Uniform Metasurface Research Articles

Spatially Dispersive Metasurfaces—Part II: IE-GSTC-SD Field Solver With Extended GSTCs

An integral equation (IE) based field solver to compute the scattered fields from spatially dispersive metasurfaces is proposed and numerically confirmed using various examples involving physical unit cells. The work is a continuation of Part-1 [1], which proposed the basic methodology of representing spatially dispersive metasurface structure in the spatial frequency domain, k||. By representing the angular dependence of the surface susceptibilities in k|| as a ratio of two polynomials, the standard generalized sheet transition conditions (GSTCs) have been extended to include the spatial derivatives of both the difference and average fields around the metasurface. These extended boundary conditions are successfully integrated here into a standard IE-GSTC solver, which leads to the new IE-GSTC-SD simulation framework. The proposed IE-GSTC-SD platform is applied to various uniform metasurfaces, including a practical short conducting wire unit cell, as a representative practical example, for various cases of finite-sized flat and curvilinear surfaces. In all cases, computed field distributions are successfully validated, either against the semi-analytical Fourier decomposition method or the brute-force full-wave simulation of volumetric metasurfaces in the commercial Ansys FEM-HFSS simulator.

Metasurface Optical Characterization Using Quadriwave Lateral Shearing Interferometry

An optical metasurface consists of a dense and usually nonuniform layer of scattering nanostructures behaving as a continuous and extremely thin optical component with predefined phase, transmission and reflection profiles. To date, various sorts of metasurfaces (metallic, dielectric, Huygens-like, Pancharatman-Berry, etc.) have been introduced to design ultrathin lenses, beam deflectors, holograms, or polarizing interfaces. Their actual efficiencies depend on the ability to predict their optical properties and to fabricate nonuniform assemblies of billions of nanoscale structures on macroscopic surfaces. To further help improve the design of metasurfaces, precise and versatile postcharacterization techniques need to be developed. Today, most of the techniques used to characterize metasurfaces rely on light intensity measurements. Here, we demonstrate how quadriwave lateral shearing interferometry (QLSI), a quantitative phase microscopy technique, can achieve full optical characterization of metasurfaces of any kind, as it can probe the local phase imparted by a metasurface with high sensitivity and a spatial resolution that reaches the diffraction limit. As a means to illustrate the versatility of this technique, we present measurements on two types of metasurfaces, namely, Pancharatnam-Berry and effective-refractive-index metasurfaces, and present results on uniform metasurfaces, metalenses, and deflectors.

Возбуждение анизотропной импедансной метаповерхности в виде эллиптического цилиндра.

The problem of arbitrary excitation of waves by a system of external sources near an anisotropic metasurface in the form of an elliptical cylinder with a surface homogenized impedance tensor of general form is solved. The solution to the problem is written as a superposition of E- and H-waves in elliptical coordinates. The partial reflection coefficients of waves were found from the boundary conditions using the orthogonality of the Mathieu angular functions. For these coefficients, four coupled infinite systems of linear algebraic equations of the second kind are obtained. The conditions under which the solution of the excitation problem by the method of eigenfunctions is obtained in an explicit form are found and analyzed. It is shown that for this, the surface impedance tensor of a uniform metasurface must belong to a class of deviators (have zero diagonal elements). In the particular case of a mutual (most easily realized) metasurface, its impedance tensor should only be reactance. In another special case, the impedance tensor of a set of deviators describes a class of anisotropic nonreciprocal metasurfaces with the so-called perfect electromagnetic conductivity (PEMC).

Miniature Wideband Non-Uniform Metasurface Antenna Using Equivalent Circuit Model

A miniaturized wideband non-uniform metasurface antenna is proposed and analyzed using an equivalent circuit model. The nonuniform metasurface consists of an array of rectangular metallic patches separated by varying gaps. The effects of the varying gap widths on the resonant frequencies of the dual-mode non-uniform metasurface antenna are studied using the proposed equivalent circuit model. The resonant frequency of the first TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> mode decreases with narrowing of either the center or side gaps; the resonant frequency of the antiphase TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> mode is hardly affected by the center gap width but decreases while reducing the side gap width. The miniaturization of the metasurface antenna is therefore achieved by narrowing the side gaps, and the wide bandwidth with good impedance matching is realized by widening the center gap. A proof-of-concept prototype demonstrates a peak gain of 7.2 dBi over a -10 dB reflection coefficient bandwidth of 21.8%, with a miniaturized non-uniform metasurface of 0.46λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> × 0.49λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> × 0.06λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> (λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> is the center operating wavelength in free space) which achieves 54% size reduction compared with the earlier developed wideband antenna with a uniform metasurface of 0.7λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> × 0.7λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> × 0.06λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> .

