Electromagnetic Response and Energy Conversion for Functions and Devices in Low‐Dimensional Materials

In this paper, a double V-shaped metasurface that can efficiently convert linear polarizations of electromagnetic (EM) waves in wideband is proposed. Based on the electric and magnetic resonant features of a single V-shaped particle, four EM resonances are generated in a V-shaped pair, leading to significant bandwidth expansion of cross-polarized reflections. The simulation results show that the proposed metasurface is able to convert linearly polarized waves into cross-polarized waves in ultrawideband from 12.4 to 27.96 GHz, with an average polarization conversion ratio (PCR) of 90%. The experimental results are in good agreement with the numerical simulations. Compared to published designs, the proposed polarization converter has a simple geometry but an ultrawideband and hence can be used in many applications, such as reflector antennas, imaging systems, remote sensors, and radiometers. The method can also be extended to the terahertz band.

Ultrawideband and High-Efficiency Linear Polarization Converter Based on Double V-Shaped Metasurface

In the wake of intense research on metamaterials the two-dimensional analogue, known as metasurfaces, has attracted progressively increasing attention in recent years due to the ease of fabrication and smaller insertion losses, while enabling an unprecedented control over spatial distributions of transmitted and reflected optical fields. Metasurfaces represent optically thin planar arrays of resonant subwavelength elements that can be arranged in a strictly or quasi periodic fashion, or even in an aperiodic manner, depending on targeted optical wavefronts to be molded with their help. This paper reviews a broad subclass of metasurfaces, viz. gradient metasurfaces, which are devised to exhibit spatially varying optical responses resulting in spatially varying amplitudes, phases and polarizations of scattered fields. Starting with introducing the concept of gradient metasurfaces, we present classification of different metasurfaces from the viewpoint of their responses, differentiating electrical-dipole, geometric, reflective and Huygens' metasurfaces. The fundamental building blocks essential for the realization of metasurfaces are then discussed in order to elucidate the underlying physics of various physical realizations of both plasmonic and purely dielectric metasurfaces. We then overview the main applications of gradient metasurfaces, including waveplates, flat lenses, spiral phase plates, broadband absorbers, color printing, holograms, polarimeters and surface wave couplers. The review is terminated with a short section on recently developed nonlinear metasurfaces, followed by the outlook presenting our view on possible future developments and perspectives for future applications.

Gradient metasurfaces: a review of fundamentals and applications.

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

Traditionally, “metamaterials” have been described by effective medium parameters due to the subwavelength nature of unit particles. The continuous nature of medium parameters makes traditional metamaterials behave as analog metamaterials. Recently, the concept of coding metamaterials or “metasurfaces” has been proposed, in which metamaterials are characterized by digital coding particles of “0” and “1” with opposite phase responses. It has been demonstrated that electromagnetic waves can be manipulated by changing the coding sequences of “0” and “1”. The coding particles provide a link between the physical world and digital world, leading to digital metamaterials and even field programmable metamaterials, which can be used to control electromagnetic waves in real time. The digital coding representation of metamaterials or metasurfaces can also allow the concepts and signal processing methods in information science to be introduced to physical metamaterials, thereby realizing extreme control of electromagnetic waves. Such studies have set the foundation of information metamaterials and metasurfaces. In this review article, the coding, digital, and field programmable metamaterials and metasurfaces are systematically summarized and analyzed with particular emphases on the information and digital convolution aspects. The future trend of information metamaterial/metasurface is predicted, including software-defined metamaterials/metasurfaces and cognitive metamaterials/metasurfaces.

Information metamaterials and metasurfaces

Achieving multiple diversified functionalities in a single flat device is crucial for electromagnetic (EM) integration, but available efforts suffer the issues of device thickness, low efficiency, and restricted functionalities. Here, a general strategy to design high-efficiency bifunctional devices based on metasurfaces composed by anisotropic meta-atoms with polarization-dependent phase responses is described. Based on the derived general criterions, two bifunctional metadevices, working in reflection and transmission modes, respectively, that can realize two distinct functionalities with very high efficiencies (≈90% in reflection geometry and ≈72% in transmission one) are designed and fabricated. Microwave experiments, including both far-field and near-field characterizations, are performed to demonstrate the predicted effects, which are in excellent agreement with numerical simulations. The findings in this study can motivate the realizations of high-performance bifunctional metadevices in other frequency domains and with different functionalities, which are of crucial importance in EM integration.

