Description and explanation of electromagnetic behaviors in artificial metamaterials based on effective medium theory
TL;DR: A general theory of effective media to set up the relationship between the particle responses and the macroscopic system behaviors for artificial metamaterials composed of periodic resonant structures and proposes a more advanced form of the fitting formulas for the effective electromagnetic parameters of metamMaterials.
Abstract: We present a general theory of effective media to set up the relationship between the particle responses and the macroscopic system behaviors for artificial metamaterials composed of periodic resonant structures. By treating the unit cell of the periodic structure as a particle, we define the average permittivity and permeability for different unit structures and derive a general form of discrete Maxwell's equations on the macroscale, from which we obtain different wave modes in metamaterials including the propagation mode, pure plasma mode, and resonant crystal band-gap mode. We explain unfamiliar behaviors of metamaterials from the numerical S parameter retrieval approach. The excellent agreement between theoretical predictions and retrieval results indicates that the defined model and method of analysis fit the physical structures very well. Thereafter, we propose a more advanced form of the fitting formulas for the effective electromagnetic parameters of metamaterials.
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TL;DR: An experimental realization of a cloak design that conceals a perturbation on a flat conducting plane, under which an object can be hidden, and results indicate that this type of cloak should scale well toward optical wavelengths.
Abstract: The possibility of cloaking an object from detection by electromagnetic waves has recently become a topic of considerable interest. The design of a cloak uses transformation optics, in which a conformal coordinate transformation is applied to Maxwell's equations to obtain a spatially distributed set of constitutive parameters that define the cloak. Here, we present an experimental realization of a cloak design that conceals a perturbation on a flat conducting plane, under which an object can be hidden. To match the complex spatial distribution of the required constitutive parameters, we constructed a metamaterial consisting of thousands of elements, the geometry of each element determined by an automated design process. The ground-plane cloak can be realized with the use of nonresonant metamaterial elements, resulting in a structure having a broad operational bandwidth (covering the range of 13 to 16 gigahertz in our experiment) and exhibiting extremely low loss. Our experimental results indicate that this type of cloak should scale well toward optical wavelengths.
1,405 citations
TL;DR: It is demonstrated that a broadband, extremely high index of refraction can be realized from large-area, free-standing, flexible terahertz metamaterials composed of strongly coupled unit cells.
Abstract: Controlling the electromagnetic properties of materials, going beyond the limit that is attainable with naturally existing substances, has become a reality with the advent of metamaterials. The range of various structured artificial 'atoms' has promised a vast variety of otherwise unexpected physical phenomena, among which the experimental realization of a negative refractive index has been one of the main foci thus far. Expanding the refractive index into a high positive regime will complete the spectrum of achievable refractive index and provide more design flexibility for transformation optics. Naturally existing transparent materials possess small positive indices of refraction, except for a few semiconductors and insulators, such as lead sulphide or strontium titanate, that exhibit a rather high peak refractive index at mid- and far-infrared frequencies. Previous approaches using metamaterials were not successful in realizing broadband high refractive indices. A broadband high-refractive-index metamaterial structure was theoretically investigated only recently, but the proposed structure does not lend itself to easy implementation. Here we demonstrate that a broadband, extremely high index of refraction can be realized from large-area, free-standing, flexible terahertz metamaterials composed of strongly coupled unit cells. By drastically increasing the effective permittivity through strong capacitive coupling and decreasing the diamagnetic response with a thin metallic structure in the unit cell, a peak refractive index of 38.6 along with a low-frequency quasi-static value of over 20 were experimentally realized for a single-layer terahertz metamaterial, while maintaining low losses. As a natural extension of these single-layer metamaterials, we fabricated quasi-three-dimensional high-refractive-index metamaterials, and obtained a maximum bulk refractive index of 33.2 along with a value of around 8 at the quasi-static limit.
572 citations
TL;DR: In this paper, the authors proposed ultrathin multiband metamaterial absorbers in the microwave frequencies in which the design, analysis, fabrication, and measurement of the absorbers working in multiple bands are presented.
Abstract: We propose ultrathin multiband metamaterial absorbers in the microwave frequencies in which the design, analysis, fabrication, and measurement of the absorbers working in multiple bands are presented. The metamaterial absorbers consist of a periodic arrangement of different scales of electric-field-coupled-LC (ELC) resonators and a metallic background plane, separated by only 1 mm dielectric spacer. By tuning the scale factor of the ELC unit cells, we achieve independently multiple absorptions at different customized frequencies. Experiments demonstrate excellent absorption rates in the designed frequency bands over wide angles of incident waves for both transverse electric and magnetic polarizations. The explanation to the physical mechanism of the multiband metamaterial absorber is presented and verified.
385 citations
TL;DR: The ability of the anisotropic coding metasurfaces to generate a beam splitter and realize simultaneous anomalous reflections and polarization conversions, thus providing powerful control of differently polarized electromagnetic waves is demonstrated.
Abstract: Metamaterials based on effective media can be used to produce a number of unusual physical properties (for example, negative refraction and invisibility cloaking) because they can be tailored with effective medium parameters that do not occur in nature. Recently, the use of coding metamaterials has been suggested for the control of electromagnetic waves through the design of coding sequences using digital elements ‘0’ and ‘1,' which possess opposite phase responses. Here we propose the concept of an anisotropic coding metamaterial in which the coding behaviors in different directions are dependent on the polarization status of the electromagnetic waves. We experimentally demonstrate an ultrathin and flexible polarization-controlled anisotropic coding metasurface that functions in the terahertz regime using specially designed coding elements. By encoding the elements with elaborately designed coding sequences (both 1-bit and 2-bit sequences), the x- and y-polarized waves can be anomalously reflected or independently diffused in three dimensions. The simulated far-field scattering patterns and near-field distributions are presented to illustrate the dual-functional performance of the encoded metasurface, and the results are consistent with the measured results. We further demonstrate the ability of the anisotropic coding metasurfaces to generate a beam splitter and realize simultaneous anomalous reflections and polarization conversions, thus providing powerful control of differently polarized electromagnetic waves. The proposed method enables versatile beam behaviors under orthogonal polarizations using a single metasurface and has the potential for use in the development of interesting terahertz devices. An artificial material that controls electromagnetic waves of different polarization independently has been demonstrated by a team in China. Tie Jun Cui from the Southeast University and co-workers have created a metamaterial that can, for example, split incoming unpolarized radiation so that horizontally polarized light goes one way while vertically polarized light goes the other. Metamaterials are structures that can be engineered to have optical properties not found in natural materials, and they consist of a repeated pattern of elements that are smaller than the wavelength of light. The researchers used two types of element, simple squares and dumbbells, which enabled them to independently control beams of long-wavelength radiation known as terahertz waves having differing polarizations. By reducing the size of the metamaterial elements, the same idea could also be applied to visible light.
382 citations
TL;DR: In this paper, a hybrid-metamaterial architecture is proposed for tuning a far-infrared resonance frequency with vanadium dioxide (VO2), a material whose optical properties can be strongly and quickly changed via external stimulus.
Abstract: We demonstrate a metamaterial device whose far-infrared resonance frequency can be dynamically tuned. Dynamic tuning should alleviate many bandwidth-related roadblocks to metamaterial application by granting a wide matrix of selectable electromagnetic properties. This tuning effect is achieved via a hybrid-metamaterial architecture; intertwining split ring resonator metamaterial elements with vanadium dioxide (VO2)-a material whose optical properties can be strongly and quickly changed via external stimulus. This hybrid structure concept opens a fresh dimension in both exploring and exploiting the intriguing electromagnetic behavior of metamaterials.
318 citations