Incident Circular Polarization Research Articles

A single dual-frequency reflective metasurface for simultaneous multi-mode orbital angular momentum multiplexing

In this paper, we propose a dual-frequency reflective metasurface that utilizes geometric phase to achieve multiple modes of orbital angular momentum (OAM). The metasurface consists of two metal layers and a dielectric layer sandwiched between them, with independent control of amplitude and phase modulation at two frequency points by rotating the outer split ring and inner cross structures on the top layer of the metasurface. When circularly polarized (CP) terahertz waves impinge on the metasurface, it is possible to achieve a single-beam normal reflection, single-beam anomalous reflection, and dual-beam reflection in the form of OAM vortex beams. Specifically, for right-hand circular polarization (RCP) incidence, at the frequency of 140 GHz, its vertical reflection is the OAM beam of mode +2, and at the frequency of 200 GHz, its vertical reflection is the OAM beam of mode +1; for left-hand circular polarization (LCP) incidence, at the frequency of 140 GHz, its vertical reflection is the OAM beam of mode −2, and at the frequency of 200 GHz, the corresponding vertical reflection is the OAM beam of mode −1. Additionally, anomalously reflected OAM beams are designed, which, after RCP incidence on the metasurface, produce an OAM beam with mode −1 and a reflection angle of 17° at 140 GHz, and an OAM beam with mode +1 and a reflection angle of 12° at 200 GHz. Finally, a dual-beam OAM reflective metasurface is designed, which generates two vortex beams with mode +2 at 140 GHz and two vortex beams with mode +1 at 200 GHz under RCP incidence. Therefore, the metasurface designed in this paper has broad application prospects in multi-channel transmission for future terahertz communication systems.

A Novel Incident Circular Polarized Light Raman Optical Activity (ICP‐ROA) Spectrometer With Advanced Polarization Control

ABSTRACTThe design and setup of a novel and simple backscatter Raman optical activity (ROA) spectrometer with incident light circular polarization (ICP) is presented, constructed from commercially available components. Incident light polarization is controlled using a combination of waveplates, compensating for unwanted birefringence and beam offsets. Realignment of the spectra in post‐processing reduced artifacts caused by spatial offsets. Spurious signals from achiral solvents like toluene and water are almost completely removed. The setup was validated by measuring references samples, including α‐pinene, carvone, and glucose in aqueous solution. The spectra show very good agreement with previously published results.

Multi-mode non-diffraction vortex beams enabled by polarization-frequency multiplexing transmissive terahertz metasurfaces

In terahertz (THz) wireless communication systems, non-diffraction vortex beams carrying an orbital angular momentum (OAM) have attracted extensive attention due to their ability to transmit information over long distances with high capacity. However, existing metasurfaces can only generate a single OAM mode non-diffracting vortex beam at reflection space for circular polarization (CP) incidence, limiting practical applications. To address this issue, we propose and design a polarization-frequency multiplexing transmissive THz metasurface to realize multi-mode non-diffracting vortex beams at linear polarization (LP) incidence. The meta-atom of this metasurface is composed of three anisotropic rectangular metallic structures embedded in vanadium dioxide (VO2) square rings, two circular aperture metallic grid layers, and four dielectric layers. By reasonably designing the size of the metal patch and the state of VO2, the designed metasurface can achieve polarization multiplexing and frequency multiplexing for LP incidence. Based on the phase response of the proposed meta-atoms, the transmissive metasurface can implement not only multi-mode non-diffraction vortex beams but also their space separation at two frequency ranges of 0.80–0.90 THz and 1.50–1.80 THz by changing the state of VO2. Therefore, the proposed multiple multiplexing metasurfaces can effectively shape the wavefront of non-diffraction vortex beams, which have broad application prospects in 6G THz communication.

