A Reconfigurable Active Huygens' Metalens

Dynamic Hyperspectral Holography Enabled by Inverse-Designed Metasurfaces with Oblique Helicoidal Cholesterics

Metasurfaces are flat arrays of nanostructures that allow exquisite control of phase and amplitude of incident light. Although metasurfaces offer new active element for both fundamental science and applications, the challenge still remains to overcome their low information capacity and passive nature. Here, by integrating an inverse-designed-metasurface with oblique helicoidal cholesteric liquid crystal (ChOH), simultaneous spatial and spectral tunable metasurfaces with a high information capacity for dynamic hyperspectral holography, are demonstrated. The inverse design facilitates a single-phase map encoding of ten independent holographic images at different wavelengths. ChOH provides precise spectral modulation with narrow bandwidth and wide tunable regime in response to programmed stimuli, thus enabling dynamic switching of the multicolor holography. The results provide simple and generalizable principles for the rational design of interactive metasurfaces that will find numerous applications, including security platform.

An Ultrathin, Fast-Response, Large-Scale Liquid-Crystal-Facilitated Multi-Functional Reconfigurable Metasurface for Comprehensive Wavefront Modulation

The rapid advancement of prevailing communication/sensing technologies necessitates cost-effective millimeter-wave arrays equipped with a massive number of phase-shifting cells to perform complicated beamforming tasks. Conventional approaches employing semiconductor switch/varactor components or tunable materials encounter obstacles such as quantization loss, high cost, high complexity, and limited adaptability for realizing large-scale arrays. Here, a low-cost, ultrathin, fast-response, and large-scale solution relying on metasurface concepts combined together with liquid crystal (LC) materials requiring a layer thickness of only 5 µm is reported. Rather than immersing resonant structures in LCs, a joint material-circuit-based strategy is devised, via integrating deep-subwavelength-thick LCs into slow-wave structures, to achieve constitutive metacells with continuous phase shifting and stable reflectivity. An LC-facilitated reconfigurable metasurface sub-system containing more than 2300 metacells is realized with its unprecedented comprehensive wavefront manipulation capacity validated through various beamforming functions, including beam focusing/steering, reconfigurable vortex beams, and tunable holograms, demonstrating a milli-second-level function-switching speed. The proposed methodology offers a paradigm shift for modulating electromagnetic waves in a non-resonating broadband fashion with fast-response and low-cost properties by exploiting functionalized LC-enabled metasurfaces. Moreover, this extremely agile metasurface-enabled antenna technology will facilitate a transformative impact on communication/sensing systems and empower new possibilities for wavefront engineering and diffractive wave calculation/inference.

Broadband generation of a multi-polarization multi-beam using a receiving-transmitting metasurface

Generating multiple beams in distinct polarization states is promising in multi-mode wireless communication but still remains challenging in metasurface design. Here, we theoretically and experimentally demonstrate a concept of broadband receiving-transmitting metasurface and its application to the generation of multi-polarization multi-beam. By employing U-slot patch, an efficient receiving-transmitting element with full phase coverage is designed within a wide bandwidth. Based on this architecture, a methodology is proposed to generate dual spin-decoupled beams and then developed into the strategy of generating multiple beams at different linear polarizations. To verify our strategy, two lens antennas, respectively radiating dual-spin dual-beam and quad-polarization quad-beam, are devised. With multi-polarization multi-beam radiated, the two lens antennas are both with whole aperture efficiency above 40% within the bandwidth of 10.6-12.3 GHz (14.8%), firmly validating our strategy and design.

Full-Color Imaging System Based on the Joint Integration of a Metalens and Neural Network

Lenses have been a cornerstone of optical systems for centuries; however, they are inherently limited by the laws of physics, particularly in terms of size and weight. Because of their characteristic light weight, small size, and subwavelength modulation, metalenses have the potential to miniaturize and integrate imaging systems. However, metalenses still face the problem that chromatic aberration affects the clarity and accuracy of images. A high-quality image system based on the end-to-end joint optimization of a neural network and an achromatic metalens is demonstrated in this paper. In the multi-scale encoder-decoder network, both the phase characteristics of the metalens and the hyperparameters of the neural network are optimized to obtain high-resolution images. The average peak-signal-to-noise ratio (PSNR) and average structure similarity (SSIM) of the recovered images reach 28.53 and 0.83. This method enables full-color and high-performance imaging in the visible band. Our approach holds promise for a wide range of applications, including medical imaging, remote sensing, and consumer electronics.

Independent Manipulations of Transmitting and Receiving Channels by Nonreciprocal Programmable Metasurface

The electromagnetic (EM) beam manipulations such as spatial scanning have always been the focus in information science and technology. Generally, the transmitting and receiving (T/R) beams of the same aperture should be coincident due to the reciprocal theory, and hence, more flexible controls of the spatial information are limited accordingly. Here, we propose a new approach to achieve independent controls of beam scanning in spatial T/R channels based on one aperture made by a nonreciprocal programmable metasurface. The meta-atom is designed to have independent propagation chains for T/R waves by introducing dual-direction power amplifiers (PAs) as the isolators for one-way transparency. A programmable phase shifter with a 360° coverage is loaded with the PA device in the transmitting or receiving chain to realize independent beam scanning in the T/R channels. A prototype of the proposed metasurface is fabricated, and independent beam scanning in the T/R channels is directly acquired with good performance in our measurements. In addition, a proof of concept of integrated sensing and auxiliary communications is accomplished to verify the validity of the presented method.

High-Resolution Metalens Imaging with Sequential Artificial Intelligence Models

An analysis of the optical response of a GaN-based metalens was conducted alongside the utilization of two sequential artificial intelligence (AI) models in addressing the occasional issues of blurriness and color cast in captured images. The optical loss of the metalens in the blue spectral range was found to have resulted in the color cast of images. Autoencoder and CodeFormer sequential models were employed in order to correct the color cast and reconstruct image details, respectively. Said sequential models successfully addressed the color cast and reconstructed details for all of the allocated face image categories. Subsequently, the CIE 1931 chromaticity diagrams and peak signal-to-noise ratio analysis provided numerical evidence of the AI models' effectiveness in image reconstruction. Furthermore, the AI models can still repair the image without blue information. Overall, the integration of metalens and artificial intelligence models marks a breakthrough in enhancing the performance of full-color metalens-based imaging systems.

