Broadband Polarization Conversion Research Articles

Multifunctional metasurface for broadband absorption and polarization conversion based on graphene-VO2

A multifunctional metasurface based on graphene and vanadium dioxide (VO2), which can function as a broadband absorber, half-wave plate (HWP), and quarter-wave plate (QWP) in the terahertz frequency band, is designed in this study. When VO2 is in the metal phase, the multifunctional metasurface operated as a broadband absorber in the range of 0.89–2.36 THz. The designed multifunctional metasurface can switch between HWP and QWP by adjusting the Fermi energy of graphene through voltage biasing when VO2 is in the insulated phase. When the Fermi level of graphene is 0 eV, the designed structure acts as an HWP to achieve linear-to-linear (LTL) polarization conversion and preserve the reflection chirality of circular polarization at 0.68–2.64 THz. When Ef = 0.3 eV, the designed structure functions as a QWP in 0.89–2.51 THz to realize the interconversion of linear polarization and circular polarization. Furthermore, the effects of the polarization and incidence angles on the operating performance are discussed. This study demonstrates that the multifunctional metasurface has promising applications in terahertz optical switches, electromagnetic stealth, modulators, and communication systems.

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.

Fast decomposed method to devise broadband polarization-conversion metasurface

Designing a broadband, wide-angle, and high-efficient polarization converter with a simple geometry remains challenging. This work proposes a simple and computationally inexpensive method for devising broadband polarization conversion metasurfaces. We focus on a cross-shape configuration consisting of two bars of different lengths connected at the center. To design the metasurface, we decompose the system into two parts with two orthogonally polarized responses and calculate the response of each part separately. By selecting the parameters with a proper phase difference in the response between the two parts, we can determine the dimensions of the system. For designing broadband polarization conversion metasurfaces, we define a fitness function to optimize the bandwidth of the linear polarization conversion. Numerical results demonstrate that the proposed method can be used to design a metasurface that achieves a relative bandwidth of 97% for converting linearly polarized waves into cross-polarized waves. Additionally, the average polarization conversion ratio of the designed metasurface is greater than 91% over the frequency range of 10.9–28.5 GHz. This method significantly reduces the computational expense compared to the traditional method and can be easily extended to other complex structures and configurations.

Broadband and high-efficiency polarization conversion with a nano-kirigami based metasurface

Nano-kirigami metasurfaces have attracted increasing attention due to their ease of three-dimension (3D) nanofabrication, versatile shape transformations, appealing manipulation capabilities and rich potential applications in nanophotonic devices. Through adding an out-of-plane degree of freedom to the double split-ring resonators (DSRR) by using nano-kirigami method, in this work we demonstrate the broadband and high-efficiency linear polarization conversion in the near-infrared wavelength band. Specifically, when the two-dimensional DSRR precursors are transformed into 3D counterparts, a polarization conversion ratio (PCR) of more than 90% is realized in wide spectral range from 1160 to 2030 nm. Furthermore, we demonstrate that the high-performance and broadband PCR can be readily tailored by deliberately deforming the vertical displacement or adjusting the structural parameters. Finally, as a proof-of-concept demonstration, the proposal is successfully verified by adopting the nano-kirigami fabrication method. The studied nano-kirigami based polymorphic DSRR mimic a sequence of discrete bulk optical components with multifunction, thereby eliminating the need for their mutual alignment and opening new possibilities.

Broadband and high-efficient reflective linear–circular polarization convertor based on three-dimensional all-metal anisotropic metamaterial at terahertz frequencies

In this paper, we proposed a broadband and high-efficient reflective linear–circular​ polarization convertor (LCPC) based on three-dimensional (3D) all-metal anisotropic metamaterial (AMM) in terahertz (THz) region. The LCPC unit-cell is composed of the stand-up inverted U-shaped resonator (USR) deposited on a ground plane. This LCPC design can convert both incident x- and y-polarized (y-pol) waves from linear to circular polarization after reflection in a broadband frequency range of 1.98-4.12THz with an axial ratio (AR) below 3 dB and polarization conversion efficiency (PCE) over 98% on average. The numerical simulation results show that the broadband polarization conversion of the designed LCPC is caused by the overlaps of stronger electric and magnetic dipolar coupling resonances. The polarization conversion properties of the designed LCPC can be adjusted by varying the geometric parameters of the USR. The proposed LCPC design has potential applications in many areas such as spectroscopy, detection, imaging, and communications in the THz region.

