Linear-to-circular Polarization Research Articles

Multi-functional switchable terahertz metasurface device prediction by K-nearest neighbor

In this paper, we present the design of a versatile terahertz (THz) device characterized by its multi-functional capabilities including broadband absorption and polarization modulation, achieved through leveraging the temperature-induced phase transition behavior of vanadium dioxide (VO2). Operating in the metallic state of VO2, the device exhibits broadband absorption properties, delivering a remarkable total effective absorption of 2.871 THz with absorption rates exceeding 90 % across the frequency ranges of 8.140∼9.405 THz and 12.740∼14.346 THz. Furthermore, it demonstrates excellent adaptability to large angle incidence. Conversely, in the dielectric state of VO2, the device transitions into a polarization modulation mode, facilitating linear-to-cross-polarization (LTX) and linear-to-circular-polarization (LTC) conversions. Notably, for LTX polarization, the conversion efficiency exceeds 90 % within the 8-11.5 THz range, while for LTC polarization, the ellipticity surpasses 0.8 within the 7.861∼8 THz range. Additionally, we introduce a machine learning (ML) approach to optimize the device parameters, presenting a novel strategy for enhancing the design and optimization of future multifunctional devices.

Optimal design of broadband linear-to-circular polarization conversion metasurface

Due to their inherent defects, circular polarization discrimination methods involving amplitude-phase or axial ratio are obstacles of the computer software-based optimal design of broadband linear-to-circular (LTC) polarization conversion metasurfaces. Therefore, based on the proposed circular polarization discrimination method relating to ellipticity angle β, this study exploits Python-programmed elitist preserving genetic algorithm to optimally design a reflective-type broadband LTC polarization conversion metasurface. The simulation results demonstrate that the final metasurface can realize the wideband LTC polarization conversion; the measured results are in excellent agreement with the simulations, thus validating the proposed design method. It provides an efficient alternative for broadband LTC polarization conversion metasurface designs, which drives the development of the design of LTC polarization conversion metasurfaces from manual methods towards automation. And this work can be applied in antenna designs, optical devices, and remote sensing.

A Low Profile Wideband Linear to Circular Polarization Converter Metasurface with Wide Axial Ratio and High Ellipticity

This paper introduces an ultra-wideband (UWB) reflective metasurface that exhibits the characteristics of a linear to circular (LTC) polarization conversion. The LTC polarization conversion is an orthotropic pattern comprising two equal axes, v and u, which are mutually orthogonal. Additionally, it possesses a 45° rotation with respect to the y-axis which extends vertically. The observed unit cell of the metasurface resembles a basic dipole shape. The converter has the capability to transform LP (linear polarized) waves into CP (circular polarized) waves within the frequency range 15.41–25.23 GHz. The band that contains its 3dB axial ratio lies within 15.41–25.23 GHz, which corresponds to an axial ratio (AR) bandwidth of 49.1%, and the resulting circular polarized wave is specifically a right-hand circular polarization (RHCP). Additionally, an LTC polarization conversion ratio (PCR) of over 98% is achieved within the frequency range between 15 and 24 GHz. A thorough theoretical investigation was performed to discover the underlying mechanism of the LTC polarization conversion. The phase difference Δφμν among the reflection coefficients of both the v- as well as the u-polarized incidences is approximately ±90° that is accurately predictive of the AR of the reflected wave. This study highlights that the reflective metasurfaces can be used as an efficient LTC polarization conversion when the Δφμν approaches ±90°. The performance of the proposed metasurface enables versatile applications, especially in antenna design and polarization devices, through LTC polarization conversion.

Metasurface-based conformal linear-to-circular polarization converter for emerging wireless applications

This paper proposes the analysis and design aspects of a metasurface-based reflective type linear-to-circular polarization (LCP) converter in both planar and conformal profiles. The metasurface unit cell consists of a diagonally placed split circular metallic ring on top of a metal backed dielectric substrate. The structure provides LCP conversion with axial ratio (AR) < 3 dB at two frequency bands, which are 27.38–28.61GHz and 33.1–52GHz corresponding to fractional bandwidths of 4.4% and 44.5%, respectively. It exhibits two different handedness at the aforementioned two bands for a TE (or TM) linearly polarized incident wave. Moreover, it shows dual polarized and angularly stable (up to 25°) characteristics. It is observed from the far-field characteristics that the LCP converter with an array of 29 × 29 unit cells (12λ×12λ) produces peak gain of 26dBi, cross polarization level 20 dB below the maximum main lobe level when illuminated by a linearly polarized source at far-field. Subsequently, the same unit cell is used to design a cylindrical conformal metasurface with different radius of curvatures (r). It provides quite stable AR, gain and radiation patterns till r=100mm. The array has been fabricated to experimentally validate the numerical results of both the planar and conformal geometries.

