Circular Polarization Conversion Research Articles

Multifunctional multi-band metasurface for linear to circular polarization conversion in transmission and reflection modes

Compact devices encompassing multiple functionalities in multiple bands of operation has become a new and emerging research area for the microwave and optical regime. In this work, design, fabrication and experimental verification of a triple band linear to circular polarization converter operating multiple functions of transmission and reflection is demonstrated. The device is based on single substrate-layered metasurfaces based structure. It performs transmitted linear to circular polarization conversion from 12.95 to 14.85 GHz, 24.89 to 32.75 GHz, and 44.9 to 52.5 GHz. Furthermore, it performs reflected near to circular polarization conversion from 13.32 ∼ 15.6 GHz, 26.5 ∼ 32.6 GHz, and 45.85 ∼ 53.5 GHz.

Wideband Linear-to-Multi-Polarization Converter Based on Active Metasurface

This paper proposes a transmission-type linear-to-multi-polarization converter based on cascading four-layer metasurface integrated with PIN diodes. By controlling the states of the PIN diodes, the polarization maintaining (PM) which means transmission without conversion, linear to linear (LTL), linear to right-hand circular polarization (LTRCP), linear to left-hand circular polarization (LTLCP) conversions can be realized. When the PM is realized, the incident wave will pass through the polarizer with low loss over wideband, the isolation is higher than 30 dB. When the linear-to-circular polarization conversion is realized, the incident linearly polarized (LP) wave can be converted to left-hand circularly polarized (LCP) or right-hand circularly polarized (RCP) wave with a relative bandwidth over 40%. When the LTL polarization conversion is realized, the polarization conversion ratio (PCR) is greater than 0.9 from 2.58 to 3.89 GHz. The overlapping bandwidth of the four operating states is 36%. The detailed design process and equivalent circuit of the proposed polarizer are analyzed in this paper. A sample consisting of 10 × 10 elements is fabricated. The measured and simulated results are in good agreement. To the authors’ best knowledge, this is the first reported reconfigurable polarizer capable of PM, LTLCP, LTRCP, and LTL polarization conversions within wideband. The proposed device can play an important role in polarization diversity and RF-integrated aperture of wideband channel multiplexing technology.

Broadband and high-efficiency thermal switchable InSb metasurface for terahertz wave anomalous reflection and focusing effect

In this paper, a reflective metasurface composed of a dielectric substrate sandwiched with a double-split-ring (DSR) structure InSb and a metal ground plane was proposed in terahertz (THz) region, which can realize a high-efficient thermal switchability of circular-polarization (CP) conversion, anomalous reflection, and planar reflective focusing effect in a broadband manner. Simulation results demonstrated that the designed InSb metasurface structure can achieve high-efficient reflective CP conversion, with the magnitude of the reflected orthogonal CP wave being over 0.8 on average from 0.7 THz to 1.55 THz under the external higher thermal (T = 320 K, “On” state). Conversely, the magnitude of the reflected orthogonal CP wave is nearly zero under external lower thermal conditions (T = 240 K, “Off” state). Furthermore, the proposed InSb metasurface structure can achieve a high-efficient reflective CP conversion with an efficiency of about 0.7 under the “On” state, and a 2π phase full coverage by adjusting the oriented rotation angle (α) of the DSR structure InSb. As evidences of the concept, two InSb metasurfaces have been constructed to achieve the broadband switchable anomalous reflection phenomenon and reflective focusing effect based on Pancharatnam-Berry (PB) phase principle. The proposed metasurface has great potential in developing a dynamically tunable device for THz wavefront modulation applications in the future.

Dual-mode bidirectional multifunctional chiral metamaterial based on self-complementary resonators

In this paper, we propose an intrinsic chiral metamaterial (ICM) consisting of two metal self-complementary resonators, dielectric layer, and an embedded continuous vanadium dioxide (VO2) layer, which can operate in reflection and transmission modes and tailor different polarization manipulations for circular and linear polarized waves in opposite incident directions. When VO2 is in metallic state, the ICM can achieve broadband and narrowband circular polarization conversions at 1.52–2.50 THz and 2.93 THz for opposite propagating directions, respectively; when VO2 is in an insulating state, it shows a strong asymmetric transmission (AT) effect at 2.43 and 3.19 THz for forward and backward linear polarized waves. In addition, the physical mechanisms of different polarization operations are explained by analyzing the surface current and electric field distributions, multiple interference model, and near-field distributions in detail, and the circuit models are introduced to verify the simulated results. Furthermore, we investigate the influence of structural parameters on performance. The proposed ICM has important implications for the development of polarization detectors, frequency selectors, etc.

