Cross-polarized Transmission Research Articles

Constraints on co- and cross-polarization reflection and transmission of Babinet-complementary metasurfaces

A study of constraints on reflection and transmission coefficients for Babinet-complementary metasurfaces is presented in this work. These coefficients have been found to describe circular paths in the complex plane, whose centers and radii are affected by losses. Additionally, it has been shown that it is possible to fully control the cross-polarization coefficients by rotating the metasurface with respect to the polarization direction. Our findings have been verified through numerical simulations and experiments. The properties discussed in this paper could be useful in limiting the types of possible responses of metasurfaces.

Transmission-type frequency selective surface with polarization modulation

In this work, a transmission-type frequency selective surface (FSS) with polarization modulation is proposed. This design achieves cross-polarized transmission under both x- and y-polarized normal incidences, while supporting co-polarized transmission at a 45° incidence polarization angle. Consequently, the structure can serve as a polarization rotating FSS (PR-FSS) or a conventional bandpass FSS. The design consists of two fundamental units, corresponding to x- and y-polarized PR-FSS, arranged according to a specific group array paradigm. This arrangement preserves the cross-polarization transmission functionality of each unit. In addition, at a 45° polarization angle, the symmetry of the paradigm with respect to the electric field enables it to support co-polarized transmission. A prototype containing a 12 × 12 element array is constructed and tested. The measured results demonstrate good agreement with the simulation results.

Multi-fold geometric phase metasurface with versatile operations for transmission and reflection

We propose a high efficiency wideband three-fold geometric phase metasurface for versatile operation of transmission and reflection. The transmission coefficient as high as 87 % is achieved in the frequency range of f1 (15.4–15.8 GHz), while equal transmission and reflection are achieved in two frequency bands represented by f2 (14.6–15.2 GHz & 16–17 GHz) with maximum coefficient reaches 49 %. With geometric rotation, the phase shifts of the cross-polarized transmission and co-polarized reflection are six times the rotation angle within the frequency range of 14.6–17 GHz. Furthermore, by elaborately breaking the mirror symmetry while preserving rotational symmetry, interesting features of resonance frequency shift and mode splitting are observed, offering a more fruitful approach for versatile operations. To substantiate the proposed design, a metasurface prototype for vortex beam generation is fabricated and verified by microwave measurement.

Few-layer bifunctional metasurfaces enabling asymmetric and symmetric polarization-plane rotation at the subwavelength scale

We introduce and numerically validate the concept of few-layer bifunctional metasurfaces comprising two arrays of quasiplanar subwavelength resonators and a middle grid (array of rectangular holes) that offer both symmetric and asymmetric transmissions connected, respectively, with symmetric and asymmetric polarization-plane rotation functionalities. The proposed structures are thinner than λ/7\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\lambda /7$$\\end{document} and free of diffractions. Usually, the structure’s symmetry or asymmetry, i.e. unbroken or broken spatial inversion symmetries, are considered for metasurfaces as prerequisites of the capability of symmetric or asymmetric conversion of linearly polarized waves, respectively. Due to the achieved adjustment of the resonances enabling the rotation of the polarization plane simultaneously for both orthogonal polarizations of the incident wave, the symmetric polarization-plane rotation functionality can be obtained within one subwavelength band, whereas the asymmetric polarization-plane rotation functionality associated with the asymmetric transmission is obtained within another subwavelength band. This combination of the functionalities in one subdiffraction structure is possible due to the optimal choice of the grid parameters, since they may strongly affect the coupling between the two resonator arrays. Although normal incidence is required for the targeted bifunctionality, the variations of the incidence angle can also be exploited for the enrichment of the overall functional capability. Variations of the polarization angle give another important degree of freedom. The connection between the polarization-angle dependence of cross-polarized transmission and capability of symmetric and asymmetric polarization-plane rotation functionalities is highlighted. The feasible designs of the bifunctional metasurfaces are discussed.

