Cross-polarization Conversion Research Articles

Total cross-polarization conversion and polarization-sensitive amplification of terahertz radiation in graphene with direct electric current

Total cross-polarization conversion and polarization-sensitive amplification of terahertz radiation in graphene with direct electric current

A tunable versatile metasurface in terahertz frequency based on graphene–metal hybrid structures

The increasing demands for device integration and reusability have led to the development of freely switchable and dynamically tunable diverse functions in both modern optics and wireless communications. This study proposes a versatile metasurface that integrates with four tunable functions in the terahertz region based on graphene patch and metal split-ring hybrid structures. By adjusting the Fermi level of the graphene patches through electrostatic gating, the proposed metasurface can effectively achieve free switching among four different functions and dynamic tuning of each function's characteristics over a broad frequency band. Moreover, the metasurface operates smoothly in reflection mode for x-polarization, y-polarization, and left-hand and right-hand circular polarization illuminations. All expected performances, including resonant absorption with tunable frequency (relative bandwidth is about 20 %) and regulable absorbance (from 10 % to 100 %), arbitrary interconversion of four polarization states (relative bandwidth is over 48 %), regulation of cross-polarization conversion rate (from 0 % to 100 %), tunable spatial power division (three-way outputs), and dual-channel high-efficiency signal modulation (modulation depth is over 81 %), are demonstrated through full-wave simulation experiments. This study opens up a new avenue for exploring tunable multifunctional integrated devices with graphene materials, with potential applications in various domains, such as wireless communication, terahertz sensing, and imaging.

Switchable bi-functional water-based metasurface for high efficiency and wideband polarization conversion and absorption

A reconfigurable metasurface with a switchable function, wideband, and high efficiency has gained a lot of attention for the creation of compact and efficient devices. Here, for the first time, we propose a bi-functional metasurface that utilizes a water-based resonator to achieve wideband and high-efficiency absorption and cross polarization conversion in the microwave range. By controlling the water injection, the functionality of the metasurface can switch between absorption and cross-polarized conversion. Via water injection, the designed water-based metasurface can act as a wideband absorber with an absorptivity of above 90% in the broadband range of 16.5–24 GHz. Meanwhile, without water injection, it exhibits the ability for high efficiency cross-polarization conversion across a wideband in the range of 4.38–11.9 GHz. The proposed metasurface is fabricated and measured to verify its ability to switch between the dual functions of high efficiency wideband absorption and cross-polarization conversion. We believe that the metasurface platform shown here will open up new possibilities for creating compact multifunctional devices for applications in the microwave region.

Tunable terahertz polarization conversion and absorption devices based on metamaterials

Metamaterials, as novel materials, have electromagnetic behaviors that can be realized by rationally designing the unit structure. It plays an important role in the research of terahertz functional devices. Switchable terahertz devices are constructed by flexibly combining the excellent properties of graphene and vanadium dioxide (VO2) with the application potential of terahertz technology. When the VO2 is in the metallic state and the graphene Fermi energy level is 0 eV, the device behaves as a broadband polarization switching device. It can realize cross-polarization conversion for the incident electromagnetic wave in the range of 2–4.2 THz. When the VO2 is in the insulating state and the graphene Fermi energy level is 0.9 eV, the device is an absorption device. The absorption rate can reach more than 99% at 4 THz. In addition, the polarization conversion ratio (PCR) as well as the absorption rate of the device can be tuned independently. This actively tunable switchable terahertz device provides ideas for dynamically controlling terahertz, bringing more possibilities for various applications such as terahertz imaging, stealth, and communication.

