Tunable Polarization Converter Research Articles

Graphene-Based Tunable Polarization Conversion Metasurface for Array Antenna Radar Cross-Section Reduction.

A graphene-based tunable polarization conversion metasurface (PCM) was designed and analyzed for the purpose of reducing the radar cross-section (RCS) of array antennas. The metasurface comprises periodic shuttle-shaped metal patches, square-patterned graphene, and inclined grating-patterned graphene. By adjusting the Fermi energy levels of the upper (μ1) and lower (μ2) graphene layers, different states were achieved. In State 1, with μ1 = 0 eV and μ2 = 0.5 eV, the polarization conversion ratio (PCR) exceeded 0.9 in the bandwidths of 1.65-2.19 THz and 2.29-2.45 THz. In State 2, with μ1 = μ2 = 0.5 eV, the PCR was greater than 0.9 in the 1.23-1.85 THz and 2.24-2.60 THz bands. In State 3, with μ1 = μ2 = 1 eV, the PCR exceeded 0.9 in the 2.56-2.75 THz and 3.73-4.05 THz bands. By integrating the PCM with the array antenna, tunable RCS reduction was obtained without affecting the basic radiation functionality of the antenna. In State 1, RCS reduction was greater than 10 dB in the 1.60-2.43 THz and 3.63-3.72 THz frequency ranges. In State 2, the RCS reduction exceeded 10 dB in the 2.07-2.53 THz, 2.78-2.98 THz, and 3.70-3.81 THz bands. In State 3, RCS reduction was greater than 10 dB in the 1.32-1.43 THz, 2.51-2.76 THz, and 3.76-4.13 THz frequency ranges. This polarization conversion metasurface shows significant potential for applications in switchable and tunable antenna RCS reduction.

Ultrawideband Tunable Polarization Converter Based on Metamaterials

In order to overcome the shortcomings of the traditional polarization conversion device, a symmetric “E” type metamaterial resonance cell structure is proposed in the paper for the design of polarization conversion devices. Two kinds of polarization converter models are designed in this paper, and their polarization coefficient, phase and polarization conversion ratio (PCR) are simulated and analyzed. The polarization coefficients at different incident angles and different phase transitions of VO2 are simulated and analyzed. The study indicates that the polarization conversion efficiency of the polarization converter reaches 90% in the 0–2.0 THz frequency band, and the performance can be maintained within the incident angle of 45 degrees, covering the whole X band, realizing the broadband line polarization conversion with extremely high polarization conversion rate. Moreover, the polarization performance is tunable with the phase transition of VO2, and the research results provide new ideas for the realization of ultrawideband tunable polarization conversion devices and the application of metamaterials.

Hybrid dual-mode tunable polarization conversion metasurface based on graphene and vanadium dioxide.

We present and numerically verify a functionally hybrid dual-mode tunable polarization conversion metasurface based on graphene and vanadium dioxide (VO2). The tunable polarization converter consists of two patterned graphene layers separated by grating which is composed of gold and VO2. Due to the existence of phase change material VO2, the polarization conversion mode can be switched flexibly between the transmission and reflection modes. Theoretical calculations show the proposed polarization conversion metasurface can obtain giant asymmetric transmission (AT) at 0.42 and 0.77 THz when VO2 is in the insulating state. Conversely, when VO2 is in the metallic state, the converter switches to the reflection mode, demonstrating broadband polarization conversion for both forward and backward incidences. Furthermore, the conductivity of graphene can be modulated by changing the gate voltage, which allows dynamic control polarization conversion bandwidth of the reflection mode as well as the AT of the transmission mode. The robustness of the metasurface has also been verified, the high polarization conversion efficiency and AT can be maintained over wide incidence angles up to 65° for both the xoz plane and yoz plane. These advantages make the proposed hybrid tunable polarization conversion metasurface a promising candidate for THz radiation switching and modulation.

Tunable reflective tri-band polarization converter based on graphene composite metasurface

A graphene composite metasurface-based reflective tri-band polarization converter working in the terahertz frequency regime is proposed and investigated by CST Microwave Studio software. The polarization converter achieves high polarization conversion ratios (above 99%) for linear-to-linear polarization conversion at 6.21, 9.11, and 11.66 THz, and for linear-to-circular polarization conversion at 5.14 THz, with a fixed Fermi energy level (Ef) of 1.0 ev due to the excitation of localized surface plasmons. Subsequently, the reason for cross-polarization conversion at three resonant frequencies and the incident angle sensitive property of the converter were discussed. Based on the graphene component, a tunable tri-band polarization converter was obtained and investigated; this converter could be applied in the active modulation area of wireless communication.