Triple-broadband asymmetric transmission via cross-polarization conversion based on composite chiral metasurface structure

In this paper, a novel composite chiral metasurface (CCMS) was proposed and investigated, which is composed of a rectangle-patch-ring resonator (RPRR) structure sandwiched between two twisted sub-wavelength metal gratings separated by a dielectric substrate. Both simulation and experimental results demonstrate that the proposed CCMS achieves a significant asymmetric transmission (AT) effect and efficient cross-polarization conversion for the incident linear polarization wave in a triple-broadband range. Simulation results are in good agreement with the experiments. Specifically, the CCMS converts normal impinging y-/x-polarization wave along the forward/backward (-z/+z) direction into transmitted x-/y-polarization wave with the cross-polarization transmission coefficient over 0.7 at 4.15–5.5 GHz, 7.29–10.77 GHz and 12.49–16.59 GHz, respectively. In addition, the AT coefficient (Δlin) and the total transmittance (Tx) of the x-polarization wave propagation along the –z axis direction are both over 0.5 in the triple-broadband frequency range. The simulated polarization azimuth rotation and ellipticity angles, induced surface current, and electric field vector distributions further confirm the characteristics of the triple-broadband high-efficiency AT effect and cross-polarization conversion of the designed CCMS. The proposed CCMS design serves as an important reference for practical applications of microwave devices.

Dual-band transmission polarization conversion metasurface and its reconfigurable-phase design

Although abundant transmission polarization conversion metasurfaces (T-PCMs) have been reported, a dual-band T-PCM has only received little attention. Moreover, a reconfigurable-phase design of the T-PCMs is limited to a single-frequency band operation. There is hardly any literature to integrate dual-band and reconfigurable-phase features into a T-PCM. Herein, a dual-band T-PCM is presented based on the Fabry-Pérot-like resonant cavity. Particularly, the T-PCM meta-atom contains dual anisotropic patches distributed in different layers, and the novel dual-patch structure introduces multiple current modes to implement dual-band operation. Further, a reconfigurable-phase design of the dual-band meta-atom is performed, and four PIN diodes are embedded in each meta-atom to realize a 1-bit reconfigurable phase. In this way, the T-PCM meta-atom can simultaneously achieve high-efficiency 90° polarization rotation and 1-bit dynamic phase manipulation in 2.3–3.0 GHz and 4.4–7.5 GHz. By changing the encoding of the T-PCM, both cross-polarized transmission and beam manipulation are demonstrated in two divided bands. A sample is fabricated and measured to validate our design, and the results confirm the simulations. With the advantages of ingenious design and a clear operation mechanism, the novel dual-patch structure can be extended to the terahertz and optics regimes.

Tunable enhanced transmitted group delays of circular polarization enabled by resonant tunneling in Ag/multi-Weyl semimetal/Ag trilayers

The 4 × 4 magneto-optical matrix is theoretically established to investigate the group delays of circular polarization, which are generated by the linearly polarized wave crossing through a sandwich composition that consists of a multi-Weyl semimetal (mWSM) layer and two identical Ag layers. Results denote that the cross-polarized transmission of right hand circularly polarized (RCP) and left hand circularly polarized (LCP) waves experience sharp increases from zero transmission to total transmission at their respective resonant tunneling wavelengths because of the nonzero off-diagonal components of mWSM. Based on the steep change of cross-polarized transmissions, the giant enlarged group delays of several picoseconds can be obtained. Furthermore, the resonant tunneling wavelength of RCP and LCP waves exhibit the dependence on tilt degree of Weyl cones, Fermi energy, Weyl nodes separation and topological charge, which provide feasible strategies to achieve the regulable enhanced group delays. Our findings reveal effective methods to acquire the tunable enlarged group delays of circular polarization with mWSM.

Integrated metamaterial with the functionalities of an absorption and polarization converter.

A switchable and tunable dual-function absorber/polarization converter is presented in this work. The constitution of the structure, which incorporates patterned graphene and photosensitive silicon (Si), can minimize undesired optical losses. Simulated results show that when the Si is metallic, the structure behaves as a broadband absorption of more than 90% in the range of 1.45-3.36THz. Its peak absorption can be tuned from 22% to 99.8% by changing the Fermi energy of graphene. Furthermore, the interference theory analyzes the physical mechanism for broadband absorption. When the Si is in the dielectric state, the structure has a transmission polarization conversion function, which realizes the conversion from linear to cross-polarized waves. The polarization conversion ratio (PCR) is greater than 90% in the 3.82-4.43THz range. Meanwhile, the cross-polarization transmission can be dynamically tuned from 28% to 97%, and the PCR can also be tuned from 17% to 99.9% by adjusting the conductivity of the Si. The reason for realizing polarization conversion is explained by the polarization decomposition method. This study provides a design opinion of high-performance multifunctional tunable terahertz devices.