Tunable broadband absorption and broadband linear polarization converter based on vanadium dioxide

A composite dielectric metamaterial based on vanadium dioxide (VO2) is proposed to achieve flexible switching between two functions, broadband absorption, and polarization conversion, by adjusting the VO2 conductivity. The designed metamaterial functions as a broadband absorber when VO2 is in the metal phase. The absorber consists of a VO2 top structure, a silicon dioxide (SiO2) dielectric layer, and a VO2 thin film. Numerical simulation ns show that the absorber can absorb up to more than 90% in the frequency range of 3.22 ∼ 8.51 THz, and due to the symmetry of the structure, the absorber is characterized by polarization-insensitive properties and good absorption over a wider incidence angle. When VO2 is in the insulator phase, the designed metamaterial has a cross-polarization conversion function. The linear polarization converter primarily comprises an I-beam metal, a SiO2 dielectric layer, and a gold substrate layer. Numerical simulations demonstrate that the linear polarization converter accomplishes a line polarization conversion rate (PCR) greater than 90% within the 1.40 ∼ 4.11 THz frequency range, attains a close to 100% cross-polarization conversion rate (PCR) at 1.46, 1.95, 3.0, and 3.97 THz. To confirm the wave absorption mechanism of the absorber, we utilize the impedance matching theory to analyze it. The proposed switchable bifunctional metamaterials present significant potential for broader applications in future terahertz communication, imaging, stealth technology, and related fields.

WITHDRAWN: Cross polarization converter with high transmittance and near unity polarization conversion efficiency for visible light based on bilayer all-dielectric metasurface

It is highly desired to achieve high efficiency polarization converters in visible wavelength region for ultra-compact optical systems which have shown potential applications in optical sensing, communication and imaging. Due to the intrinsic property of low loss, all dielectric metasurfaces are more competitive candidates to be utilized for fabrication of polarization converters than the counterparts based on metals. In this paper, we design a bilayer all dielectric metasurface polarization converter composed of a Si double-rod resonator placed on the top of a glass block with a polymer substrate for linearly polarized visible light. The proposed bilayer metasurface structure can convert linearly polarized light to the cross-polarized light efficiently within the visible region from 542 nm to 563 nm in the transmission mode with high transmittance and near unity polarization conversion efficiency. The transmittance is larger than 0.8 in the mentioned wavelength range with a maximum transmission of 0.86 at 556 nm and polarization conversion ratio is larger than 90% in the same wavelength range with a maximum polarization conversion ratio of 99.01% at 556 nm. The underline physical mechanisms of high conversion efficiency and high transmittance are analyzed in detail with simulated distributions of surface current, electrical field distributions and the calculated multipole decompositions of the scattering power.

Noninvasive inset-integrated meta-atom for achieving single-layer metasurface simultaneously with coded microwave reflectivity and digitalized infrared emissivity

Abstract With the rapid improvement of equipment integration technology, multi-spectrum detectors are integrated into compact volumes and widely used for object detection. Confront with this challenge, it is essential to propose a strategy to design a single-layer metasurface with multi-spectrum responses in microwave and infrared ranges. In this work, we proposed a method of designing meta-atoms, which is capable of achieving functional electromagnetic response at microwave and infrared individually. As a demonstration, a metasurface with four different occupation ratios and coding permutation features is designed, fabricated, and tested. In the microwave band, the pixel meta-atom is designed to realize highly efficient cross-polarization conversion between 5.0 and 10.0 GHz, which shows the metasurface can behave as ultra-low Radar Cross Section (RCS) reflectors in the working band; In the infrared band, different occupation ratio of meta-atoms are designed to realize the infrared emissivity from 0.60 to 0.80 in 3–14 μm, which can be used to exhibit digital infrared camouflage pattern. This work promotes the ability to use single-layer design to achieve digital infrared camouflage and microwave RCS reduction simultaneously. The one-layer design is simple in geometry, simplified in process, low cost in economy, and large scale in fabrication, which can promote practical use in compatible microwave stealth and infrared camouflage.

High-efficiency reflective terahertz cross-polarization converter with broadband and wide-angle response

In this paper, a broadband terahertz metasurface dedicated to cross-polarization conversion was designed, fabricated, and assessed. The metasurface, comprising two nested double-split rings, features an inherent insensitivity to the angle of incidence. Simulations reveal that the converter achieves a >99% polarization conversion efficiency across the 90–140 GHz range. Moreover, it maintains a >90% polarization conversion ratio (PCR), even at a 50° incidence angle. The sample, featuring 50×70 arrays, was fabricated, and the relevant experimental results align closely with the simulated outcomes. The metasurface characteristics can markedly enhance the performance of cross-polarization converters operating in the terahertz range.