Design of All‐Metal 3D Anisotropic Metamaterial for Ultrabroadband Terahertz Reflective Linear Polarization Conversion

Herein, a novel and simple design of all‐metal 3D anisotropic metamaterial (3DAMM) is proposed and investigated numerically, which can achieve a high‐efficient and wide‐angle ultrabroadband reflective linear–linear and dual‐band linear–circular polarization conversion in the terahertz (THz) region. The 3DAMM is composed of a periodic array of copper stand‐up split ring resonator (SRR) adhered on a copper film ground plane. Incident ultrabroadband linear polarization waves can be converted to its orthogonal counterpart after reflection due to the three neighboring plasmon resonances of the 3DAMM structure. The proposed design demonstrates a high conversion efficiency of over 90% within a relative bandwidth of 88.7% in the range of 0.62–1.61 THz, and its efficiency is validated for both transverse electric and transverse magnetic modes and a wide range of incident angles (0°–50°). This ultrabroadband and high‐efficient linear polarization conversion of the 3DAMM is attributed to multiple plasmon resonances within the operating frequency range. The surface current distributions on the unit cell show that the ultra‐broadband conversion behaviors result from the decomposed electric field components coupling with different resonance modes. Further simulation results suggest that the polarization conversion properties of the 3DAMM can be adjusted by changing the geometric parameters of the unit cell. The proposed 3DAMM‐based reflective linear polarization convertor can find potential applications in remote sensors, imaging systems, and reflector antennas at THz frequencies.

Multi-functional metamaterial polarization convertor in S/C/X bands

In this study, a reflective multi-functional metamaterial polarization converter printing two connected metal rings and two unconnected metal square patches on a metal backed FR4 substrate is designed in S/C/X Bands. The lowest operating frequency of the proposed polarizer is starting from 2.41 GHz. Linear to linear (LTL) polarization conversion at three bands: 2.41–2.52 GHz, 3.77–4.16 GHz, and 7.02–7.37 GHz as well as linear to circular (LTC) polarization conversion at 7 bands: 2.35–2.38 GHz, 2.58–2.65 GHz, 3.49–3.63 GHz, 4.32–4.36 GHz, 6.73–6.86 GHz, 7.5–7.62 GHz, 9.77–9.87 GHz are achieved in this design. Both numerical simulations and experimental validation are provided for this polarization converter. Performance of the proposed and some referenced polarization converters are also compared to represent the advantages of the proposed design.

Gain enhancement of circularly polarized Fabry-Perot resonator antenna using simple superstrate

This communication presents a method to enhance the gain of a circularly polarized (CP) Fabry-Perot resonator antenna (FPRA) by using a simple superstrate. The proposed superstrate is a polarization conversion metasurface (PCM) which is capable of partial reflection and CP transmission phase manipulation. The partial reflection property of PCM is used to form FPRA with a feed, and the linear to circular polarization conversion capacity make the antenna realize CP radiation. Furthermore, the PCM can also correct the aperture field phase distribution of the antenna through tuning the CP transmission phase of each unit. Then the gain of the CP FPRA can be further improved. Finally, the CP radiation and gain enhancement of the FPRA are achieved by a simple superstrate without the need to add additional antenna structure. This makes the proposed antenna keep a simple structure. The performance of the antenna is verified by comparing the test results of the fabricated antenna with the simulation results. Measured results show that the antenna can realize well CP radiation at 10 GHz. After phase correction, the measured peak gain of the antenna is increased by 1 dB, reaches 18.2 dBic. The method proposed in this communication provides a new idea for improving the gain of CP FPRA while maintaining simple antenna structure.

Terahertz linear to circular polarization converter based on reflective metasurface

Metasurfaces are two-dimensional artificial structures which have extraordinary electromagnetic properties. They have been used in myriad of devices such as nano-antennas, cloaking coatings, imaging devices, flat lenses, and polarization converters over a wide range of frequency. Due to high dependency of many devices on incident wave polarization, manipulating the polarization of electromagnetic waves would be useful, especially in the THz regime. In this study, we propose a linear to circular polarization converter (LTC-PC) based on a THz reflective metasurface. For a TE linear polarization incident wave, this structure has two distinct bands; the first one lays in a wideband frequency range of 0.5-1.41 THz, in which the reflected wave would be a left-handed circular polarization (LHCP) with minimum efficiency of 89% and maximum efficiency of more than 95% in 80% of the bandwidth. The second band lays in the narrowband frequency range of 1.45-1.55 THz, resulting a right-handed circular polarization (RHCP) wave with a minimum efficiency of 82%. The proposed polarization converter can be used in optical communication and electronic devices.

