Cross-polarization Conversion Efficiency Research Articles

Multifunctional Reconfigurable Vanadium Dioxide Integrated Metasurface for Reflection, Asymmetric Transmission and Cross‐Polarization Conversion in Terahertz Region

AbstractIntegrating reconfigurable and diverse functionalities into a single metasurface at terahertz (THz) frequencies is an emerging research topic that faces significant challenges. Here, a reconfigurable THz metasurface is proposed, offering diversified functionalities based on the phase transition of vanadium dioxide (). The proposed metasurface can switch from wideband reflection to wideband cross‐polarization conversion (CPC) and asymmetric transmission (AT) for linearly polarized waves. When is in its fully metallic state, the metasurface efficiently reflects normally incident waves ranging from 0.2 to 2.8 THz, with total reflection exceeding 0.8. When is in its insulating state, the metasurface achieves nearly perfect wideband cross‐polarization conversion with CPC efficiency above 0.99 from 0.46 to 2.67 THz, and excellent AT effect with efficiency over 0.9 from 0.65 to 2.29 THz for normal incidence of linearly polarized waves. Moreover, the high efficiency of CPC and AT effects is maintained over a wide range of incident angles. The proposed switchable metasurface with diverse functionalities is expected to enable cutting‐edge research and innovative applications in THz communication, sensing, and imaging.

Active anisotropic polarization conversion meta-surface for 6G communication bandgap in the reflection mode

This paper investigates a single-layered active anisotropic meta-surface design capable of achieving high polarization conversion efficiency in the 6 G communication band gap. Additionally, the designed structure can convert the linear polarization of the incoming wave fronts to its cross-polarization and linear polarization to circular polarization in the reflection mode. The polarization conversion characteristic is achieved due to the strong anisotropic behavior of the meta-surface. Such a simple design can achieve a 90% to 100% cross-polarization conversion efficiency over a frequency region of [0.28–0.36] THz. The linear to circular polarization conversion is performed near 0.26 THz and 0.40 THz. Between these two frequencies, band linear to cross-polarization conversion is more prominent. The proposed design can be tuned from a half-wave plate (HWP) or linear polarization converter to a quarter-wave plate (QWP) or circular polarization converter using the chemical potential of graphene. The optical response of the meta-surface behaves differently from different incident angles. The proposed polarization converter can contribute to the development of active polarization device.

Tunable broadband transmissive terahertz cross-polarization converter enabled by a hybrid metal-graphene metasurface

Graphene has shown potential in terahertz (THz) polarization modulation due to highly tunable optoelectronic properties, fast photoelectric response, and ease of integration. However, the performance of polarization converters based on graphene metasurfaces is often limited by the achievable carrier mobility of large-area graphene. In this paper, a flexible and tunable broadband transmissive THz cross-polarization converter based on a hybrid metal-graphene metasurface is proposed. It is composed of two metal grating layers with a graphene-loaded 45-degree antenna array sandwiched between them. The THz response of the antenna can be tuned by adjusting the graphene Fermi level, which further alters the cross-polarization conversion efficiency (CPCE) of the device. The average CPCE can be continuously tuned from 80.3% to 4.5% within a broadband from 0.6 to 2.0 THz, and the average modulation depth of the whole band is 94.2%. The mechanisms of this highly efficient polarization conversion and dynamic modulation are explained with a transfer matrix method and an equivalent circuit model. Furthermore, the proposed structure has a low requirement on the graphene mobility, which is only 500 cm2/(V·s) here. This work provides a new approach to highly efficient tunable cross-polarization conversion over a broadband in THz, which will promote the application of graphene-based polarization modulators in THz sensing, imaging and communication.

Low-profile linear polarization conversion metasurfaces using degenerate modes for high selectivity

In this paper, two ultrathin polarization conversion metasurfaces (PCMs) were designedfor linear polarization of electromagnetic waves. A method of controlling degenerate modes was applied for enhanced selectivity. The surface resonator is a square patch etched with orthogonal cross-slots. Adjustment of the cross-slot size is beneficial as it introduces a perturbation factor and controls the splitting of the degenerate modes. The constructed dual-mode single-band and four-mode dual-band PCMs with different pairs of degenerate modes can efficiently reflect linear polarized waves to the orthogonal polarization counterpart in the range of 5.89–6.4 GHz and 5.88–6.30 GHz/8.19–8.59 GHz, respectively. Current distribution and an equivalent circuit model were studied to reveal the polarization conversion mechanism. Finally, experiments were implemented to verify the near unity cross-polarization conversion efficiency at the corresponding bands. Given the PCM’s advanced performance of low profile and high selectivity, it has great potential applications in controlling the polarization state of microwaves.

