Passive Metasurfaces Research Articles

Passive time-varying waveform-selective metasurfaces for attainment of magnetic property control

Passive time-varying waveform-selective metasurfaces for attainment of magnetic property control

Universal strategy study of applying passive metasurfaces to an intra-DC wireless-optical interconnection

Free-space optical (FSO) communication can reduce the routing complexity of data center networks (DCNs), not only ensuring high-capacity optical switching, but also owning similar flexibility to the wireless connectivity. Presently, mainstream wireless-optical switching units (WOSUs) have adopted serial beam control for unicasting. As a two-dimensional planar structure composed of meta-atoms with special electromagnetic properties arranged in a certain way, a passive metasurface (PMF) has strong parallel beam regulating capability. In this paper, we propose a wireless-optical intra-DC interconnection scheme based on PMFs. Through a real PMF chip, by adjusting the polarization of an incident beam, the power distribution between normal and abnormal reflected beams can be controlled. When both reflected beams are assigned with power, we obtain a pair of beams reflected in parallel, thus simultaneously communicating with two racks. In fact, we can perform 1-to-N (N≥2) multicast by using cascaded PMFs, and 1-to-N means that each source rack can communicate with N destination racks, i.e., a wide communication coverage range. However, the cascaded PMFs may result in huge chip costs and high accumulated power loss. In this paper, we only demonstrate the 1-to-2 communication ability of our PMFs, so the communication coverage may be still limited. Hence, by introducing electrostatic drive to control the PMF rotation, each source rack can achieve a wider communication coverage range than before. As a result, all racks within the coverage range have the ability to establish communication links with the source rack. To this end, we also design a neural network to mimic the PMF rotation, further improving the communication capability between racks. We then propose a DCN-topology reconfiguration algorithm supporting the coexistence of unicasting and multicasting, in order to make real-time decisions on the matching between FSO ports with minimized latency. Finally, we established a proof-of-concept prototype system to demonstrate the parallel beam control of our PMF. The test results show that the angle error is less than 0.1°, satisfying the accuracy requirements of WOSUs, and our PMF always has a good polarization response with the insertion loss of less than 2 dB under various azimuth angles or rotation planes.

Continuous manipulation of electromagnetic radiation based on ultrathin flexible frequency coding metasurface

The physical characteristics of electromagnetic waves are combined with digital information in coding metasurfaces. Coding metasurfaces enable precise control of beams by flexibly designing coding sequences. However, achieving continuous multivariate modulation of electromagnetic waves on passive flexible coded metasurfaces remains a challenge. Previous passive coding metasurfaces have a fixed phase difference between adjacent coding units throughout the operating frequency band, and when the coding pattern is defined, the coded metasurface can only achieve a single electromagnetic function. Our proposed frequency coding metasurface units vary linearly in phase difference over the operating frequency band with different phase sensitivities. Frequency coding metarsurfaces enable a wide range of tunable and versatile electromagnetic energy radiation, without introducing any active devices and changing the coding pattern. As a demonstration of the concept, we have shown theoretically and numerically that frequency coding metasurface can achieve successive transformations of electromagnetic functions, including multi-beam generation, anomalous deflection and diffuse scattering. In addition, beam sweeping function is achieved by means of spatially non-periodically distributed frequency coding metasurface. When the frequency of the incident wave is changed, the deflection angle of the beam is also changed. In addition to the tunability of properties, research on coding metasurfaces has tended to be limited to rigid materials. Flexible coding metasurfaces have potential applications in microwave antennas, radar and aircraft. The passive flexible frequency coding metasurfaces provide a novel approach to manipulating electromagnetic waves with increased design flexibility. This promises applications in microwave antennas, radar, aircraft, and satellite communications.

Magneto-optical chiral metasurfaces for achieving polarization-independent nonreciprocal transmission.