Inverse design of polarization conversion metasurfaces by deep neural networks.

To address the problem of multiple solutions and improve the calculating speed, we construct a tandem architecture consisting of a forward modeling network and an inverse design network. Using this combined network, we inversely design the circular polarization converter and analyze the effect of different design parameters on the prediction accuracy of the polarization conversion rate. The average mean square error of the circular polarization converter is 0.00121 at an average prediction time of 1.56×10-2 s. If only the forward modeling process is considered, it takes 6.15×10-4 s, which is 2.1×105 times faster than that using the traditional numerical full-wave simulation method. By slightly resizing the network input and output layers, the network is adaptable to the design of both the linear cross-polarization and linear-to-circular polarization converters.

Numerical Simulations of Circular Dichroism and Polarization Conversion in VO2-Based Terahertz Metamaterials

Metamaterials with actively tunable functionalities are highly desirable for applications of advanced optoelectronic devices. In this paper, we theoretically present a metamaterial with diversified functionalities by availing of the phase transition characteristics of vanadium dioxide (VO2) in terahertz frequency regions. The research results demonstrate that the function of the designed metamaterial can be switched from giant circular dichroism (CD) to a reflecting broadband half-wave plate (HWP) and a quarter-wave plate (QWP). When VO2 is in the isolating state, the metamaterial exhibits a quite distinct transmission efficiency for circularly polarized lights, thus resulting in a maximum CD value ~0.97 at the resonant frequency. When VO2 is operating in the metallic state, the metamaterial performs like a broadband HWP, in which the nearly perfect linear polarization conversion can be achieved at the frequency range from 3 to 7 THz. Moreover, the structure can play a role of a high-efficiency QWP that can simultaneously convert the incident linear polarized light to left-handed and right-handed circularly polarized light. The calculated ellipticity indicates a good polarization conversion at the frequency of 2.4 THz and 7.4 THz, respectively. The physical mechanism of the discussed features and effects can be explained by exploring the electric field distributions. Furthermore, the structural parameters also exert great influences for achieving giant CD and HWP as well as QWP. The proposed metamaterial may offer a new approach for designing metamaterial devices with multi-functions in THz regions.

Metasurface assisted wideband multifunctional polarizer

A wideband reflective type metasurface with both functionality cross and circular polarization conversions at different frequencies is presented in this paper. The meta-atom of the designed metasurface consists of six L-shaped metallic patches. A patching arrangement for the meta-atom yielded both polarization conversion ratio (PCR) > 0.9 over the 10.3–15.26 GHz band and axial ratio (AR) < 3 dB over the 8.97–9.75 GHz and 17.51–23.93 GHz bands. More than 30% fractional bandwidth is achieved for both functions. The physical mechanism of linear and circular polarization conversion has been analyzed by the spatial profile of electric and magnetic fields. The prototype of the designed metasurface consisting of 25×25 meta-atoms is fabricated and experimental results are in reasonable agreement with the simulated results. The designed metasurface when integrated with antennas can be extensively applied for diverse microwave applications for polarization manipulation devices.

Design of a high-speed circular polarization converter with a large field of view and wavelength range.

A high-speed circular polarization converter (CPC) with a wide field of view (FOV) and wavelength range is designed and fabricated in this paper. The multi-waveplate combined structure is applied to constitute the basic configuration of the CPC for broadening the wavelength range. An electrically suppressed helix ferroelectric liquid crystal (ESHFLC) material with fast response is used as a medium for dynamic polarization operation. The compensation films are used to expand the FOV by attaching to the configuration. The simulation results demonstrate that the optimized CPC structure can achieve over 97% orthogonal circular polarization conversion efficiency in 300 nm bandwidth at a 90° viewing cone for both working states. Finally, we have experiments and the results show well consistency with the theoretical results.