High efficiency and broadband wide-angle linear polarization converter based on dumbbell-like resonant structure

In this paper, a broadband wide-angle reflective linear polarization converter is designed, of which a dumbbell-like resonant structure is applied to form the unit cell of the periodic metasurface. Research results show that characteristics of broadband polarization conversion with higher polarization conversion ratio (PCR) in the range of 13.3 GHz to 24.1 GHz come from the multiple electric and magnetic resonances along the dumbbell-like resonant unit. Meanwhile, the entire conversion performance can be well controlled through changing the structural parameters. In addition, the property of wide-angle polarization conversion can make it convenient for the real application. Due to the simple structure together with the excellent performance of the proposed linear polarization converter, it has promising values in polarization converting, detecting, imaging and so on in the GHz frequency range.

Deep neural network training method based on vectorgraphs for designing of metamaterial broadband polarization converters

In this work, we proposed a method of extracting feature parameters for deep neural network prediction based on the vectorgraph storage format, which can be applied to the design of electromagnetic metamaterials with sandwich structures. Compared to current methods of manually extracting feature parameters, this method can automatically and precisely extract the feature parameters of arbitrary two-dimensional surface patterns of the sandwich structure. The position and size of surface patterns can be freely defined, and the surface patterns can be easily scaled, rotated, translated, or transformed in other ways. Compared to the pixel graph feature extraction method, this method can adapt to very complex surface pattern design in a more efficient way. And the response band can be easily shifted by scaling the designed surface pattern. To illustrate and verify the method, a 7-layer deep neural network was built to design a metamaterial broadband polarization converter. Prototype samples were fabricated and tested to verify the accuracy of the prediction results. In general, the method is potentially applicable to the design of different kinds of sandwich-structure metamaterials, with different functions and in different frequency bands.

Dual-functional metamaterial with ultra-broadband polarization conversion and narrowband absorption based on vanadium dioxide

In this paper, a three-dimensional dual-functional metamaterial (MM) is proposed. The polarization conversion part is entirely composed of vanadium dioxide (VO2), and the absorption part is composed of graphite and silica materials. By regulating the phase transition characteristics of VO2, the switching between dual-functional polarization conversion and absorption is realized. Through simulation and theoretical calculation, when VO2 is in the metallic state, the anisotropic polarization converter composed of VO2 plays a major role. In the frequency range of 0.67–1.99 THz, more than 90% linear polarization conversion rate (PCR) is obtained, and its relative bandwidth (RB) reaches 99.3%. The formation mechanism of broadband polarization conversion is explained by conducting current distribution. However, when VO2 is in dielectric state, the absorptance is nearly to 100% at 1.6 THz, and the formation mechanism of absorption is also explained by the distribution of electric and magnetic fields on the surface. The design of the two functions is carried out independently, and the reconfiguration and tuning of the two functions does not affect the working frequency band of the other function, thus achieving functional isolation.

Broadband terahertz linear cross-polarization conversion in transmission mode using planar coupled metamaterials

We demonstrate a metamaterial (MM) design capable of showing linear broadband polarization conversion over the terahertz (THz) frequency range. The building block of the proposed MM structure is composed of a strip and four split ring resonators (SRRs), which are coupled through their near fields. To examine co- and cross-polarization transmission amplitudes, we gradually increase the distance between the strip and SRRs. When the SRRs are near (S = 2 μm) the strip, maximum cross-polarization conversion is attained with a resonance mode hybridization effect in the co-polarization transmission due to strong near-field coupling between the strip and SRRs. When the SRRs moved away from the strip (S = 22 μm), minimum cross-polarization conversion is attained due to weak coupling between the strip and SRRs. This MM system exhibits a transition from a strongly coupled state to a weakly coupled state with the rise in displacement between the strip and SRRs. The ability to tune the linear polarization conversion can be useful in the improvement of efficient THz polarization rotation devices. The proposed MM structure can be used in other frequency domains, like the microwave and visible range, by scaling up/down the structure.