A Cross-Polarization Converter Using Guided Wave Inspired Metasurface With an In-Band Notch and High Frequency Selectivity

A Cross-Polarization Converter Using Guided Wave Inspired Metasurface With an In-Band Notch and High Frequency Selectivity

Multifunction and switchable hybrid metasurface based on graphene and gold

This paper presents the design and numerical investigation of a graphene-based switchable terahertz (THz) metasurface for ultrawideband absorption and multiband cross polarization conversion. The device’s operational mode can be altered to absorption or reflection by using the electrostatic capabilities exhibited by graphene. The numerical calculations yield results indicating that in absorption mode, it maintains a bandwidth ratio of 152% within the frequency range of 1.04–7.74 THz. In polarization conversion mode, the design demonstrates the ability to convert linearly polarized and circularly polarized waves to their respective cross-polarization states within the frequency ranges of 1.8–2.5 THz, 3.5–4.1 THz, and 5.6 THz. Moreover, calculated results indicate that linear-to-circular conversion can be realized at 2.64, 3.45, and 5.4 THz frequencies. In addition, the absorption and polarization conversion ratio parameters were analyzed using the multiple reflection theory, which demonstrated significant agreement with the simulation results. The designed metasurface exhibits significant potential in the field of terahertz devices, including stealth technology, smart switches, and other related applications.

Comment on Lu et al. Ultrathin Terahertz Dual-Band Perfect Metamaterial Absorber Using Asymmetric Double-Split Rings Resonator. Symmetry 2018, 10, 293

In this study, the design of a dual-band terahertz absorber, previously published by Lu et al. (Symmetry 2018, 10, 293), was re-simulated. Our findings showed significantly different absorption results from those published in the article. A detailed analysis was conducted to explain this discrepancy, which was attributed to the reflection of an unaccounted orthogonal component of the waves from the design, rather than absorption. The metasurface design has two resonances at 4.48 THz and 4.76 THz, respectively. It was reported that at these frequencies, the structure achieved absorption of 98.6% and 98.5%, respectively. However, in our results, it was found that at the second resonance (4.76 THz), the structure acted as a strong cross-polarization converter, reflecting a significant amount of incident energy in the cross-polarization component of the reflected wave. When this component is considered in the reflection coefficient calculations, the absorption reduces to 41% (from 98.5%), which is not an acceptable level for an absorber. In addition, the structure was simulated for both lossy and lossless (FR4) substrate cases to understand the effect of substrate losses. The results showed that the absorption response significantly deteriorates at the first resonance (4.48 THz) in the case of a lossy FR4 substrate.

A tunable terahertz metasurface with function of polarization conversion and circular dichroism

A tunable and multifunctional electromagnetic control metasurface is proposed. The metasurface comprises the graphene sheet and gold open ring, enabling multiband cross-polarization conversion (CPC) and line-to-circular polarization conversion (LTCPC). Simultaneously, it can achieve circular dichroism (CD) with amplitude of 0.99 at 2.28 THz, along with the capability for near-field imaging. The physical mechanism of operation is studied by analyzing surface current distribution and electric field. In addition, because the metasurface exhibits chiral characteristics, appropriate rotation can induce a phase gradient, facilitating a phase coverage of 0–360°. Based on the tunable phase gradient of metasurface, anomalous reflection and vortex beams can be realized.

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.

Design of a Dual-Band Metasurface Cross-Polarization Converter for Cancer Detection in the Terahertz Band

We present the design of a metasurface-based, cross-polarization converter as a sensor for label-free detection of cancerous cells in the terahertz band. The device is designed to work on the principle of refractive index (RI) sensing wherein the frequency shift of the maximum polarization conversion peak in the presence of the target analyte is measured. This shift is proportional to the RI difference between healthy and cancerous cells thereby enabling classification with high sensitivity. Our metasurface is designed to support two resonances which significantly reduces the measurement errors, thereby increasing the detection accuracy. We report sensitivities of 155 GHz/RIU and 350 GHz/RIU at the first and second resonant frequencies respectively. The proposed structure is simulated using a finite element method and validated with an equivalent circuit model.