A One-Bit Programmable Multi-Functional Metasurface for Microwave Beam Shaping

In this paper, we demonstrate a multi-functional metasurface for microwave beam-shaping application. The metasurface consists of an array of programmable unit cells, and each unit cell is integrated with one varactor diode. By turning the electrical bias on the diode on and off, the phase delay of the microwave reflected by the metasurface can be switched between 0 and π at a 6.2 GHz frequency, which makes the metasurface 1-bit-coded. By programming the 1-bit-coded metasurface, the generation of a single-focus beam, a double-focus beam and a focused vortex beam was experimentally demonstrated. Furthermore, the single-focus beam with tunable focal lengths of 54 mm, 103 mm and 152 mm was experimentally observed at 5.7 GHz. The proposed programmable metasurface manifests robust and flexible beam-shaping ability which allows its application to microwave imaging, information transmission and sensing applications.

High-efficiency plasmonic vortex generation with near-infrared bifunctional metasurfaces

Plasmonic vortices have shown a wide range of applications in on-chip photonics due to their fascinating properties of the orbital angular momenta (OAM) and phase singularity. However, conventional devices to generate them suffer from issues of low efficiencies and limited functionalities. Here, we establish a systematic scheme to construct high-efficiency bifunctional metasurfaces that can generate two plasmonic vortices exhibiting distinct topological charges, based on a series of reflective meta-atoms exhibiting tailored reflection-phases dictated by both resonant and geometric origins. As a benchmark test, we first construct a meta-coupler with meta-atoms exhibiting geometric phases only, and experimentally demonstrate that it can generate a pre-designed plasmonic vortex at the wavelength of 1064 nm with an efficiency of 27% (56% in simulation). Next, we design/fabricate two bifunctional metasurfaces with meta-atoms integrated with both resonant and geometric phases, and experimentally demonstrate that they can generate divergent (or focused) or convergent (or defocused) plasmonic vortices with district OAM as shined by circularly polarized light with opposite helicity at 1064 nm wavelength. Our work provides an efficient platform to generate plasmonic vortices as desired, which can find many applications in on-chip photonics.

Programmable multifunctional metasurface for polarization, phase, and amplitude manipulation

Metasurfaces have shown extraordinary capability in individually manipulating various electromagnetic (EM) properties, including polarization, phase, and amplitude. However, it is still a challenge to manipulate these EM properties in one metasurface simultaneously. In this paper, a programmable multifunctional metasurface (PMFMS) is demonstrated with polarization, phase, and amplitude manipulation abilities. By controlling tunable coding states and changing the direction of incident waves, the PMFMS can operate as a transmission cross-polarization converter, spatial wave manipulator, and low-RCS radome. Besides, the PMFMS possesses an ultra-wideband property, which can operate from 6.5 to 10.2 GHz with 44.3% relative bandwidth. More importantly, multiple functionalities can also be achieved in reflection operating mode by reassembling the PMFMS. As a proof of concept, the PMFMS is fabricated and experimentally verified. Measured results are in good agreement with simulated results. Benefiting from multifunctional EM manipulations in an ultra-wideband, such a design can be applied in wireless communication systems, radar detection, and EM stealth platform.

Controlling the polarization dependence of the complex transmission spectrum using the Brewster effect in metafilms

We propose a method for controlling the polarization dependence of the complex transmission spectrum using the Brewster effect in a two-dimensional array of meta-atoms with finite thickness, which we refer to as a metafilm. We show that the complex transmission spectra of the orthogonal linear polarization components can be controlled independently without reflection by stacking multiple metafilms that exhibit only an electric dipole resonance. As a proof-of-concept numerical demonstration, we design several broadband waveplates with high transmission efficiency based on simple design principles. The proposed method would enable us to easily design anisotropic metamaterials with various complex transmission spectra.

Active polarization-converting metasurface with electrically controlled magnitude amplification

Recently, reconfigurable polarization-manipulation metasurfaces controlled with active components have gained widespread interest due to their adaptability, compact configuration, and low cost. However, due to the inherent non-negligible ohmic loss, the output energy of these tunable metasurfaces is typically diminished, particularly in the microwave region. To surmount the loss problem, herein, we propose an active polarization-converting metasurface with non-reciprocal polarization responses that is integrated with amplifying transistors. In addition, we provide a design strategy for a polarizer that is insensitive to polarization and has energy amplification capabilities. Experiments are conducted in the microwave region, and amplification of the polarization-converting behaviors is observed around 3.95 GHz. The proposed metasurface is prospective for applications in future wireless communication systems, such as spatial isolation, signal enhancement, and electromagnetic environment shaping.

Taming Fabry-Pérot resonances in a dual-metasurface multiband antenna with beam steering in one of the bands

Metasurfaces are artificial materials that can provide properties not readily available in nature for the interaction with acoustic, elastodynamic, or electromagnetic waves. In Electromagnetics, metasurfaces allow particular functionalities to antennas, which in turn lately have been increasingly pushed to a multiband operation. To fully exploit metasurfaces' capabilities, the use of a metasurface reflector and a metasurface superstrate surrounding a radiating element in multiband antennas is interesting. However, such topology generally creates multiple reflections inside the formed cavity, known as Fabry-Pérot resonances. Here we show that one should tame this phenomenon to use two parallel metasurfaces surrounding a planar radiating element. We present the conditions to obtain directive, multiband antennas under such circumstances. The concepts are validated with a compact device for 5G/4G/Wi-Fi 2.4/5/6E performing a beam steering in the 5G without disturbing the radiation patterns of the other bands. This device demonstrates that the functionalities of two metasurfaces may be exploited in a single design if the presented conditions are respected. We also anticipate our work to be a starting point for other studies in the wave domain. For example, compact, multiband, beam-steerable microphones or sonar transducers with two parallel metasurfaces could be investigated in the future.