Programmable graphene metasurface for terahertz propagation control based on electromagnetically induced transparency

Metasurfaces with programmable functions possess great potential in developing propagation control devices for electromagnetic waves. However, in the terahertz (THz) regime, most related metasurfaces are mainly focused on strength tuning and dual-function switching, otherwise relatively complex working conditions are required and the available controlling degree of freedom over the THz field is limited. Here, we propose a programmable graphene metasurface based on the quantum effect analogue, electromagnetically induced transparency. By varying the bias voltages applied to the graphene patterns connected to the dark resonators, the amplitude and phase of the unit cell's cross-polarized transmission can be continuously tuned and robustly switched between 0 and π, respectively. Based on this, flexible manipulations over the diffraction angles and focal lengths of the transmitted THz beams are achieved in either an amplitude or phase control manner. The proposed programmable scheme may provide new inspirations for designing THz programmable metasurface devices.

Broadband terahertz linear cross-polarization conversion in transmission mode using planar coupled metamaterials

We demonstrate a metamaterial (MM) design capable of showing linear broadband polarization conversion over the terahertz (THz) frequency range. The building block of the proposed MM structure is composed of a strip and four split ring resonators (SRRs), which are coupled through their near fields. To examine co- and cross-polarization transmission amplitudes, we gradually increase the distance between the strip and SRRs. When the SRRs are near (S = 2 μm) the strip, maximum cross-polarization conversion is attained with a resonance mode hybridization effect in the co-polarization transmission due to strong near-field coupling between the strip and SRRs. When the SRRs moved away from the strip (S = 22 μm), minimum cross-polarization conversion is attained due to weak coupling between the strip and SRRs. This MM system exhibits a transition from a strongly coupled state to a weakly coupled state with the rise in displacement between the strip and SRRs. The ability to tune the linear polarization conversion can be useful in the improvement of efficient THz polarization rotation devices. The proposed MM structure can be used in other frequency domains, like the microwave and visible range, by scaling up/down the structure.

Double-layer broadband transmission metasurface and its application in low sidelobe antenna

This study proposes a double-layer metasurface which is capable of conducting independent amplitude and phase modulation for transmitting x-polarized waves under y-polarized incidence. The phase modulation is controlled by regulating the opening angle of the middle metal ring. Meanwhile, amplitude modulation is achieved by adjusting the rotation angle of the opening ring structure. The cross-polarized transmission response achieves complete phase and amplitude coverage and exhibits wideband characteristic with which a 3 dB gain bandwidth of 54.6% (5.4–9.5 GHz). The polarization isolation level is lower than −20 dB. In the wideband frequency range, the average sidelobe level of phase-only modulated metalens antenna is less than −20 dB with the lowest value of −22.5 dB, and the intermediate sidelobe level of amplitude-phase independent modulated metalens antenna is less than −25 dB with the lowest value of −27.4 dB. Both numerical simulated and experimental results verify that the proposed metalens antenna realizes a 4.8 dB averages of the sidelobe level suppression compared with the phase-only modulation metalens antenna. Holding strong anti-interference performance due to its superiority in polarization isolation level and side lobe level, the proposed antenna embraces broad application prospects in point-to-point communication systems in jamming environments.

Spin-selected bifunctional metasurface for grayscale image and metalens.

With the extensive research on the Pancharatnam-Berry phase, metasurfaces have been widely designed as various cross-polarized nanodevices for circularly polarized (CP) illumination. However, co- and cross-polarized lights are rarely co-modulated by the metasurface. To fully utilize the transmitted light, we propose a spin-selected bifunctional metasurface composed of arrayed silver nanorods, integrating an amplitude-based grayscale imaging for co-polarized transmission and a phase-based metalens for cross-polarized transmission, under left-handed CP incidence. Moreover, such dual functionalities work well under right-handed CP incidence. Both experiments and simulations demonstrate the bifunctional performance as potential meta-devices.