Tunable VO2 metasurface for reflective terahertz linear and circular polarization wavefront manipulation at two frequencies independently

In this work, a tunable VO2 metasurface (MS) is proposed and numerically demonstrated in the terahertz (THz) region, which can independently manipulate reflected linearly polarized (LP) and circularly polarized (CP) wavefronts at two frequencies independently. The unit-cell of the proposed VO2 MS consists of a metallic circular ring (CR) structure, rectangular-shaped-patch (RSP) and circular-double-split-ring (CDSR) structure VO2 adhered on a dielectric substrate by a bottom metallic ground plane. The proposed MS structure enables the reflected cross-polarization conversion of LP waves at lower frequencies (fL = 1.34 THz) and CP waves at higher frequencies (fH = 2.53 THz), respectively. By changing the external temperature, the conductivity of the VO2 RSP and CDSR structures can be adjusted dynamically. By varying the conductivity of the VO2, the proposed MS can achieve a reflected LP conversion with efficiency from 0% to 98.86% at lower frequency of 1.34 THz and the reflected CP conversion with efficiency from 0% to 81.86% at higher frequency of 2.53 THz. Moreover, the corresponding modulation depths of this MS are as high as 98.86% and 81.86% at two different frequencies, respectively. Moreover, a 2π phase shift can be realized by the transmission phase and geometric phase principles at lower and higher frequencies, respectively. Finally, two VO2 MSs have been constructed to achieve the reflection deflection for both CP and LP waves, and reflective CP focusing and LP vortex beam generation, respectively. These findings are favorable for expanding the range of temperature-controlled VO2 MS for wavefront manipulation, thereby opening up new possibilities for the development of advanced, tunable, and multifunctional THz devices that can handle both LP and CP waves.

A Multiband and Multifunctional Metasurface for Linear and Circular Polarization Conversion in Reflection Modes

Multifunctional integrated meta-devices are the demand of modern communication systems and are given a lot of attention nowadays. Most of the research has focused on either cross-polarization conversion (CPC) or linear-to-circular (LP–CP) conversion. However, simultaneously realizing multiple bands with good conversion efficiency remains crucial. This paper proposes a multiband and multifunctional dual reflective polarization converter surface capable of converting a linearly polarized (LP) wave into a circularly polarized (CP) wave, in frequency bands of 12.29–12.63 GHz, 16.08–24.16 GHz, 27.82–32.21 GHz, 33.75–38.74 GHz, and 39.70–39.79 GHz, with 3 dB axial ratio bandwidths of 2.7%, 40.15%, 14.6%, 13.76%, and 0.2%, respectively. Moreover, the converter is capable of achieving CPC with a polarization conversion ratio (PCR) that exceeds 95%, within the frequency ranges of 13.10–14.72 GHz, 25.43–26.00, 32.46–32.56 GHz, and 39.14–39.59 GHz. In addition, to identify the fundamental cause of the CPC and LP–CP conversion, a comprehensive theoretical investigation is provided. Furthermore, the surface current distribution patterns at different frequencies are investigated to analyze the conversion phenomena. A sample prototype consisting of 20 × 20 unit cells was fabricated and measured, verifying our design and the simulated results. The proposed structure has potential applications in satellite communications, radar, stealth technologies, and reflector antennas.

Multi-functional terahertz metasurface for a vortex beam, multi-channel focusing, polarization conversion, and broadband absorption based on vanadium dioxide.