Eye accommodation-inspired neuro-metasurface focusing

The human eye, which relies on a flexible and controllable lens to focus light onto the retina, has inspired many scientific researchers to understand better and imitate the biological vision system. However, real-time environmental adaptability presents an enormous challenge for artificial eye-like focusing systems. Inspired by the mechanism of eye accommodation, we propose a supervised-evolving learning algorithm and design a neuro-metasurface focusing system. Driven by on-site learning, the system exhibits a rapid response to ever-changing incident waves and surrounding environments without any human intervention. Adaptive focusing is achieved in several scenarios with multiple incident wave sources and scattering obstacles. Our work demonstrates the unprecedented potential for real-time, fast, and complex electromagnetic (EM) wave manipulation for various purposes, such as achromatic, beam shaping, 6 G communication, and intelligent imaging.

A Double-Layer Dual-Polarized Huygens Metasurface and Its Meta-Lens Antenna Applications

In this paper, a dual-polarized Huygens unit is proposed, which has a double-layer metallic pattern etched on both sides of one dielectric substrate. Induced magnetism enables the structure to support Huygens' resonance, thus obtaining nearly complete available transmission phase coverage. By optimizing the structural parameters, a better transmission performance can be achieved. When the Huygens metasurface was used for the design of a meta-lens, good radiation performance was exhibited, with a maximum gain of 31.15 dBi at 28 GHz, an aperture efficiency of 42.7% and a 3 dB gain bandwidth of 26.4 GHz to 30 GHz (12.86%). Due to its excellent radiation performance and very simple fabrication, this Huygens meta-lens has important applications in millimeter-wave communication systems.

Direction-Duplex Janus Metasurface for Full-Space Electromagnetic Wave Manipulation and Holography

Janus metasurfaces, a category of two-faced two-dimensional (2D) materials, are emerging as a promising platform for designing multifunctional metasurfaces by exploring the intrinsic propagation direction (k-direction) of electromagnetic waves. Their out-of-plane asymmetry is utilized for achieving distinct functions selectively excited by choosing the propagation directions, providing an effective strategy to meet the growing demand for the integration of more functionalities into a single optoelectronic device. Here, we propose the concept of direction-duplex Janus metasurface for full-space wave control yielding drastically different transmission and reflection wavefronts for the same polarized incidence with opposite k-directions. A series of Janus metasurface devices that enable asymmetric full-space wave manipulations, such as integrated metalens, beam generators, and fully direction-duplex meta-holography, are experimentally demonstrated. We envision the Janus metasurface platform proposed here to open new possibilities toward a broader exploration of creating sophisticated multifunctional meta-devices ranging from microwaves to optical systems.

Scalable manufacturing of high-index atomic layer-polymer hybrid metasurfaces for metaphotonics in the visible

Metalenses are attractive alternatives to conventional bulky refractive lenses owing to their superior light-modulating performance and sub-micrometre-scale thicknesses; however, limitations in existing fabrication techniques, including high cost, low throughput and small patterning area, have hindered their mass production. Here we demonstrate low-cost and high-throughput mass production of large-aperture visible metalenses using deep-ultraviolet argon fluoride immersion lithography and wafer-scale nanoimprint lithography. Once a 12″ master stamp is imprinted, hundreds of centimetre-scale metalenses can be fabricated using a thinly coated high-index film to enhance light confinement, resulting in a substantial increase in conversion efficiency. As a proof of concept, an ultrathin virtual reality device created with the printed metalens demonstrates its potential towards the scalable manufacturing of metaphotonic devices.

Intelligent metasurface system for automatic tracking of moving targets and wireless communications based on computer vision

The fifth-generation (5G) wireless communication has an urgent need for target tracking. Digital programmable metasurface (DPM) may offer an intelligent and efficient solution owing to its powerful and flexible controls of electromagnetic waves and advantages of lower cost, less complexity and smaller size than the traditional antenna array. Here, we report an intelligent metasurface system to perform target tracking and wireless communications, in which computer vision integrated with a convolutional neural network (CNN) is used to automatically detect the locations of moving targets, and the dual-polarized DPM integrated with a pre-trained artificial neural network (ANN) serves to realize the smart beam tracking and wireless communications. Three groups of experiments are conducted for demonstrating the intelligent system: detection and identification of moving targets, detection of radio-frequency signals, and real-time wireless communications. The proposed method sets the stage for an integrated implementation of target identification, radio environment tracking, and wireless communications. This strategy opens up an avenue for intelligent wireless networks and self-adaptive systems.

Soft Actor-Critic-Driven Adaptive Focusing under Obstacles

Electromagnetic (EM) waves that bypass obstacles to achieve focus at arbitrary positions are of immense significance to communication and radar technologies. Small-sized and low-cost metasurfaces enable the accomplishment of this function. However, the magnitude-phase characteristics are challenging to analyze when there are obstacles between the metasurface and the EM wave. In this study, we creatively combined the deep reinforcement learning algorithm soft actor-critic (SAC) with a reconfigurable metasurface to construct an SAC-driven metasurface architecture that realizes focusing at any position under obstacles using real-time simulation data. The agent learns the optimal policy to achieve focus while interacting with a complex environment, and the framework proves to be effective even in complex scenes with multiple objects. Driven by real-time reinforcement learning, the knowledge learned from one environment can be flexibly transferred to another environment to maximize information utilization and save considerable iteration time. In the context of future 6G communications development, the proposed method may significantly reduce the path loss of users in an occluded state, thereby solving the open challenge of poor signal penetration. Our study may also inspire the implementation of other intelligent devices.