Evaluation of Fossil Amber Birefringence and Inclusions Using Terahertz Time-Domain Spectroscopy.

Using a cross-polarization transmission geometry, stress maps for the normalized birefringence and intrinsic stress direction of polymeric materials may be obtained using terahertz nondestructive evaluation. The analysis method utilizes a deconvolution method to determine the arrival times and amplitude of the cross-polarized terahertz pulses through a birefringent material. Using amber (a naturally occurring polymer) as a material of interest, stress maps show that inclusion-free Lebanese amber samples behave as classic uniaxial birefringent (photoelastic) materials whose principal stress directions, as inferred in the terahertz spectral range, agree well with visible photoelasticity measurements. Since amber samples, depending upon their source, may be either transparent or opaque to visible light, comparing birefringence measurements in the visible and terahertz spectral ranges cross-validates the stress measurements, thereby establishing a strong and unique stress analysis methodology for visibly opaque samples. While the material of interest for this paper is amber, the method is generally applicable for any terahertz-transparent polymer. The cross-polarization experimental configuration enables stress levels within the amber matrix to be visualized while also outlining highly localized regions of stress surrounding inclusions. Birefringence stress maps clearly show localized increases in stress magnitude and directional changes surrounding inclusions.

Radiation pressure of a hybrid bianisotropic chiral structure

Abstract To obtain radiation pressure of a hybrid chiral structure, we derive the transfer matrix method and optical force densities from the Maxwell stress tensor for a planar layered bianisotropic media. The key derivations are how to get wavevectors and field components of each layer, transfer matrices connecting four eigenwaves of adjacent chiral media, as well as the force densities expressed by co- and cross-polarized reflection and transmission coefficients. After the validation of the methods and programs is performed, the radiation pressure of a layered biaxial bianisotropic chiral slab is studied. The effects of linearly and circularly polarization incident waves, incident angle, thickness, opposite handedness (optical activity), anisotropy, gain and loss of chiral media on the radiation pressure are discussed. Our work elucidates the mechanism of light-chiral media interactions, provides better understanding of chiral detection, optical trapping, and biophysics.

Topology optimization of the azimuth-rotation-independent polarization conversion metasurface for bandwidth enhancement.

Metasurfaces offer an unprecedented opportunity for flexible manipulation of electromagnetic wave. The azimuth-rotation-independent (ARI) polarization conversion metasurface (PCM) is an ultrathin device, which could convert an arbitrary linearly-polarized incident wave to its cross-polarized state. However, the bandwidth of an ARI PCM with a high cross-polarized transmission is usually limited. Here, a topology optimization method of multi-feature points based on the differential evolution (DE) algorithm is adopted to enhance the bandwidth of the traditional ARI PCM while maintaining a high transmission and polarization conversion ratio. The simulated results of the optimized structure indicate a 2.08 times bandwidth expansion in the cross-polarization conversion compared with the original structure. In addition, the measured results are consistent with the simulated ones and the ARI characteristic is validated. The proposed method provides a promising route for efficient high-performance metasurface designs.

Highly efficient and tunable terahertz polarization converter based on double subwavelength metallic gratings infiltrated with liquid crystal.

In this work, a tunable cross-polarized transmission structure at the terahertz frequency was demonstrated, and the polarization state during modulation was investigated. The proposed structure can significantly enhance the polarization conversion performance of nematic liquid crystals by leveraging the formation of a Fabry-Perot-like resonant cavity that consists of two metal gratings. As a result, the incident waves are continuously reflected in the liquid crystal layer to complete the accumulation of polarization angle changes. From the experimental results acquired, it was concluded that the insertion loss of the cross-polarization transmission was less than 3 dB and the extinction ratio was larger than 28 dB in the frequency range of 388-426 GHz. Our work provides useful insights for improving the efficiency of cross-polarization conversion by enhancing the resonance process in a Fabry-Perot-like resonant cavity and, thus, significantly extending the equivalent optical path.

