arXiv subjects

Andrea Alu

Publications and source records attributed to Andrea Alu.

At least 91 records · Page 5Linked to original sources

arXiv2013

Broadening the Bandwidth of Metamaterial Cloaks with Non-Foster Metasurfaces

We introduce the concept and practical design of broadband, ultrathin cloaks based on non-Foster, negatively capacitive metasurfaces. By using properly tailored, active frequency-selective screens conformal to an object, within the realm of practical realization, is shown to enable drastically reduced scattering over a wide frequency range in the microwave regime, orders of magnitude broader than any available passive cloaking technology. The proposed active cloak may impact not only invisibility and camouflaging, but also practical antenna and sensing applications.

physics.optics↗

arXiv2013

Generalized Retrieval Method for Metamaterial Constitutive Parameters Based on a Physically-Driven Homogenization Approach

Based on the recently introduced homogenization theory developed in [Phys. Rev. B 84, 075153 (2011)], we propose a generalized retrieval method that allows extracting physically meaningful bulk effective parameters from conventional scattering measurements of periodic metamaterial samples composed of subwavelength inclusions. We show that, compared to conventional approaches, our method is able to capture the anomalous physics in the wave interaction with resonant metamaterials and return physically meaningful homogenized parameters that retain local properties in the long-wavelength limit. As a byproduct, we are also able to retrieve the polarizabilities of the constituent inclusions, which are shown to satisfy expected dispersion properties for passive inclusions, in contrast with conventional retrieval approaches.

cond-mat.mes-hall↗

arXiv2013

Enhanced Super-Radiance in Epsilon-Near-Zero Plasmonic Channels

We describe the possibility of drastically boosting the spontaneous emission of a collection of two-level quantum emitters by embedding them in an epsilon-near-zero (ENZ) environment, consisting of a plasmonic waveguide operated at cut-off. This phenomenon relies on the combination of Purcell enhancement and Dicke superradiance effects, exploiting the large and uniform local density of states in ENZ channels, which is shown to significantly extend the spatial extent of these quantum effects. We envision exciting applications in optical sensing, molecular detection and low-threshold lasing.

cond-mat.mes-hall↗

arXiv2013

Furtive Quantum Sensing Using Matter-Wave Cloaks

We introduce the concept of furtive quantum sensing, demonstrating the possibility of concealing quantum objects from matter-waves, while maintaining their ability to interact and get excited by the impinging particles. This is obtained by cloaking, with tailored homogeneous metamaterial layers, quantum systems having internal degrees of freedom. The effect is a low-observable quantum sensor with drastically reduced elastic scattering and optimized absorption levels, a concept that opens interesting venues in particle detection, high-efficiency electrical pumping and quantum supercomputing.

cond-mat.mes-hall↗

arXiv2013

Manipulating the Nanoscale Optical Transmission with a Meta-Transmitarray

By applying the optical nanocircuit concepts to metasurfaces, we propose an effective route to locally control light transmission over a deeply subwavelength scale. This concept realizes the optical equivalent of a transmitarray, whose use is demonstrated for light bending and focusing with unprecedented efficiency over a subwavelength distance, with crucial benefits for nano-optics applications. These findings may lead to large improvements in the manipulation of optical transmission and processing of nanoscale optical signals over conformal and Si-compatible substrates.

cond-mat.mes-hall↗

arXiv2013

Multi-Layered Plasmonic Covers for Comb-Like Scattering Response and Optical Tagging

We discuss the potential of multilayered plasmonic particles to tailor the optical scattering response. The interplay of plasmons localized in thin stacked shells realizes peculiar degenerate cloaking and resonant states occurring at arbitrarily close frequencies. These concepts are applied to realize ultrasharp comb-like scattering responses and synthesize staggered, ideally strong super-scattering states closely coupled to invisible states. We demonstrate robustness to material losses and to variations in the background medium, properties that make these structures ideal for optical tagging.

cond-mat.mtrl-sci↗

arXiv2012

Broadband Absorbers and Selective Emitters based on Plasmonic Brewster Metasurfaces

We discuss the possibility of realizing utlrabroadband omnidirectional absorbers and angularly selective coherent thermal emitters based on properly patterned plasmonic metastructures. Instead of relying on resonant concentration effects that inherently limit the bandwidth, we base our design on the combination of two inherently nonresonant effects: plasmonic Brewster funneling and adiabatic plasmonic focusing. With this approach, we demonstrate compact, broadband absorption and emission spanning terahertz, infrared and optical frequencies, ideal for various energy and defense applications.