Bifunctional Manipulation of Terahertz Waves with High-Efficiency Transmissive Dielectric Metasurfaces

Multifunctional terahertz (THz) devices in transmission mode are highly desired in integration-optics applications, but conventional devices are bulky in size and inefficient. While ultra-thin multifunctional THz devices are recently demonstrated based on reflective metasurfaces, their transmissive counterparts suffer from severe limitations in efficiency and functionality. Here, based on high aspect-ratio silicon micropillars exhibiting wide transmission-phase tuning ranges with high transmission-amplitudes, a set of dielectric metasurfaces is designed and fabricated to achieve efficient spin-multiplexed wavefront controls on THz waves. As a benchmark test, the photonic-spin-Hall-effect is experimentally demonstrated with a record high absolute efficiency of 92% using a dielectric metasurface encoded with geometric phases only. Next, spin-multiplexed controls on circularly polarized THz beams (e.g., anomalous refraction and focusing) are experimentally demonstrated with experimental efficiency reaching 88%, based on a dielectric meta-device encoded with both spin-independent resonant phases and spin-dependent geometric phases. Finally, high-efficiency spin-multiplexed dual holographic images are experimentally realized with the third meta-device encoded with both resonant and geometric phases. Both near-field and far-field measurements are performed to characterize these devices, yielding results in agreement with full-wave simulations. The study paves the way to realize multifunctional, high-performance, and ultra-compact THz devices for applications in biology sensing, communications, and so on.

Recent Advances in Reconfigurable Metasurfaces: Principle and Applications

Metasurfaces have shown their great capability to manipulate electromagnetic waves. As a new concept, reconfigurable metasurfaces attract researchers' attention. There are many kinds of reconfigurable components, devices and materials that can be loaded on metasurfaces. When cooperating with reconfigurable structures, dynamic control of the responses of metasurfaces are realized under external excitations, offering new opportunities to manipulate electromagnetic waves dynamically. This review introduces some common methods to design reconfigurable metasurfaces classified by the techniques they use, such as special materials, semiconductor components and mechanical devices. Specifically, this review provides a comparison among all the methods mentioned and discusses their pros and cons. Finally, based on the unsolved problems in the designs and applications, the challenges and possible developments in the future are discussed.

A 6G meta-device for 3D varifocal

The sixth-generation (6G) communication technology is being developed in full swing and is expected to be faster and better than the fifth generation. The precise information transfer directivity and the concentration of signal strength are the key topics of 6G technology. We report the synthetic phase design of rotary doublet Airy beam and triplet Gaussian beam varifocal meta-devices to fully control the terahertz beam's propagation direction and coverage area. The focusing spot can be delivered to arbitrary positions in a two-dimensional plane or a three-dimensional space. The highly concentrated signal can be delivered to a specific position, and the transmission direction can be adjusted freely to enable secure, flexible, and high-directivity 6G communication systems. This technology avoids the high costs associated with extensive use of active components. 6G communication systems, wireless power transfer, zoom imaging, and remote sensing will benefit from large-scale adoption of such a technology.

Metasurface-Assisted Wireless Communication with Physical Level Information Encryption

Since the discovery of wireless telegraphy in 1897, wireless communication via electromagnetic (EM) signals has become a standard solution to address increasing demand for information transfer in modern society. With the rapid growth of EM wave manipulation technique, programmable metasurface (PM) has emerged as a new type of wireless transmitter by directly modulating digital information without complex microwave components, thus providing an alternative to simplify the conventional wireless communication system. However, the challenges of improving information security and spectrum utilization still exist. Here, a dual-band metasurface-assisted wireless communication scheme is introduced to provide additional physical channels for the enhancement of information security. The information is divided into several parts and transmitted through different physical channels to accomplish information encryption, greatly reducing the possibility of eavesdropping. As the proof of concept, a dual-channel and high-security wireless communication system based on a 1-bit PM is established to simultaneously transmit two different parts of a picture to two receivers. Experiments show that the transmitted picture can be successfully retrieved only if the received signals of different receivers are synthetized as predefined. The proposed scheme provides a new route of employing PM in information encryption and spectrum utilization of wireless communication.

Fast Topology Optimization for Near-Field Focusing All-Dielectric Metasurfaces Using the Discrete Dipole Approximation

Using an efficient implementation of the discrete dipole approximation and topology optimization, we design all-dielectric metasurfaces capable of focusing light into intense deep subwavelength hotspots. The light focusing of these metasurfaces far outweighs conventional lenses and can provide dramatic enhancements of processes that depend superlinearly on light intensity, such as light-powered membrane distillation and photocatalysis. Our approach can easily be generalized to optimize metasurfaces for other functionalities, such as nonlinear optics or photothermal conversion.

Tailorable Switching of Transmission Modes between Waveguide and Spoof Surface Plasmon Polariton for Flexible and Dynamic Control of Phase Shift

Programmable metamaterials are suitable for their dynamic and real-time control capabilities of electromagnetic (EM) functions in radars and antenna communications, but it remains a challenge to achieve dynamic modulation of arbitrary transmission phase with high transmission efficiency. Here, we propose a paradigm to tailor transmission phase shift in real time by switching modes between waveguide and SSPP based on the voltage-driven PIN diodes. Step-like phase shift is achieved by the "ON" and "OFF" states of PIN diodes, while continuous phase regulation is by the characteristic of the nonlinear region between those two states. As validations, three systems with programmable functionalities are implemented, including the multibeam generator, the dual-beam scanner, and the active phased-array antenna. The experimental results are consistent with simulation, which verify the feasibility of the proposed approach. Our work offers an alternative route for transmission full-phase modulation and provides unprecedented potential for high-gain, real-time, and multidimensional EM capabilities in applications such as active phased array radars, self-adaption radomes, smart beam shaping.

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.