A Wideband Polarization-Insensitive Bistatic Radar Cross-Section Reduction Design Based on Hybrid Spherical Phase-Chessboard Metasurfaces

A wideband polarization-insensitive bistatic radar cross-section (RCS) reduction design under linear and circular polarization incidence is proposed based on spherical-chessboard metasurfaces. A new metasurface element with wideband characteristics was designed, including a double split-ring structure, single-layer media, and metal board. In the proposed RCS-reduction design, the Pancharatnam–Berry (P-B) phase theory is applied with the designed metasurface element to realize phase distribution mimicking the low-scattering sphere, and thus realizing RCS reduction. In addition, the chessboard configuration is combined with spherical phase distribution to further improve the performance of monostatic and bistatic RCS reduction. Finally, the proposed RCS reduction design can not only realize wideband RCS reduction but also exhibit polarization-insensitive characteristics. It realized 10 dB monostatic and bistatic RCS reduction in a frequency band ranging from 8.5 to 21 GHz (84.8% relative bandwidth) under linear polarization (LP) and circular polarization (CP) incidence. The straightforward and efficient design method of the hybrid spherical chessboard can effectively avoid the complex and time-consuming optimization process in RCS-reduction design.

Broadband intelligent programmable metasurface with polarization-modulated self-adaptive electromagnetic functionality switching

Programmable metasurfaces have received a great deal of attention due to their ability to dynamically manipulate electromagnetic (EM) waves. Despite the rapid growth, most of the existing metasurfaces require manual control to switch among different functionalities, which poses severe limitations on practical applications. Here, we put forth an intelligent metasurface that has self-adaptive EM functionality switching in broadband without human participation. It is equipped with polarization discrimination antennas (PDAs) and feedback components to automatically adjust functionalities for the different incident polarization information. The PDA module can first perceive the polarization of incident EM waves, e.g., linear or circular polarization, and then provide the feedback signal to the controlling platform for switching the EM functionality. As exemplary demonstrations, a series of functionalities in the 9–22 GHz band has been realized, including beam scanning for x-polarization, specular reflection for y-polarization, diffuse scattering for left-handed circular polarization (LCP), and vortex beam generation for right-handed circular polarization (RCP) waves. Experiments verify the good self-adaptive reaction capability of the intelligent metasurface and are in good agreement with the designs. Our strategy provides an avenue toward future unmanned devices that are consistent with the ambient environment.

An ultra-wideband reflective polarization conversion metasurface and its application in RCS reduction

ABSTRACT In this work, an ultra-wideband reflective polarization conversion metasurface (PCM) is proposed at first. Because the PCM is an anisotropic structure that is symmetric with respect to both x- and y-axes, and the reflection phase difference under x- and y-polarized incidences is close to 180° in an ultra-wide frequency range, the PCM can achieve both linear polarization conversion and circular-polarization (CP) maintaining reflection in the ultra-wide frequency band from 8.3 to 41.3 GHz except for near the frequency point of 38.8 GHz. Moreover, when its unit cell structure is rotated by 90°, its cross-polarized reflection coefficient under LP incidence, together with the co-polarized reflection coefficient under CP incidence, will be changed by almost 180° in phase. Thus, based on the PCM, an ultra-wideband coding diffusion metasurface (CDM) is further proposed for radar cross section (RCS) reduction. The simulation and experiment results indicate that the CDM can achieve effective RCS reduction in the ultra-wide frequency band of 8.2–41.7 GHz under normal incidence with arbitrary polarization; in addition, an ultra-wideband RCS reduction can still be realized under oblique incidence with an incident angle less than 45°, which shows that the CDM is of good application value in radar stealth technology.