Nonreciprocal transmission, resulting from the breaking of Lorentz reciprocity, plays a pivotal role in nonreciprocal communication systems by enabling asymmetric forward and backward propagations. Metasurfaces endowed with nonreciprocity represent a compact and facile platform for manipulating electromagnetic waves in an unprecedented manner. However, most passive metasurfaces that achieve nonreciprocal transmissions are polarization dependent. While incorporation of active elements or nonlinear materials can achieve polarization-independent nonreciprocal metasurfaces, the complicated configurations limit their practical applications. To address this issue, we propose and demonstrate a passive and linear metasurface that combines magneto-optical and chiral effects, enabling polarization-independent isolation. The designed metasurface achieves a transmittance of up to 80%, with a high contrast between forward and backward propagations. Our work introduces a novel mechanism for nonreciprocal transmission and lays the foundation for the development of compact, polarization-insensitive nonreciprocal devices.

Inverse Equivalent-Source Techniques for Lossless and Passive Metasurfaces With Noncanonical Shapes and Geometries

Inverse Equivalent-Source Techniques for Lossless and Passive Metasurfaces With Noncanonical Shapes and Geometries

Transmissive reconfigurable metasurface enabling independent control of active and passive modules through weak coupling

Metasurfaces have demonstrated rich electromagnetic control capabilities and degrees of freedom in past years. As is well known, for passive metasurfaces, their functionalities cannot be further expanded accordingly once prototypes are established. Therefore, reconfigurable metasurfaces, utilizing active devices to replace geometric changes in passive structures, have received widespread attention, especially with the development of wireless communication recently. In reconfigurable metasurfaces, artificial meta-atoms are composed of active devices and passive structures combined together. However, these two modules are usually utilized as a whole due to the tight coupling of the active devices and the passive structures, which results in passive structures not receiving sufficient attention and being utilized as independent degrees of freedom. In this article, we propose the concept of weakly coupled reconfigurable metasurfaces in transmissive systems, enabling independent control of active and passive modules through weak coupling. As the proof of concept, a simple weakly coupled system is proposed, which can realize the transmission wavefront engineering through the geometric changes of meta-structures in passive mode, while achieving switching between transmission and reflection states in active mode, respectively. Our exploration lies in making use of the physical structure, which is easily neglected in traditional reconfigurable metasurface design, emphasizing the collaborative work of active and passive modules, exploring more available variables within the same aperture, and providing a potential solution for balancing functionality and resource consumption in practical applications.

Complex Refraction Metasurfaces for Locally Enhanced Propagation Through Opaque Media

AbstractMetasurfaces with linear phase gradients can redirect light beams. Controlling both phase and amplitude of a metasurface is proposed to extend Snell's law to the realm of complex angles, enabling a non‐decaying transmission through opaque media with complex refractive indices. This leads to the discovery of non‐diffracting and non‐decaying solutions to the wave equation in opaque media, in the form of generalized cosine and Bessel‐beams with a complex argument. While these solutions present nonphysical exponentially growing side tails, this is addressed via a windowing process, removing the side tails of the field profile while preserving significant transmission enhancement through an opaque slab on a small localized region. Such refined beam profiles may be synthesized by passive metasurfaces with phase and amplitude control at the opaque material's interface. The findings, derived from rigorous solutions of the wave equation, promise new insights and enhanced control of light propagation in opaque media.

Passive bias-free non-reciprocal metasurfaces based on thermally nonlinear quasi-bound states in the continuum

Passive bias-free non-reciprocal metasurfaces based on thermally nonlinear quasi-bound states in the continuum

Deterministic approach to decoupling amplitude from phase via a single-layered metasurface

