Transparent dynamic metasurface for a visually unaffected reconfigurable intelligent surface: controlling transmission/reflection and making a window into an RF lens

Optically-transparent meta-window for wireless communication

Circumventing the attenuation of microwaves during the propagation is of prime importance to wireless communication towards higher carrier frequencies. Here, we propose a scheme of wireless communications via a functionalized meta-window constructed by an optically-transparent metasurface (OTM) consisting of indium tin oxide (ITO) patterns. When the signal is weak, the OTM can significantly strengthen the signal by focusing the incoming waves towards the windowsill, thus substantially enhancing the network speed. The intensity enhancement of microwaves at 5 GHz via an OTM is verified by both numerical simulations and experiments. Furthermore, the ability to increase the data transfer rate in a 5-GHz-WiFi environment is directly demonstrated. Our work demonstrates the feasibility of applying an optically-transparent meta-window for enhancing wireless communications.

Metasurface-Assisted Terahertz Sensing

Terahertz (THz) waves, which fall between microwaves and infrared bands, possess intriguing electromagnetic properties of non-ionizing radiation, low photon energy, being highly sensitive to weak resonances, and non-polar material penetrability. Therefore, THz waves are extremely suitable for sensing and detecting chemical, pharmaceutical, and biological molecules. However, the relatively long wavelength of THz waves (30~3000 μm) compared to the size of analytes (1~100 nm for biomolecules, <10 μm for microorganisms) constrains the development of THz-based sensors. To circumvent this problem, metasurface technology, by engineering subwavelength periodic resonators, has gained a great deal of attention to enhance the resonance response of THz waves. Those metasurface-based THz sensors exhibit high sensitivity for label-free sensing, making them appealing for a variety of applications in security, medical applications, and detection. The performance of metasurface-based THz sensors is controlled by geometric structure and material parameters. The operating mechanism is divided into two main categories, passive and active. To have a profound understanding of these metasurface-assisted THz sensing technologies, we review and categorize those THz sensors, based on their operating mechanisms, including resonators for frequency shift sensing, nanogaps for enhanced field confinement, chirality for handedness detection, and active elements (such as graphene and MEMS) for advanced tunable sensing. This comprehensive review can serve as a guideline for future metasurfaces design to assist THz sensing and detection.

Alignment-free twisted-split-ring metasurface on single substrate with 2π phase range for linearly polarized sub-terahertz wave

To achieve high-speed, large-capacity communication, next-generation mobile communication systems will require manipulation of the propagation of sub-terahertz waves in the propagation channel. In this paper, we propose the use of a novel split-ring resonator (SRR) structure as a metasurface unit cell for manipulating the linearly polarized incident and transmission waves used in mobile communication systems. In this SRR structure, the gap is twisted by 90° to efficiently use cross-polarized scattered waves. By changing the twist direction and gap size of the unit cell, 2π phase designability can be achieved, which enables linear polarization conversion efficiencies of -2 dB with a backside polarizer and -0.2 dB with two polarizers. In addition, a complementary pattern of the unit cell was fabricated, and a measured conversion efficiency of more than -1 dB at the peak with only the backside polarizer on a single substrate was verified. In the proposed structure, the 2π phase designability and efficiency gain are obtained independently by the unit cell and polarizer, respectively, thus enabling alignment-free characteristics, which are highly advantageous from an industrial viewpoint. Metasurface lenses with binary phase profiles of 0 and π were fabricated using the proposed structure with a backside polarizer on a single substrate. The lenses' focusing, deflection, and collimation operations were experimentally verified with a lens gain of 20.8 dB, which agreed well with our calculated results. Our metasurface lens has the great advantages of easy fabrication and implementation, and it has the potential to enable dynamic control by combining it with active devices because of the simple design methodology, which entails only changing the twist direction and the gap's capacitance component.

Recent Progress in Reconfigurable and Intelligent Metasurfaces: A Comprehensive Review of Tuning Mechanisms, Hardware Designs, and Applications

Intelligent metasurfaces have gained significant importance in recent years due to their ability to dynamically manipulate electromagnetic (EM) waves. Their multifunctional characteristics, realized by incorporating active elements into the metasurface designs, have huge potential in numerous novel devices and exciting applications. In this article, recent progress in the field of intelligent metasurfaces are reviewed, focusing particularly on tuning mechanisms, hardware designs, and applications. Reconfigurable and programmable metasurfaces, classified as space gradient, time modulated, and space-time modulated metasurfaces, are discussed. Then, reconfigurable intelligent surfaces (RISs) that can alter their wireless environments, and are considered as a promising technology for sixth-generation communication networks, are explored. Next, the recent progress made in simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs) that can achieve full-space EM wave control are summarized. Finally, the perspective on the challenges and future directions of intelligent metasurfaces are presented.

Analysis of Asymmetry in Active Split-Ring Resonators to Design Circulating-Current Eigenmode: Demonstration of Beamsteering and Focal-Length Control toward Reconfigurable Intelligent Surface

In this work, toward an intelligent radio environment for 5G/6G, design methodologies of active split-ring resonators (SRRs) for more efficient dynamic control of metasurfaces are investigated. The relationship between the excitation of circulating-current eigenmode and the asymmetric structure of SRRs is numerically analyzed, and it is clarified that the excitation of the circulating-current mode is difficult when the level of asymmetry of the current path is decreased by the addition of large capacitance such as from semiconductor-based devices. To avoid change in the asymmetry, we incorporated an additional gap (slit) in the SRRs, which enabled us to excite the circulating-current mode even when a large capacitance was implemented. Prototype devices were fabricated according to this design methodology, and by the control of the intensity/phase distribution, the variable focal-length and beamsteering capabilities of the transmitted waves were demonstrated, indicating the high effectiveness of the design. The presented design methodology can be applied not only to the demonstrated case of discrete varactors, but also to various other active metamaterials, such as semiconductor-integrated types for operating in the millimeter and submillimeter frequency bands as potential candidates for future 6G systems.