Linear-to-circular Polarization Conversion Research Articles

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.

An optimized metastructure switchable between ultra-wideband angle-insensitive absorption and transmissive polarization conversion: a theoretical study.

An optimized metastructure (MS) switchable between ultra-wideband (UWB) angle-insensitive absorption, and transmissive linear-to-circular (LTC) polarization conversion (PC), is proposed, which is a theoretical study. The structural parameters of this MS are optimized by the thermal exchange optimization algorithm. By modulating the chemical potential (μc) of the graphene-based hyperbolic metamaterial embedded in the MS, the MS can achieve UWB absorption in the absorption state and LTC PC in the transmission state. At normal incidence, in the absorption state, the MS exhibits absorptivity exceeding 0.9 within 7-15.45 THz, with a relative bandwidth (RBW) of 75.28%. By elevating μc, an UWB LTC PC is realized, with a RBW of 118.8%, achieving transmittance above 0.9 and the axial ratio below 3 dB. When prioritizing the angular stability, in the absorption state, the MS secures the angular stability of 75° for TE waves and 65° for TM ones. In the transmission state, the angular stability of PC reaches 60°, with RBW = 100.7%. Moreover, by manipulating μc, the tunability of UWB absorption is realized. The optimized MS provides a reference for designing multifunctional intelligent terahertz modulators, with promising application potential in domains like electromagnetic shielding, communication systems, and THz modulation.

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.

Bifunctional metasurface for high-efficiency terahertz absorption and polarization conversion.

A reconfigurable metasurface with a switchable function, broad band, high efficiency, and ultra-compact size is crucial for the development of efficient and compact devices. We propose a bifunctional metasurface that utilizes vanadium dioxide (V O 2) and graphene to achieve high-efficiency absorption and polarization conversion (PC) in the terahertz (THz) range. In our design, an extra dielectric layer is added on the top of V O 2 and graphene. It is worth pointing out that the presence of the additional dielectric layer greatly enhances the coupling of the wave in the Fabry-Perot cavity, resulting in remarkable improvement in absorption and PC efficiency. Furthermore, by controlling the working state of V O 2 and graphene, the functionality of the metasurface can be flexibly switched among absorption, cross-polarized conversion, and linear-to-circular PC (LTC). Simulation results indicate that the metasurface works in the absorption mode when V O 2 is in a metal state, and it can efficiently absorb THz waves at 2.0-7.0THz with a remarkable relative bandwidth of 111.1%. Furthermore, the absorption is over 98.4% under a normal incident case and still maintains over 90% with an incident angle of 50° at 2.8-7.0THz. Importantly, by changing the conductivity of V O 2, the absorption can be flexibly adjusted, allowing for tuning the absorption between 10% and 98.4%. When V O 2 is in an insulator state, the function of the designed metasurface is altered to PC mode, and it can efficiently convert incident linearly polarized (LP) waves into cross-polarized waves with a PC ratio exceeding 95% at 1.8-3.4THz when the Fermi level of graphene is 1eV. When switched to the LTC mode, it can convert incident LP waves into right-circularly polarized waves with ellipticity less than -0.95 at 1.7-2.1THz and into left-circularly polarized waves with ellipticity greater than 0.90 at 2.7-3.0THz when the Fermi level of graphene is 0.55eV.

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.

Polarization angle tailored multitasking metasturcure: electromagnetically induced transparency, electromagnetically induced absorption, linear-to-circular polarization conversion