Deep neural network-aided design of terahertz bifunctional metasurface

Based on the insulator-to-metal phase transition of vanadium dioxide (VO2), we present a bifunctional metasurface (MS), which can be switched between the two working states, i.e., a broadband absorber and a broadband polarization converter. When the top VO2 is in the metallic state, the MS can work as an absorber, which has an absorption bandwidth of 3.53 THz with an ≥90% absorption rate; and conversely, when the top VO2 is in the insulating state, the structure can function as a polarization converter, which exhibits a bandwidth of 3.0 THz with a ≥90% conversion efficiency. The overlapped bandwidth of the two states is the widest when compared with the other bifunctional counterparts. Meanwhile, this bifunctional MS has good angular and parametric tolerance characteristics. In the MS’s design, the deep neural network (DNN) is also utilized to assist us in optimizing the structural parameters efficiently. The proposed structure and design method of the bifunctional MS may provide a valuable reference for new multifunctional terahertz devices.

Dual function tunable THz metamaterial device possessing broadband absorption and polarization conversion

Based on the phase transition properties of vanadium dioxide (VO2), a novel tunable metamaterial device with functions of broadband absorption and broadband polarization conversion is constructed. The proposed structure operates in the broadband absorption mode when the VO2 is in the metallic state. The simulation results demonstrated that the absorbance can achieve more than 90% with a relative bandwidth of 76.9% in the frequency range of 1.80-4.05THz. When VO2 is in the insulating state, the proposed structure operates in the broadband polarization conversion mode. In the frequency range of 1.68-3.79THz, it can efficiently convert the incident linearly polarized terahertz (THz) wave into its cross-polarized state. The polarization conversion ratio (PCR) is above 90% and the relative conversion bandwidth is larger than 77.1%. Furthermore, for the two operating modes, it has acceptable tolerance of incident angles and polarization angles. These advantages make the proposed VO2-based dual-functional metamaterial device have potential applications in many THz areas, such as absorption, polarization conversion, imaging, and so on.

Multifunctional terahertz device based on a VO2-assisted metamaterial for broadband absorption, polarization conversion, and filtering

This paper proposes a multifunctional terahertz device based on VO2 with a simple structure that needs only one step lithography. The designed device can realize broadband absorption, reflective broadband cross-polarization conversion, reflective linear-to-circular polarization conversion, transmissive narrowband cross-polarization conversion, and filtering under different working conditions. When VO2 is in a metallic state, the device can perform two different functions depending on the direction of the incident wave. When the electromagnetic wave is a forward incident, the device is a broadband absorber in the frequency range from 3.53 THz to 9.68 THz with a corresponding absorption efficiency above 90% and a relative bandwidth of 93%. When the electromagnetic wave is a backward incident, the device can work as a reflective broadband cross-polarization converter in the frequency range from 0.77 THz to 1.79 THz with a polarization conversion rate greater than 97% and a relative bandwidth of 80% and a reflective linear-to-circular polarization converter at 0.66 THz and 1.86 THz. While for VO2in the insulating state, the device exhibits the transmissive narrowband cross-polarization converter with polarization conversion rate greater than 90% at 1.32 THz and the function of the transmissive narrowband filter with transmittance more than 60% at 1.84 THz for both forward and backward incident waves. This multifunctional device may have great potential in miniaturized terahertz systems.

Dual-function switchable terahertz metamaterial device with dynamic tuning characteristics

A dual-function switchable terahertz (THz) device is proposed. Based on the phase transition of vanadium dioxide (VO2), dynamic switching between broadband absorption and polarization conversion is obtained. By manipulating the conductivity of photosensitive silicon through laser pumping, dynamic modulation of the amplitude in both functions is realized at the same time. Unlike the previous ones, the entire structure of the device consists of seven layers. When VO2 is in the metallic phase, our device is a broadband absorber. In frequency range of 0.78–1.81 THz, a broadband absorption with a relative bandwidth of 80% is achieved. Dynamic tuning of the absorption amplitude is achieved by changing the conductivity of the silicon, and the modulation depth is 74%. When VO2 is in the insulating phase, our device can be used as a polarization converter. In frequency range of 0.51–1.45 THz, cross-polarization conversion ratio is over 90%, and the relative bandwidth is up to 96%. Similarly, dynamic tuning of the polarization intensity is obtained, and the modulation depth is 89%. Compared with the previously reported ones, our device can achieve dynamical switching between the two functions, as well as dynamic tuning of performance at the same time, which offers a wide range of possible applications in sensors, detectors, solar cells, etc.