cond-mat.mes-hall↗

arXiv2012

The Physics of Unbounded, Broadband Absorption/Gain Efficiency in Plasmonic Nanoparticles

Anomalous resonances in properly shaped plasmonic nanostructures can in principle lead to infinite absorption/gain efficiencies over broad bandwidths. By developing a closed-form analytical solution for the fields scattered by conjoined semicircles, we outline the fundamental physics behind these phenomena, associated with broadband adiabatic focusing of surface plasmons at the nanoscale. We are able to justify the apparent paradox of finite absorption/gain in the limit of infinitesimally small material loss/gain, and we explore the potential of these phenomena in nonlinear optics, spasing, energy-harvesting and sensing.

cond-mat.mes-hall↗

arXiv2012

Extraordinary Sound Transmission through Density-Near-Zero Ultranarrow Channels

We introduce the acoustic equivalent of 'supercoupling' by studying the anomalous sound transmission and uniform energy squeezing through ultranarrow acoustic channels filled with zero-density metamaterials. We propose their realization by inserting transverse membranes with a subwavelength period along the channel, and we prove a novel form of acoustic tunneling based on impedance matching and infinite phase velocity at the zero-density operation. We envision applications in sensing, noise control, cloaking and energy harvesting.

cond-mat.mtrl-sci↗

arXiv2012

Comment on 'Propagation and Negative Refraction,' IEEE Microwave Magazine, pp. 58-65, July/August 2012

We discuss and clarify some misleading statements appeared in a recent article on negative refraction in metamaterials. In this comment, we straighten the physical meaning and underlying phenomena behind negative refraction and negative index metamaterials and show that these phenomena are perfectly consistent with causality. It is hoped that this comment draws the attention of authors, reviewers and readers to the large body of rigorous analytical, numerical, and experimental evidence in favor of negative refraction at a single interface, so as to avoid reviving settled debates which mislead, rather than educate, the physics and engineering communities.

physics.optics↗

arXiv2011

First-Principle Homogenization Theory for Periodic Metamaterials

We derive from first principles an accurate homogenized description of periodic metamaterials made of magnetodielectric inclusions, highlighting and overcoming relevant limitations of standard homogenization methods. We obtain closed-form expressions for the effective constitutive parameters, pointing out the relevance of inherent spatial dispersion effects, present even in the long-wavelength limit. Our results clarify the limitations of quasi-static homogenization models, restore the physical meaning of homogenized metamaterial parameters and outline the reasons behind magnetoelectric coupling effects that may arise also in the case of center-symmetric inclusions.

cond-mat.mes-hall↗

arXiv2010

Restoring the Physical Meaning of Metamaterial Constitutive Parameters

Metamaterial homogenization is often based on implicit assumptions inspired to natural material models. Retrieved effective permittivity and permeability, however, are often non-physical, especially near the array resonances, of most interest for metamaterial applications. We explain here the nature of typical homogenization artifacts, relating them to an inherent form of magneto-electric coupling associated with the finite phase velocity along metamaterial arrays. Our findings allow restoring the proper definition and physical meaning of local constitutive parameters for metamaterials.

cond-mat.mes-hall↗

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

Plasmonic Cloaking of Cylinders: Finite Length, Oblique Illumination and Cross-Polarization Coupling

Metamaterial cloaking has been proposed and studied in recent years following several interesting approaches. One of them, the scattering-cancellation technique, or plasmonic cloaking, exploits the plasmonic effects of suitably designed thin homogeneous metamaterial covers to drastically suppress the scattering of moderately sized objects within specific frequency ranges of interest. Besides its inherent simplicity, this technique also holds the promise of isotropic response and weak polarization dependence. Its theory has been applied extensively to symmetrical geometries and canonical 3D shapes, but its application to elongated objects has not been explored with the same level of detail. We derive here closed-form theoretical formulas for infinite cylinders under arbitrary wave incidence, and validate their performance with full-wave numerical simulations, also considering the effects of finite lengths and truncation effects in cylindrical objects. In particular, we find that a single isotropic (idealized) cloaking layer may successfully suppress the dominant scattering coefficients of moderately thin elongated objects, even for finite lengths comparable with the incident wavelength, providing a weak dependence on the incidence angle. These results may pave the way for application of plasmonic cloaking in a variety of practical scenarios of interest.

physics.class-ph↗

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↗

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↗