Svetlana N. Tcvetkova

Perfect control of reflection and refraction using spatially dispersive metasurfaces

Nonuniform metasurfaces (electrically thin composite layers) can be used for shaping refracted and reflected electromagnetic waves. However, known design approaches based on the generalized refraction and reflection laws do not allow realization of perfectly performing devices: there are always some parasitic reflections into undesired directions. In this paper we introduce and discuss a general approach to the synthesis of metasurfaces for full control of transmitted and reflected plane waves and show that perfect performance can be realized. The method is based on the use of an equivalent impedance matrix model which connects the tangential field components at the two sides on the metasurface. With this approach we are able to understand what physical properties of the metasurface are needed in order to perfectly realize the desired response. Furthermore, we determine the required polarizabilities of the metasurface unit cells and discuss suitable cell structures. It appears that only spatially dispersive metasurfaces allow realization of perfect refraction and reflection of incident plane waves into arbitrary directions. In particular, ideal refraction is possible only if the metasurface is bianisotropic (weak spatial dispersion), and ideal reflection without polarization transformation requires spatial dispersion with a specific, strongly nonlocal response to the fields.

Flat Engineered Multichannel Reflectors

Recent advances in engineered gradient metasurfaces have enabled unprecedented opportunities for light manipulation using optically thin sheets, such as anomalous refraction, reflection, or focusing of an incident beam. Here we introduce a concept of multi-channel functional metasurfaces, which are able to control incoming and outgoing waves in a number of propagation directions simultaneously. In particular, we reveal a possibility to engineer multi-channel reflectors. Under the assumption of reciprocity and energy conservation, we find that there exist three basic functionalities of such reflectors: Specular, anomalous, and retro reflections. Multi-channel response of a general flat reflector can be described by a combination of these functionalities. To demonstrate the potential of the introduced concept, we design and experimentally test three different multi-channel reflectors: Three- and five-channel retro-reflectors and a three-channel power splitter. Furthermore, by extending the concept to reflectors supporting higher-order Floquet harmonics, we forecast the emergence of other multiple-channel flat devices, such as isolating mirrors, complex splitters, and multi-functional gratings.

Multifunctional Cascaded Metamaterials: Integrated Transmitarrays

Control of electromagnetic waves using engineered materials is very important in a wide range of applications, therefore there is always a continuous need for new and more efficient solutions. Known natural and artificial materials and surfaces provide a particular functionality in the frequency range they operate but cast a “shadow” and produce reflections at other frequencies. Here, we introduce a concept of multifunctional engineered materials that possess different predetermined functionalities at different frequencies. Such response can be accomplished by cascading metasurfaces (thin composite layers) that are designed to perform a single operation at the desired frequency and are transparent elsewhere. Previously, out-of-band transparent metasurfaces for control over reflection and absorption were proposed. In this paper, to complete the full set of functionalities for wave control, we synthesize transmitarrays that tailor transmission in a desired way, being “invisible” beyond the operational band. The designed transmitarrays for wavefront shaping and anomalous refraction are tested numerically and experimentally. To demonstrate our concept of multifunctional engineered materials, we have designed and measured a cascade of three metasurfaces that performs three different functions for waves at different frequencies. Remarkably, applied to volumetric metamaterials, our concept can enable a single composite possessing desired multifunctional response.

Scanning Characteristics of Metamirror Antennas With Subwavelength Focal Distance

We investigate beam scanning by lateral feed displacement in novel metasurface-based reflector antennas with extremely short focal distances. Electric field distributions of the waves reflected from the antenna are studied numerically and experimentally for defocusing angles up to 24°. The results show that despite their subwavelength focal distances, the scanning ability of the metamirrors is similar to that of short-focus reflectarrays (focal distance about several wavelengths). In addition to offering a possibility to realize extremely small focal distances, metamirror antennas are practically penetrable and invisible for any radiation outside of the operating frequency range.

Nearly perfect conversion of a propagating wave into a surface wave

In this paper the problem of propagating wave to surface wave conversion by using metasurfaces is studied theoretically and numerically. We consider the problem of determining the electromagnetic properties of an inhomogeneous lossless boundary which would fully convert an incident plane wave into a surface wave propagating along the boundary. An approximate design solution, which satisfies the energy conservation law and produces a slowly growing surface wave, is discussed. The results of the study are relevant for the future development of such devices as leaky-wave antennas and can potentially lead to other novel applications.

Multi-channel reflectors: Versatile performance experimentally tested

We investigate multi-channel reflectors, such as a three-channel power splitter and a five-channel isolating mirror. These metasurface reflectors are able to control reflections from and into several directions while possessing a flat surface. We design, fabricate, and experimentally study these new devices, confirming that the performance is nearly perfect.

General approach to the synthesis of perfectly refractive metasurfaces

In this presentation we will introduce and discuss a general approach to the synthesis of metasurfaces for full control of reflected and transmitted fields. The method, which is applicable for any linear metasurface, is based on the use of an equivalent Z-matrix, connecting the tangential field components at the two sides on the metasurface. Finding the impedance matrix components, we are able to understand what physical properties of metasurface are needed in order to realize the desired response. Furthermore, we can find the required polarizabilities and/or susceptibilities of the metasurface unit cells and design the cell structures. In particular, we will discuss metasurfaces for perfect refraction into an arbitrary direction, explain possible alternative physical realizations and reveal the crucial role of bianisotropic coupling for design of perfectly matched metasurfaces for transmission control.