Effect of Twisting Phases on Linear–Circular Polarization and Spin–Orbital Angular Momentum Conversions in Tightly Focused Vector and Scalar Beams

We theoretically investigated the effect of a new type of twisting phase on the polarization dynamics and spin–orbital angular momentum conversion of tightly focused scalar and vector beams. It was found that the existence of twisting phases gives rise to the conversion between the linear and circular polarizations in both scalar and vector beams during focusing. The linear–circular polarization conversion further leads to an optical spin–orbital angular momentum transformation in the longitudinal component (LC). Therefore, even in a scalar optical field with a uniform linear polarization distribution, a circular polarization (spin angular momentum), and an orbital angular momentum (OAM) can appear in the cross-section and the longitudinal component, respectively, while being tightly focused. The novel distributions of the optical field, state of polarization (SOP) and OAM in the focal region are sensitively dependent on the twisted strength of the twisting phase. These results provide a more flexible manipulation of a structured optical field in the aspects of the optical field, SOP, and OAM.

A High Angular Stability, Single-Layer Transmission Linear-to-Circular Polarization Converter for Dual ISM-Band Operation

This paper presents a planar metasurface structure for linear to circular polarization (CP) conversion at the industrial, scientific, and medical bands of 2.4 GHz and 5.8 GHz. The new unit-cell structure consists of two loaded symmetric cross-shaped stubs with a size of 0.26 λ × 0.26 λ in a single thin substrate with a thickness of 0.0064 λ. The design process and parametric study are presented to clarify the operating mechanism for each frequency. The unit-cell element is analyzed and simulated, which exhibits an axial ratio (AR) bandwidth of 15.8% and 12.6% at a 3-dB criterion with the same polarization mode at 2.4 GHz and 5.8 GHz, respectively. The proposed LP-to-CP converter achieves a stable AR bandwidth with an oblique incident angle up to 45°. Moreover, a dual-band linear polarization (LP) antenna is combined with an array of 4 × 4 unit cells to validate the polarization converting ability of the proposed metasurface. The LP-to-CP converter and the antenna are fabricated and measured. The measurement results show that the antenna obtains a good impedance matching and CP waves radiation at 2.33–2.51 GHz and 5.55–5.89 GHz. The integrated antenna with dual-band CP waves radiation is suitable for improving the transmission efficiency in communication and wireless power transfer applications.

A Low-Profile Multifunctional Metasurface Reflector for Multiband Polarization Transformation

This brief proposes a low-profile multifunctional metasurface reflector for simultaneous linear to linear (LP-to-LP) and linear to circular (LP-to-CP) polarization conversion over multiple bands. The metasurface unit cell comprises a split ring resonator-loaded cross-diagonal pattern printed at the top of a dielectric substrate and a thin air space that separates the metallic bottom plate. The proposed reflector exhibits tri-band LP-to-LP conversion at 5.47-5.68, 7.86-8.84, 14.68-16.83 GHz, and quad-band LP-to-CP conversion at 5.30-5.41, 5.77-7.58, 9.27-13.91, 17.53-19.59 GHz frequency range. The response of the surface is analyzed through an equivalent circuit model (ECM), which gives physical insight into the metasurface. Moreover, the metasurface is explored through the characteristics mode (CM), where significant modes are determined based on the modal behaviors. Thereafter, modal behaviors are further investigated for simultaneous linear and circular polarization. Furthermore, the proposed structure and its mirror symmetric configuration are employed to develop the checkerboard surface. Such surface essentially realizes the phase gradient mechanism, which results in radar cross-section (RCS) reduction in the frequency range of 5.85-5.95, 9.2-10.3, and 16.6-18.5 GHz. The measured results are in good agreement with the simulated response.

Dual-Band Wide-Angle Reflective Circular Polarization Converter with Orthogonal Polarization Modes.

Herein a dual-band wide-angle reflective circular polarization converter, based on a metasurface was developed. The unit cell is composed of a split square ring and a nested square patch. The split square ring plays the role of creating polarization conversion. The square patch is useful for improving the quality of axial ratio. It was verified that the structure could transform the x-polarized incident wave into left-hand circular polarization in the lower frequency band, and to right-hand circular polarization in the higher frequency band. For y-polarized incidence, the transformation has orthogonal modes to that for x-polarized incidence. Moreover, the 3 dB axial ratio takes place in the ranges of 8.42-12.32 GHz and 18.74-29.73 GHz, corresponding to a relative bandwidth of 37.61%, and 45.35%, respectively. In addition, the polarization conversion efficiency is greater than 99% in the ranges of 8.65-11.83 GHz and 19.55-29.36 GHz. Furthermore, for oblique incidence, the axial ratio remains stable, even at 50° incidence, for the lower frequency band. Lastly, a prototype is fabricated and measured for experimental verification. The measured and simulated results were in good agreement. Compared with other designs in the literature, the proposed converter operates with good performance in dual-band, with high-efficiency, and with angular stability.