Controlling the phase and amplitude of reflected waves has been challenging due to the inherent coupling between them. Changes in the phase of a wave often result in changes in its amplitude, and vice versa. This coupling makes it difficult to control both parameters independently. Here we report a deterministic approach, illustrated by an equivalent circuit model, to decouple the phase from the amplitude of reflected waves based on a single-layer metasurface. With the variables conversion and appropriate impedance patterns, this approach allows for the generation of 1-bit phase meta-atoms with a natural phase difference of 180° and controllable amplitude ranging from 0 to 0.9. Furthermore, the decoupling frequency can be customized by using different permittivities and dielectric thickness. These 1-bit phase meta-atoms, whose amplitude can be independently controlled, have potential in wavefront manipulation such as multi-beam antennas and hybrid radar cross section reduction metasurfaces. As a proof of concept, a metasurface for backscatter suppression is demonstrated and measured. The reflected wave is scattered by wavefront modulation with phase control in the spatial domain, while the energy is absorbed by amplitude control in the energy domain, resulting in extremely low backward scatterings in a wideband. In addition to the discussion of passive metasurfaces, the active case is also expanded on. The results of our paper demonstrate the potential of the phase- and amplitude-decoupled approach in designing multi-function metasurfaces for wavefront manipulation and radar applications.

Indoor Non‐Line‐of‐Sight Millimeter‐Wave Coverage Enhancements by Field‐Reorganizable Passive Digital Coding Metasurfaces

Digital metasurfaces define a novel methodology for metasurface designs by adopting discrete coding meta‐atoms to engineer electromagnetic waves in programmable ways. Herein, a novel field‐reorganizable digital metasurface (FRDM) that can be used to enhance the indoor non‐line‐of‐sight (NLOS) millimeter‐wave (mmWave) signal coverage is presented. The passive binary supercells which can be reorganized with an in situ optimization characteristic to deflect the incoming mmWaves into the blind area with adjustable angles are proposed. The indoor L‐shaped corridor signal coverage simulated by the ray‐tracing technique confirms that the NLOS blind area is effectively communicated using the passive FRDM. Practical environment experiments using FRDMs with different coding sequences are conducted, indicating that the averaged signal intensity is increased by over 10 dB in the NLOS blind area in a wide operating frequency band from 26 to 30 GHz by reorganizing the passive digital metasurfaces. The results suggest that the proposed FRDM enabled by the reorganizable binary supercells is a highly flexible, low‐cost, and extensible solution for B5G and 6G mmWave wireless communications.

Metasurfaces: physics and applications in wireless communications.

The ever increasing number of wireless devices and systems has led to a crowded spectrum and increased the demand for versatile and multi-functional wireless apparatuses. Recently, metasurfaces have been explored as a prominent technological solution to the current paradigm of spectrum scarcity by opportunistically sharing the spectrum with various users. In general, metasurfaces are passive/dynamic, ultra-compact, multi-functional and programmable structures that are capable of both reciprocal and nonreciprocal signal-wave transmissions. The controllability and programmability of such metasurfaces are governed through DC bias and occasionally a radio-frequency modulation applied to the active components of the unit cells of the metasurface, e.g.diodes and transistors. This article overviews some of the recently proposed passive and dynamic metasurfaces and shows that metasurfaces can enhance the performance of wireless communication systems thanks to their unique physical features such as real-time signal coding, nonreciprocal-beam radiation, nonreciprocal beamsteering amplification and advanced pattern-coding multiple access communication.

Deterministic approach to design passive anomalous-diffraction metasurfaces with nearly 100% efficiency.

Designing perfect anomalous reflectors is crucial for achieving many metasurface-based applications, but available design approaches for the cases of extremely large bending angles either require unrealistic gain-loss materials or rely on brute-force optimizations lacking physical guidance. Here, we propose a deterministic approach to design passive metasurfaces that can reflect impinging light to arbitrary nonspecular directions with almost 100% efficiencies. With both incident and out-going far-field waves given, we can retrieve the surface-impedance profile of the target metadevice by matching boundary conditions with all allowed near-field modes added self-consistently and then construct the metadevices deterministically based on passive meta-atoms exhibiting local responses. We design/fabricate two proof-of-concept microwave metadevices and experimentally demonstrate that the first one achieves anomalous reflection to a 70° angle with efficiency ∼98%, and the second one can generate multiple reflected beams with desired bending angles and power allocations. Our findings pave the way for realizing high-efficiency wave-control metadevices with desired functionalities.