Deep-Learning-Empowered Holographic Metasurface with Simultaneously Customized Phase and Amplitude

Metasurfaces with simultaneously and independently controllable amplitude and phase have provided a higher degree of freedom in manipulating electromagnetic (EM) waves. Compared with phase- or amplitude-only modulation, the capability of simultaneously controlling the phase and amplitude of EM waves can enable holography with a higher resolution. However, this drastically increases the design complexity of holographic metasurfaces, and the design process is usually quite time-consuming. In this paper, we propose an inverse design of meta-atoms that can simultaneously and independently tailor the phase and amplitude of transmitted waves using customized deep ResNet while eliminating the coupling of parameters. To demonstrate the design method, two holographic metasurfaces were designed using the trained network without the need for parameter sweeping, which will significantly enhance design efficiency. Prototypes were fabricated and measured. Both the simulated and measured results show that high-resolution holography is obtained, which sufficiently verifies the reliability of the design method. Our work paves the way for the intelligent design of metasurfaces and can also be applied to the design of other artificial materials or surfaces.

Efficient generation of a dual-polarized vortex wave with an ultrathin Huygens' metasurface

In this paper, an ultrathin Huygens' metasurface is designed for generating an orbital angular momentum (OAM) beam. The Huygens' metasurface is a double-layered metallic structure on a single-layer PCB. Based on induced magnetism, the Huygens' metasurface achieves the abilities of available near-complete transmission phase shift around 28 GHz. According to the principle of vortex wave generation, a Huygens' metasurface is designed, implemented and measured. The simulated and measured results show that the dual-polarized OAM transmitted waves with the mode l = 1 can be efficiently generated on a double-layered Huygens' metasurface around 28 GHz. The measured peak gain is 23.4 dBi at 28 GHz, and the divergence angle is 3.5°. Compared with conventional configurations of OAM transmitted beam generation, this configuration has the advantages of high gain, narrow divergence angle, and low assembly cost. This investigation will provide a new perspective for engineering application of OAM beams.

Reconfigurable Radiation Angle Continuous Deflection of All-Dielectric Phase-Change V-Shaped Antenna

All-dielectric optical antenna with multiple Mie modes and lower inherent ohmic loss can achieve high efficiency of light manipulation. However, the silicon-based optical antenna is not reconfigurable for specific scenarios. The refractive index of optical phase-change materials can be reconfigured under stimulus, and this singular behavior makes it a good candidate for making reconfigurable passive optical devices. Here, the optical radiation characteristics of the V-shaped phase-change antenna are investigated theoretically. The results show that with increasing crystallinity, the maximum radiation direction of the V-shaped phase-change antenna can be continuously deflected by 90°. The exact multipole decomposition analysis reveals that the modulus and interference phase difference of the main multipole moments change with the crystallinity, resulting in a continuous deflection of the maximum radiation direction. Thus, the power ratio in the two vertical radiation directions can be monotonically reversed from -12 to 7 dB between 20% and 80% crystallinity. The V-shaped phase-change antenna exhibits the potential to act as the basic structural unit to construct a reconfigurable passive spatial angular power splitter or wavelength multiplexer. The mechanism analysis of radiation directivity involving the modulus and interference phase difference of the multipole moments will provide a reference for the design and optimization of the phase-change antenna.

Full-space dual-helicity decoupled metasurface for a high-efficiency multi-folded reflective antenna

The independent tailoring of electromagnetic waves with different circular-polarized (CP) wavefront in both reflection and transmission channels is of broad scientific and technical interest, offering ultimate degrees of freedom in designing advanced devices with the merits of functionality integration and spatial exploitation. However, most metasurfaces only provide dependent wavefront control of dual-helicity in a single channel, restricting their applications to limited practical scenarios. Herein, we propose a full-space dual-helicity decoupled metasurface and apply it to assemble a multi-folded reflective antenna (MFRA) in the microwave regime. A multilayered chiral meta-atom is designed and optimized to reflect a particular helical wave while allowing the orthogonal helical wave to penetrate through, with simultaneous full span of phase modulations in both channels. When a uniform reflection and a hyperbolic transmission phase profile is imposed simultaneously on the metasurface in a polarization-selective manner, it can be engineered to conduct specular reflection for one helical wave and convergent transmission of the other helical wave. Combining the proposed metasurface with a metallic plate as a bottom reflector and an integrated microstrip patch antenna in the center of metasurface as a feed, a MFRA is realized with a low profile, high efficiency, and high polarization purity in a broad frequency band. The proposed design method of the dual-helicity decoupled metasurface and its antenna application provide opportunities for high-performance functional devices, promising more potential in future communication and detection systems.

Intelligent metasurfaces: digitalized, programmable, and intelligent platforms

Distinguished from conventional ones, intelligent metasurfaces are endowed with three important characteristics: digitalization, programmability, and intelligence, which can be further integrated with detection, artificial neural networks, and feedback systems into a smart platform. Metasurface-based smart systems are expected to play an important role in wireless communications, advanced sensing technologies and artificial intelligence.

Ultra-Thin/Wide-Band Polarization Conversion Metasurface and Its Applications in Anomalous Reflection and RCS Reduction

In this paper, the design of an ultra-wideband polarizer based on a metasurface with high-performance is reported and demonstrated. The polarizer is composed of a dielectric substrate with double semicircular gap patches and a metal film. Multiple strong resonance points enable the design to convert the incident linearly polarized waves into cross-polarized waves in the 14.8–28.0 GHz range, with a fractional bandwidth of 61.7% and a corresponding polarization conversion rate (PCR) above 95%. Further simulated results show that the PCR remains above 87% in the 14.37–24.75 GHz range when the incident angle of the electromagnetic (EM) waves is between 0–30°, and the physical mechanism is explained by the surface current distribution. In addition, the gradient metasurface is designed according to the Pancharatnam–Berry phase principle to achieve anomalous reflection, and the 1-bit metasurface is coded to reduce the Radar Cross Section (RCS). The EM waves reach an anomalous reflection of −23° at 15 GHz normal incidence, and the RCS is reduced by 10 dB in the range of 15.3–28.0 GHz. These findings have potential application value in stealth and antenna design.