Ultra-wideband RCS reduction realized by a coding diffusion metasurface

In this study, a coding diffusion metasurface (CDMS) based on a circular-polarization (CP)-maintaining metasurface is proposed for radar cross-section (RCS) reduction. The CP-maintaining metasurface can achieve ultra-wideband CP-maintaining reflection, and its main reflection phase under CP incidence can be adjusted at will by the Pancharatnam-Berry phase generated by the rotation of its unit cell, so it can be used to design CDMS. Thus, based on the CP-maintaining metasurface, the 2-bit CDMS is proposed. The simulated and measured results demonstrate that the CDMS can achieve highly effective and ultra-wideband RCS reduction under arbitrarily polarized incidences, compared with a pure metallic plate of the same size, its RCS can be reduced by more than 10 dB under normal incidence in the ultra-wide frequency band of 7.1-33.6 GHz with a relative bandwidth of 130.2%; moreover, when the incident angle is increased to 40°, an ultra-wideband RCS reduction can still be achieved.

Terahertz spin-selective metasurface for multichannel switching of OAM

Thanks to switchable wavefronts, design methods of diverse functions are greatly enriched. However, less attention has been paid to multichannel switchable orbital angular momentums (OAMs) of metasurfaces in previous reports. In this work, a spin-selective metasurface enabling multichannel switchable OAMs is proposed in terahertz band. This metasurface is designed based on the phase change property of vanadium dioxide (VO2). Firstly, when VO2 is metal, vortex beam with topological charge of l = 2 is achieved under left-handed circular polarization (LCP) incidence. Under right-handed circular polarization (RCP) incidence, the obtained vortex beam has a topological charge of l = -3. Secondly, when VO2 is changed to insulator, function of the metasurface is correspondingly switched. Under LCP and RCP incidences, vortex beams are realized with topological charges of l = 3 and l = -2. Therefore, the designed metasurface as a multichannel vortex beam generator has switchable characteristic. Our design shows great promise in the integration of multifunctional devices.

Localized spin-selective interference assisted broadband tunable chirality with diatomic all-dielectric metasurfaces

Chiral metasurfaces with spin-selective properties have shown an unprecedented ability to modulate optical properties on the subwavelength scale. Alternatively, the simultaneous implementation of broadband tunable chiral response and wavefront manipulation on minimalistically constructed monolayer metasurfaces can lead to a plethora of applications such as optical converter diodes, chiral imaging, and sensing. Benefiting from a localized interference between two pairs of achiral meta-atoms whose optical behavior is similar to that of a half-wave plate (HWP), we theoretically provide a design strategy for constructing meta-molecules for asymmetric transmission (AT) parameters using interference mechanisms, each of which achieves a specific modulation function in the terahertz (THz) band. Localized interference effects with polarization-selective properties permit the incident circular polarization to be transformed into its orthogonal polarization mode. Benefiting from spin-selective transmission, the proposed metasurface simultaneously implements polarization filtering and wavefront manipulation with the assistance of a geometric phase mechanism. Not only that, chiral imaging schemes are also evaluated under such a design mechanism. As a result, this generalized assembly scheme can not only be extended to arbitrary working bands, but also find a variety of applications in photodetectors.

Wideband single-feed dual circularly polarized multi-beam transmitarray antenna based on receiver–transmitter metasurface

In this paper, a novel metasurface (MS) as well as a generalized dual-circular-polarized (CP) decoupled modulating method are proposed. The proposed MS can realize high-efficiency dual-spin transmissions with equal-magnitude upon left handed circular polarization (LCP) incidence across 9–12.5 GHz. This MS is constructed based on the receiver–transmitter configuration, and the receiver layer composed of an LCP patch resonator while the transmitter layer is a linearly polarized patch resonator. Rotating the receiver and transmitter patches separately can impose identical and conjugated geometric phase modulations for the dual-CP transmission waves, respectively. Hence, through an elaborate combination of the dual geometric phases, totally decoupled and dispersion-less phase modulations for transmitted dual-spin waves can be obtained. As the proof of its uses in practical applications, we have designed a single-feed dual-CP dual-beam transmitarray antenna (TA) and a dual-CP quad-beam TA. The second TA is also fabricated and tested. The measured results are consistent with the simulated results, showing that the TA achieves peak aperture efficiency of 49% at 9.5 GHz, with 1 dB gain bandwidths (BWs) of the dual-CP quad-beams all larger than 24.5% while 3 dB axial ratio BWs all larger than 54.5%. The proposed wideband and high-gain dual-CP multi-beam TAs may have promising prospects for many applications such as vehicle-satellite communication systems, satellite broadcasting and coverage systems.