Tunable dual-band dual-polarization terahertz polarization converter and coding metasurfaces based on Weyl semimetals

Tunable dual-band dual-polarization terahertz polarization converter and coding metasurfaces based on Weyl semimetals

Broadband electrically tunable linear polarization converter based on a graphene metasurface.

In this study, a broadband tunable reflective graphene-based linear polarization converter (GLPC) is proposed based on the graphene-ionic liquid-ITO structure (GIIS) integrated with a periodic double split ring resonator (DSRR) in the millimeter-wave regime. The tuning characteristic of the designed GLPC is analyzed using full-wave simulations and the equivalent circuit model method (ECM), which is based on multi-section transmission lines. There is a good agreement between ECM and simulation results. A comprehensive physical mechanism for the proposed broadband GLPC is then achieved by analyzing the surface current distributions. After manufacturing, the GLPC prototype's co- and cross-polarized reflection coefficients were measured using various bias voltages. The reflectivity can be controlled from -4.5 to -20 dB by changing the bias voltage in the range of +1.1 to -3.3 V. The designed GLPC can provide a tunable polarization conversion within the frequency range of 15.5∼35 GHz and shows a more than 75% conversion efficiency. The results of the simulation and the measurement are also in good agreement. The designed GLPC has potential applications in radar cross-section reduction, antenna design, and stealth technology by reconfiguring its polarized reflection characteristic dynamically.

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.

Tunable polarization converter with high polarization isolation based on metasurface and its application on horn antenna

Tunable polarization converter with high polarization isolation based on metasurface and its application on horn antenna

Thermally tunable THz polarization converter based on Babinet-inverted metasurface

Thermally tunable THz polarization converter based on Babinet-inverted metasurface

Broadband and tunable terahertz polarization converter based on graphene composite metasurface

In this paper, a broadband and tunable terahertz polarization converter metasurface is proposed, which is composed of an open metal ring embedded with graphenes and a columnar metal on polyimide (PI) dielectric–metal substrate. The polarization conversion rate (PCR) converted to cross-polarized in reflection mode can be regulated from 95% to 20% in a wide band from 2.1 to 3.6 THz by adjusting the Fermi energy of graphene. Simulation results show that broadband polarization conversion results from three resonances in neighboring frequencies, and the physical mechanism underlying tunable function is explained by the distribution of surface current in different states. In addition, the reflection coefficient and reflection phase in orthogonal component are calculated to further reveal basic principle of proposed polarization converter. This study could continue the process of developing compact and active polarization and wave front control application in the THz, and may be applied in various areas such as wireless communication, terahertz sensing and imaging.

A tunable circularly polarized antenna in THz regime

A transmissive type single layered tunable linear to circular polarization converter is realized by conglomerating both metallic and graphene patches on the same layer over a polyimide substrate. The tunability has been realized in the proposed metasurface by varying the chemical potential of the L-shaped graphene patches at the diagonal ends. Quadrature phase shift between the orthogonal components of the electric field is produced by the anisotropy caused by the double T-shaped copper patch. A tunable axial ratio (≤3 dB) bandwidth ranging from 2.57 to 3.06 THz is exhibited. To demonstrate the polarization conversion capability of the proposed tunable polarization converter (PC), a linearly polarized microstrip patch antenna (MSA) operating in the same frequency band has been designed and integrated with it. The radiated linearly polarized electromagnetic waves from the antenna get transformed into circularly polarized waves after passing through the PC. Thus, a circularly polarized antenna is realized whose 3-dB axial ratio bandwidth is tunable across 2.6–3 THz. The transmissive type PC has been placed 10 μm above the antenna to generate left-hand circularly polarized (LHCP) waves across a bandwidth of 400 GHz. The proposed simple structured linear to circular PC provides the advantage of dynamic regulation of the CP band without making any alteration in the physical structure of the antenna. Most of the reported CP antennas in THz range constitute graphene based radiating patches. They do not involve the use of metasurface for polarization conversion. For the first time, the PC integrated microstrip patch antenna has been analyzed in this work. The principle of polarization conversion has been numerically studied through full-wave simulations in CST.

