On Differentiation Among Pilot Signals of Multiple Mobile Targets in Retro-Reflective Beamforming for Wireless Power Transmission

An experimental study is conducted on several retro-reflective beamforming schemes for wireless power transmission to multiple wireless power receivers(referred to herein as“targets”).The experimental results demonstrate that,when multiple targets broadcast continuous-wave pilot signals at respective frequencies,a retro-reflective wireless power transmitter is capable of generating multiple wireless power beams aiming at the respective targets as long as the multiple pilot signals are explicitly separated from one another by the wireless power transmitter.However,various practical complications are identified when the pilot signals of multiple targets are not appropriately differentiated from each other by the wireless power transmitter.Specifically,when multiple pilot signals are considered to be carried by the same frequency,the wireless power transmission performance becomes heavily dependent on the interaction among the pilot signals,which is highly undesirable in practice.In conclusion,it is essential for a retro-reflective wireless power transmitter to explicitly discriminate multiple targets’pilot signals among each other.

Artificial intelligence-assisted chiral nanophotonic designs

Chiral nanostructures can enhance the weak inherent chiral effects of biomolecules and highlight the important roles in chiral detection.However,the design of the chiral nanostructures is challenged by extensive theoretical simulations and explorative experiments.Recently,Zheyu Fang’s group proposed a chiral nanostructure design method based on reinforcement learning,which can find out metallic chiral nanostructures with a sharp peak in circular dichroism spectra and enhance the chiral detection signals.This work envisions the powerful roles of artificial intelligence in nanophotonic designs.

Wireless Communications with Programmable Metasurface:Transceiver Design and Experimental Results

Metasurfaces have drawn significant attentions due to their superior capability in tailoring electromagnetic waves with a wide frequency range, from microwave to visible light. Recently, programmable metasurfaces have demonstrated the ability of manipulating the amplitude or phase of electromagnetic waves in a programmable manner in real time, which renders them especially appealing in the applications of wireless communications. In this paper, we present the fundamental principle of applying programmable metasurface as transmitter for wireless communications. Then, we establish a prototype system of metasurface-based transmitter to conduct several experiments and measurements over the air, which practically demonstrate the feasibility of using programmable metasurfaces in future communication systems. By exploiting the dynamically controllable property of programmable metasurface, the design, implementation and experimental evaluation of the proposed metasurface-based wireless communication system are presented with the prototype, which realizes single carrier quadrature phase shift keying(QPSK) transmission over the air. In the developed prototype, the phase of the reflected electromagnetic wave of programma-ble metasurface is directly manipulated in real time according to the baseband control signal, which achieves 2.048 Mbps data transfer rate with video streaming transmission over the air. In addition, experimental result is provided to compare the performance of the proposed metasurface-based architecture against the conventional one. With the slight increase of the transmit power by 5 dB, the same bit error rate(BER) performance can be achieved as the conventional system in the absence of channel coding. Such a result is encouraging considering that the metasurface-based system has the advantages of low hardware cost and simple structure, thus leading to a promising new architecture for wireless communications.

A novel spoof surface plasmon polariton structure to reach ultra-strong field confinements

Ultrathin corrugated metallic structures have been proved to support spoof surface plasmon polariton (SPP) modes on two-dimension (2D) planar microwave circuits.However,to provide stronger field confinement,larger width of strip is required to load deeper grooves,which is cumbersome in modern large-scale integrated circuits and chips.In this work,a new spoof SPP transmission line (TL) with zigzag grooves is proposed.This new structure can achieve stronger field confinement compared to conventional one with the same strip width.In other words,the proposed spoof SPP TL behaves equivalently to a conventional one with much larger size.Dispersion analysis theoretically indicates the negative correlation between the ability of field confinement and cutoff frequencies of spoof SPP TLs.Numerical simulations indicate that the cutoff frequency of the proposed TL is lower than the conventional one and can be easily modified with the fixed size.Furthermore,two samples of the new and conventional spoof SPP TLs are fabricated for experimental demonstration.Measured S-parameters and field distributions verify the ultra-strong ability of field confinement of the proposed spoof SPP TL.Hence,this novel spoof SPP structure with ultra-strong field confinement may find wide applications in microwave and terahertz engineering.