Intelligent Reflecting Surfaces Assisted UAV Communications for Massive Networks: Current Trends, Challenges, and Research Directions

An intelligent reflecting surface (IRS) can intelligently configure wavefronts such as amplitude, frequency, phase, and even polarization through passive reflections and without requiring any radio frequency (RF) chains. It is predicted to be a revolutionizing technology with the capability to alter wireless communication to enhance both spectrum and energy efficiencies with low expenditure and low energy consumption. Similarly, unmanned aerial vehicle (UAV) communication has attained a significant interest by research fraternity due to high mobility, flexible deployment, and easy integration with other technologies. However, UAV communication can face obstructions and eavesdropping in real-time scenarios. Recently, it is envisaged that IRS and UAV can combine together to achieve unparalleled opportunities in difficult environments. Both technologies can achieve enhanced performance by proactively altering the wireless propagation through maneuver control and smart signal reflections in three-dimensional space. This study briefly discusses IRS-assisted UAV communications. We survey the existing literature on this emerging research topic for both ground and airborne scenarios. We highlight several emerging technologies and application scenarios for future wireless networks. This study goes one step further to elaborate research opportunities to design and optimize wireless systems with low energy footprint and at low cost. Finally, we shed some light on open challenges and future research directions for IRS-assisted UAV communication.

Bandpass Filter Integrated Metalens Based on Electromagnetically Induced Transparency

A bandpass filter integrated metalens based on electromagnetically induced transparency (EIT) for long-wavelength infrared (LWIR) imaging is designed in this paper. The bandwidth of the metalens, which is a diffractive optical element, decreases significantly with the increase of the aperture size to a fixed f-number, which leads to the decline of imaging performance. The same material composition and preparation process of the metalens and the EIT metasurface in the long-wavelength infrared make it feasible that the abilities of focusing imaging and filtering are integrated into a metasurface device. With the purpose of validating the feasibility of this design method, we have designed a 300-μm-diameter integrated metalens whose f-number is 0.8 and the simulation was carried out. The introduction of EIT metasurface does not affect the focusing near the diffraction limit at the target wavelength, and greatly reduces the influence of stray light caused by non-target wavelength incident light. This bandpass filter integrated metalens design method may have a great potential in the field of LWIR compact optical systems.

Extremely angle-stable transparent window for TE-polarized waves empowered by anisotropic metasurfaces

Impedance mismatch generally exists upon interfaces between different media. This is especially true for TE-polarized waves with large incident angles since there is no Brewster effect. As a result, high-efficiency transmission can only be guaranteed within limited incident angle range. It is desirable that transparent windows possess robust angle-stability. In this work, we propose a strategy of realizing transparent windows with extreme angle-stability using anisotropic metasurfaces. Different from traditional isotropic materials, anisotropic metasurfaces require specific three-dimensional permittivity and permeability parameters. Theoretical formulas are derived to realize a highly efficient transmission response without angular dispersion. To validate our design concept, a two-layer cascaded electromagnetic anti-reflector is designed, and it exhibits a characteristic impedance matching for nearly all incidence angles under TE-polarization illumination. As a proof-of-concept, a prototype of extremely angle-stable transparent window is fabricated and measured. Compared with the pure dielectric plate, the reflection coefficients are on average reduced by 40% at 13.5 GHz for TE-polarized waves from 0° to 80°. Therefore, we think, anisotropic cascaded electromagnetic transparent windows are capable of tailoring the electromagnetic parameter tensors as desired, and provide more adjustable degrees of freedom for manipulating electromagnetic wavefronts, which might open up a promising way for electromagnetic antireflection and find applications in radomes, IR windows and others.

Terahertz Airy beam generated by Pancharatnam-Berry phases in guided wave-driven metasurfaces

Metasurface antennas scatter traveling guided waves into spatial waves, which act as extendable subsources to overcome the size limitation on emission sources. With the use of a Pancharatnam-Berry phase metasurface stimulated by a circularly polarized wave in a waveguide, the local phase distributions of scattered spatial waves can be made consistent with those of an Airy beam, thereby allowing the generation of high-quality Airy beams. In a slab waveguide, circularly polarized waves are synthesized through superposition of in-plane transverse electric modes. Simulations demonstrate that a 20 mm × 20 mm footprint all-dielectric guided wave-driven metasurface generates a 2D Airy beam at a frequency of 0.6 THz. Furthermore, we employ a metasurface deposited on a strip waveguide to generate a 1D Airy beam under direct stimulation by the fundamental transverse electric mode. Our work not only provides a large-scale emitter, but it also suggests promising potential applications in on-chip imaging and holography.

Ultra-thin 2-bit anisotropic Huygens coding metasurface for terahertz wave manipulation

In this work, we design an ultrathin 2-bit anisotropic Huygens coding metasurface (AHCM) composed by bilayer metallic square-ring structures for flexible manipulation of the terahertz wave. Based on the polarized-dependent components of electric surface admittance and magnetic surface impedance, we confirm that both the electric and magnetic resonances on coding meta-atoms are excited, so as to provide a full phase coverage and significantly low reflection. By encoding the elements with distinct coding sequences, the x- and y-polarized incident waves are anomalously refracted into opposite directions. More uniquely, we also demonstrate that the designed AHCM can be utilized as a transmission-type quarter-wave plate. The proposed metasurface paves a new way toward multifunctional terahertz wavefront manipulation.

Spintronic terahertz emission with manipulated polarization (STEMP)

Highly efficient generation and arbitrary manipulation of spin-polarized terahertz (THz) radiation will enable chiral lightwave driven quantum nonequilibrium state regulation, induce new electronic structures, consequently provide a powerful experimental tool for investigation of nonlinear THz optics and extreme THz science and applications. THz circular dichromic spectroscopy, ultrafast electron bunch manipulation, as well as THz imaging, sensing, and telecommunication, also need chiral THz waves. Here we review optical generation of circularly-polarized THz radiation but focus on recently emerged polarization tunable spintronic THz emission techniques, which possess many advantages of ultra-broadband, high efficiency, low cost, easy for integration and so on. We believe that chiral THz sources based on the combination of electron spin, ultrafast optical techniques and material structure engineering will accelerate the development of THz science and applications.

