High Polarization Conversion Efficiency Research Articles

Simple and hybrid metalens with high polarization conversion efficiency for near-infrared spectrum

Thanks to their ability to control light, research on metalenses is developing rapidly. However, it is still quite difficult to design broadband metalenses with high polarization conversion efficiency. In this study, an alternative plasmonic material, aluminum-doped zinc oxide, is presented for metalens operating in near-infrared regime. We propose simple and hybrid metalenses with high polarization conversion efficiency and high transmission values that can focus efficiently in a wide near-infrared bandwidth (700–1000 nm). We design metalens consisting of subwavelength aluminum-doped zinc oxide nanoblocks based the Pancharatnam-Berry phase method and utilizing the finite-difference time-domain method. The polarization conversion efficiency (minimum 87 %) and transmission values (minimum 85 %) calculated for the metalens unit cell are higher than those previously obtained over the entire 300 nm bandwidth. In addition, we propose hybrid metalens to focus the incident beam in right and left handed circular polarized states in the studied frequency range. The presented aluminum-doped zinc oxide metalens with high polarization conversion efficiency and transmission values can find a place in the applications of near-infrared nanophotonic systems.

Vanadium dioxide-assisted switching of a half-wave and quarter-wave plate to a dual-band absorber for the terahertz wave

In this study, we present a broadband, multi-functional, and switchable metamaterial for terahertz (THz) waves. The metamaterial design achieves high polarization conversion efficiency and can seamlessly transition between a polarization converter (PC) and a dual-band absorber through the phase transition behavior of vanadium dioxide (VO2). Our design facilitates linear to-cross (Half-wave plate) and linear to-circular (Quarter-wave plate) polarization conversion across a bandwidth extending from 0.58 THz to 0.88 THz, and 0.35 THz to 0.50 THz respectively. Near-perfect absorption at 0.35 THz and 0.99 THz was observed when VO2 changed its phase from an insulator to a conductor. These broadband features are realized through a straightforward metamaterial design incorporating only a reflector, a dielectric spacer, and an antenna (meta-atom resonator). Our designed structure demonstrates impressive angular stability for incident angles ranging from 0° to 45°. The proposed switchable metamaterial holds promise for advancing the development of tunable polarization rotating devices and on/off switching LPC devices, offering broad applications in THz detection, sensing, and communications.

Ultra-broadband and thin switchable multifunctional metamaterial for terahertz wave

An ultra-broadband thin multi-functional and switchable metamaterial is examined in the terahertz (THz) regime. The proposed design achieved high polarization conversion efficiency and can be switched from a polarization converter to an absorber using the phase change transitioning of vanadium dioxide (VO2). The linear polarization conversion is achieved from 6.0 THz to 15.0 THz, achieving a bandwidth of 9.0 THz, and the absorption is realized from 5.0 THz to 16.0 THz with a bandwidth of 11.0 THz. These broadband characteristics were achieved by a simple metamaterial design incorporating a few layers. The relative bandwidth was 85% for the polarization conversion and 105% for the absorption. Moreover, the angular stability of the designed structure is impressive for various incident angles from 0° to 45°. The proposed switchable design has the potential to contribute to the development of tunable polarization rotating devices, on/off switching LPC devices, which have wide application potentials in THz detection, sensing, adaptive optics, and communications.

Ultrathin Bi‐Switchable Vanadium Dioxide‐Based Multifunctional Metamaterial for Terahertz Polarization Modulation

AbstractA broadband multi‐functional bi‐switchable ultrathin optical metamaterial design is investigated, which can achieve a high polarization conversion efficiency over a wide band of frequencies in the terahertz (THz) regime. The proposed design switches from a linear and circular polarization converter (LPC/CPC) to an efficient absorber by using the phase change property of a functional material, vanadium dioxide (VO2). The proposed design shows switching from an absorber to a reflector using the mechanical configuration of the functional material, and therefore the design can be used as a bi‐switchable device. The first type of switching is accomplished by changing the conductivity of the functional material, while the second type of switching is achieved with the help of the mechanical configuration of the functional material. Moreover, the structure is responsive to wide incident angles. The proposed switchable design has the potential to contribute to the development of tunable polarization rotating devices and on/off switching LPC devices, which have wide applications in detection, sensing, and communications.

A metasurface-based full-color circular auto-focusing Airy beam transmitter for stable high-speed underwater wireless optical communications

Due to its unique intensity distribution, self-acceleration, and beam self-healing properties, Airy beam holds great potential for optical wireless communications in challenging channels, such as underwater environments. As a vital part of 6G wireless network, the Internet of Underwater Things requires high-stability, low-latency, and high-capacity underwater wireless optical communication (UWOC). Currently, the primary challenge of UWOC lies in the prevalent time-varying and complex channel characteristics. Conventional blue Gaussian beam-based systems face difficulties in underwater randomly perturbed links. In this work, we report a full-color circular auto-focusing Airy beams metasurface transmitter for reliable, large-capacity and long-distance UWOC links. The metasurface is designed to exhibits high polarization conversion efficiency over a wide band (440-640 nm), enabling an increased data transmission rate of 91% and reliable 4 K video transmission in wavelength division multiplexing (WDM) based UWOC data link. The successful application of this metasurface in challenging UWOC links establishes a foundation for underwater interconnection scenarios in 6G communication.