A Planar 4-Bit Reconfigurable Antenna Array Based on the Design Philosophy of Information Metasurfaces

Inspired by the design philosophy of information metasurfaces based on the digital coding concept,a planar 4-bit reconfigurable antenna array with low profile of 0.15λ0(whereλ0is the free-space wavelength)is presented.The array is based on a digital coding radiation element consisting of a 1-bit magnetoelectric(ME)dipole and a miniaturized reflection-type phase shifter(RTPS).The proposed 1-bit ME dipole can provide two digital states of"0"and"1"(with 0°and 180°phase responses)over a wide frequency band by individually exciting its two symmetrical feeding ports.The designed RTPS is able to realize a relative phase shift of 173°.By digitally quantizing its phase in the range of 157.5°,additional eight digital states at intervals of 22.5°are obtained.To achieve low sidelobe levels,a 1:16 power divider based on the Taylor line source method is employed to feed the array,A prototype of the proposed 4-bit antenna array has been fabricated and tested,and the experimental results are in good agreement with the simulations.Scanning beams within a±45°range were measured with a maximum realized gain of 13.4 dBi at12 GHz.The sidelobe and cross-polarization levels are below-14.3 and-23.0 dB,respectively.Furthermore,the beam pointing error is within 0.8°,and the 3 dB gain bandwidth of the broadside beam is 25%.Due to its outstanding performance,the array holds potential for significant applications in radar and wireless communication systems.

Active odd-mode-metachannel for single-conductor systems

Although tremendous efforts have been devoted to investigating planar single-conductor circuits,it remains challenging to provide tight confinement of electromagnetic field and compatibility with active semi-conductor components such as amplifier,harmonic generator and mixers.Single-conductor spoof surface plasmon polariton(SSPP)structure,which is one of the most promising planar single-conductor transmission media due to the outstanding field confinement,still suf-fers from the difficulty in integrating with the active semi-conductor components.In this paper,a new kind of odd-mode-metachannel(OMM)that can support odd-mode SSPPs is proposed to perform as the fundamental transmission chan-nel of the single-conductor systems.By introducing zigzag decoration,the OMM can strengthen the field confinement and broaden the bandwidth of odd-mode SSPPs simultaneously.More importantly,the active semi-conductor amplifier chip integration is achieved by utilizing the intrinsic potential difference on OMM,which breaks the major obstacle in im-plementing the single-conductor systems.As an instance,an amplifier is successfully integrated on the single-conductor OMM,which can realize both loss compensation and signal amplification.Meanwhile,the merits of OMM including crosstalk suppression,low radar cross section(RCS),and flexibility are comprehensively demonstrated.Hence,the pro-posed OMM and its capability to integrate with the active semi-conductor components may provide a new avenue to fu-ture single-conductor conformal systems and smart skins.

Polarization singularities in planar electromagnetic resonators with rotation and mirror symmetries

In this work,we apply the group representation theory to systematically study polarization singularities in the inplane components of the electric fields supported by a planar electromagnetic(EM)resonator with generic rotation and reflection symmetries.We reveal the intrinsic connections between the symmetries and the topological features,i.e.,the spatial configuration of the in-plane fields and the associated polarization singularities.The connections are substantiated by a simple relation that links the topological charges of the singularities and the symmetries of the resonator.To verify,a microwave planar resonator with the D8group symmetries is designed and numerically simulated,which demonstrates the theoretical findings well.Our discussions can be applied to generic EM resonators working in a wide EM spectrum,such as circular antenna arrays,microring resonators,and photonic quasi-crystals,and provide a unique symmetry perspective on many effects in singular optics and topological photonics.

Topologically Protected Edge State in Two-Dimensional Su–Schrieffer–Heeger Circuit

Topological circuits,an exciting feld just emerged over the last two years,have become a very accessible platform for realizing and exploring topological physics,with many of their physical phenomena and potential applications as yet to be discovered.In this work,we design and experimentally demonstrate a topologically nontrivial band structure and the associated topologically protected edge states in an RF circuit,which is composed of a collection of grounded capacitors connected by alternating inductors in the x and y directions,in analogy to the Su–Schriefer–Heeger model.We take full control of the topological invariant(i.e.,Zak phase)as well as the gap width of the band structure by simply tuning the circuit parameters.Excellent agreement is found between the experimental and simulation results,both showing obvious nontrivial edge state that is tightly bound to the circuit boundaries with extreme robustness against various types of defects.Te demonstration of topological properties in circuits provides a convenient and fexible platform for studying topological materials and the possibility for developing fexible circuits with highly robust circuit performance.