In this work, simultaneous implementation of electromagnetically induced transparency (EIT), electromagnetic induced absorption (EIA), and linear-to-circular polarization conversion (LTCPC) in the terahertz band is proposed in a metasturcture (MS) associated with tailoring the polarization angle (θ). The linear polarization (LP) waves with different θ can trigger different phenomena. EIT can be achieved with the incidence of transverse electric (TE) waves (θ = 0°), transverse magnetic (TM) waves (θ = 90°) cause the appearance of EIA, while a LP wave with θ = 45° induces the appearance of the LTCPC phenomenon. For TE waves, the EIT transparent window accompanied by the slow light effect has emerged from 0.288 to 0.797 THz, where the maximum group delay has arrived at 2.51 ps. For TM waves, an absorption peak of EIA is observed at 0.497 THz. The LTCPC phenomenon is well obtained with θ = 45° at 0.399 and 0.634 THz, where the transmission coefficient highly reaches 0.571 and 0.650. The optimal axis ratios (ARs) of LTCPC are 2.08 dB at 0.389 THz and 2.18 dB at 0.634 THz, respectively. Noteworthy, the two-oscillator and circuit models are adopted for the theoretical analysis of the EIT and EIA, and the conclusions obtained are in general agreement with the simulation results.

The bifunctional tunable metasurface based on bilayer graphene

In this study, a bifunctional tunable metasurface based on graphene is designed to realize circular dichroism and linear-to-circular polarization conversion. The bifunctional tunable metasurface shows significant spin-selective absorption with the incidence of circularly polarized waves, resulting in a huge circular dichroism. Then, with the incidence of linearly polarized waves, the circularly polarized waves are reflected, realizing the linear-to-circular polarization conversion (LTC-PC) function. The switch between “off” and “on” states of the circular dichroism as well as the switch between “off” and “on” states of the polarization conversion can be achieved by altering the Fermi levels of graphene. Furthermore, when circularly polarized waves (linearly polarized waves) are incident at a wide angle on the metasurface in the “on” state, the metasurface maintains consistent circular dichroism (polarization conversion) properties. This design can be applied to active spin detectors, terahertz waveplates, signal processing and many other fields.

Bifunctional Terahertz Metasurface for Transmissive Broadband Linear-to-Circular and Linear Polarization Conversion

In this work, we experimentally demonstrate a bifunctional terahertz (THz) metasurface that can achieve broadband linear-to-circular polarization conversion (LCPC) and linear polarization conversion (LPC) in two different bands, respectively. The measured 3-dB axial ratio relative bandwidth is 34.2% for the LCPC in the lower band from 0.34 THz to 0.48 THz, and the measured polarization conversion ratio is over 80% for the LPC in the higher band from 0.53 THz to 0.85 THz. The physical mechanism of the LCPC and LPC is explained by the surface current distribution and is further discussed by employing the multiple interference model. This bifunctional THz metasurface polarization converter has promising applications in THz systems, communications, spectroscopy, etc., and the design strategy can also be extended to microwave and millimeter bands.

A reconfigurable polarization converter and related application as horn antenna cladding

In this paper, a transmissive reconfigurable polarization converter (RPC) based on an active metasurface is proposed, which can regulate the ideal plane wave and the spherical wave as a cladding of the horn antenna. The metasurface unit is composed of a single-layer F4B dielectric substrate and two double-cross metal strips with a p–i–n diode on the top and seven vertical metal strips on the bottom. One RPC prototype with 15 × 15 units has been fabricated and measured when it regulates an ideal plane wave. By controlling the states of diodes, the RPC can realize the linear-to-circular polarization conversion (LCPC) or the linear polarization maintaining (LPM) function from 2.5 to 3.64 GHz. Furthermore, the RPC is used as the cladding of a horn antenna for high integration in practical applications. The measured results show that this RPC-antenna system can still achieve LCPC or LPM function from 3.1 to 4.5 GHz. It is worth mentioning that the 3 dB axial ratio beam width of RPC can cover from −65° to 65° in LCPC. The proposal has advantages of a wideband, simple biasing network, and easy fabrication and integration, which can be widely applied in modern wireless communication systems.

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.

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.

Tailoring dual-band electromagnetically induced transparency with polarization conversions in a dielectric-metal hybrid metastructure.