Switchable visible and near-infrared light bifocal lens based on hybrid graphene-metasurface structures

We propose a wideband visible (VIS) and near infrared (NIR) light reflective flat lens with switching capability base on hybrid graphene-metasurface structure. This tunable flat lens structure consists of a slotted gold reflector substrate, a graphene monolayer and TiO2 nanofins that provide excellent focusing performance and tunability in the VIS and NIR range. In order to achieve the switching feature, a bi-functional device capable of switching reflection/absorption modes is required. To this end, the gold substrate with two cross slots has been used to create absorption properties and a graphene layer has been used to change the intensity of absorption and reflection. The switching property for incident light with circular polarization arises from uniformly changing the chemical potential of the graphene layer. By setting the chemical potential of graphene, μc, equal to 1.1 eV, we obtained the reflection property. The reflection mode exhibits a broadband (715–800 nm) optical response with high polarization conversion efficiency (more than 70 %). On the other hand, by choosing μc = 0.75 eV, the absorption feature is created at the wavelength of 700–780 nm. The reflected light can be focused on an arbitrary position above the structure by rotating the TiO2 nanofins with the help of Pancharatnam-Berry (PB) phase theory. In addition to obtaining the single and dual focus tunable flat lenses, we have also presented a multifunction configuration that can switch between different modes, such as non-focus, single and dual focus modes. Such switchable flat lenses can have various applications in imaging and sensing systems.

Active anisotropic polarization conversion meta-surface for 6G communication bandgap in the reflection mode

This paper investigates a single-layered active anisotropic meta-surface design capable of achieving high polarization conversion efficiency in the 6 G communication band gap. Additionally, the designed structure can convert the linear polarization of the incoming wave fronts to its cross-polarization and linear polarization to circular polarization in the reflection mode. The polarization conversion characteristic is achieved due to the strong anisotropic behavior of the meta-surface. Such a simple design can achieve a 90% to 100% cross-polarization conversion efficiency over a frequency region of [0.28–0.36] THz. The linear to circular polarization conversion is performed near 0.26 THz and 0.40 THz. Between these two frequencies, band linear to cross-polarization conversion is more prominent. The proposed design can be tuned from a half-wave plate (HWP) or linear polarization converter to a quarter-wave plate (QWP) or circular polarization converter using the chemical potential of graphene. The optical response of the meta-surface behaves differently from different incident angles. The proposed polarization converter can contribute to the development of active polarization device.

Broadband terahertz metasurface with three switchable functions based on complementary structure

In this work, we propose a switchable multifunctional metasurface based on complementary VO2-graphene structure. Tuning the state of VO2 and the Fermi level of graphene, the metasurface can act as broadband half-wave plate (HWP), quarter-wave plate (QWP) or absorber in the same frequency range of 0.65–1.27THz, respectively. In the V1G0 state, the metasurface behaves as a broadband HWP to realize linear-to-linear polarization conversion with the polarization conversion ratio >0.9 in the range of 0.64–1.37THz. In the V0G1 state, the metasurface acts as a broadband QWP achieving linear-to-circular polarization conversion with the ellipticity below −0.9 in the range of 0.65–1.27THz. In the V1G1 state, the metasurface is a broadband absorber with the absorption over 0.9 in the 0.63–1.32THz range. In addition, the physical mechanisms of the metasurface in different states are analyzed through the field and current distributions. Such a switchable multi-functional metasurface can simultaneously achieve high efficiency polarization conversion and absorption, which has potential applications in the integrated terahertz devices.

A Metasurface-Based LTC Polarization Converter with S-Shaped Split Ring Resonator Structure for Flexible Applications.

This paper presents a metasurface-based linear-to-circular polarization converter with a flexible structure for conformal and wearable applications. The converter consists of nested S- and C-shaped split ring resonators in the unit cell and can convert linearly polarized incident waves into left-handed circularly polarized ones at 12.4 GHz. Simulation results show that the proposed design has a high polarization conversion rate and efficiency at the operating frequency. Conformal tests are also conducted to evaluate the performance under curvature circumstances. A minor shift in the operating frequency is observed when the converter is applied on a sinusoidal wavy surface.