Scattering Properties of Non-Gaussian Ocean Surface with the SSA Model Applied to GNSS-R

Global Navigation Satellite System Reflectometry (GNSS-R) is an emerging earth observation method for remote sensing of feature parameters using reflected signals from navigation satellites, and is a purely specular bistatic forward scattering observation means with special right-handed circular polarization incident wave. In this paper, the small-slope approximation model of non-Gaussian sea surface is used as the basis to construct the scattering model for the observation geometry of GNSS-R as well as the L-band characteristics, and the fully-polarization normalized bistatic radar scattering cross section (NBRCS) are simulated by the method of polarization synthesis to analyze the scattering characteristics under different wind speeds and directions on the ocean surface, which highlights the variation of NBRCS with wind direction, and the scattering modeling accuracy is improved by comparing with the data of CYGNSS. In addition, we adopt the observation geometry deviating from purely specular geometry, discuss the scattering azimuth angle, scattering influence, and the relative relationship between different polarizations of the scattering angle under the non-specular geometry.

High-performance full-Stokes polarization detection at exceptional point in a non-Hermitian metasurface

Polarization and its detection have attracted tremendous attention due to the rich information it provides and the practical applications it enables. Here, we propose a high-performance reflective pixelated metasurface with full-Stokes detection in the visible wavelength, which integrates both linear polarization (LP) and circular polarization (CP) filters on one platform. The system achieves a high extinction ratio for CP incidence in a non-Hermitian system, generated from the exceptional point (EP) effect. With integrated LP filters, full-Stokes detection can be obtained with accuracy and efficiency in real time. The EP-based innovation also promises other practical applications, such as polarization imaging technologies and optical computing.

Tailoring spin-sensitive focusings by optical metasurfaces

Metalens is composed of artificially subwavelength metal or dielectric meta-atoms arranged according to phase distribution of lens, and it can focus wave in a compact and flexible manner. However, conventional geometric phase-based metalens cannot focus spin-sensitive splitting beams, hindering the simultaneous control of two spins. Here, spin-sensitive plasmonic metasurfaces are presented with independent phase modulation capability for circularly polarized channels. These metasurfaces encode phase information by integrating propagating phase and geometric phase to achieve arbitrarily independent control of focal length and focal position for circularly polarized waves (CPWs). Three metasurfaces, including longitudinally splitting focusing, multidimensional spin-sensitive splitting focusing, and multifocal metasurface, are numerically displayed to confirm these predictions. Firstly, a longitudinally splitting focusing is designed with focal length 3 μm under left-handed circular polarization (LCP) incidence and focal length 5 μm under right-handed circular polarization (RCP) incidence. Secondly, two anomalously reflected converging beams are formed by superimposing different gradient phases on a multidimensional spin-sensitive splitting focusing under LCP and RCP illuminations. Finally, a multifocal metasurface is presented, and it has bifocal points and four focal points under LCP and RCP incidences. This work provides novel methods for flexible control of circular polarization-sensitive focusing.

An ultra-wideband phase gradient metasurface for anomalous reflection and RCS reduction