Octupole corner state in a three-dimensional topological circuit

Higher-order topological insulators(HOTIs)represent a new family of topological materials featuring quantized bulk polarizations and zero-dimensional corner states.In recent years,zero-dimensional corner states have been demonstrated in two-dimensional systems in the form of quadrupole modes or dipole modes.Due to the challenges in designing and constructing three-dimensional systems,octupole corner modes in 3D have not been observed.In this work,we experimentally investigate octupole topological phases in a three-dimensional electrical circuit,which can be viewed as a cubic lattice version of the Hofstadter model with aπ-flux threading each plaquette.We experimentally observe in our higher-order topological circuit a 0D corner state manifested as a localized impedance peak.The observed corner state in the electrical circuit is induced by the octupole moment of the bulk circuit and is topologically protected by anticommuting spatial symmetries of the circuit lattice.Our work provides a platform for investigating higher-order topological effects in three-dimensional electrical circuits.

Non-Hermitian Skin Effect in a Non-Hermitian Electrical Circuit

The conventional bulk-boundary correspondence directly connects the number of topological edge states in a finite system with the topological invariant in the bulk band structure with periodic boundary condition(PBC).However,recent studies show that this principle fails in certain non-Hermitian systems with broken reciprocity,which stems from the non-Hermitian skin effect(NHSE)in the finite system where most of the eigenstates decay exponentially from the system boundary.In this work,we experimentally demonstrate a 1D non-Hermitian topological circuit with broken reciprocity by utilizing the unidirectional coupling feature of the voltage follower module.The topological edge state is observed at the boundary of an open circuit through an impedance spectra measurement between adjacent circuit nodes.We confirm the inapplicability of the conventional bulk-boundary correspondence by comparing the circuit Laplacian between the periodic boundary condition(PBC)and open boundary condition(OBC).Instead,a recently proposed non-Bloch bulk-boundary condition based on a non-Bloch winding number faithfully predicts the number of topological edge states.

Topological metasurface:from passive toward active and beyond

Metasurfaces are subwavelength structured thin films consisting of arrays of units that allow the control of polarization,phase,and amplitude of light over a subwavelength thickness.Recent developments in topological photonics have greatly broadened the horizon in designing metasurfaces for novel functional applications.In this review,we summarize recent progress in the research field of topological metasurfaces,first from the perspectives of passive and active in the classical regime,and then in the quantum regime.More specifically,we begin by examining the passive topological phenomena in two-dimensional photonic systems,including both time-reversal broken systems and time-reversal preserved systems.Subsequently,we discuss the cutting-edge studies of active topological metasurfaces,including nonlinear topological metasurfaces and reconfigurable topological metasurfaces.After overviewing topological metasurfaces in the classical regime,we show how they could provide a new platform for quantum information and quantum many-body physics.Finally,we conclude and describe some challenges and future directions of this fast-evolving field.

Realizing complex beams via amplitude-phase digital coding metasurfaces and semidefinite relaxation optimization

Complex beams play important roles in wireless communications,radar,and satellites,and have attracted great interest in recent years.In light of this background,we present a fast and efficient approach to realize complex beams by using semidefinite relaxation(SDR)optimization and amplitude-phase digital coding metasurfaces.As the application examples of this approach,complex beam patterns with cosecant,flat-top,and double shapes are designed and verified using full-wave simulations and experimental measurements.The results show excellent main lobes and low-level side lobes and demonstrate the effectiveness of the approach.Compared with previous works,this approach can solve the complex beam-forming problem more rapidly and effectively.Therefore,the approach will be of great significance in the design of beam-forming systems in wireless applications.

Reconfigurable intelligent surfaces for wireless communications

Reconfigurable intelligent surface(RIS)is a two-dimensional artificial material with reconfigurable electromagnetic characteristics.Since the phase,amplitude,polarization,and frequency responses of electromagnetic waves at each element can be independently adjusted by changing the biasing signals of tunable devices embedded in the RIS elements,it is possible to reshape the wavefront of the spatial electromagnetic waves in a programmable way.