Tunable Transmissive Metastructure for Precise or Broadband Polarization Conversion Modulation Based on Graphene

AbstractBased on the principle of Fabry–Perot (F–P) cavity resonance and the selective permeability of gratings to specific electromagnetic waves, a graphene‐based metastructure (MS) is proposed for transmissive polarization conversion (PC). Using the full‐wave numerical simulation, it is found that by varying the Fermi energy of graphene, the effective resonance range of the suggested MS can be dynamically adjusted from 0.47 to 0.348–0.714 THz, achieving the target of precise to ultra‐broadband polarization modulation. In this paper, the plausibility of the structure is verified from multiple perspectives, and the correlation analyses of the electric and magnetic fields are the supporting illustrations. Additionally, the triggering mechanism of PC is visually illustrated in the study of the surface currents distributions. Simulation results reveal that the MS is superior in performance, functionality, and principle, and it is foreseen to hold excellent promise for integrated equipment in the terahertz (THz) band.

Wideband linear cross-polarization conversion with 3D metamaterial configuration.

In this Letter, a broadband and wide-angle linear polarization (LP) converter based on a 3D resonator for cross-polarization conversion (CPC) is presented. The designed structure performs a CPC with a polarization conversion ratio (PCR) of more than 90% in the frequency range 16.32 to 34.12 GHz, corresponding to a relative frequency bandwidth of 70.6%. Moreover, the presented structure possesses broad angle stability and miniaturized configuration. The efficiency of CPC remains over 90% in the operational frequency band with the incident angles up to 40°, and the cell size is 0.18λ for the lowest frequency of the CPC operational band. The proposed 3D structure is fabricated using the multi-material hybrid microdroplet jetting modeling (MHMJM) technique, and the experiments agree closely with the simulated results. Compared with traditional polarization converters based on a planar resonant structure, the proposed design shows excellent bandwidth, wide-angle performance, and miniaturization advantages.

Graphene-Based Active Tunable Metasurfaces for Dynamic Terahertz Absorption and Polarization Conversion

Simultaneous broadband absorption and polarization conversion are crucial in many practical applications, especially in terahertz communications. Thus, actively tunable metamaterial systems can exploit the graphene-based nanomaterials derived from renewable resources because of the flexible surface conductivity and selective permeability of the nanomaterials at terahertz frequencies. In this paper, we propose a graphene-based active tunable bifunctional metasurface for dynamic terahertz absorption and polarization conversion. The graphene ring presents a certain opening angle (A) along the diagonal of the xoy plane. When A = 0°, the proposed metasurface behaves as a broadband absorber. Numerical results show the feasibility of achieving this polarization-insensitive absorber with nearly 100% absorptance, and the bandwidth of its 90% absorptance is 1.22 THz under normal incidence. Alternatively, when A = 40° after optimization, the proposed metasurface serves as a broadband polarization convertor, resulting in robust broadband polarization conversion ratio (PCR) curves with a bandwidth surpassing 0.5 THz in the reflection spectrum. To tune the PCR response or the broad absorption spectrum of graphene, we change the Fermi energy of graphene dynamically from 0 to 0.9 eV. Furthermore, both the broadband absorption and the linear polarization conversion spectra of the proposed metasurface exhibit insensitivity to the incident angle, allowing large incident angles within 40° under high-performance operating conditions. To demonstrate the physical process, we present the impedance matching theory and measure electric field distributions. This architecture in the THz frequency range has several applications, such as in modulators, sensors, stealth, and optoelectronic switches. THz wave polarization and beam steering also have broad application prospects in the field of intelligent systems.