Dual-broadband high-efficiency polarization conversion metasurface based on asymmetric transmission

A dual-broadband and high-efficiency polarization conversion metasurface based on asymmetric transmission is proposed, which is composed of two orthogonal metal gratings, metal resonator and FR-4 dielectric spacer. The simulated results show that the designed metasurface can convert x-polarized incident waves into y-polarized waves in the frequency ranges of 4.64–14.37 GHz and 17.02–37.23 GHz, and the corresponding polarization conversion ratio (PCR) exceeds 90%. The relative bandwidths of the two bands are 102.37% and 74.51%, respectively. The influence of structural parameters on the transmissive polarization conversion performance is studied. The bandwidths of the two bands can be easily modulated by varying the structural parameters of l, n, m and w. The excellent performance of dual-broadband and high-efficiency covers C, X, Ku, K and Ka bands, which can also be well maintained in the oblique incidence range of 0°–50°. Moreover, the mechanism of transmissive polarization conversion is analyzed. The prototype is fabricated and the experimental results are basically consistent with the numerical simulation. The proposed metasurface has potential application value in polarization imaging radar or radiometer, radar, transmission array antennas and filter.

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.

Wideband circular polarizer for a photoconductive antenna.

We report a thin-film circular polarizer consisting of three metal-grid layers to be used with a photoconductive antenna (PCA) to generate terahertz (THz) circularly polarized (CP) radiation. The polarizer has a high transmission with a measured 3 dB axial-ratio bandwidth of 54.7% from 0.57 to 1 THz. We further developed a generalized scattering matrix approach to provide insight into the underlying physical mechanism of the polarizer. We revealed that the Fabry-Pérot-like multi-reflection among gratings enables the high-efficiency polarization conversion. The successful realization of the CP PCA can find widespread application, such as THz circular dichroism spectroscopy, THz Mueller imaging, and ultrahigh-speed THz wireless communications.

Multifunctional and switchable metamaterial for terahertz polarization modulation in the reflection mode.

In this paper, a broadband multi-layered active metamaterial design is investigated, which can achieve a high polarization conversion efficiency over a wide band of frequencies in the terahertz regime. The design can be switched to an efficient metamaterial absorber using the phase transition property of vanadium dioxide (V O 2). Additionally, the designed structure can convert the linear polarization of the incoming wavefronts to its cross-polarization and linear polarization to circular polarization in the reflection mode. The broadband characteristic is achieved due to the strong anisotropic behavior of the metasurface. The structure is robust to a wide range of incident angles as well. The proposed switchable multifunctional design can contribute to the development of active plasmonic polarization devices and metamaterial absorbers.

Ge3Sb2Te6-based mid-infrared metasurfaces for polarization control and wavefront steering

Benefiting from excellent properties in wavefront control, germanium antimony telluride (GeSbTe)-based photonic devices provide new opportunities for manipulating electromagnetic wave. In this paper, Ge3Sb2Te6 meta-atoms are presented to realize polarization switching for mid-infrared wave through the state transition from the crystalline Ge3Sb2Te6 to the amorphous Ge3Sb2Te6. When the crystalline Ge3Sb2Te6 is involved, the proposed meta-atoms with 90° phase shift achieve high-efficiency polarization conversion and 360° phase coverage. As Ge3Sb2Te6 is changed to the amorphous state, phase coverage drops to only 36°, and most of the cross-polarized wave vanishes. Using these designed meta-atoms, three metasurfaces are implemented at 76.5 THz. Firstly, a gradient metasurface is constructed, and it dynamically switches between specular reflection and anomalous reflection. Next, a reflective metalens is proposed to realize switching between focusing and defocusing under different states of Ge3Sb2Te6. Lastly, a focused vortex beam is presented to reconstruct the mode of orbital angular momentum (OAM). All designs realize the switching between cross-polarization and co-polarization. Our work could have possible applications in fields such as mid-infrared switching, focusing, and wireless communication.

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.

Self-design of arbitrary polarization-control waveplates via deep neural networks

The manipulation of polarization states beyond the optical limit presents advantages in various applications. Considerable progress has been made in the design of meta-waveplates for on-demand polarization transformation, realized by numerical simulations and parameter sweep methodologies. However, due to the limited freedom in these classical strategies, particular challenges arise from the emerging requirement for multiplex optical devices and multidimensional manipulation of light, which urge for a large number of different nanostructures with great polarization control capability. Here, we demonstrate a set of self-designed arbitrary wave plates with a high polarization conversion efficiency. We combine Bayesian optimization and deep neural networks to design perfect half- and quarter-waveplates based on metallic nanostructures, which experimentally demonstrate excellent polarization control functionalities with the conversion ratios of 85% and 90%. More broadly, we develop a comprehensive wave plate database consisting of various metallic nanostructures with high polarization conversion efficiency, accompanying a flexible tuning of phase shifts (0–2π) and group delays (0–10 fs), and construct an achromatic metalens based on this database. Owing to the versatility and excellent performance, our self-designed wave plates can promote the performance of multiplexed broadband metasurfaces and find potential applications in compact optical devices and polarization division multiplexing optical communications.