Although terahertz metasurface devices have been widely studied, thus far, metasurfaces can rarely manipulate both circularly and linearly polarized incident waves. In this paper, taking advantage of the phase transition characteristics of vanadium dioxide (V O 2), a multi-functional terahertz metasurface for a vortex beam, multi-channel focusing, polarization conversion, and broadband absorption is proposed. When V O 2 is in the insulating state, a vortex beam is generated at 1.2THz when the circularly polarized wave is incident on the metasurface. Meanwhile, the multi-channel focusing is realized at 1.0THz, and the cross-polarization conversion rate can reach more than 90% at the frequencies of 0.6THz, 1.1THz, and 1.6THz when the y-polarized wave is incident vertically. When V O 2 is in the metallic state, the metasurface achieves close to 95% absorption in the range of 0.8-1.5THz. The designed metasurface has tunability and multi-functional characteristics, which have potential applications in wireless communication.

A low-side-lobe folded transmitarray antenna based on metasurface with independent amplitude/phase control

In this paper, a low-side-lobe folded transmitarray antenna (LSLFTA) based on metasurface (MS) that can independently control amplitude and phase is proposed. An MS, composed of two vertically placed polarization gates, can realize the efficient reflection of y-polarized waves, efficient transmission of x-polarized waves, and independently control of phase and amplitude is designed for the top surface. A polarization conversion surface (PCS) using an open resonant ring that can efficiently achieve cross polarization conversion is presented. The feed antenna is placed on the center of the top surface to obtain an accurate electromagnetic amplitude distribution. To the best of our knowledge, our work is thus the first one to achieve low-side-lobe FTA. The LSLFTA operating at 14.5 GHz is fabricated and measured to verify the design. After adding amplitude modulation (Taylor distribution), the side lobe level (SLL) is 5.2 dB and 7.5 lower than that of only phase modulation in the yoz plane and xoz plane, respectively. The 3 dB gain bandwidth of the antenna is 19.4% (13.5∼16.4 GHz), and the maximum aperture efficiency is 25.8%. Our work is a promising candidate in the design of high-gain, low SLL, and low-profile antenna.

An angular stable ultra-broadband asymmetric transmission chiral metasurface with efficient linear-polarization conversion

Abstract In this paper, a chiral metasurface is presented to effectively attain an ultra-broadband operation of asymmetric transmission with highly efficient cross-polarization conversion. The efficiency of the asymmetric transmission operation in the frequency band ranging from 6.5 to 13 GHz is above 70%. Additionally, in the frequency range from 3.7 to 14 GHz, the cross-polarization conversion realizes greater than 90% polarization conversion ratio. Asymmetric transmission as well as cross-polarization conversion show angular stability against the incident angle of electromagnetic waves up to 45°. To validate the simulation results, the fabricated chiral metasurface is tested and compared with simulation results. The concepts of asymmetric transmission and linear-polarization conversion are demonstrated and physically explained through the distributions of surface currents over metallic layers. This anticipated work has significant applications in telecommunications and radar, as well as potential use in equipment designed for the manipulation of electromagnetic waves.

Multipath-controlled bidirectional metasurface for multitasking polarization regulation and absorption.

In the design of metasurfaces, integrating multiple tasks into a single small unit cell and achieving regulation through various paths pose a serious challenge. In this paper, a multipath-controlled bidirectional metasurface (MCBM) is designed to achieve polarization regulation, perfect absorption and total reflection as multitasking functions. The findings demonstrate that under different excitation conditions, when co-planar polarized terahertz (THz) waves are incident normally on the metasurface, the MCBM can convert co-planar polarization to cross-polarization, co-planar polarization to circular polarization wave in reflection mode, and co-planar polarization to cross-polarization in transmission, respectively. When co-planar polarized THz waves are incident from the back side of the metasurface, the tasks of MCBM change to broadband perfect absorption, total reflection, and transmission co-planar polarization to cross-polarization conversion. Remarkably, all operating frequency bands of these tasks are very approximate. Additionally, the multitasking functions can be switched by altering the excitation conditions, and their performance can be regulated through multipath controls, such as the temperature, voltage, and polarization status. Our design provides an effective strategy for multipath-controlled multitasking integrated devices in the THz band.