In this work, an ultra-wideband phase gradient metasurface (PGM) based on a novel polarization conversion metasurface (PCM) is proposed for anomalous reflection and radar cross section (RCS) reduction. Because the PCM can achieve ultra-wideband circular-polarization(CP)-maintaining reflection in the frequency band from 8.9 to 34.9 GHz, through the rotation of its unit cell structure, the reflection phase in the ultra-wide frequency band can be adjusted at will by the Pancharatnam–Berry (P-B) phase only generated in its co-polarized reflection coefficient under CP incidence. Thus, a PGM is proposed by gradually rotating each unit-cell structure of the PCM, and the proposed PGM can realize ultra-wideband anomalous reflection to reduce the specular RCS. In addition, when the PGM is divided into multiple square elements with different phase gradients, it can work as a diffusion metasurface to reduce the specular RCS and maximum bistatic RCS simultaneously. The simulated results show that the PGM can achieve more than 10 dB specular RCS reduction in the frequency band from 10.0 to 33.5 GHz, but its maximum bistatic RCS is very large; however, when it is changed as a diffusion metasurface, it can reduce not only the specular RCS but also the maximum bistatic RCS in the ultra-wide frequency band of 10.1–34.3 GHz with a relative band-width of 109.5%; in addition, when the incident angle is increased to 45°, the RCS reduction performance of the diffusion metasurface can still be kept well. Finally, an effective experimental verification is carried out for the diffusion metasurface.

Spin‐Selective Absorption and Geometric‐Phase Modulation via Chiral Metasurface in Triple Bands

AbstractMetasurfaces with spin‐selective reflection or transmission functions for circularly‐polarized (CP) incident waves have aroused researchers' great attention. However, current spin‐selective metasurface designs have yet to mature into a practical method due to the limitations of single operating band or inefficient phase modulation. Herein, a novel chiral meta‐atom is proposed, exhibiting spin‐selective absorption/reflection functions for dual‐orthogonal CP incident waves at three different operating bands. More significantly, the Pancharatnam–Berry phase theory is introduced at all working frequencies to generate a full 2π reflection phase modulation by rotating the chiral resonator, and the three distinct absorption bands remain almost constant with the rotation. Theoretical results indicate that high absorption can separately act on left‐handed or right‐handed CP incident waves at triple bands, and meanwhile, the other incident wave with different handedness is efficiently reflected without changing the handedness. As a proof of concept, two tri‐band meta‐devices with spin‐selective anomalous reflection and spin‐selective orbital angular momentum generating functions are designed and experimentally characterized, both of which exhibit excellent spin‐dependent bi‐functional performances at three different operating bands. The research results provide a promising route to realize spin‐selective meta‐devices with multi‐band operating modes in wireless communication systems and multispectral imaging and information encryption fields.

Ultra-wideband radar cross-section reduction based on phase cancellation

ABSTRACT In this study, a 2-bit coding metasurface is proposed, which can achieve ultra-wideband radar cross section (RCS) reduction under arbitrary polarized incidences due to phase cancellation. In the design process of the 2-dit coding metasurface, a novel polarization conversion metasurface (PCM) is proposed at first, which can realize ultra-wideband circular-polarization (CP) maintaining reflection; moreover, when its unit cell structure is rotated by an angle ψ, ±2ψ Pancharatnam-Berry (PB) phase will be generated in its co-polarized reflection coefficients under CP incidence. Thus, based on the PCM, an ultra-wideband 2-dit coding metasurface is conveniently proposed. Numerical simulations demonstrate that the coding metasurface has excellent RCS reduction performance. Compared with a pure metallic plate with the same size, its RCS can be reduced more than 10 dB under normal incidences with arbitrary polarizations in the ultra-wide frequency band of 6.9–33.1 GHz with a relative bandwidth of 131%. Moreover, when the incident angle is increased to 45°, the RCS reduction performance can be kept well. Finally, one laboratory sample of the coding metasurface is fabricated and measured, a reasonable agreement is observed between the measured and simulated results.