Dynamic Control of Light Chirality with Nanostructured Monolayer Black Phosphorus for Broadband Terahertz Applications

AbstractDynamically tunable polarization conversion at the nanoscale is essential for the realization of novel on‐chip photonic devices. To achieve the desired polarization control, typically, nanopatterning of resonant metallic or dielectric structures in anisotropic or chiral geometries is required, in many cases limiting the performance to narrowband, nontunable operation. In this work, by taking advantage of the strong anisotropic surface conductivity of black phosphorus (BP), polarization conversion is demonstrated via the coherent excitation of localized surface plasmons (LSPs) in symmetrically patterned monolayer BP nanosquare arrays. Using finite‐difference time‐domain simulations, linear‐to‐circular and circular‐to‐linear polarization conversion, dynamic tunability and switching, and broadband operation in the terahertz (THz) regime, all enabled by means of electrostatic control of the carrier concentration of BP, are demonstrated. Last, it is demonstrated how near‐field coupling between the LSPs and dipole emitters positioned at chosen hotspots can mediate the emission of circularly polarized light (CPL), also providing enhanced emission rates. The proposed design offers a flexible, ultrathin, platform for wave manipulation in the THz regime, enabling active control of light polarization by nanoscopic on‐chip optical sources.

A high-efficiency wideband tunable polarization conversion metasurface assisted by the localized surface plasmon resonances

Combined with the Dirac semimetals (DSMs), which is a new type of material and also called as 3D graphene, a tunable wideband terahertz polarization conversion metasurface based on an anisotropic configuration is studied, in which the DSMs wire array is beneficial to the regulation of Fermi energy. The results show that the metasurface can realize wideband and highly efficient polarization conversion, and has the property of half wave plate at the resonant modes. This characteristics are derived from the excitation of Localized Surface Plasmon Resonances (LSPRs) and the anisotropy of structure itself. When the incident angle changes in the range of 0°~40°, the high efficiency of wideband polarization conversion can be maintained. When it is greater than 40°, the wideband polarization conversion gradually changes to the dual-band or the multi-band conversion. Furthermore, it is found that in the process of increasing the Fermi energy of AlCuFe from 65 meV to 140 meV, the polarization conversion ratio can be maintained at a high level, and the conversion performance changes from single-band conversion to wideband conversion, and then to wideband conversion with wider band and single-band conversion with narrower band. At the same time, by discussing the metasurface combined with the different DSMs, it is concluded that the better the metallic property of DSMs is, the better the wideband polarization conversion performance of the corresponding metasurface is. At last, the numerical results are verified by the Multiple Interference Theory (MIT) based on the Fabry-Pérot-like resonance cavity.

Gaussian to Vector Vortex Beams with Tunable Polarization Converter Based on Nematic Liquid Crystals

Gaussian to Vector Vortex Beams with Tunable Polarization Converter Based on Nematic Liquid Crystals

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.

Mechanical modulation of multifunctional responses in three-dimensional terahertz metamaterials.

Reconfigurable metamaterials have attracted a surge of attention for their formidable capability to dynamically manipulate the electromagnetic wave. Among the multifarious modulation methods, mechanical deformation is widely adopted to tune the electromagnetic response of the stereotype metamaterial owing to its straightforward and continuous controllability on the metamaterial structure. However, previous morphologic reconfigurations of metamaterials are typically confined in planar deformation that renders limited tunable functionalities. Here we have proposed a novel concept of out-of-plane deformation to broaden the functionalities of mechanically reconfigurable metamaterials via introducing a cross-shaped metamaterial. Our results show that the out-of-plane mechanical modulation dramatically enhances the magnetic response of the pristine metamaterial. Furthermore, by uncrossing the bars of cross-shaped meta-atoms, a L-shaped metamaterial is proposed to verify the effectiveness of such a mechanical method on the handedness switching via changing mechanical loading-paths. More importantly, the differential transmission for circularly polarized incidences can be continuously modulated from -0.45 to 0.45, and the polarization states of the transmission wave can be dynamically manipulated under the linearly polarized illumination. Our proposed mechanical modulation principle might open a novel avenue toward the three-dimensional reconfigurable metamaterials and shows their ample applications in the areas of chiroptical control, tunable polarization rotator and converter.