Wide-angle chiral polarization converter for reflection and transmission modes

Metamaterials provide high degrees of freedom for the design and manipulation of electromagnetic (EM) waves such as polarization conversion due to their unique controllable capabilities. In this paper, we propose a tunable wide-angle chiral metamaterial incorporating with vanadium dioxide (VO2), which can realize different operating modes and polarization conversions. When working in reflection mode, it can achieve circular polarization conversion at 3.35 THz and 3.07 THz for two different incident directions, the corresponding polarization conversion rate (PCR) can reach 93.5% and 97.1%, respectively. When operating in transmission mode, the incident x (y)-polarized waves can be converted to the orthogonal counterparts in the range of 2.24–2.88 THz, the asymmetric transmission (AT) parameter is over 0.6. Moreover, the good PCR and large AT effect can still be maintained in wide angle range. The proposed structure has potential application prospects in controllable switch, isolation device, communication system and so on.

Design and analysis of a multi-band and dual-functional terahertz wave polarization converter based on asymmetric cross-shaped metasurface

In this paper, a reflective multi-band and dual-functional terahertz wave polarization converter based on a metasurface is proposed. The unit cell of the proposed metasurface consists of a periodic array of two asymmetric Jerusalem cross-shaped resonators. The proposed metasurface behaves as a linear-cross polarization (LCP) converter at dual frequency bands of 0.911∼0.926 THz and 1.088∼1.093 THz. The polarization conversion rate (PCR) of the converter is above 95%, and the ellipticity of the reflected wave is about 0 within these two operating bands. Moreover, the proposed metasurface behaves as a linear to circular polarization (LTCP) converter at dual bands of 0.461∼0.862 THz and 0.968∼1.068 THz. The ellipticity of reflected wave approaches ±1 within these two operating bands. Wide angular stability up to 50° oblique along with high efficiency reveals the good robustness of the converter. The root cause of the polarization conversion has been analyzed through surface current distribution and multiple interference theory as well. We also analyzed the effects of asymmetry and different size structures on polarization conversion, which can provide some reference experience for the design of metasurface polarization converters in the future. The dual-functional, angular stability and simple structure make this converter an outstanding applicant for terahertz detection, dual-link satellite communication, and terahertz imaging.

Terahertz Wave Conversion From Linear to Circular Polarization by Graphene Metasurface Featuring Ultrawideband Tunability

This paper reports a detailed study of a graphene-based metasurface to function as tunable broadband linear to circular polarization converter in the terahertz gap. This device provides a wide polarization conversion band of 2.25 THz (3.75 THz to 6 THz) with an enhanced fractional 3-dB axial ratio bandwidth (FARBW) of 46.15%. The proposed structure consists of a simple tri-layer configuration where the meta-atom comprises a patterned graphene layer on a gold-backed silicon dioxide substrate. The proposed reflective-type linear to circular polarization converter (RLCPC) exhibits both right-handed and left-handed circular orientations of the reflected wave in two halves of the operating spectrum, further validated in terms of polarization ellipses developed by an inhouse code. The lack of experimental setup at the above said frequencies available to the authors in their country has been paid off by verifying the response using a detailed circuit model. The RLCPC maintains a thickness of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ_o</i> /7.69 with a lattice size of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ_o</i> /8.33. The structure also promises a stable wideband circular polarization conversion up to 20° and a dual-band circular polarization conversion up to 40° incident angle of the electromagnetic wave. This device can be useful for THz beam scanning antennas, RCS reduction and THz antennas for space and military radar applications.

Broadband absorption and polarization conversion switchable terahertz metamaterial device based on vanadium dioxide

A broadband absorption and polarization conversion switchable terahertz metamaterial device can be obtained by introducing vanadium dioxide with the metal to insulate phase transition. As the vanadium dioxide is in the metallic state, the device is a broadband absorber with over 90% absorption from 1.03 THz to 2.62 THz, and the broadband absorption performance is not sensitive to both the oblique incident angle and polarization. When vanadium dioxide is in insulate state, the device performs as a polarization converter with two functions of linear to cross polarization and linear to circular polarization conversion. The polarization conversion efficiency of linear to cross polarization conversion is above 90% from 1.81 THz to 2.39 THz, while the ellipticity of linear to circular polarization conversion is over 0.9 in the ranges of 0.75–1.69 THz, 2.48–2.59 THz and 2.81–2.90 THz, respectively. The proposed bifunctional device provides the potential in THz devices with tunable and diverse functionalities.