Dual-band chiral metasurface for independent controls of spin-selective reflections

The development of chiral metasurfaces with spin-selective reflection or transmission provides a new way to control the circularly polarized (CP) waves. However, it is still a great challenge to independently manipulate the polarization, frequency, and phase of the spin-selective reflected waves in different operating bands, which may have potential applications in improving the data capacity of microwave and optical communication systems. Here, a dual-band chiral metasurface is proposed to generate gigantic intrinsic chirality with strong circular dichroism (CD) in two different frequency bands by piecing two typical mono-chiral units together. The polarization, frequency and phase of the spin-selective reflected waves can also be independently designed in the two operating bands by adjusting the configuration of the chiral unit structures. Based on the proposed chiral structures, a dual-band chiral metasurface with spin-selective anomalous reflections is designed and demonstrated by both simulations and experiments. The results show that the polarization of spin-selective reflected waves can be customized by selecting appreciate chiral structures, while the wavefront of the spin-selective reflected waves can be further controlled by designing their arrangement.

Design of the all-silicon long-wavelength infrared achromatic metalens based on deep silicon etching

An all-silicon long-wavelength infrared (LWIR) achromatic metalens based on deep silicon etching is designed in this paper. With a fixed aperture size, the value range of the equivalent optical thickness of the non-dispersive meta-atoms constructing the achromatic metalens determines the minimum f-number. The fabrication characteristic with high aspect ratio of deep silicon etching amplifies the difference value of optical thickness between different meta-atoms by increasing the propagation distance of the propagation mode, which ensures a small f-number to obtain a better imaging resolution. A 280-µm-diameter silicon achromatic metalens with a f-number of 1 and the average focusing efficiency of 27.66% has been designed and simulated to validate the feasibility of this strategy. The simulation results show that the maximum focal length deviation percentage from the target value between the wavelength of 8.6 and 11.4 µm is 1.61%. This achromatic metalens design is expected to play a role in the field of LWIR integrated optical system.

The first family of application-specific integrated circuits for programmable and reconfigurable metasurfaces

Reconfigurable metasurfaces are man-made surfaces, which consist of sub-wavelength periodic elements-meta-atoms-that can be reconfigured to manipulate incoming electromagnetic waves. However, reconfigurable metasurfaces developed to-date, have limitations in terms of loading impedance range, reconfiguration delay and power consumption. Also, these systems are costly and they require bulky electronics and complex control circuits, which makes them unattractive for commercial use. Here, we report the first family of CMOS application-specific integrated circuits that enable microsecond and microwatt reconfiguration of complex impedances at microwave frequencies. Our approach utilizes asynchronous digital control circuitry with chip-to-chip communication capabilities, allowing simple and fast reconfiguration via digital devices and user-friendly software. Our solution is low-cost and can cover arbitrary board-to-board metasurfaces, with different sizes and shapes.

Ultracompact Nanophotonics: Light Emission and Manipulation with Metasurfaces

Internet of Things (IoT) technology is prosperous for the betterment of human well-being. With the expeditious needs of miniature functional devices and systems for adaptive optics and light manipulation at will, relevant sensing techniques are thus in the urgent stage of development. Extensive developments in ultrathin artificial structures, namely metasurfaces, are paving the way for the next-generation devices. A bunch of tunable and reconfigurable metasurfaces with diversified catalogs of mechanisms have been developed recently, enabling dynamic light modulation on demand. On the other hand, monolithic integration of metasurfaces and light-emitting sources form ultracompact meta-devices as well as exhibiting desired functionalities. Photon-matter interaction provides revolution in more compact meta-devices, manipulating light directly at the source. This study presents an outlook on this merging paradigm for ultracompact nanophotonics with metasurfaces, also known as metaphotonics. Recent advances in the field hold great promise for the novel photonic devices with light emission and manipulation in simplicity.

Nanotubes as sinks for quantum particles

Nanotubes with proper thickness, size, and texture make ultra-efficient sinks for quantum particles traveling into specific background media. Several optimal semiconducting cylindrical layers are reported to achieve enhancement in the trapping of matter waves by two to three orders of magnitude. The identified shells can be used as pieces in quantum devices that involve the focusing of incident beams, spanning from charge pumps and superconducting capacitors to radiation pattern controllers and matter-wave lenses.

Reconfigurable terahertz metamaterials: From fundamental principles to advanced 6G applications

Terahertz (THz) electromagnetic spectrum ranging from 0.1THz to 10THz has become critical for sixth generation (6G) applications, such as high-speed communication, fingerprint chemical sensing, non-destructive biosensing, and bioimaging. However, the limited response of naturally existing materials THz waves has induced a gap in the electromagnetic spectrum, where a lack of THz functional devices using natural materials has occurred in this gap. Metamaterials, artificially composed structures that can engineer the electromagnetic properties to manipulate the waves, have enabled the development of many THz devices, known as "metadevices". Besides, the tunability of THz metadevices can be achieved by tunable structures using microelectromechanical system (MEMS) technologies, as well as tunable materials including phase change materials (PCMs), electro-optical materials (EOMs), and thermo-optical materials (TOMs). Leveraging various tuning mechanisms together with metamaterials, tremendous research works have demonstrated reconfigurable functional THz devices, playing an important role to fill the THz gap toward the 6G applications. This review introduces reconfigurable metadevices from fundamental principles of metamaterial resonant system to the design mechanisms of functional THz metamaterial devices and their related applications. Moreover, we provide perspectives on the future development of THz photonic devices for state-of-the-art applications.