A terahertz wideband linear-to-circular polarization converter based on meandering-structured metasurface

In the terahertz (THz) band, this paper proposes a transmissive linear-to-circular (LTC) polarization converter based on a bilayer metallic meandering-structured metasurface. In this design, the LTC polarization conversion is achieved by utilizing the meandering structure, and the bandwidth is greatly expanded by inserting a wire grid into the y direction of the back metal layer. Furthermore, this metasurface converter is characterized by its simple structure and ultra-thin thickness. In detail, the axial ratio of the proposed converter is less than 3 dB in the frequency range of 0.4–1.21 THz signifying 100% relative bandwidth. Meanwhile, the emulation polarization conversion efficiency exceeds 60% in the frequency range of 0.68–1.21 THz, and the experimental results are in general agreement with the simulation ones. The proposed converter provides a new idea for the LTC polarization conversion in a wide THz band.

A wide-incidence-angle insensitive multi-polarization converter based on reflective metasurface

In this paper, we propose a wide-incidence-angle insensitive reflective metasurface multi-polarization converter, using split-ring-resonators and cross oscillators structure. The simulated results indicate that the polarization conversion ratio for linear to linear (LTL) polarization conversion is greater than 90% in three frequency bands of 5.03–5.19 GHz (3.13%), 6.38–6.82 GHz (6.67%) and 7.38–7.44 GHz (0.81%). Meanwhile, the axial ratio for linear to circular (LTC) polarization conversion is less than 3 dB in four frequency bands of 4.79–5 GHz (4.29%), 5.24–6.27 GHz (17.90%), 6.94–7.35 GHz (5.74%) and 7.45–7.48 GHz (0.40%). LTL or LTC polarization conversion will not be affected when the incidence angle is tilted by 45° under both transverse electric and transverse magnetic modes. To verify the polarization conversion performance, a sample consisting of 18 × 18 unit cells is fabricated and measured. The measured results are found to be in reasonable agreement with the simulated results, demonstrating the potential application of this design for future microwave polarization state manipulation.

Multifunctional metamaterial for realizing absorption and polarization conversion on the basis of vanadium dioxide

In this paper, the phase change material vanadium dioxide (VO2) is introduced to propose a tunable multifunctional metamaterial device, which treats the phase change property of VO2 to realize the switching of absorption, linear to linear (LTL) polarization conversion, and linear to circular (LTC) polarization conversion functions. When VO2 is in metal state, the structure can be applied as an absorber to achieve the wide-band and narrow-band absorption. In the broad frequency area of 2.007–2.803 THz, the absorption is above 90%. And the narrow-band absorption near the center frequency 1.0126 THz reaches 95.04%. When VO2 is in the insulator state, the studied device can be treated as a polarization conversion device to achieve LTL and LTC polarization conversion. In the low frequency range, the device achieves LTL with polarization conversion efficiency (PCR) above 90% when the frequency is between 0.439 and 0.907 THz. In the high frequency range, the structure can be treated as the LTC polarization converter to achieve right-hand circular polarization, left-hand circular polarization (LHCP), and LHCP in the ranges of 2.317–2.329 THz, 2.356–2.531 THz and 2.582–2.633 THz, respectively. In addition, the ellipticity is above 0.9. Finally, the effects of geometric parameters, angle of incidence and polarization angle on the absorption and PCR are also discussed. The proposed structure has great potential for advanced technologies such as imaging, sensing, communication, and stealth.

A Broadband Multifunctional Reconfigurable Polarization Conversion Metasurface

In this work, a reflective polarization conversion metasurface (PCM) with symmetrical L-shaped patches is proposed which enables to switch between different polarization modes by PIN diodes, i.e., linear-to-linear (LTL) cross-polarization and linear-to-circular (LTC) polarization. Because of multimodes activated in the structure, the PCM shows an ultrawide bandwidth both for LTL and LTC polarization conversion. A microstrip line-based DC biasing network composed of radial stubs is introduced to isolate RF currents, which can resolve problems raised by RF lumped choke inductors in the ultrawide bandwidth. A prototype with 15 × 15 units is fabricated. The simulated and measured results show that when the PIN diodes are forward biased, the PCM achieves LTC polarization conversion in the range of 5.96–15.34 GHz (relative bandwidth of 88.4%) with the axial ratio (AR) less than 3 dB; when the PIN diodes are reverse biased, the PCM achieves LTL polarization conversion in the range of 6.05–14.76 GHz (relative bandwidth of 83.75%) with the polarization conversion ratio (PCR) higher than 90%. The proposed PCM is expected to be potentially applied to a broad scanning range array, intelligent reflecting surfaces (IRS), and remote sensing.

