Metamaterials最新文献

Abstract

In this paper we suggest and study a design solution of metamaterial made of raspberry-like clusters of silver triangular nanoprisms. We show that this design theoretically allows one to obtain isotropic negative effective permeability in the visible range even taking into account real dissipative losses in silver. To estimate the magnetic response of the structure two independent methods are used. The study is presented in view of prospective for isotropic doubly-negative metamaterials operating in the visible range.

We correct a statement of paper [1].

We report on the optical response and, in particular, on the refractive properties of an fcc crystal of metallic nanoshells by means of full-electrodynamic layer-multiple-scattering simulations. Exact numerical calculations of the isofrequency surfaces reveal the existence of two frequency regions where negative refraction occurs. A thorough analysis of the photonic band structure, in conjunction with corresponding transmission diagrams, attributes this behavior to the excitation of collective modes, which stem from dipole particle-plasmon resonances, and shows that only in one of the two frequency regions negative refraction without birefringence can be obtained. In addition, we discuss the effect of absorptive losses, and reveal the existence of narrow bands of slab modes in a finite slab of the crystal that can transfer the evanescent components of an incident wave field.

Abstract

In this paper the concept of wave-guiding medium, recently introduced for planar structures, is defined for the spherically symmetric case. It is shown that a quarter-wavelength layer of such a medium serves as a transformer of boundary conditions between the two interfaces. As an application, the D′B′-boundary condition, requiring vanishing of normal derivatives of the normal components of D and B field vectors, is realized by transforming the DB-boundary conditions. To test the theory, scattering from a spherical DB object covered by a layer of wave-guiding material is compared to the corresponding scattering from an ideal D′B′ sphere, for varying medium parameters of the layer.

In this work, we discuss the analytical modeling of single and cascaded plasmonic metasurfaces excited by normally incident plane waves using a transmission-line shunt admittance model. We derive an analytical model that relates the effective surface optical admittance of a single metasurface to its inclusion polarizability and its plane wave reflection and transmission coefficients, and we apply these concepts to predict the wave interaction of more complicated setups in which two stacked surfaces are separated by a varying distance, and possibly rotated with respect to one another, for normal incidence excitation. We show that the analytical model holds very well under suitable conditions, and we verify our analytical results with full-wave numerical simulations, including material dispersion and loss of optical materials.

We present a theoretical and experimental study of the effective permittivity of a photonic crystal (PC) made of parallel dielectric rods in the long-wavelength limit. Using the plane-wave expansion method we derive an analytical expression for the imaginary part of the dielectric tensor. Angular dependence of the dissipative part of permittivity turns out to be more complicated than that for the real part. In particular, it exhibits stronger anisotropy for a PC with rectangular unit cell. Theoretical and numerical results are compared to the experimental measurements of microwave reflection and transmission through a PC fabricated from low-loss alumina rods. A good agreement between the theory and experiment was obtained in the limit where the effective medium approximation is valid.

Here, we present a numerical and experimental parametric analysis of anomalous enhanced transmission supported by dielectric-loaded holey metals. We focus the study on circular and square subwavelength hole apertures (arranged in a rectangular lattice) both under normal and oblique incidence within the millimetre-wave regime. It is experimentally confirmed that the thickness of the dielectric slab plays a role: the thicker the slab is, the higher the transmission is, which is in good agreement with the interpretation founded on grounded dielectric slab mode and circuit models. Also, from the experimental results, it is shown that finiteness effects arisen in the real experiments have significant consequences. Among them, it is outstanding the considerable penalty on the transmission through circular holes, which can be explained intuitively by the smaller clear aperture per unit area with respect to the square holes. In addition to transmission results, the TE nature of the grounded dielectric slab mode is stressed by the field distribution calculated numerically. The results presented here enlarge the knowledge about anomalous enhanced transmission and establish a platform from which polarization- and frequency-dependent devices can be foreseen.

In this work, we discuss the homogenization of a metamaterial geometry composed of periodic arrays of densely packed subwavelength magnetodielectric spheres, in order to study whether a local quasi-isotropic homogenization model may accurately describe its wave interaction in its negative-index or zero-index operation. We analyze and compare the electromagnetic response of these arrays with their retrieved metamaterial model, for frequency regimes in which positive or negative values of effective index of refraction are expected. We pay special attention to the effects of array truncation and complex forms of excitation, showing that it is possible to realize quasi-isotropic negative-index or zero-index metamaterials with negligible spatial dispersion effects in certain frequency bands. We then apply these concepts to specific configurations of interest for metamaterial devices, showing that, despite their finite size and complex operation, their response is consistent with the one associated with their homogenized local description.

This article reviews some of the recent progress made in the field of near-field focusing and near-field imaging. In particular, it addresses the “meta-screen” that produces a subwavelength focal spot from an incident plane wave, as well as the near-field antenna arrays that have been used to demonstrate two-dimensional subwavelength imaging. The significance of these contributions is that subwavelength operation occurs at a relatively long working distance (a quarter-wavelength) from the screen or array elements. To analyze and design such structures, a spatially shifted beam theory was introduced for enhancing the evanescent field. Several meta-screen and antenna array structures will be discussed, with simulation and experimental results presented to illustrate the focusing and imaging behavior.