Polarization-Selective Bifunctional Metasurface for High-Efficiency Millimeter-Wave Folded Transmitarray Antenna With Circular Polarization

A novel high-efficiency polarization-selective bifunctional metasurface (MS) is proposed, which can realize cross-polarized focusing transmission for left-handed circular polarization (LHCP) incidence and co-polarized specular reflection for right-handed circular polarization (RHCP) incidence. As an example of its practical application, the designed MS is applied to form an RHCP folded transmitarray antenna (FTA) whose feed source (FS) is integrated with the bottom metallic surface. To prove the concept, a prototype of millimeter-wave (MMW) FTA based on the proposed MS is numerically simulated, physically implemented, and experimentally measured. The measured results coincide well with the simulated ones, showing a peak gain of 23.6 dBi at 27.7 GHz with the peak aperture efficiency of 43.1%, a 3 dB gain bandwidth of 15.8% (25–29.3 GHz), and a 3 dB axial-ratio (AR) bandwidth over 29.6%. The proposed high-performance antenna has the advantages of low profile, high efficiency, integration design, and broad bandwidth, which may have promising applications in the fifth-generation MMW communication system. The proposed MS and its application to high-gain antenna may promote the uses of MS to real-world applications in satellite communication, radar detection, and wireless communication.

A non-interleaved bidirectional Janus metasurface with full-space scattering channels

Abstract Metasurfaces have attracted broad interest thanks to their unprecedented capacity for electromagnetic wavefront manipulation. The compact, ultrathin and multifunctional metasurface calls for novel design principles. Here, we propose and experimentally demonstrate a non-interleaved and non-segmented bidirectional Janus metasurface that encodes multiple functionalities in full-space scattering channels with different propagation directions and polarization in the microwave region. Specifically, by rotating and adjusting the elementary double-arrow-shaped structure within the same meta-atom, the independent phase control can be achieved in both cross-polarized transmission and co-polarized reflection components under oppositely directed incident waves. Our metasurface with broken mirror symmetry can fully exploit four independent information channels under opposite propagation directions. A series of proof-of-concept is constructed to validity of our methodology, and the simulations and experimental results further show that the proposed non-interleaved bidirectional metasurface can provide an attractive platform for various applications, ranging from structured light conversion, optical imaging, multifunctional optical information processing and others.

Ultrathin chiral metasurface with multiple functionalities

This paper presents an ultrathin multiband chiral metasurface having good angular stability. The metasurface consists of two patterned metallic sheets placed on opposite sides of a microwave substrate. It has twelve operational bands. It achieves asymmetric linear-to-circular polarization transmission within three frequency bands and asymmetric cross-polarized transmission of linearly polarized incident waves within seven frequency bands. It also exhibits two symmetric co-polarized transmission bands for both x- and y-directed incident E-field orientations. The structure gives favorable response for incident angles up to 60° till 11 GHz and up to 40° from 11 to 13 GHz for both transverse-electric and transverse-magnetic incidence. Its ultrathin nature makes it lightweight. The twelve operational frequency bands of the metasurface can be tuned and adjusted to meet user specifications.

Enhanced and tunable asymmetric Imbert–Fedorov and Goos–Hänchen shifts based on epsilon-near-zero response of Weyl semi-metal

In this paper, we discuss the Imbert–Fedorov (IF) and Goos–Hänchen (GH) shifts induced by the transverse magnetic polarized light penetrating a bulk Weyl semi-metal (WSM). It is found that the asymmetric IF and GH shifts emerge due to the different energies of left- and right-handed circular polarization components caused by the cross-polarized transmission coefficient t ps. What is more, the asymmetric IF and GH shifts are enhanced significantly at the epsilon-near-zero (ENZ) frequency, where the condition of the transmission coefficients |t ps| = |t pp| is realized because of the decreases in |t pp|. In particular, the ENZ effect can be controlled with the tilt degree of Weyl cones and Fermi energy, thus leading to the enhanced asymmetric IF and GH shifts at different ENZ frequencies. Additionally, the enhanced asymmetric IF and GH shifts show the tunability of the WSM thickness and Weyl node separation. Finally, the enhanced asymmetric IF and GH shifts in the untilted WSM can also be adjusted with the Weyl node separation. Our findings provide easy and available methods to enhance and control the asymmetric IF and GH shifts with a WSM.