Broadband and high-efficiency photonic spin-Hall effect with all-metallic metasurfaces.

In this paper, all-metallic reflective metasurfaces comprising S-shape streamline structures are proposed to achieve the photonic spin-Hall effect with average cross-polarization conversion efficiency exceeding ∼84% in the range of 8-14 µm. By comparing with all-metallic nanobricks, it is demonstrated that the electric field coupling could be enhanced by constructing a similar split ring resonator between adjacent unit elements to further improve its efficiency and bandwidth. As a proof of concept, the photonic spin Hall effect and spin-to-orbit angular momentum conversion could be observed by two metadevices with the maximum diffraction efficiency of ∼95.7%. Such an all-metallic configuration may provide a platform for various high-efficiency electromagnetic components, catenary optics, and practical applications.

All-metallic high-efficiency generalized Pancharatnam-Berry phase metasurface with chiral meta-atoms.

Metasurfaces based on the Pancharatnam-Berry (PB) phase have attracted significant attention in the domains of subwavelength optics and electromagnetics. Conventional theory predicts that the PB phase is exactly twice the rotation angle of the anisotropic meta-atoms. Differently, a recent advance has demonstrated that the generalized PB phase representing multiple times of the rotation angle could be obtained with high-fold rotational symmetry meta-atoms, but it suffers from the low cross-polarization conversion efficiency (the theoretical upper limit of 25%) that impedes its further applications, especially for meta-atoms with rotational symmetry≥3. Here, we verify that the chiral meta-atoms with high-fold rotational symmetries could produce the generalized PB phase.Besides, the all-metallic configuration is utilized to design C2, C3, and C5 chiral catenary meta-atoms to improve their efficiency and bandwidth. The equivalent air waveguide with low loss between two adjacent meta-atoms is formed to analyse the higher performances of the all-metallic scheme for the realization of the generalized PB phase compared with the metal-insulator-metal and all-dielectric C3 meta-atoms. As a proof of concept, four metadevices including two spin-Hall metadevices and two holograms are experimentally demonstrated and their maximum efficiency could exceed 83% in simulation. This work could provide a high-efficiency platform for the study of the generalized PB phase in linear and nonlinear optics.

Tunable and reflective polarization converter based on single-layer vanadium dioxide-integrated metasurface in terahertz region

A single-layer and actively tunable reflective polarization converter is proposed, which is composed of periodical two-dimensional unit cell of inclined split circular ring filled into by vanadium oxide (VO2) and central rod. Through numerical simulation obtained by the CST Microwave Studio, we find that the broad bandwidth of reflective cross-polarization conversion above 90% is normally obtained from 2.03 to 5 THz at the temperature about 25 °C for both the linearly and circularly polarized wave incidence. At the same time, the polarization conversion ratio (PCR) and energy conversion ratio (ECR) have been furtherly investigated, which reflect high cross-polarization conversion efficiency with nearly no energy loss at room temperature of 25 °C. Thereafter, namely, the cross-polarization conversion can be achieved by theoretical analysis from the views on qualitative variables of polarization azimuth angle (θ), ellipticity (η) and symmetrical polarization decomposition. What is more, the device is sensitive to the incident angles which result into the split reflected frequency spectrum. Afterwards, the actively dynamic responses for reflective frequency spectrum depending on the insulator-to-metal phase transition of VO2 have been investigated. To be demonstrated, the physical mechanism of changeable polarization conversion has been discussed by the distributions of current densities. Lastly, also, the influences of structural parameter variation can be further studied. According to the interesting results, the method provides a strategy that manipulate active and passive metadevices for polarization converter, wireless communication in THz wavelength domain.

A High-Efficiency and Broadband Folded Reflectarray Based on an Anisotropic Metasurface for Generating Orbital Angular Momentum Vortex Beams

In this paper, a low-profile folded reflectarray (FRA) assembled by an anisotropic metasurface is proposed to generate high-efficiency and broadband orbital angular momentum (OAM) vortex waves in the Ka band. In order to suppress the mutual coupling and increase the polarization conversion efficiency, a dual-polarization phase matching method (DPPMM) for helical wavefront construction is developed to achieve minimum phase compensation error in the orthogonal direction. A compact subwavelength anisotropic meta-atom with orthogonal || |-shaped structure is designed to achieve flexible wavefront manipulation and high-resolution phase compensation for x and y-polarization incident waves. After the optimization design, nearly 100% cross-polarization conversion efficiency for the anisotropic metasurface can be achieved at the working frequency of 12 GHz. To validate the proposed design method, an OAM FRA with 24×24 meta-atoms is fabricated and measured. The measurement results show that the aperture efficiency of the proposed FRA carried with the OAM mode l = 1 is significantly improved and reaches 20.2%. Moreover, the OAM bandwidth and 3-dB gain bandwidth reach 22.5% and 25%, respectively.

