) slab paired with bi-layer made of double-positive (i.e., positive permittivity and permeability) media. In particular, one of the two double-positive media can be chosen arbitrarily (even vacuum), while the other may exhibit extreme (either near-zero or very high) permittivity/permeability values. Our results on this counterintuitive tunneling phenomenon also demonstrate the possibility of synthesizing double-positive slabs that effectively exhibit single-negative-like wave-impedance properties within a moderately wide frequency range.

physics.optics↗

arXiv2011

One-Atom-Thick IR Metamaterials and Transformation Optics Using Graphene

Here we theoretically show, by designing and manipulating spatially inhomogeneous, non-uniform conductivity patterns across a single flake of graphene, that this single-atom-layered material can serve as a "one-atom-thick" platform for infrared metamaterials and transformation optical devices. It is known that by varying the chemical potential using gate electric and/or magnetic fields, the graphene conductivity in the THz and IR frequencies can be changed. This versatility provides the possibility that different "patches" on a single flake of graphene possess different conductivities, suggesting a mechanism to construct "single-atom-thick" IR metamaterials and transformation optical structures. Our computer simulation results pave the way for envisioning numerous IR photonic functions and metamaterial concepts-all on a "one-atom-thick" platform-of such we list a few here: edge waveguides, bent ribbon-like paths guiding light, photonic splitters and combiners, "one-atom-thick" IR scattering elements as building blocks for "flatland" metamaterials, thin strips as flatland superlenses, and "one-atom-thick" subwavelength IR lenses as tools for Fourier and transformation optics.

physics.optics↗

arXiv2010

Coupling and Guided Propagation along Parallel Chains of Plasmonic Nanoparticles

Here, extending our previous work on this topic, we derive a dynamic closed-form dispersion relation for a rigorous analysis of guided wave propagation along coupled parallel linear arrays of plasmonic nanoparticles, operating as optical 'two-line' waveguides. Compared to linear arrays of nanoparticles, our results suggest that these waveguides may support longer propagation lengths and more confined beams, operating analogously to transmission-line segments at lower frequencies. Our formulation fully takes into account the whole dynamic interaction among the infinite number of nanoparticles composing the parallel arrays, considering also realistic presence of losses and the frequency dispersion of the involved plasmonic materials, providing further physical insights into the guidance properties that characterize this geometry.

cond-mat.mes-hall↗

arXiv2010

Power scattering and absorption mediated by cloak/anti-cloak interactions: A transformation-optics route towards invisible sensors

The suggestive idea of "cloaking" an electromagnetic sensor, i.e., strongly reducing its visibility (scattering) while maintaining its field-sensing (absorption) capabilities, has recently been proposed in the literature, based on scattering-cancellation, Fano-resonance, or transformation-optics approaches. In this paper, we explore an alternative, transformation-optics-based route, which relies on the recently-introduced concept of "anti-cloaking." More specifically, our proposed approach relies on a suitable tailoring of the competing cloaking and anti-cloaking mechanisms, interacting in a two-dimensional cylindrical scenario. Via analytical and parametric studies, we illustrate the underlying phenomenology, identify the critical design parameters, and address the relevant optimality and tradeoff issues, taking also into account the effect of material losses. Our results confirm the envisaged potentials of the proposed transformation-optics approach as an attractive alternative route to sensor cloaking.

physics.optics↗

arXiv2010

Transformation-optics generalization of tunnelling effects in bi-layers made of paired pseudo-epsilon-negative/mu-negative media

Transformation-media designed by standard transformation-optics (TO) approaches, based on real-valued coordinate-mapping, cannot exhibit single-negative (SNG) character unless such character is already possessed by the domain that is being transformed. In this paper, we show that, for a given field polarisation, pseudo-SNG transformation media can be obtained by transforming a domain featuring double positive (or double-negative) character, via complex analytic continuation of the coordinate transformation rules. Moreover, we apply this concept to the TO-based interpretation of phenomena analogous to the tunnelling effects observable in bi-layers made of complementary epsilon-negative (ENG) and mu-negative (MNG) media, and explore their possible TO-inspired extensions and generalizations.

physics.optics↗

arXiv2010

A General Class of Metamaterial Transformation Slabs

In this paper, we apply transformation-based optics to the derivation of a general class of transparent metamaterial slabs. By means of analytical and numerical full-wave studies, we explore their image displacement/formation capabilities, and establish intriguing connections with configurations already known in the literature. Starting from these revisitations, we develop a number of nontrivial extensions, and illustrate their possible applications to the design of perfect radomes, anti-cloaking devices, and focusing devices based on double-positive (possibly nonmagnetic) media. These designs show that such anomalous features may be achieved without necessarily relying on negative-index or strongly resonant metamaterials, suggesting more practical venues for the realization of these devices.