Dual functional metasurface with broadband polarization conversion and perfect absorption

A dual-functional metasurface based on vanadium dioxide (VO2) and operating in the infrared region is proposed in this paper. The photonic metasurface consists of a double-split ring resonator and the gaps are loaded with VO2 to achieve dual-functionality. The proposed metasurface functions as a polarization converter at a temperature (T) of 358 K when VO2 is in insulating state. In this state, broadband linear to cross polarization is achieved with an excellent polarization conversion ratio (PCR) of > 98% in the 2702–4687 nm wavelength range. At a lower temperature (303 K), VO2 is in the metallic state and the metasurface functions as a wideband absorber with an absorptivity of > 90% in the wavelength range from 3896 − 4615 nm. There is a significant overlap in the frequency responses of the absorber and polarization converter leading to dual-functional behavior of the metasurface. Therefore, the structure is thermally tunable and suitable for optical and photonic applications.

Ultra-Wide Band Polarization Converter Based on Ultra-Thin Bi-Layer Slot Structures

An ultrathin linear-to-linear polarization converter exhibiting a high polarization conversion ratio (PCR) in an ultrawide bandwidth is fabricated on a printed circuit board (PCB). It is based on a bilayer slot structure that is electrically connected by via holes. Front- and back-sided slot structures are arranged in mutually orthogonal directions and function as receivers and radiators, respectively. The via holes provide a route for the transfer of electromagnetic energy received by the front-sided slot structures to the back-sided slot structures and radiate an electromagnetic wave having polarization orthogonal to that of the input electromagnetic wave. Moreover, two slots are employed for each unit cell design, and the locations of the via holes are carefully determined to achieve broadband polarization conversion. The via-hole positions that change the surface current distribution around the slot apertures enable the control of multiple resonance frequencies. Via holes are positioned to make first and second resonance frequencies that are blue-shifted toward the third resonance frequency, which broadens the operational band of the perfect polarization conversion. The 0.8 mm thick polarization converter exhibits PCR greater than 99% in the frequency band from 24.5 to 47 GHz.

A study of linear-to-linear (LTL) cross-polarization conversion metasurface (CPCM) in microwave regime

An efficient new polarization convertor was considered in this paper. It contains semi-circular cutting head coupling, V-shaped head, and diagonal metal strip with the same picture rotation 180⁰ based on the unit cell’s electromagnetic resonant features. This article shows a linear polarization convertor that correctly simulates a metasurface which can be converted to an ultra-wideband frequency with a wide-angle of the incident. To achieve the broad-band polarization conversion, it operates x-and y-linear polarization waves in the specular reflection mode. Four resonances lead to an increase in bandwidth and cross-polarization reflection. The simulated results indicate that it is possible to convert a bandwidth of 92 % from 6.1 to 16.5 GHz with a conversion ratio of 78 % to the reflected waves of x or y. The cross-polarized reflection phase and magnitude can be controlled separately by the geometry and rotation angle. The proposed metasurface polarization capability shows wide-angle incidents from 0⁰ to 45⁰. We designed a model to simulate the ultra-broadband results for x-and y-polarized normal incident waves. The usual metasurface can be discovered promising applications in remote sensing, imaging technology, and reflector antennas operating at microwave frequency.

Switchable multi-functional broadband polarization converter in terahertz band.

In this work, we propose a multi-functional broadband terahertz polarization converter based on graphene-VO2 hybrid metamaterial, which can switch between transmissive linear-to-linear conversion and reflective linear-to-circular conversion. The function of the metamaterial can be controlled by both the temperature and the Fermi energy of the graphene. At 298K, the metamaterial converts the y-polarized wave into x-polarized wave in 0.39-1.22THz. In the meanwhile, changing the Fermi energy of graphene, the converted polarization angle can be tuned from 90° to 45°. Increasing the temperature to 358K, the incident linearly polarized wave is reflected into circularly polarized wave. On this condition, tuning the Fermi energy, the metamaterial can separately convert the linear polarization wave into left-circularly polarized wave in 1.57-2.74THz and right-circularly polarized wave in 1.13-1.59THz. Such a switchable multi-functional broadband polarization converter may achieve potential applications in compact terahertz devices and integrated terahertz circuits.