An X-shaped metasurface for high-efficiency ultra-wideband linear-to-circular and circular-to-linear polarization conversions

In this study, an X-shaped metasurface composed of an array of X-shaped metal patches printed on a grounded dielectric substrate and covered by a top dielectric layer is proposed for both linear-to-circular (LTC) and circular-to-linear (CTL) polarization conversions. Theoretical analysis and numerical simulations indicate that the metasurface can be used as both LTC and CTL reflective polarization converters. Under u- and v-polarized incidences, it can achieve LTC polarization conversion in the ultra-wide frequency band from 7.5 to 25.8 GHz with a relative bandwidth of 110%, and the polarization conversion rate (PCR) can be kept larger than 99.7% in the frequency range of 7.9-24.7 GHz. Moreover, under CP incidence, a high-efficiency ultra-wideband CTL polarization conversion can be achieved, the bandwidth and PCR are almost the same as those of the LTC polarization conversion. Finally, one experimental verification is carried out, and a reasonable agreement is observed between the simulation and experimental results.

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.

Transmission-type reconfigurable metasurface for linear-to-circular and linear-to-linear polarization conversions

We present a transmission-type polarization conversion metamaterial (PCM) whose functions can be dynamically switched among the linear-to-circular (LTC) and linear-to-linear (LTL) polarization conversions. The proposed PCM consists of a grating, a polarization conversion surface and a reconfigurable polarization selective surface incorporated with PIN diodes. By changing the states of diodes, the PCM can achieve the reconfigurable manipulations for incident waves. The Fabry–Pérot (F–P) resonances excited by the PCM contribute to the polarization conversions, as is illustrated. Moreover, through establishing the F–P-like cavity model and analyzing the electric field components of the transmitted waves, the conditions for realizing LTC polarization conversion are revealed, which can guide the construction of PCM. The prototype of PCM is fabricated and measured, which can achieve LTC and LTL polarization conversions within 3.31–3.56 GHz and 2.76–4.24 GHz, respectively, the polarization conversion ratios of two functions are higher than 0.95. The measurement results are in agreement with the simulation data.

Realization of Multifunctional Metamaterial Structure Based on the Combination of Vanadium Dioxide and Graphene.

Combining tunable properties and various functionalities into a single metamaterial structure has become a novel research hotspot and can be used to tackle great challenges. The multifunctional metamaterial structure that combines absorption, linear-to-circular (LTC) polarization conversion, filtering and switching functions into a single metamaterial device was designed and investigated in this study. The switching of different functions can be achieved based on the phase transition of vanadium dioxide (VO2) and change of graphene chemical potential. When VO2 is in a metal state, the multi-frequency absorption and LTC polarization conversion can be achieved with different chemical potentials. When VO2 is in the insulator state and the polarization angle of incident wave is 45°, the device can be used to select or isolate the incident waves with different polarization states in the frequency region of 1.2–1.8 THz. Furthermore, when the chemical potentials are 0.05 eV and 1.2 eV, the corresponding transmissions of the TE-polarized wave demonstrate the opposite results, realizing the switching functions in the frequency region of 0.88–1.34 THz. In the frequency region above 2 THz, the multi-frequency rejection filter can be achieved. The designed switchable multifunctional metamaterial device can be widely implemented in radar monitoring and communication systems.

Vanadium dioxide-assisted switchable multifunctional metamaterial structure.

A multifunctional design based on vanadium dioxide (VO2) metamaterial structure is proposed. Broadband absorption, linear-to-linear (LTL) polarization conversion, linear-to-circular (LTC) polarization conversion, and total reflection can be achieved based on the insulator-to-metal transition (IMT) of VO2. When the VO2 is in the metallic state, the multifunctional structure can be used as a broadband absorber. The results show that the absorption rate exceeds 90% in the frequency band of 2.17 - 4.94 THz, and the bandwidth ratio is 77.8%. When VO2 is in the insulator state, for the incident terahertz waves with a polarization angle of 45°, the structure works as a polarization converter. In this case, LTC polarization conversion can be obtained in the frequency band of 0.1 - 3.5 THz, and LTL polarization conversion also can be obtained in the frequency band of 3.5 - 6 THz, especially in the 3.755 - 4.856 THz band that the polarization conversion rate is over 90%. For the incident terahertz waves with a polarization angle of 0°, the metamaterial structure can be used as a total reflector. Additionally, impacts of geometrical parameters, incidence angle and polarization angle on the operating characteristics have also been investigated. The designed switchable multifunctional metasurfaces are promising for a wide range of applications in advanced terahertz research and smart applications.