Research on the reflection-type ELC-based optomechanical metamaterial

In this paper, we propose a new kind of optomechanical metamaterial based on a planar ELC-type absorbing structure fabricated on the low-loss flexible substrate. The nonlinear coupling mechanism and nonlinear response phenomenon of the proposed optomechanical metamaterial driven by electromagnetic induced force are analyzed theoretically. The mechanical deformation/displacement and the mechanical resonance frequency shift of the metamaterial unit deposed on the flexible substrate are also numerically and experimentally demonstrated to reveal the coupling phenomenon of electromagnetic field and mechanical field. These results will help researchers to further understand the multi-physics interactions of optomechanical metamaterials and will promote the developments of new type of metasurface for high-efficiency dynamic electromagnetic wave controlling and formatting.

Broadband tunable metasurface platform enabled by dynamic phase compensation

Broadband metasurfaces have attracted significant attention for a variety of applications in imaging and communication systems. Here, a method to alleviate the chromatic aberrations issue is proposed in the microwave region using dynamic phase compensation enabled by a reconfigurable metasurface. The dispersion characteristic of the meta-atom implemented with varactor diodes can be flexibly manipulated electronically, such that the dispersion-induced phase distortions over a wide frequency band can be compensated dynamically to achieve broadband performances. Various aberration-free functionalities can be realized with the proposed active metasurface. Near-field measurements are performed on a fabricated prototype to demonstrate aberration-free beam bending and hologram imaging, showing good agreement with simulation results. Such an active metasurface platform paves the way to efficient devices for wireless power transfer, sensors, and communication and antenna systems at radio or much higher frequencies.

A Time-Modulated Transparent Nonlinear Active Metasurface for Spatial Frequency Mixing

In this article, a time-modulated transparent nonlinear active metasurface loaded with varactor diodes was proposed to realize spatial electromagnetic (EM) wave frequency mixing. The nonlinear transmission characteristic of the active metasurface was designed and measured under time-modulated biasing signals. The transmission phase can be continuously controlled across a full 360° range at 5 GHz when the bias voltage of the varactor diodes changes from 0 V to 25.5 V, while the transmission amplitude is between −2.1 dB to −2.7 dB. By applying the bias voltage in time-modulated sequences, frequency mixing can be achieved. Due to the nonlinearity of the transmission amplitude and transmission phase of the metasurface versus a time-modulated bias voltage, harmonics of the fundamental mode were observed using an upper triangle bias voltage. Furthermore, with a carefully designed bias voltage sequence, unwanted higher order harmonics were suppressed. The proposed theoretical results are validated with the measured results.

Angular-Adaptive Reconfigurable Spin-Locked Metasurface Retroreflector

Metasurface retroreflectors, which scatter the incident electromagnetic wave back to incoming direction, have received significant attention due to their compelling advantages of low profile and light weight compared with conventional bulky retroreflection devices. However, the current metasurface retroreflectors still have limitations in wide-angle and omnidirectional operations. This work proposes a high-efficiency, wide-angle, reconfigurable, and omnidirectional retroreflector composed of spin-locked phase gradient metasurface with a thickness of only 5.2 mm or 0.07 operating wavelength. The reflection phase of constituent meta-atoms can be controlled dynamically and continuously by altering their orientation states through individually addressing each mechanically rotational meta-atom, whereas the reflection handedness is kept the same as incidence. Therefore, adaptive and arbitrary momentum can be imparted to the incident wave, providing high-efficiency retroreflection over a wide continuous range from -47° to 47°. Moreover, such high-performance retroreflection is extended to omnidirectional level, enabling great degrees of freedom that are unavailable by previous researches. As a proof of concept, a retroreflective metasurface is fabricated and experimentally demonstrated at microwave frequencies. The proposed thin thickness, high efficiency, and reconfigurable metasurface retroreflector can be extended to other frequencies that may offer an untapped platform toward reconfigurable spin-based retroreflection devices for electromagnetic signal processing.

Electrically Tunable Bifocal Metalens with Diffraction-Limited Focusing and Imaging at Visible Wavelengths

Tunable optical devices powered by metasurfaces provide a new path for functional planar optics. In particular, lenses with tunable focal lengths can play a key role in various fields with applications in imaging, displays, and augmented and virtual reality devices. Here, the authors demonstrate an electrically controllable bifocal metalens at visible wavelengths by incorporating a metasurface designed to focus light at two different focal lengths, with liquid crystals to actively manipulate the focal length of the metalens through the application of an external bias. By utilizing hydrogenated amorphous silicon that is optimized to provide an extremely low extinction coefficient in the visible regime, the metalens is highly efficient with measured focusing efficiencies of around 44%. They numerically design and experimentally realize and characterize tunable focusing and demonstrate electrically tunable active imaging at visible wavelengths using the bifocal metalens combined with liquid crystals. Diffraction limited focusing and imaging is verified through the analysis of the measured optical intensities at the focal points and the modulation transfer function. The bifocal metalens is used to demonstrate electrically modulated focus switching between the two designed focal planes, to display images of positive and negative target objects.

Adaptable Invisibility Management Using Kirigami-Inspired Transformable Metamaterials

Many real-world applications, including adaptive radar scanning and smart stealth, require reconfigurable multifunctional devices to simultaneously manipulate multiple degrees of freedom of electromagnetic (EM) waves in an on-demand manner. Recently, kirigami technique, affording versatile and unconventional structural transformation, has been introduced to endow metamaterials with the capability of controlling EM waves in a reconfigurable manner. Here, we report for a kirigami-inspired sparse meta-architecture, with structural density of 1.5% in terms of the occupation space, for adaptive invisibility based on independent operations of frequency, bandwidth, and amplitude. Based on the general principle of dipolar management via structural reconstruction of kirigami-inspired meta-architectures, we demonstrate reconfigurable invisibility management with abundant EM functions and a wide tuning range using three enantiomers (A, B, and C) of different geometries characterized by the folding angle β. Our strategy circumvents issues of limited abilities, narrow tuning range, extreme condition, and high cost raised by available reconfigurable metamaterials, providing a new avenue toward multifunctional smart devices.