Switchable broadband and wide-angular terahertz asymmetric transmission based on a hybrid metal-VO2 metasurface

Multi-functional terahertz metasurface for a vortex beam, multi-channel focusing, polarization conversion, and broadband absorption based on vanadium dioxide

Although terahertz metasurface devices have been widely studied, thus far, metasurfaces can rarely manipulate both circularly and linearly polarized incident waves. In this paper, taking advantage of the phase transition characteristics of vanadium dioxide (V O 2), a multi-functional terahertz metasurface for a vortex beam, multi-channel focusing, polarization conversion, and broadband absorption is proposed. When V O 2 is in the insulating state, a vortex beam is generated at 1.2 THz when the circularly polarized wave is incident on the metasurface. Meanwhile, the multi-channel focusing is realized at 1.0 THz, and the cross-polarization conversion rate can reach more than 90% at the frequencies of 0.6 THz, 1.1 THz, and 1.6 THz when the y-polarized wave is incident vertically. When V O 2 is in the metallic state, the metasurface achieves close to 95% absorption in the range of 0.8-1.5 THz. The designed metasurface has tunability and multi-functional characteristics, which have potential applications in wireless communication.

Multi-functional device: manipulating linear and circular-polarization conversion in a terahertz chiral metamaterial

We propose a terahertz chiral metamaterial as a multi-functional device to manipulate asymmetry transmission of linear polarized waves, linear-to-elliptical polarization conversion and circular dichroism in transmission mode while asymmetry reflection of circular polarized waves. For incidence of linear polarized waves, dual-band asymmetry transmission is shown around 0.42 THz and 1.04 THz where asymmetry transmission factors reach up to two peak values: ∼0.51 and ∼0.55, respectively. Intense linear-to-elliptical polarization conversion occurs at 0.81 THz and 0.97 THz, respectively. For incidence of circular polarized waves, a strong circular dichroism appears at 0.36 THz where circular dichroism parameter reaches to ∼0.64 and asymmetry reflection is displayed around 0.36 THz with the maximum of asymmetry reflection factors approaching to 0.55.

Parity-time symmetry transition and exceptional points in terahertz metal-graphene hybrid metasurface with switchable transmission and reflection characteristics

Non-Hermitian metasurfaces provide an excellent platform for the study of parity-time (PT) symmetry transition. The exceptional point (EP) in the transition process exhibits peculiar physical phenomena and enriches the development of metasurfaces. In this study, a terahertz metal-graphene hybrid metasurface that can study PT symmetry transition and EP in transmission and reflection polarization channels is designed by using the phase transition characteristics of VO₂. The tunable asymmetric loss and PT symmetry transition can be actively controlled by changing the Fermi energy of the graphene strip. Interestingly, owing to the special chirality of the structure, the original metasurface, and the mirrored metasurface degenerate into a circularly polarized state with opposite rotations at the same Fermi energy. The π-phase mutation at EP is observed following the interaction of circularly polarized waves and the metasurface and is expected to have good application prospects in environmental monitoring and gas sensing.

Bidirectional Terahertz Vortex Beam Regulator

Most of the reported vortex beam generators with orbital angular momentum (OAM) in the terahertz region only operate in either the reflection mode or the transmission mode, which greatly limits the integration and application in terahertz technology systems. Herein, we propose a full-space vortex beam regulator at two different frequencies. By changing the VO₂ phase transition state, the transmission and reflection mode OAM beams can be flexibly controlled by a single metasurface. For the transmission mode, the proposed structure realizes an OAM beam at the topological charges of l = 1 and 2 at 0.6 THz and 1.4 THz. For the reflection mode, our structure generates an OAM beam at the topological charges of l = 1 and 2 at 0.9 THz and 1.5 THz. Based on the superposition theorem and convolution operation principle, the regulation of an OAM vortex beam with a specific deflection angle and a symmetrical deflection OAM vortex beam are realized. The designed metasurface integrates multiple transmitted and reflected vortex beam functions in full space and has potential application in different terahertz systems.

Nonvolatile chirality switching in terahertz chalcogenide metasurfaces

Actively controlling the polarization states of terahertz (THz) waves is essential for polarization-sensitive spectroscopy, which has various applications in anisotropy imaging, noncontact Hall measurement, and vibrational circular dichroism. In the THz regime, the lack of a polarization modulator hinders the development of this spectroscopy. We theoretically and experimentally demonstrate that conjugated bilayer chiral metamaterials (CMMs) integrated with Ge₂Sb₂Te₅ (GST225) active components can achieve nonvolatile and continuously tunable optical activity in the THz region. A THz time-domain spectroscopic system was used to characterize the device, showing a tunable ellipticity (from ‒36° to 0°) and rotation of the plane polarization (from 32° to 0°) at approximately 0.73 THz by varying the GST225 state from amorphous (AM) to crystalline (CR). Moreover, a continuously tunable chiroptical response was experimentally observed by partially crystallizing the GST225, which can create intermediate states, having regions of both AM and CR states. Note that the GST225 has an advantage of nonvolatility over the other active elements and does not require any energy to retain its structural state. Our work allows the development of THz metadevices capable of actively manipulating the polarization of THz waves and may find applications for dynamically tunable THz circular polarizers and polarization modulators for THz emissions.