Inverse-designed single-phase elastic metasurfaces for underwater acoustic vortex beams

During the past decade, the passive metasurfaces enabling vortex beams carrying orbital angular momentum have drawn great interest in the fields of electromagnetic and acoustic wave. In the underwater environment, however, the elastic metasurface with a certain bandwidth is more difficult to be realized due to the strong fluid-solid interaction and rigorous impedance matching. To overcome this challenge, an inverse-design strategy is proposed to systematically construct a single-phase underwater metasurface for converting the incident plane waves into vortex beams on demand. All topology-optimized metasurface elements, possessing both the positive/negative effective dynamic properties for the full phase control, can support the desired phase modulation and high transmission. We numerically and experimentally demonstrate the acoustic functionality of converting the plane waves in free space to the first-order vortex modes. The present passive single-phase metasurface by inverse design may offer opportunities for underwater acoustic communication.

Active control of metasurface via integrated spintronic terahertz emitter

Active metasurfaces have attracted increased attention due to their capabilities in function switching and wavefront shaping. Here we develop a new paradigm for active control of metasurfaces via integrating a tunable and programmable spintronic terahertz emitter (STE). While compatible with almost all conventional materials for metasurfaces, the STE can empower the passive metasurfaces to be active with increased flexibility. For the sake of illustration, a STE integrated metasurface quarter-wave plate is demonstrated, which enables broadband full polarization control over the entire Poincaré sphere. We also share a future perspective that the STE integrated metasurface can be readily programmed by using a commercial spatial light modulator. This work bridges the studies of metasurfaces and spintronic THz emitters, and may inspire more fruitful active metasurface designs and applications.

Directional Lasing in Coupled InP Microring/Nanowire Systems

AbstractThe realization of on‐chip microlasers with out‐of‐plane, controllable emission direction has been one of the core research themes in modern photonics. Traditional approaches for directional light scattering, including plasmonic nanoantennas and passive dielectric metasurfaces, are not suitable for on‐chip integration due to potential metallic contamination and reliance on external light sources. Meanwhile, achieving directional emission in active III–V microcavity lasers remains challenging due to geometrical symmetry and large free‐space coupling losses. Here, a novel approach is presented to realize directional lasing in an all‐dielectric, bottom‐up grown material system by coupling the laser emission from an InP microring cavity into a vertical nanowire at the ring center, which function as the photon source and directional antenna in the system, respectively. Efficient optical coupling is facilitated by enhanced light scattering at specific sidewall facets of the microring cavity, which can be engineered deterministically during epitaxial growth. Through Fourier imaging, out‐of‐plane laser emission with antenna‐like far‐field directivity in the coupled system is demonstrated. Furthermore, the emission directivity and side mode suppression in the coupled system can be improved significantly by tuning the geometric parameters of the system. The way for low power consumption, on‐chip microlasers with tunable emission directionality is paved here.

On the Design of Multibeam Digital Metasurfaces With Multiple Feeds

AbstractIn this paper, an analytical design procedure for multibeam metasurfaces with multiple feeds is presented. Considering arbitrary number of input and output beams, the required amplitude and phase distributions on the metasurface plane are theoretically derived without any optimization process. How the limitation of using only passive metasurfaces poses different challenges in the practical implementation of the amplitude profile is discussed. Then, to resolve these problems, a filtering procedure is proposed which effectively produces the desired output pattern. Moreover, the design method is verified through introducing a reflective metasurface with three input beams and sixteen output beams in arbitrary directions. It utilizes a graphene‐based unit cell providing simultaneous 2/2‐bit phase/amplitude modulation, that is, totally 16 states. For validation purposes, the analytical results are compared with numerical full‐wave simulation ones in which a good agreement between them is observed. In addition, by using the proposed design method, three different practical cases for wave manipulation functionalities, including power control, beam steering, and radar cross section (RCS) reduction capabilities are demonstrated. The presented approach paves the way for efficient design of the metasurfaces for a multitude of applications, including reconfigurable intelligent surfaces and multifunctional surfaces.