Metastructure analogs of electromagnetically induced transparency (EIT) provide a new approach for engineering realizations of nonlinear optical manipulations regardless of harsh conditions; further can be employed in polarization conversions for its low-loss transmission and phase modulation. In this work, dual-band EIT in a dielectric-metal hybrid metasurface achieved via providing different coupling channels is theoretically investigated with a maximum group delay of 404 ps. The linear-to-circular polarization conversion (LCPC) behaviors are observed respectively holding the transmittance of 0.58 at 0.68 THz, 0.73 at 0.76 THz, 0.61 at 0.90 THz, 0.53 at 0.99 THz, owning to the asymmetric EIT responses in the transverse magnetic (TM) and transverse electric (TE) modes incidence. On the other hand, phase-transition VO2 is doped to perturb the dark mode resonances. With its conductivity σ = 105 S/m, dual transparency peaks transform into unimodal broadband transmission windows with relative bandwidths of 17.1% and 9.1% under the TE and TM excitations apart. Induced LCPC possesses a bandwidth of 10.4% centered at 0.76 THz attributed to the drastic dispersion. The as-proposed design exploits pattern asymmetry of EIT responses to realize LCPC, promising the wide prospect of reconfigurable multiplexings.

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.

Tunable dual-band linear-to-circular polarization conversion based on the electromagnetically induced transparency utilizing the graphene metamaterial

A tunable dual-band linear-to-circular polarization conversion (LTCPC) related to electromagnetically induced transparency (EIT) is theoretically proposed by employing the graphene metamaterial in the terahertz (THz) regime. Since the electric resonance (bright mode) and the magnetic resonance (quasi-dark mode) for the x - and y -polarized (TM and TE) waves are mutually coupled, the EIT behavior is realized due to the destructive interference. Two transparent EIT windows have emerged in 1.452–1.661 THz and 1.348–1.683 THz when the TM and TE waves are incidents. The corresponding values of the maximum group delay and group index respectively are 225 ps, 162 ps, 1636, and 1177. The LTCPC is achieved at 1.432 THz and 1.676 THz when the graphene does not exist. While the graphene is introduced, the LTCPC is dynamically adjustable by controlling the Fermi energy ( E f ) of the graphene. The optimal ARs can dynamically change between 1.438 (1.689) THz and 1.452 (1.696) THz while the E f changes from 0.1 eV to 0.9 eV. A theoretical investigation of two-oscillator model further confirms the effectiveness and consistency of the simulation results. The presented metamaterial with a thickness of subwavelength and high transparency for the electromagnetic waves opens a new path to the applications in beam steering and polarization controls. • The dual-band linear-to-circular PC of EIT phenomenon can be simultaneously realized. • The structure is equipped with graphene which can adjust the EIT behavior and circular polarization features. • The presented structure is great transparency for the EM wave, and its thickness is subwavelength. • The proposed structure provides a new idea to manipulate the polarization state of the EM wave. • The device is stable for the circular polarization feature within 10°.

Achieving polarization control by utilizing electromagnetically induced transparency based on metasurface

In this study, a design of transmissive linear-to-circular polarization conversion (LCPC) related to electromagnetically induced transparency (EIT) by arranging four sets of asymmetric resonators on a metasurface in a cross-shaped distribution is theoretically reported in the terahertz (THz) regime. It has numerically demonstrated that the bright modes and quasi-dark modes can also be mutually coupled when the x-polarized (TM) and y-polarized (TE) waves are incidents, resulting in a famous EIT phenomenon. Subsequently, when the direction of the electric fields of the electromagnetic (EM) waves are incident along the x-axis with an angle of 45°, the electric fields can be decomposed into the x- and y-directions. At this time, the corresponding amplitude and phase meet the conditions of the LCPC. Furthermore, for the TM and TE waves, the values of the maximum transmission coefficients respectively reach up to 0.910 at 1.384 THz and 0.890 at 1.277 THz. Correspondingly, two brand-new bright transparent windows are located in 1.276-1.454 THz for the TM waves and 1.206-1.373 THz for the TE waves. Concurrently, the band of the maximum 3 dB axial ratio is obtained from 1.308 THz to 1.343 THz with a relative bandwidth of 3%.