Generation and regulation of two-dimensional autofocusing Airy beams based on holographic metasurfaces

This paper proposes a new method to generate a two-dimensional(2D)Airy beam and Airy autofocusing beam by using the scalar holographic metasurface with amplitude-phase modulation in the microwave band.The proposed holographic metasurface comprises subwavelength patch unit cells with a period of fewer than 1/8 wavelengths,which means that it has the finer sampling for electromagnetic waves and can simultaneously achieve precise modulations for the amplitude and phase of electromagnetic waves.Firstly,the 2D-Airy beam with quasi-non-diffraction and selfbending characteristics is generated,from which the holographic metasurface is designed to realize four different 2D-Airy beams with the same focus,achieving the 2D-Airy autofocusing beam in the microwave frequency.The holographic metasurface for Airy beam generation has high efficiency and an ultra-lower profile.Meanwhile,for applying the Airy beam in wireless power transfer(WPT),the efficiency of the generated Airy beam and Airy autofocusing beam is calculated for the first time in the microwave field.The simulation results show that the efficiency of the 2D-Airy beam can reach 66%at 150 mm away from the metasurface,while the efficiency of the 2D-Airy autofocusing beam at the focus,which is 280 mm from the metasurface,can reach 35%.The theoretical,simulated,and measured results show that the proposed method and holographic metasurfaces can flexibly achieve the special characteristics of self-autofocusing and self-bending Airy beams in the microwave domain,providing an effective path for wireless power transfer(WPT)scenario with radial obstructions.

Reconfigurable intelligent surface with high optical-transparency based on metalmesh

Reconfigurable intelligent surfaces(RISs)have aroused extensive attentions from academic and wireless communication communities due to their abilities to customize the electromagnetic(EM)characteristics of the propagation channels flexibly and rapidly.Recent advances in theoretical innovations and prototype systems have demonstrated the advantages of RISs in terms of low cost,low power consumption,and easy deployment.Meanwhile,the optically transparent RISs are demanded in some application scenarios.In this paper,we propose a 2-bit metalmesh-based RIS with high optical-transparency.By analyzing the surface current distributions on the element,we employ the metalmesh-grid patterns and metalmesh-stripe patterns on the top and ground layers respectively.The metalmesh patterns can help improve the optical transparency of RISs,while maintaining similar microwave characteristics.The RIS can reach the optical transparency of 79%,and the reflection amplitude is greater than3.2 dB within the operating band.Finally,to verify the capability of the proposed RIS in wavefront controls,the far-field scattering patterns of the RIS with different coding sequences are investigated and the simulation results are in good agreement with the theoretical results.

Information metasurfaces and reconfigurable intelligent

Information metamaterials and metasurfaces are artificial structures composed of meta-atoms integrated with active tunable devices,which can tightly combine the physical and digital spaces.The digital coding representation of the information metamaterials and metasurfaces can also introduce the concepts,theories,and signal processing methods in information science to the physical metamaterials and metasurfaces,thereby realizing effective controls of electromagnetic waves.In addition to manipulating the electromagnetic waves,the information metamaterials and metasurfaces can also process and modulate digital information.Hence,this concept has been emerging as an entirely new system of metamaterials and metasurfaces,which can provide unprecedented opportunities for achieving theoretical breakthroughs and new research methodologies,and building up novel electronic information platforms to revolutionize the traditional methods and functionalities in the applications of wireless communication,sensing,imaging,and radar systems.

