Xing-Xiang Liu

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.

Limitations and potentials of metamaterial lenses

The seminal discovery that an ideal negative-index lens may overcome Abbes diffraction limit has raised enormous interest in the field of metamaterials and of subwavelength focusing. This finding is based on the anomalous wave propagation in ideally isotropic and homogeneous metamaterials with negative index of refraction and low loss, provided they are available. We have designed a metamaterial lens based on one of the simplest metamaterial geometries, a cubic array of spheres, with the aim of verifying its imaging properties in a practical configuration. After a rigorous homogenization, we have shown that, for suitable designs, the effective bulk parameters may indeed provide a quasi-isotropic negative-index response, ideal for imaging applications. We have then tested the imaging properties for finite-size lenses, analyzing challenges and potentials of going beyond the diffraction limit in a practical setup. We have also explored an alternative venue to exploit the negative-index property of the designed metamaterial in a concave lens, in order to resolve subwavelength features in the far-field. Our results indicate that, although subwavelength resolution and evanescent-wave amplification are possible in metamaterial arrays, practical imaging beyond the diffraction limit is challenging and a careful design should consider the granularity, degree of isotropy, and transverse size of the metamaterial lens.

Polarizability Tensor Retrieval for Subwavelength Particles of Arbitrary Shape

We present a semianalytical method to determine the full polarizability tensor of subwavelength particles with arbitrary shape and composition. This method extracts both co- and cross-coupling terms of the polarizability tensor, retrieving the full 6 × polarizability tensor with three measurements, by appropriately rotating the inclusions, when embedded in a planar periodic array, with respect to their principle axes. In order to validate this method, we present four examples, including a magnetodielectric sphere, a magnetodielectric spheroid, a conducting helix and pairs of split-ring resonators, and discuss the relation with fundamental constraints such as reciprocity and Onsagers relations. We also discuss how this retrieval method may be useful for the design and homogenization of periodic metamaterials and two-dimensional (2-D) metasurfaces formed by subwavelength inclusions.

Homogenization of three-dimensional metamaterial objects and validation by a fast surface-integral equation solver.

A homogenization model is applied to describe the wave interaction with finite three-dimensional metamaterial objects composed of periodic arrays of magnetodielectric spheres and is validated with full-wave numerical simulations. The homogenization is based on a dipolar model of the inclusions, which is shown to hold even in the case of densely packed arrays once weak forms of spatial dispersion and the full dynamic array coupling are taken into account. The numerical simulations are based on a fast surface-integral equation solver that enables the analysis of scattering from complex piecewise homogeneous objects. We validate the homogenization model by considering electrically large disk- and cube-shaped arrays and quantify the accuracy of the transition from an array of spheres to a homogeneous object as a function of the array size. Simulation results show that the fields scattered from large arrays with up to one thousand spheres and equivalent homogeneous objects agree well, not only far away from the arrays but also near them.

First-principle homogenization of magnetodielectric metamaterial arrays

We develop here a rigorous homogenization approach applied to dense cubic arrays of magnetodielectric spheres, in order to determine their first-principle bulk constitutive parameters. We carefully model their electromagnetic response and assess the validity and limitations of commonly used approximate homogenization models for metamaterials. In particular, we show that, in regimes for which exotic wave propagation is obtained, near the array resonances and for especially dense arrays, more rigorous homogenization models are necessary, including magneto-electric coupling and weak spatial dispersion effects. We apply our findings to the design of negative-index metamaterial devices, and compare their scattering properties with those of our homogenized model. The comparison of near-field distributions shows remarkable agreement and confirms that a proper homogenization model can indeed correctly capture the physics and complex wave propagation in exotic metamaterial arrays.

Chirality and bianisotropy effects in plasmonic metasurfaces and their application to realize ultrathin optical circular polarizers

In this paper we develop a rigorous analytical theory relating the effective impedance of plasmonic metasurfaces to a generalized form of polarizability, which compactly describes the electric, magnetic and magneto-electric response of the individual inclusions and the overall array coupling. We apply this theory to the design of plasmonic metasurfaces composed of lithographically printed planar inclusions, showing that their inherent chiral and bianisotropic response may be exploited to produce ultrathin optical circular polarizers. Bianistropic effects, particularly relevant to enhance the response to circularly polarized light, may be maximized in specific incidence planes, as a function of the inclusion symmetries.

Optical nanontennas for enhancing nonlinearities, sensing, optical communications and energy harvesting

We discuss the use of plasmonic nanoantennas to drastically enhance and tailor the linear and nonlinear response of optical materials to realize novel optical nanodevices for optical communications and computing, energy harvesting and sensing applications. By translating some of the familiar concepts of radio-frequency antennas to the visible spectrum, we propose a variety of exciting designs to tailor the nanoscale interaction of nanoparticles with light, in order to realize nanodevices with linear and nonlinear properties not available in conventional optical materials and systems. We propose individual and collections of plasmonic nanoparticles that may realize the bridge between unconventional nanoscale optical processing and far-field propagation and radiation.

Plasmonic low-profile nanoantenna reflectors

We describe here the possibility to realize ultralow profile nanoantenna reflectors, which may manipulate optical radiation and emission due to their plasmonic interaction with light. Classic concepts at microwave frequencies, such as reflectarray and holographic antennas are applied here to plasmonic materials, with the goal to achieve a large degree of flexibility and tunability for low-profile reflectors, with potential applications in nano-photonics and optical communications.