Flexible Metamaterial Quarter-Wave Plate and Its Application in Blocking the Backward Reflection of Terahertz Waves.

A terahertz flexible metamaterial quarter-wave plate (QWP) is designed and fabricated using polyimide as the substrate in this paper, with a 3 dB axial ratio bandwidth of 0.51 THz and high polarization conversion efficiency and transmittance. The effect of the incidence angle on the polarization conversion performance of the QWP is discussed by measuring the transmissions at multiple incidence angles. The blocking effect of this QWP combined with a polarizer on the backward reflection of terahertz waves is investigated by terahertz time-domain spectral transmission experiments. By adjusting the angle of the QWP and polarizer with respect to the incident light in the optical path, a blocking efficiency of 20 dB can be achieved at a 20° incidence angle, with a bandwidth of 0.25 THz, a maximum blocking efficiency of 58 dB at 1.73 THz, and an insertion loss of only 1.4 dB. Flexible terahertz metamaterial QWPs and polarizers can effectively block harmful reflected waves in terahertz communication and other systems. They have the advantages of a simple structure, ultra-thinness and flexibility, easy integration, no external magnetic field, and no low-temperature and other environmental requirements, thus having broad application prospects for terahertz on-chip integrated systems.

A reconfigurable asymmetric-transmission metasurface for dynamic manipulation of transmission, reflection, and polarization

A broadband reconfigurable metasurface consisting of perpendicular metallic gratings and a PIN diode-driven split circular ring for polarization and propagation modes control is demonstrated. The designed metasurface possesses tunable properties in two propagation modes. In the “OFF” state, it can convert incident linear-polarized waves into transmitted cross-polarized waves with full transmission. In the “ON” state, it can reflect incident linear-polarized waves into co-polarized waves with full reflection. Without the increasing number of active elements commonly utilized in chirality-type metasurface polarizers, the equivalent transformation of the split circular ring can be achieved by switching the single PIN diode, and high polarization-conversion efficiency is guaranteed. A metasurface array with 10 × 10 unit cells is fabricated and measured. The experimental results show that the tunable transmission, reflection, and polarization properties of the proposed reconfigurable metasurface can be achieved from 4.9 to 7 GHz.

Vanadium dioxide-based terahertz metasurfaces for manipulating wavefronts with switchable polarization

As a unique phase transition material, vanadium dioxide (VO2) has been widely applied in switchable and modulating metasurfaces. In this work, VO2-based reflective meta-atoms are designed to achieve wavefront reconfiguration in terahertz frequency. When VO2 is metal, four optimized meta-atoms with 90o phase difference provide a full phase coverage, and they achieve high polarization conversion efficiency. After phase transition of VO2, the capability in the metallic state is lost. Thus, the corresponding phase coverage is switched from 360° to 0°, and the phenomenon of polarization conversion almost vanishes. Assembled by these meta-atoms, three metasurfaces are numerically presented. When VO2 is in the state of metal, they are anomalous reflector, metalens, and vortex beam generator. The presented metasurfaces own the abilities of wavefront manipulation and polarization conversion. When VO2 is in the insulating state, three metasurfaces are switched to specular reflector. Simulations are conducted to verify their abilities of wavefront modulation and polarization control. The proposed metasurfaces may have potential applications in the fields of terahertz switching, communication, and focusing.

Reconfigurable broadband metasurfaces with nearly perfect absorption and high efficiency polarization conversion in THz range

Reconfigurable metasurfaces (RMSs) that enable the switching function of absorption and polarization conversion have attracted increasing attention. However, the design of RMSs to achieve wideband and high efficiency for both absorption and polarization conversion functions simultaneously remains a great challenge. Here, we propose the design of a RMS structure with a high-efficiency cross-polarization conversion and nearly perfect absorption. The reconfiguration between different functions of polarization conversion and absorption is obtained based on the reversible insulator-to-metal phase transition of Vanadium dioxide (VO_{2}). When the VO_{2} is in insulator state, the RMS realizes the cross-polarization conversion function in the wideband of 1.04–3.75 THz with a relative bandwidth up to 113 % due to the multi-resonant modes of electric and magnetic resonances. Meanwhile, the nearly-perfect absorption is achieved in the range of 1.36–3.38 THz with the corresponding relative bandwidth up to 85 % for the VO_{2} in metallic state. Specially, the wideband and high-efficiency performance of these functionalities is maintained for a wide angle incidence. The capability of bi-functional switch and integration with polarization conversion and absorption in a single metasurface structure endowed with both wideband and high-efficiency characteristics for a wide incident angle is very promising for emerging RMS devices in the terahertz region.