Modified Explicit Finite-Difference Time-Domain Method for Nonparaxial Wave Scattering From Electromagnetic Metasurfaces

An updated method for simulating the scattered fields from a dispersive Huygens’ metasurface using the explicit finite-difference time-domain technique has been proposed and numerically demonstrated. The method uses a spatial–temporal averaging of the electric and magnetic fields, using virtual sources in a standard Yee cell grid. This creates a nonparaxial implementation of the generalized sheet transition conditions in the time domain and rigorously solves the scattered fields in reflection and transmission regions separately. The metasurfaces are solved for Lorentzian susceptibilities, and the proposed method is successfully demonstrated using two examples: a uniform metasurface with a strongly divergent beam and a space-modulated metasurface emulating a diffraction grating.

Investigation of various cavity configurations for metamaterial-enhanced field-localizing wireless power transfer

Controlling power to an unintended area is an important issue for enabling wireless power transfer (WPT) systems. The control allows us to enhance efficiency as well as suppress unnecessary flux leakage. The flux leakage from WPT can be reduced effectively via selective field localization. To realize field localization, we propose the use of cavities formed on a single metamaterial slab that acts as a defected metasurface. The cavity is formed by strong field confinement using a hybridization bandgap (HBG), which is created by wave interaction with a two-dimensional array of local resonators on the metasurface. This approach using an HBG demonstrates strong field localization around the cavity regions. Motivated by this result, we further investigate various cavity configurations for different sizes of the transmitter (Tx) and receiver (Rx) resonators. Experiments show that the area of field localization increases with the number of cavities, confirming the successful control of different cavity configurations on the metasurface. Transmission measurements of different cavities show that the number of cavities is an important parameter for efficiency, and excess cavities do not enhance the efficiency but increase unnecessary power leakage. Thus, there exists an optimum number of cavities for a given size ratio between the Tx and Rx resonators. For a 6:1 size ratio, this approach achieves efficiency improvements of 3.69× and 1.59× compared to free space and a uniform metasurface, respectively. For 10:1 and 10:2 size ratios, the efficiency improvements are 3.26× and 1.98× compared to free space and a uniform metasurface, respectively.

Anomalous refraction and asymmetric transmission of SV-waves through elastic metasurfaces

Recent advances in acoustic metasurface design make it possible to manipulate sound waves in an almost arbitrary way. Here, we present several elastic metasurfaces comprised of an array of subwavelength plates for controlling SV-wave in solids. The underlying physics are the coupling between the SV-wave in the elastic body and the flexural wave in plates, and the coupling between the P-wave in the elastic body and the longitudinal wave in plates. By varying the thicknesses of the plates, a wide range of phase delay for flexural waves can be obtained, while keeping constant the phase delay for longitudinal waves. The anomalous refraction of SV-waves is achieved by selecting the thickness of each plate to engineer the phase change according to the generalized Snell’s law. This metasurface has another feature that it redirects SV-waves only, which enables it to be used to split SV- and P-wavefronts into different directions. In addition, this metasurface can be paired with a uniform metasurface to break spatial symmetry and achieve asymmetric transmission for normally incident SV-waves. Other potential applications, such as focusing and negative refraction, will also be discussed. [Work supported through ONR MURI.]

OMNIDIRECTIONAL RADIATION IN THE PRESENCE OF HOMOGENIZED METASURFACES

Analytical and numerical approaches are presented for modeling the interaction of azimuthally symmetric fields with omnidirectional metasurfaces, based on the use of locally homogenized equivalent sheet impedances. Radially uniform metasurfaces on layered dielectric media are described in terms of a spectral impedance dyadic, thus allowing for the derivation of the field excited by omnidirectional sources through a simple transmission-line model. In a first approximation, the effect of circular edges in laterally truncated structures is taken into account through an efficient physical- optics method. Then, truncated and radially non-uniform homogenized layered structures are treated numerically with the method of moments, by suitably extending a recently developed spectral-domain formulation. Numerical results are presented for planar radiating structures based on omnidirectional metasurfaces, comparing the radiation patterns obtained through the proposed homogenized models with those calculated by means of full-wave simulations. The discussion emphasizes the validity of the proposed approaches and their usefulness in the analysis of two-dimensional leaky-wave antennas based on printed omnidirectional metasurfaces.