Corrections to “Optimization of Wireless Power Transfer Systems Enhanced by Passive Elements and Metasurfaces”

In the above article <xref ref-type="bibr" rid="ref1" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">[1]</xref> , the authors have since become aware of two typos that require correction. They concern a missing operator and a missing factor in a derivation made in the appendix, notably <xref ref-type="disp-formula" rid="deqn32" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">equations (32)</xref> and <xref ref-type="disp-formula" rid="deqn33" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(33)</xref> in the very last paragraph. Note that these typos do not in any way affect the results and conclusions in <xref ref-type="bibr" rid="ref1" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">[1]</xref> . However, they may make it impossible for others to reproduce the presented results or use the proposed method for their own applications.

On the Design of Modular Reflecting EM Skins for Enhanced Urban Wireless Coverage

The design of modular, passive, and static artificial metasurfaces to be used as electromagnetic skins (EMSs) of buildings for improving the coverage in urban millimeter-wave communication scenarios is addressed. Towards this end, an ad-hoc design strategy is presented to determine optimal trade-off implementative solutions that assure a suitable coverage of the areas of interest, where the signal from the base station is too weak, with the minimum complexity. More specifically, the admissible surface in the building facade is first partitioned into tiles, which are the minimum-size elements of the artificial coating (i.e., the building block of an EMS). Then, the search for the optimal EMS layout (i.e., the minimum number and the positions of the tiles to be installed) is carried out with a binary multi-objective optimization method. Representative numerical results are reported and discussed to point out the features and the potentialities of the EMS solution in the smart electromagnetic environment (SEME) as well as the effectiveness of the proposed design method.

Fast Analysis and Optimization of Sparsely Distributed Partial Modification Problems

This article addresses the efficient numerical analysis of sparsely distributed small modifications in a large structure, that is, the sparsely distributed partial modification problem (SDPMP). A hierarchical matrix (H-matrix)-based fast direct solver is adopted to solve the SDPMP in a very short time for each new set of modifications. The main idea of this method is to reuse the parts of the compressed inverse matrix operator that do not change and only recompute the modified parts. In contrast with already existing algorithms designed to tackle a single localized modification, this new approach is still very fast for sparsely distributed small modifications, so it becomes a very attractive option to solve optimization problems efficiently, due to the very cheap analysis of the modified structure at each iteration of the optimization procedure. The efficiency of the solution of SDPMP is demonstrated for several cases involving passive and active metasurfaces and a reconfigurable antenna. As a practical example, a programmable metasurface reflector is optimized with particle swarm optimization to obtain a prescribed reflection pattern.

Terahertz Reconfigurable Intelligent Surfaces (RISs) for 6G Communication Links.

The forthcoming sixth generation (6G) communication network is envisioned to provide ultra-fast data transmission and ubiquitous wireless connectivity. The terahertz (THz) spectrum, with higher frequency and wider bandwidth, offers great potential for 6G wireless technologies. However, the THz links suffers from high loss and line-of-sight connectivity. To overcome these challenges, a cost-effective method to dynamically optimize the transmission path using reconfigurable intelligent surfaces (RISs) is widely proposed. RIS is constructed by embedding active elements into passive metasurfaces, which is an artificially designed periodic structure. However, the active elements (e.g., PIN diodes) used for 5G RIS are impractical for 6G RIS due to the cutoff frequency limitation and higher loss at THz frequencies. As such, various tuning elements have been explored to fill this THz gap between radio waves and infrared light. The focus of this review is on THz RISs with the potential to assist 6G communication functionalities including pixel-level amplitude modulation and dynamic beam manipulation. By reviewing a wide range of tuning mechanisms, including electronic approaches (complementary metal-oxide-semiconductor (CMOS) transistors, Schottky diodes, high electron mobility transistors (HEMTs), and graphene), optical approaches (photoactive semiconductor materials), phase-change materials (vanadium dioxide, chalcogenides, and liquid crystals), as well as microelectromechanical systems (MEMS), this review summarizes recent developments in THz RISs in support of 6G communication links and discusses future research directions in this field.