Dual-Frequency Polarized Reconfigurable Terahertz Antenna Based on Graphene Metasurface and TOPAS.

A hybrid dual-frequency polarized reconfigurable terahertz antenna is designed and studied. Graphene and TOPAS are employed as the polarization conversion metasurface and dielectric substrate, respectively, enabling tunable polarization conversion and circular polarization. TOPAS is a good substrate material for broadband THz components due to its low absorption. By adjusting the chemical potential of graphene between 0 eV and 0.5 eV, the polarization state in the band of 1 THz (0.76–1.02 THz) and 2.5 THz (2.43–2.6 THz) can be reconstructed. Thanks to the multilayer graphene structure and low absorption TOPAS, the graphene metasurface exhibits a broad bandwidth of 0.26 and 0.17 THz, respectively, in the band of 1 THz and 2.5 THz. The working state of the circularly polarized antenna and linearly polarized antenna can be switched in the bands around 1 THz (0.7–0.75 THz, 0.96–1.04 THz) and 2.5 THz (2.42–2.52 THz), respectively, without changing the physical geometry. Moreover, the graphene antenna, metasurface, and hybrid structure are tested, respectively, to verify that the components do not interfere with each other in performance. The hybrid antenna shows great potential in tunable terahertz devices and related applications.

Multiple interference theoretical model for graphene metamaterial-based tunable broadband terahertz linear polarization converter design and optimization.

Terahertz (THz) polarization converters often working as modulators and switches have many applications in THz sensing, imaging and communication, but many of them still suffer from low polarization conversion efficiency, fixed and narrow polarization conversion band, and low output polarization purity, which are mainly due to the lack of theoretical model for THz polarization converter design and optimization. In order to solve the problem, we adopt multiple interference theory to successfully design and optimize a graphene metamaterial-based tunable broadband THz linear polarization converter: it achieves polarization conversion ratio (PCR) over 0.97, polarization azimuth angle of almost ±90° and rather low ellipticity within a broad polarization conversion band of 1.25 THz; and additionally, its polarization conversion band can be actively tuned by adjusting the graphene chemical potential and almost insensitive to the incident THz radiation angle below 50°. Considering the high performance of the optimal graphene metamaterial-based tunable broadband THz linear polarization converter, this work provides an optimal design offering a way in high-quality manipulation of THz radiation polarization; but more importantly, delivers a theoretical model for tunable THz polarization converter design and optimization.

Tunable broadband cross polarization converter based on a graphene sheet with a T-shaped carved-hollow array

We propose a tunable broadband reflective cross polarization converter (CPC) composed of a graphene sheet with a T-shaped carved-hollow array. In the mid-infrared region, cross polarization conversion with 5.17 THz bandwidth is achieved due to the superimposition of the two reflection components with a phase difference of nearly 180°. The polarization conversion ratio is larger than 80% in this broadband range, and the fractional bandwidth is 40%. The physical mechanism of CPC is attributed to the excitation of symmetric and antisymmetric graphene surface plasmons. The CPC broadband can always be retained when the incident angle increases from 0° to 20°, and the polarization angle increases from 40° to 50°. Also, the tunable responses of the reflective polarizer with Fermi energy and electron scattering time are discussed. Our design can be widely applied to adjustable broadband polarization conversion.

Reflective triple-band line-to-circular polarization conversion based on diamond-shaped graphene metasurface

In this paper, we present a triple-band line-to-circular (LTC) polarization converter based on graphene-integrated metasurface, which is composed of graphene diamond patches. Finite element method results show that there are two LTC polarizations bands with 0.36 and 0.44 THz 1 dB bandwidth, which can achieve line-to-right-circular (LTRC) conversion; there is one LTC polarization band with 2.23 THz 1 dB bandwidth, which can achieve line-to-left-circular (LTLC) conversion. We found two graphene surface plasmons (GSPs) modes were excited, which were responsible for the three LTC bands. The influences of axis lengths on the performances of LTC bands are investigated. Besides, the triple-band polarization converter can be realized when the incidence angle ranges from 0° to 30° and polarization angle ranges from 0° to 9°. Also, the frequency shifts of three LTC bands are also achieved by changing Fermi level of graphene. It is believed that our proposed polarizer can be widely used for multiband and tunable polarization conversion.