Broadband Polarization-Reconfigurable Converter Using Active Metasurfaces

Polarization control of electromagnetic (EM) waves is vital in imaging and wireless communication systems. In this communication, we propose a broadband polarization-reconfigurable converter (BPRC) based on active metasurfaces, which can achieve polarization-reconfigurable conversion under both linear-polarization (LP) and circular-polarization (CP) incident waves in wide bands by changing the states of the p-i-n diodes. The BPRC is composed of three-layer metal patches, three-layer dielectric substrates, and p-i-n diodes. When the p-i-n diodes are in the OFF-state, the LP and CP incident waves can be converted into cross-polarization and co-polarization reflected waves, respectively. Whereas when the p-i-n diodes are changed from OFF-state to ON-state, the LP and CP incident waves can be converted into co-polarization and cross-polarization reflected waves, respectively. The conversion ratio is higher than −1 dB in the frequency range of 7.4–12 GHz, with a relative bandwidth of 47.4%. Furthermore, the simulated and experimental results were in good agreement, and thus verified the feasibility of the BPRC, which will find promising applications in wireless communication systems.

Polarization-switchable focal vortex beam by an Archimedean array.

Focal position control of vortex beams has tremendous applications in optical field. Herein, non-classical Archimedean arrays were proposed for optical devices with bifocal length and polarization-switchable focal length. The Archimedean arrays were constructed by rotational elliptical holes in a silver film, which were followed by two one-turned Archimedean trajectories. The elliptical holes in this Archimedean array provide the freedom of polarization control for the optical performance by their rotation status. The rotation of elliptical hole can provide additional phase to affect the shape of vortex beam (converged or diverged) under the illumination of circular polarization. The geometric phase of Archimedes trajectory will also determine the focal position of vortex beam. This Archimedean array can produce a converged vortex beam at the specific focal plane according to the handedness of the incident circular polarization and geometrical arrangement of array. The Archimedean array was also demonstrated by experiment and numerical simulation for its exotic optical performance.

Generating superposed terahertz perfect vortices via a spin-multiplexed all-dielectric metasurface

Perfect optical vortices (POVs), characterized as a ring radius independent of topological charge (TC), possess extensive application in particle manipulation and optical communication. At present, the complex and bulky optical device for generating POVs has been miniaturized by leveraging the metasurface, and either spin-dependent or spin-independent POV conversions have been further accomplished. Nevertheless, it is still challenging to generate superposed POVs for incidences with orthogonal circular polarization. Here, a spin-multiplexed all-dielectric metasurface method for generating superposed POVs in the terahertz frequency range is proposed and demonstrated. By using the multiple meta-atom comprised structure as the basic unit, the complex amplitude of two superposed POVs is modulated, decoupled, and subsequently encoded to left- and right-handed circular polarization incidences. Furthermore, two kinds of metasurfaces are fabricated and characterized to validate this controlling method. It is demonstrated that the measured intensity and phase distributions match well with the calculation of the Rayleigh–Sommerfeld diffraction integral, and the radius of superposed POVs is independent of TCs. This work provides promising opportunities for developing ultracompact terahertz functional devices applied to complex structured light generation and terahertz communication, and exploring sophisticated spin angular momentum and orbital angular momentum interactions like the photonic spin-Hall effect.

Tri-band terahertz vortex beam generator based on a completely independent geometric phase metasurface

A vortex beam carrying orbital angular momentum (OAM) is an effective way to increase wireless communication capacity. The existing terahertz (THz) metasurface (MS)-based vortex beam generator is limited to operating at a single or two frequency points, and also has wavelength-dependent behavior, thus limiting its application. Here, a transmissive MS is proposed to generate THz vortex beams carrying OAM with different topological charges l = − 1 , − 2 , and − 3 ( l = + 1 , + 2 , and + 3 ) under left/right-handed circular polarization (CP) incidence at three different frequencies. The mode purity of the generated vortex beam with topological charges of all orders is greater than 60% under CP wave incidence at 0.6 THz, 0.857 THz, and 1.892 THz, respectively. Both theoretical prediction and simulation results verify that the designed MS can independently manipulate CP waves at three different THz frequencies. The proposed unit-cell structure is preferable for increasing the number of operation frequencies while guaranteeing the manipulation of electromagnetic waves. The compact system of unit-cell structure provides ideas for the fabrication of multifunctional THz MS-based devices with higher integration and smaller size.