physics.optics↗

arXiv2009

Nonlinear Control of Tunneling Through an Epsilon-Near-Zero Channel

The epsilon-near-zero (ENZ) tunneling phenomenon allows full transmission of waves through a narrow channel even in the presence of a strong geometric mismatch. Here we experimentally demonstrate nonlinear control of the ENZ tunneling by an external field, as well as self-modulation of the transmission resonance due to the incident wave. Using a waveguide section near cut-off frequency as the ENZ system, we introduce a diode with tunable and nonlinear capacitance to demonstrate both of these effects. Our results confirm earlier theoretical ideas on using an ENZ channel for dielectric sensing, and their potential applications for tunable slow-light structures.

physics.optics↗

arXiv2009

Cloak/anti-cloak interactions

Coordinate-transformation cloaking is based on the design of a metamaterial shell made of an anisotropic, spatially inhomogeneous "transformation medium" that allows rerouting the impinging wave around a given region of space. In its original version, it is generally believed that, in the ideal limit, the radiation cannot penetrate the cloaking shell (from outside to inside, and viceversa). However, it was recently shown by Chen et al. that electromagnetic fields may actually penetrate the cloaked region, provided that this region contains double-negative transformation media which, via proper design, may be in principle used to (partially or totally) "undo" the cloaking transformation, thereby acting as an "anti-cloak." In this paper, we further elaborate this concept, by considering a more general scenario of cloak/anti-cloak interactions. Our full-wave analytical study provides new insightful results and explores the effects of departure from ideality, suggesting also some novel scenarios for potential applications.

physics.optics↗

arXiv2008

A Hertzian Plasmonic Nanodimer as an Efficient Optical Nanoantenna

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's 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.

cond-mat.mtrl-sci↗

arXiv2008

Dynamical Theory of Artificial Optical Magnetism Produced by Rings of Plasmonic Nanoparticles

We present a detailed analytical theory for the plasmonic nanoring configuration first proposed in [A. Alu, A. Salandrino, N. Engheta, Opt. Expr. 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 details and compared with other available setups.

cond-mat.mtrl-sci↗

arXiv2008

Optical Tunneling through Arbitrarily-Shaped Plasmonic Channels and Sharp Bends

We propose a mechanism for optical energy squeezing and anomalous light tunneling through arbitrarily-shaped plasmonic ultranarrow channels and bends connecting two larger plasmonic metal-insulator-metal waveguides. It is shown how a proper design of sub-wavelength optical channels at cut-off, patterned by plasmonic implants and connecting larger plasmonic waveguides, may allow enhanced resonant transmission, inspired by the anomalous properties of epsilon-near-zero (ENZ) metamaterials. The resonant tunneling is shown to be only weakly dependent on the channel length and its specific geometry, such as possible presence of abruptions and bends.

physics.optics↗

arXiv2008

Transmission-Line Analysis of Epsilon-Near-Zero (ENZ)-Filled Narrow Channels

Following 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.

cond-mat.mtrl-sci↗

arXiv2008

Dielectric Sensing in Epsilon-Near-Zero Narrow Waveguide Channels

We exploit here the dramatic field enhancement caused by energy squeezing and tunneling (i.e., 'supercoupling') in metamaterial-inspired ultranarrow waveguide channels with near zero effective permittivity in order to sense small permittivity variations in a tiny object. The supercoupling effect is accurately modeled analytically and closed form expressions are derived to describe the presence of defects or permittivity perturbations along the channel. Applications for tailoring its pass-band frequency and for high-Q sensing are proposed at microwave frequencies.

cond-mat.mtrl-sci↗

arXiv2008

Reflectionless Sharp Bends and Corners in Waveguides Using Epsilon-Near-Zero Effects

Following our recent theoretical and experimental results that show how zero-permittivity metamaterials may provide anomalous tunneling and energy squeezing through ultranarrow waveguide channels, here we report an experimental investigation of the bending features relative to this counterintuitive resonant effect. We generate the required effectively-zero permittivity using a waveguide operating at the cut-off of its dominant mode, and we show how sharp and narrow bends may be inserted within the propagation channel without causing any sensible reflection or loss.

cond-mat.mtrl-sci↗

arXiv2008

Plasmonic Resonant Optical Nanoswitch

We examine here the anomalous optical response of a nanosphere composed of two conjoined hemispheres of different materials, one of them being plasmonic. At the internal resonance, we show how the light interaction becomes dramatically dependent on the polarization of the local electric field exciting the particle. In particular, its response may be varied from the one of an ideal perfect electric to a perfect magnetic homogeneous sphere through all the possible intermediate stages, by a simple 90 deg rotation. Applications as optical switch and as a novel nanocircuit element are envisioned.

cond-mat.mtrl-sci↗