High efficiency and broad bandwidth terahertz vortex beam generation based on ultra-thin transmission Pancharatnam–Berry metasurfaces* *Project supported by the National Natural Science Foundation of China (Grant No. 62071312), the Important R&D Projects of Shanxi Province, China (Grant No. 201803D121083), and the Shanxi Scholarship Council (Grant No. 2020-135).

The terahertz (THz) vortex beam generators are designed and theoretically investigated based on single-layer ultra-thin transmission metasurfaces. Noncontinuous phase changes of metasurfaces are obtained by utilizing Pancharatnam–Berry phase elements, which possess different rotation angles and are arranged on two concentric rings centered on the origin. The circularly polarized incident THz beam could be turned into a cross-polarization transmission wave, and the orbital angular momentum (OAM) varies in value by lℏ. The l values change from ± 1 to ± 5, and the maximal cross-polarization conversion efficiency that could be achieved is 23%, which nearly reaches the theoretical limit of a single-layer structure. The frequency range of the designed vortex generator is from 1.2 THz to 1.9 THz, and the generated THz vortex beam could keep a high fidelity in the operating bandwidth. The propagation behavior of the emerged THz vortex beam is analyzed in detail. Our work offers a novel way of designing ultra-thin and single-layer vortex beam generators, which have low process complexity, high conversion efficiency and broad bandwidth.

Ultrathin and Flexible Reflective Polarization Converter Based on Metasurfaces With Overlapped Arrays

In this letter, an ultrathin reflective polarization converter with the total thickness of 1/62 wavelengths is proposed to realize the high efficiency of cross-polarization conversion at 32 GHz. Two metasurfaces overlapped on the same layer with a displacement are designed to improve its polarization conversion performance, angular stability, and conformal adaptability. Due to the flexibility of the polyimide substrate, a planar and a conformal converter are conveniently fabricated by the printed circuit board technology. Experimental results show that the maximum conversion efficiency of the planar converter is higher than 94.3%, which is rather close to the simulation prediction. Meanwhile, a cross-polarization conversion efficiency of 88.1% can be obtained by the conformal converter with a 100 mm curvature radius, which demonstrates the validity of the overlapped array design. Due to the ultrathin thickness and flexibility, the proposed converter has great application potential in polarization control devices, on-chip antennas, and wearable antennas.

Theoretical and Experimental Verification of 90° Polarization Converter Based on Chiral Metamaterials

In this paper, a chiral metamaterial (CMM) has been proposed theoretically and experimentally to realize a 90° polarization converter, whose maximum cross-polarization conversion efficiency is nearly to 1. In 0.4 GHz bandwidth around 16.8 GHz, a 90° polarization angle and negligible ellipticity can be obtained, with a minimum cross-polarization conversion efficiency of 0.96. The mechanism of the cross-polarization conversion is explained by studying the surface current and electric field of the structure. Finally, through parameter inversion, the parameter characteristics of the structure are studied.

Dual-focal metalenses based on complete decoupling of amplitude, phase, and polarization

The simultaneous control of amplitude and phase via metasurfaces affords us unprecedented degree of freedom in manipulating electromagnetic waves, but currently most designs suffer from low efficiency which raises certain concerns for real-world applications. Moreover, the complete amplitude, phase and polarization modulation is particularly challenging, which typically requires a combination of attenuators, optically thick wave plates and large dielectric lenses. Here, we propose an alternative scheme by introducing vertical mode cross-coupling for polarization control and high efficiency, while by involving spatially-varied orientations and structures for independent amplitude and phase modulation. The vertical mode cross-coupling is synthesized by stacking triple-layer twisted split ring resonators (SRRs) operated in transmissive scheme. Such tight cross-coupling and chirality-assisted coherent multiple resonances facilitates high cross-polarization conversion efficiency (~100%) and broadband transmission window with full phase cover. As a proof of concept, two dual-focal metalenses that are challenging to be actualized through conventional metasurfaces are designed, numerically and experimentally studied with a total thickness of λo/12 at microwave frequencies. Desirable dual focusing behavior with axial and lateral alignment of two foci are demonstrated. Our findings, not confined to microwave operation, opens up an alternative way in fine controlling light and can stimulate novel and high-performance versatile photonic metadevices.

