The recently proposed digital coding metasurfaces make it possible to control electromagnetic (EM) waves in real time, and allow the implementation of many different functionalities in a programmable way. However, current configurations are only space-encoded, and do not exploit the temporal dimension. Here, we propose a general theory of space-time modulated digital coding metasurfaces to obtain simultaneous manipulations of EM waves in both space and frequency domains, i.e., to control the propagation direction and harmonic power distribution simultaneously. As proof-of-principle application examples, we consider harmonic beam steering, beam shaping, and scattering-signature control. For validation, we realize a prototype controlled by a field-programmable gate array, which implements the harmonic beam steering via an optimized space-time coding sequence. Numerical and experimental results, in good agreement, demonstrate good performance of the proposed approach, with potential applications to diverse fields such as wireless communications, cognitive radars, adaptive beamforming, holographic imaging.

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Space-time-coding digital metasurfaces

Diverse electromagnetic (EM) responses of a programmable metasurface with a relatively large scale have been investigated, where multiple functionalities are obtained on the same surface. The unit cell in the metasurface is integrated with one PIN diode, and thus a binary coded phase is realized for a single polarization. Exploiting this anisotropic characteristic, reconfigurable polarization conversion is presented first. Then the dynamic scattering performance for two kinds of sources, i.e. a plane wave and a point source, is carefully elaborated. To tailor the scattering properties, genetic algorithm, normally based on binary coding, is coupled with the scattering pattern analysis to optimize the coding matrix. Besides, inverse fast Fourier transform (IFFT) technique is also introduced to expedite the optimization process of a large metasurface. Since the coding control of each unit cell allows a local and direct modulation of EM wave, various EM phenomena including anomalous reflection, diffusion, beam steering and beam forming are successfully demonstrated by both simulations and experiments. It is worthwhile to point out that a real-time switch among these functionalities is also achieved by using a field-programmable gate array (FPGA). All the results suggest that the proposed programmable metasurface has great potentials for future applications.

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A programmable metasurface with dynamic polarization, scattering and focusing control

We review the current trends in the design of Huygens’ metasurfaces (HMSs), which are planar arrays of balanced electric and magnetic polarizable particles (meta-atoms) of subwavelength size. We focus on schemes that follow the equivalence principle, as these can be rigorously incorporated into Maxwell’s equations, leading to design specifications in the form of (electric and magnetic) surface-impedance distributions. The advantages of this approach with respect to the more common phase-shift stipulation approach are highlighted and discussed. We present a (microscopic) methodology to associate a general meta-atom configuration with an equivalent surface impedance, and derive metasurface (macroscopic) design procedures for various beam forming applications. The methods and concepts developed in the paper provide the basic tools for understanding and designing scalar, passive, and lossless HMSs, and we indicate possible extensions applicable to more complex structures.

Huygens' metasurfaces via the equivalence principle: design and applications

The concept of coding metasurface makes a link between physically metamaterial particles and digital codes, and hence it is possible to perform digital signal processing on the coding metasurface to realize unusual physical phenomena. Here, this study presents to perform Fourier operations on coding metasurfaces and proposes a principle called as scattering-pattern shift using the convolution theorem, which allows steering of the scattering pattern to an arbitrarily predesigned direction. Owing to the constant reflection amplitude of coding particles, the required coding pattern can be simply achieved by the modulus of two coding matrices. This study demonstrates that the scattering patterns that are directly calculated from the coding pattern using the Fourier transform have excellent agreements to the numerical simulations based on realistic coding structures, providing an efficient method in optimizing coding patterns to achieve predesigned scattering beams. The most important advantage of this approach over the previous schemes in producing anomalous single-beam scattering is its flexible and continuous controls to arbitrary directions. This work opens a new route to study metamaterial from a fully digital perspective, predicting the possibility of combining conventional theorems in digital signal processing with the coding metasurface to realize more powerful manipulations of electromagnetic waves.