Coding metamaterials, digital metamaterials and programmable metamaterials

Metamaterials are artificial structures that are usually described by effective medium parameters on the macroscopic scale,and these metamaterials are referred to as‘analog metamaterials’.Here,we propose‘digital metamaterials’through two steps.First,we present‘coding metamaterials’that are composed of only two types of unit cells,with 0 and p phase responses,which we name‘0’and‘1’elements,respectively.By coding‘0’and‘1’elements with controlled sequences(i.e.,1-bit coding),we can manipulate electromagnetic(EM)waves and realize different functionalities.The concept of coding metamaterials can be extended from 1-bit coding to 2-bit coding or higher.In 2-bit coding,four types of unit cells,with phase responses of 0,p/2,p,and 3p/2,are required to mimic the‘00’,‘01’,‘10’and‘11’elements,respectively.The 2-bit coding has greater freedom than 1-bit coding for controlling EM waves.Second,we propose a unique metamaterial particle that has either a‘0’or‘1’response controlled by a biased diode.Based on this particle,we present‘digital metamaterials’with unit cells that possess either a‘0’or‘1’state.Using a field-programmable gate array,we realize digital control over the digital metamaterial.By programming different coding sequences,a single digital metamaterial has the ability to manipulate EM waves in different manners,thereby realizing‘programmable metamaterials’.The above concepts and physical phenomena are confirmed through numerical simulations and experiments using metasurfaces.

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

Metamaterials based on effective media can be used to produce a number of unusual physical properties(for example,negative refraction and invisibility cloaking)because they can be tailored with effective medium parameters that do not occur in nature.Recently,the use of coding metamaterials has been suggested for the control of electromagnetic waves through the design of coding sequences using digital elements‘0’and‘1,'which possess opposite phase responses.Here we propose the concept of an anisotropic coding metamaterial in which the coding behaviors in different directions are dependent on the polarization status of the electromagnetic waves.We experimentally demonstrate an ultrathin and flexible polarization-controlled anisotropic coding metasurface that functions in the terahertz regime using specially designed coding elements.By encoding the elements with elaborately designed coding sequences(both 1-bit and 2-bit sequences),the x-and y-polarized waves can be anomalously reflected or independently diffused in three dimensions.The simulated far-field scattering patterns and near-field distributions are presented to illustrate the dual-functional performance of the encoded metasurface,and the results are consistent with the measured results.We further demonstrate the ability of the anisotropic coding metasurfaces to generate a beam splitter and realize simultaneous anomalous reflections and polarization conversions,thus providing powerful control of differently polarized electromagnetic waves.The proposed method enables versatile beam behaviors under orthogonal polarizations using a single metasurface and has the potential for use in the development of interesting terahertz devices.

Independent control of harmonic amplitudes and phases via a time-domain digital coding metasurface

Harmonic manipulations are important for applications such as wireless communications,radar detection and biological monitoring.A general approach to tailor the harmonics involves the use of additional amplifiers and phase shifters for the precise control of harmonic amplitudes and phases after the mixing process;however,this approach leads to issues of high cost and system integration.Metasurfaces composed of a periodic array of subwavelength resonators provide additional degrees of freedom to realize customized responses to incident light and highlight the possibility for nonlinear control by taking advantage of time-domain properties.Here,we designed and experimentally characterized a reflective time-domain digital coding metasurface,with independent control of the harmonic amplitude and phase.As the reflection coefficient is dynamically modulated in a predefined way,a large conversion rate is observed from the carrier signal to the harmonic components,with magnitudes and phases that can be accurately and separately engineered.In addition,by encoding the reflection phases of the meta-atoms,beam scanning for multiple harmonics can be implemented via different digital coding sequences,thus removing the need for intricate phase-shift networks.This work paves the way for efficient harmonic control for applications in communications,radar,and related areas.

Negative reflection and negative surface wave conversion from obliquely incident electromagnetic waves

Complete control of spatially propagating waves(PWs)and surface waves(SWs)is an ultimate goal that scientists and engineers seek for,in which negative reflection of PW and negative surface wave are two exotic phenomena.Here,we experimentally demonstrate an anisotropic digital coding metasurface capable of controlling both PWs and SWs with a single coding pattern.On the basis of the digital description of coding metasurfaces,a simple coding method is proposed to allow dual functionalities(either PW or SW manipulations)under two orthogonal polarizations at arbitrarily oblique incidences,thus improving the adaptability of digital coding metasurfaces in more practical circumstances.With elaborately designed ellipse-shaped coding particles,we experimentally demonstrate various functions under oblique incidences,including the negative reflection of PW,negative SW,anomalous reflection and their arbitrary combinations,all having good agreements with theoretical and numerical predictions.We believe that the proposed method may enable the digital coding metasurfaces to have broad applications in radar detections,wireless communications and imaging.