Polarization manipulation associated with electromagnetically induced transparency based on metamaterials

The polarization of electromagnetic waves is a key feature in modern optics and information science research fields. How to effectively control the polarization state of electromagnetic waves in electromagnetically induced transparency (EIT) is still a challenge. Herein, the EIT behavior associated with transmissive linear-to-circular (LTC) polarization conversion is theoretically proposed in the terahertz (THz) regime by employing a metamaterial. Four asymmetric H-shaped resonators are arranged in a cross-shaped arrangement on the metamaterial. It has numerically demonstrated that the electric and magnetic resonances are mutually coupled, leading to the famous EIT phenomenon. Because of the strong dispersion properties and high transmission dominated by the EIT behavior and the anisotropy of metamaterial, the transmissive LTC polarization conversion is conspicuously realized while the x- and y-polarized waves are incidents. Furthermore, two new transparent windows are located in 1.396–1.654 THz and 1.258–1.721THz, which are higher than 0.9 can be gained from 1.409 (1.323) THz to 1.467 (1.594) THz, and the relative bandwidths are 4.0% and 18.6%, respectively. Moreover, the values of the maximum group delay and group index under the x- and y-polarized waves respectively reach 504 ps, 122 ps, and 18861, 4575, displaying a slow-light effect. In addition, the LTC polarization conversion is well obtained at 1.685 THz, where the transmission coefficient highly reaches 0.605. Based on the Stokes parameters, the corresponding ellipticity η, the band of the maximum 3 dB axial ratio, and the polarization azimuth ψ respectively are 0.961, 2.1%, and −42.5°. It is worth noting that not only the thickness of the reported metamaterial is subwavelength, but also the proposed structure with low loss is great transparency for the electromagnetic waves. Surpassing conventional EIT metamaterials and polarization converters, the proposed structure synchronously equipped with those two characteristics has numerous potential applications in the integration to the existing terahertz devices, future polarization controls, and the antenna fields.

Tunable terahertz all-dielectric linear-to-circular polarization conversion metasurface

Terahertz (THz) linear-to-circular (LTC) polarization conversion plays a crucial role in imaging and 6G wireless communication. This paper will give an account of a thermally tunable THz LTC polarization converter by using the active all-dielectric metasurface. It consists of zirconium oxide (ZrO2) microsphere resonators, active strontium titanate (STO) cladding, and flexible polyimide substrate. Through numerical simulation, the amplitude of the ellipticity of the proposed polarization converter at 0.265 THz is –1, indicating that perfect right-hand circular polarization (RHCP) wave is achieved. Meanwhile, the amplitude of the ellipticity is less than –0.8 between 0.247 and 0.278 THz (relative bandwidth is 12%). In addition, with the temperature changes of 180 K (from 200 to 380 K), the operating frequency of the converter can be tuned from 0.220 to 0.291 THz, a sensitivity about 39 GHz/100 K is achieved. Besides, the modulation depth of the ellipticity amplitude can achieve 92% at 0.220 THz, which demonstrates that the converter can output terahertz wave with different polarization states, and the device can be fabricated on a large scale. These perfect conversion performances show that the converter has potential applications in high-speed communication and imaging.

Broadband of linear-to-linear and double-band of linear-to-circular polarization converter based on a graphene sheet with a π-shaped hollow array

In this manuscript, we propose a tunable multifunctional reflection polarization converter based on a graphene sheet, which is decorated by a π-shaped carved-hollow array. Our design can achieve the linear-to-linear (LTL) polarization conversion with 3.58 THz bandwidth and two linear-to-circular (LTC) polarization conversion bands with opposite handedness, which originates from the excitation of the three graphene surface plasmons (GSPs) modes. The LTL polarization converter can be switched to the LTC polarization converter by only changing the medium height. The performances of this multifunctional reflection polarization converter under different geometric parameters, incident angles, and electron scattering time of graphene are investigated. Besides, it is found that the broadband of LTL and double bands of LTC have the blue shift with increasing the Fermi energy (Ef) of graphene. Especially, the conversion between line-to-left-circular and line-to-right-circular polarization converter can be achieved by changing Ef. All the simulation results are conducted by finite element method, and our design has potential applications in integrated terahertz systems.