Convolution Operations on Coding Metasurface to Reach Flexible and Continuous Controls of Terahertz Beams

Metasurfaces are ultrathin metamaterials consisting of planar electromagnetic (EM) microstructures (e.g., meta-atoms) with pre-determined EM responses arranged in specific sequences. Based on careful structural designs on both meta-atoms and global sequences, one can realize homogenous and inhomogeneous metasurfaces that can possess exceptional capabilities to manipulate EM waves, serving as ideal candidates to realize ultracompact and highly efficient EM devices for next-generation integration-optics applications. In this paper, we present an overview on the development of metasurfaces, including both homogeneous and inhomogeneous ones, focusing particularly on their working principles, the fascinating wave-manipulation effects achieved both statically and dynamically, and the representative applications so far realized. Finally, we also present our own perspectives on possible future directions of this fast-developing research field in the conclusion.

Electromagnetic metasurfaces: physics and applications

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.

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Anisotropic coding metamaterials and their powerful manipulation of differently polarized terahertz waves

Independent controls of orthogonally polarized transmitted waves using a Huygens metasurface

We propose to radiate spoof surface plasmon polaritons (SPPs) in the endfire direction efficiently using metamaterial particles. The printed dipole is used as the main radiator, while the spoof SPP waveguide acts as balun to feed the printed dipole. A loaded I-shaped resonator array is configured to realize a high effective index of refraction, resulting in the gain enhancement in the endfire direction. A prototype of the spoof SPP antenna is fabricated and measured. Both numerical simulations and measured results demonstrate good performance of the proposed structure, which presents a gain of about 7 dBi at the designed frequency (6 GHz). The proposed endfire SPP antenna is of great value in SPP communication systems and integrated circuits due to its compact dimension, easy fabrication, and good performance.

Endfire Radiations of Spoof Surface Plasmon Polaritons

This paper presents a novel compact meta-structure, which is characterized by left-handed properties. In the novel structure, two linear arrays of metallic vias and short-stub inductors are used to generate shunt inductors besides the series interdigital capacitors. Compared with conventional composite right/left-handed transmission-line structures, the new structure has weaker coupling to other components and is easily controlled by adjusting the shunt inductance and series capacitance to produce different left-handed properties. Due to the isolation of two via arrays, the proposed structure can also be transplanted directly from the microstrip system to the coplanar-waveguide system. We have proposed an efficient procedure to retrieve the equivalent propagating modes for both forward and backward waves based on the matrix pencil method, which provides an easy way to study the propagation characteristics. Using such a meta-structure, a series of super-wide bandpass filters are designed and fabricated at different frequency bands. Good agreements between simulation and experiment results have been achieved, and good performance in passbands and stopbands has been observed with a relative 3-dB bandwidth larger than 70%. For easy design of such super-wide bandpass filters, an experienced formula is given between the physical size and the center frequency of the 3-dB passband, which can be used to redesign any filters at other frequency bands

Design and Analysis of Super-Wide Bandpass Filters Using a Novel Compact Meta-Structure

Transformation optics has led the way in the development of electromagnetic invisibility cloaks from science fiction to engineering practice. Invisibility cloaks have been demonstrated over a wide range of the electromagnetic spectrum, and with a variety of different fabrication techniques. However, all previous schemes have relied on the use of metamaterials consisting of arrays of sub-wavelength inclusions. We report on the first cloaking structure made of a high-κ dielectric-loaded foam mixture. A polyurethane foam mixed with different ratios of barium titanate is used to produce the required range of permittivities, and the invisibility cloak is demonstrated to work for all incident angles over a wide range of microwave frequencies. This method will greatly facilitate the development and large-scale manufacture of a wide range of transformation optics-based structures.

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Di Bao

Papers

Anisotropic coding metamaterials and their powerful manipulation of differently polarized terahertz waves

Independent controls of orthogonally polarized transmitted waves using a Huygens metasurface

Endfire Radiations of Spoof Surface Plasmon Polaritons

Design and Analysis of Super-Wide Bandpass Filters Using a Novel Compact Meta-Structure

All-dielectric invisibility cloaks made of BaTiO3-loaded polyurethane foam