Broadband Spin-Decoupled Metasurface for Dual-Circularly Polarized Reflector Antenna Design

In this article, we theoretically and experimentally demonstrate a single-layered spin-decoupled metasurface and its application to a dual-circularly polarized reflector antenna. A developed method is first analyzed to guide the design of broadband spin-decoupled metasurface. Then, a quasi-I-shaped meta-atom, which exhibits a phase coverage of $2\pi $ and high cross-polarization conversion efficiency within 12–21 GHz, is accordingly designed to actualize spin-decoupled phase change by simultaneously tuning the structural parameters and self-rotation of the element. To illustrate the independent phase control of orthogonal spins, a dual-circularly polarized multiplexing reflector antenna is designed with offset configuration. The experimentally measured results of the assembled reflector antenna are in good agreement with the simulated ones, showing a peak gain of 29.5 and 29.6 dBic, peak aperture efficiency (AE) of 53% and 54%, 3 dB gain bandwidth (BW) of 13.2–20.2 GHz (41.9%) and 13.4–20.2 GHz (40.5%), and 3 dB axial ratio (AR) BW of 11.8–21 GHz (56.1%) and 11.5–22 GHz (62.7%) for left-hand circular polarization (LHCP) and right-hand circular polarization (RHCP) radiations, respectively. The design strategy and the well-performed multiplexing reflector antenna may have promising perspective in many practical applications, for example, satellite communication system and long-distance wireless communication system.

Multifunctional Single Layer Metasurface Based on Hexagonal Split Ring Resonator

We present a single-layer metasurface, which performs both cross and circular polarization conversion at multiple frequency bands. The unit cell of the proposed metasurface consists of periodic array of a uniquely designed hexagonal split ring resonator (SRR). The proposed metasurface behaves as an efficient 90° polarization rotator at dual frequency bands of 6.36-6.59 GHz and 10.54-13.56 GHz. The polarization conversion efficiency approaches 90% within these two operating bands. Moreover, linear-to-circular polarization conversion is also achieved in other three wide frequency bands from 6.10-6.20 GHz, 6.84-9.02 GHz and 14.10-15.48 GHz. The novel features of the subject hexagonal metasurface, which qualify it for numerous practical applications are high cross-polarization conversion (CPC) efficiency, wideband operation and angular stability for oblique incidence up to 45°.

0.02-wavelengths-thick transmission-type designer wave plate with high efficiency

The thin thickness and the working efficiency are always two key problems for transmission-type polarization conversion devices. Here, we propose a C-band ultrathin (0.02 λ0 in the thickness) and highly efficient circular-polarization conversion device in the transmission mode. The designer wave plate is composed of double layer surface structures printed on both sides of a single-layer substrate and the metallic layers are connected by a pair of through-via holes. The structure of the top layer is composed of a patch with a D-shape slot inside, and the bottom layer can be obtained by rotating the top layer with 180 degrees around the center. We demonstrate that the designer wave plate can achieve transmission polarization conversion from right-handed circularly polarized incident electromagnetic (EM) waves to left-handed ones or from left-handed circularly polarized incident EM waves to right-handed ones in the transmission mode. The results show that the maximum cross-polarization conversion efficiency of the designer wave plate at a working frequency of 5.8 GHz is 94.6%. This ultra-thin circular-polarization conversion device is expected to be applied to wireless systems that require circular polarization diversity.

Scattered EM-Wave Shaping Using Combination of Cross-Polarization Conversion and Reflection Phase Cancellation

The design of millimeter-wave 1 bit coding engineered reflector for backscattered electromagnetic (EM)-wave shaping by combining both cross-polarization conversion and reflection phase cancellation principles is presented in this letter. The anisotropic unit cell of the presented engineered reflector can efficiently rotate the polarization of an incident EM wave to its orthogonal one at millimeter-wave regime. Each unit cell has four inverted L-shaped copper resonators on the top side of a PEC-backed dielectric substrate. The proposed unit cell has three plasmon resonance frequencies at 87.3, 90.1, and 91.8 GHz with 100% cross-polarization conversion efficiency at these frequencies, and more than 92.1% from about 86.7 to 91.9 GHz. Moreover, if the proposed anisotropic unit cell (“0” element) along with its mirrored one (“1” element) are arranged in one engineered reflector, a 180° ± 25° reflection phase difference is valid between their reflection phases and engineered reflectors that can efficiently manipulate the reflected EM-wave and generates many kinds of scattering patterns such as two lobes, three lobes, four lobes, or even a diffuse scattering pattern can be composed using only those two unit cells. Full-wave simulation, fabrication, and experimental characterization results are presented.