Reconfigurable Multifunction Polarization Converter Integrated With PIN Diode

A novel reconfigurable polarization conversion method based on a metasurface turned by PIN diodes is proposed herein. The proposed metasurface consists of one layer of a dielectric and two layers of metal surfaces. When the diode is in an OFF-state, a wideband linear to circular (LTC) polarization converter is realized. The results show that the metasurface can achieve LTC polarization conversion in the frequency range of 11.8-24.1 GHz, and the axial ratio (AR) reveals that the 3-dB bandwidth is approximately 68.5%. When the diode is in the ON-state, a dual-band linear to linear (LTL) polarization converter is realized, with the 3-dB bandwidths of 10.5-13.9 GHz and 17.7-27.2 GHz, achieving a polarization converter ratio (PCR) greater than 90%. The dc bias circuit of the PIN diode is etched on the reflectance surface. Therefore, no additional feeding lines are introduced, which can decrease the impact on the active metasurface; this metasurface has practical applications in microwave communication systems.

Conformal Polarization Conversion Metasurface for Omni-Directional Circular Polarization Antenna Application

In this article, conformal linear-to-circular (LTC) polarization conversion metasurface (MS) in transmission mode is engineered and fabricated. The conformal MS is cylinder-shaped with curved elements bent on the cylinder wall. First, the anisotropic element with the capability of LTC polarization conversion in high transmission efficiency is designed and simulated, and the electromagnetic response on the variation of the curvature radius of the conformal element is discussed. Second, the cylinder conformal polarization conversion MS is engineered based on the anisotropic element. Then by launching a monopole antenna at the center of the cylindrical axis as the feed source, the omni-directional circular polarization antenna is implemented. More importantly, the height, radius, and element number along the longitudinal direction of the MS are optimized to obtain an optimized performance of the antenna. Finally, the antenna system is fabricated, assembled, and measured. The device can operate in 9.9–10.1 GHz with an axial ratio lower than 3 dB along the circumferential direction and at 10 GHz the radiation fluctuation along the circumference is less than 1 dB. This study opens up a new way to design conformal MS on a curved surface and affords an efficient alternative for circular polarization antenna design.

Graphene-Based High-Efficiency Broadband Tunable Linear-to-Circular Polarization Converter for Terahertz Waves

Broadband tunable terahertz (THz) polarization converter is a vivid necessity due to its great significance in functional utilization. Graphene is a promising material for the evolution of tunable terahertz devices, thanks to its tunable surface conductivity and extraordinary electrical properties. In this paper, we present a high-efficiency graphene-based ultra-thin metasurface manifesting linear-to-circular (LTC) polarization conversion for THz wave. A broadband LTC conversion with ellipticity over 0.95 can be achieved for the entire operating bandwidth of 14 THz to 40 THz for any graphene chemical potential from 0.4 eV to 0.6 eV. It is also shown that tuning the graphene chemical potential may provide much improved ellipticity and efficiency for different frequencies. The proposed LTC converter also provides over 90% efficiency for the complete operating bandwidth for graphene chemical potential from 0 eV to 0.7 eV. Simulation results also reveal that a perfect LTC converter (ellipticity = 1) with more than 90% efficiency can be realized from 16 THz to 37 THz by tuning the graphene chemical potential between 0 eV to 0.7 eV. Altogether, the proposed LTC polarization converter provides the advantage of dynamic regulation, simpler structure, higher efficiency, enhanced ellipticity and greater relative bandwidth.

Frequency-Selective Surface-Based Compact Single Substrate Layer Dual-Band Transmission-Type Linear-to-Circular Polarization Converter

In this article, a compact dual-band linear-tocircular polarization converter designed on a single-layer substrate is proposed. The unit cell of the converter has a size of 3.72 × 3.90 × 1.52 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> (0.0072λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , λo is calculated with respect to 20.67 GHz). The converter consists of split rings and a pair of metallic strips printed on both sides of a single-layer substrate for dual-band operation. The proposed converter has a measured 3-dB axial ratio bandwidth of 5.56% (20.10-21.25 GHz) in the lower band and 3.97% (29.12-30.30 GHz) at the upper band. The converter transmits a right-hand circularly polarized (RHCP) wave at the lower band and left-hand circularly polarized (LHCP) wave at the upper band for a linearly polarized (LP) incident wave. The characteristics of the polarization converter are validated by integrating an array of 7 × 7 elements of its unit cell in the endfire direction of a dual-band LP dipole antenna operating in the same frequency bands such as that of the polarization converter. The antenna integrated with the polarization converter exhibits measured circularly polarized (CP) bandwidths of 7.80% and 6.57% in the lower frequency band: 20.10-21.75 GHz and the upper frequency band: 29.4-31.4 GHz, respectively. The integrated antenna can act as a feed source of a high-gain dualband transmit array for uplink (30.0-31.0 GHz) and downlink (20.2-21.2 GHz) of Ka-band military satellite communication.