High‐Performance Bifunctional Metasurfaces in Transmission and Reflection Geometries

We propose a high-gain transmitting lens antenna by employing layered phase-gradient metasurface (MS). The MS is engineered to focus the propagating plane wave to a point with high efficiency. An X-band patch antenna is placed at the focal point of the MS as a feed source, and then the quasi-spherical wave emitted by the source is transformed to plane wave. Due to the successful conversion of quasi-spherical wave to plane wave, the beam width of the patch antenna has been decreased 66° and the gain has been enhanced 11.6 dB. The proposed lens antenna not only opens up a new route for the applications of phase-gradient MS in microwave band, but also affords an alternative for high-gain antenna.

X-Band Phase-Gradient Metasurface for High-Gain Lens Antenna Application

Although microstrip reflectarrays/transmitarrays have been extensively studied in the past decades, most previous designs were confined to monofunctional operations based on either transmission or reflection In this communication, we propose a scheme to design multifunctional arrays that can simultaneously exhibit the functionalities of a reflectarray and a transmitarray on the basis of the appealing feature of a polarizer we discovered (ie, constant phase difference between its cross-polarization transmission and copolarization reflection within a broadband) To demonstrate the proposed scheme, we designed and fabricated a multifunctional device comprising a $15 \times 15$ array of twisted complementary dual-split ring resonators, each carefully designed to exhibit the desired transmission phase satisfying a parabolic distribution Feeding the device by a Vivaldi antenna at its focus, we numerically and experimentally demonstrated that our system functioned as a directive emitter working in a transmission/reflection mode for cross-polarization/copolarization radiation at low/high frequencies, and it can radiate directively in both directions with different polarizations at intermediate frequencies The half-power beamwidth of the array antenna was $\sim 15^{\circ }$ , which is 40° narrower than that of a bare Vivaldi antenna Moreover, the gain was higher than 13 dB in all cases studied, which is at least 7 dB higher than that of the Vivaldi antenna

Multifunctional Microstrip Array Combining a Linear Polarizer and Focusing Metasurface

We propose a kind of chiral metamaterial inspired from the fractal concept The Hilbert fractal perturbation in the twisted split ring resonator element results in compact metamaterial and breaking mirror symmetry, which readily forms chirality over triple bands The discrepancy between co-polarization conversion and cross-polarization conversion over multiple bands can be explored for multifunctional devices A multiband circular polarizer is then numerically and experimentally studied in the X band based on the bilayered twisted Hilbert resonator with mutual 90° rotation The ability of transforming linearly polarized incident waves to circularly polarized waves is unambiguously demonstrated with high conversion efficiency and large polarization extinction ratio of more than 20 dB across dual bands Moreover, exceptionally strong optical activity and circular dichroism are also observed

Compact dual-band circular polarizer using twisted Hilbert-shaped chiral metamaterial.

A novel triple-layer dual-mode meta-atom wherein an H-shaped structure is combined with a pair of symmetric patches is proposed. The composite structure substantially lowered the operation frequency, and balanced phase agility with transmission magnitude bandwidth. The transmission phase limit of the proposed composite structure approaches the theoretical limit. Because this structure has appealing features, we employed a set of these ultrathin meta-atoms to implement the phase distribution of an optimized array using the alternating projection method. An X-band single-feed quad-beam transmitarray consisting of $25 \times 25$ elements, each carefully designed to exhibit the desired transmission phase, was designed, simulated, physically implemented, and measured. Feeding the meta-array using a horn at its focus, four-beam radiation patterns with satisfactory sidelobe levels and gain were numerically and experimentally demonstrated. The peak gain was found to be 18.8 dB at 9.6 GHz, and the aperture efficiency was calculated as 38.3%. Moreover, the half-power beamwidth of the array antenna was approximately 7°, which was 50° narrower than that of a bare feed horn. Moreover, the gain of each beam was higher than 17 dB in all studied cases, which was at least 7 dB higher than that of a bare feed horn.

Tong Cai

Papers

High‐Performance Bifunctional Metasurfaces in Transmission and Reflection Geometries

X-Band Phase-Gradient Metasurface for High-Gain Lens Antenna Application

Multifunctional Microstrip Array Combining a Linear Polarizer and Focusing Metasurface

Compact dual-band circular polarizer using twisted Hilbert-shaped chiral metamaterial.

Dual-Mode Transmissive Metasurface and Its Applications in Multibeam Transmitarray