Design of 1-Bit Coding Engineered Reflectors for EM-Wave Shaping and RCS Modifications

In this paper, the engineered reflectors are designed and characterized for cross polarization rotation, RCS modification, and EM-wave shaping at W-band. The multiple plasmon resonances, cross-polarization rotation, reflection phase cancellation, and coding sequence principles are combined together to design the presented reflectors. First, an anisotropic unit cell consisting of a two E-shaped metallic resonators on the top side of a PEC-backed dielectric substrate is precisely designed and optimized. The unit cell operates in a linear cross-polarization scheme from about 86 GHz to 94 GHz and has multiple plasmon resonances at 86.5 GHz, 89.2 GHz, 92.2 GHz, and 93.2 GHz with 100% cross-polarization conversion efficiency at these frequencies. Then this unit cell and its mirrored unit cell are used to compose a number of 1-bit coding reflective engineered reflectors to generate the “1” and “0” elements of the coding sequence required for EM-wave shaping. Four engineered reflectors of various coding sequences are designed to shape the backscattered energy to achieve one lobe, two lobes, three lobes, and four lobes. Furthermore, the low-scattering diffuse reflection pattern is also achieved under both normal and oblique incidence by using a random distribution (random coding sequence) of the unit cells across the engineered reflector aperture. Both 3D full wave simulations and measurement results verify the capability of the presented surfaces in shaping the backscattered EM-wave.

Microwave linear polarization rotator in a bilayered chiral metasurface based on strong asymmetric transmission

We propose and study a kind of bilayered chiral metasurface (BCM) composed of complementary L-shaped resonators with a lossless dielectric spacer that can realize linear polarization rotation with ultrahigh conversion efficiency. We present a theoretical analysis of the BCM with specific chiral geometry that enables asymmetric transmission for linear polarization only. Numerical results show that the proposed metasurface has dual-band asymmetric transmission with nearly 100% cross-polarization conversion efficiency when the loss is ignored. More importantly, depending on the incident direction, only one of the cross-polarization transmissions can approach unity while all the remaining transmissions are close to zero. As a result, nearly perfect linear polarization rotation is achieved for a particular polarization direction. We further show that the working frequency and the bandwidth of the proposed BCM can be tuned by adjusting the geometric size and spatial arrangement of the unit cell.

Polarization Filtering and Phase Controlling Metasurfaces Based on a Metal-Insulator-Metal Grating

Metasurfaces used in the manipulation of light beams have attracted growing interests owing to their unique electromagnetic properties in the subwavelength regime. However, most previously demonstrated single-layer metasurfaces are normally designed to realize one-fold function of either polarization or phase manipulation and suffer from low cross-polarization conversion efficiency and high-background, especially for transmissive metasurfaces. Here, a metasurface based on metal-insulator-metal (MIM) subwavelength grating is proposed to simultaneously achieve polarization filtering and phase controlling. The transmission coefficient reaches up to 78.9% and the polarization extinction ratio (ER = 20*log(T TM /T TE) is larger than 16.1 dB. A local abrupt phase difference covering 0–2π is introduced into transmitted light with the polarization direction vertical to the grating by artificially tailoring the geometrical parameters of MIM grating. Furthermore, background-free wavefront control and high-purity radial/azimuthal polarization are realized by the metasurfaces based on the MIM grating. This flexible and high-efficient scheme of full control wavefront and polarization promises an unprecedented progress of spatial vectorial beams modulation and enable the realization of novel optical components.

Ultra-broadband perfect cross polarization conversion metasurface

We propose a metasurface with multiple plasmon resonances that achieves an ultra-broadband perfect cross polarization conversion. The metasurface is composed of an array of unit resonators, three plasmon resonances are excited in the unit resonator, which leads to an ultra-broadband perfect cross polarization conversion. The cross polarization conversion efficiency is higher than 99%, and the bandwidth of the converter is 53.7% of the central wavelength. Both numerical and experimental results were used to validate the ultra-broadband perfect cross polarization converter presented here.