Asymmetric interface excited chirality and its applications in reconfiguration

In this paper, a chiral excitation method based on the asymmetric interface condition is proposed. The chiral characteristics of the metamaterials are affected by the difference in the environmental parameters of the front and rear surfaces. Thus, the device can achieve functional reconfiguration and two applications based on this mechanism are presented, one for sensing and the other for chiral switching. At the same time, a self-calibration measurement method that greatly simplifies the sensing system is proposed. These results have potential applications in the fields of chirality excitation, bio-sensing, and reconfigurable device.

VO2-assisted multifunctional metamaterial for polarization conversion and asymmetric transmission

A VO₂-assisted temperature-controlled multifunctional metamaterial polarization converter with large asymmetric transmission (AT) is proposed by introducing a gold-VO₂ grating. The converter can be switched between reflection mode and transmission mode by controlling the phase transition. When VO₂ is in the metallic state, the converter works in reflection mode, converting the incident forward/backward linearly/circularly polarized waves into the cross-polarized waves, and the broadband polarization conversion rates (PCRs) can reach 90% with relative bandwidth of up to 91.1% and 87.5%, respectively; when VO₂ is in the insulating state, the converter shows giant AT effect for circularly polarized waves at 0.64 THz and 1.28 THz. The multifunctional polarization converter holds great potential in the fields of communication and imaging, which provides a new way to design optical devices such as polarizers, isolators.

Enhanced circular dichroism of cantilevered nanostructures by distorted plasmon

Chiral structures have a wide range of applications, such as biometric identification, chemical analysis, and chiral sensing. The simple fabrication process of chiral nanostructures that can produce a significant circular dichroism (CD) effect remains a challenge. Here, a three-dimensional (3D) cantilever-shaped nanostructure, which inherits the chiral advantages of 3D nanostructures and simplicity of 2D nanostructures, is proposed. The nanostructure can be prepared by the combination of one-time electron beam lithography and oblique-angle deposition and consists of a thin metal film with periodic holes such that two hanging arms were attached to the edges of holes. The length of the cantilever and the height difference between the two arms can be adjusted by controlling the tilt angle of beam current during the deposition processes. Numerical calculations showed that the enhancement of CD signal was achieved by plasmon distortion on the metal film by the lower hanging part of the cantilever structure. Furthermore, signals can be actively adjusted using a temperature-sensitive polydimethylsiloxane (PDMS) material. The angle between the lower cantilever and the top metal film was regulated by the change in PDMS volume with temperature. The results provide a new way to fabricating 3D nanostructures and a new mechanism to enhance the CD signal. The proposed nanostructure may have potential applications, such as in ultra-sensitive detection and remote temperature readout, and is expected to be an ultra-compact detection tool for nanoscale structural and functional information.

Switchable tri-function terahertz metasurface based on polarization vanadium dioxide and photosensitive silicon

A terahertz switchable metasurface with the function of absorption and polarization conversion is proposed. It consists of metal pattern layer - dielectric layer - VO₂ layer - dielectric layer - metal pattern layer, and the photosensitive silicon is embedded in the metal pattern. When VO₂ is in insulated state, the metasurface behaves as a linear polarization converter. The polarization conversion rate (PCR) is more than 90% at two frequency bands of 1.64 THz ∼ 1.91 THz and 2.35 THz ∼ 2.75 THz. The polarization converter has good asymmetric transmission ability. Moreover, the polarization conversion performance can be dynamically controlled by changing the conductivity of the photosensitive silicon. When VO₂ is in metallic state, the metasurface becomes a terahertz bidirectional absorber, which exhibits different absorption properties under TE and TM waves with the maximum absorptance reaching to 100%. In addition, the absorption of TE and TM terahertz waves can be controlled at the specific frequency by changing the conductivity of photosensitive silicon. We also explore the application of dynamic control of polarization waves in the near-field image display.

Tunable and switchable multi-functional terahertz metamaterials based on a hybrid vanadium dioxide-graphene integrated configuration

In this paper, an actively tunable and switchable multi-functional terahertz metamaterial device based on a hybrid vanadium dioxide (VO₂)-graphene integrated configuration is proposed. By transiting the phase of VO₂, the functions of the proposed device can be reversibly switched between asymmetric transmission (AT) and two different polarization conversions in the terahertz region. When VO₂ operates at the isolating state, the AT effect can be achieved with a maximum value of 0.34 for linearly polarized lights due to the excitation of enantiomerically sensitive plasmons in patterned graphene nanostructures. Furthermore, when VO₂ is transited from the isolating state to the conducting state, the metamaterial does not only exhibit a linear dichroism response but also perform linear-to-linear and linear-to-circular polarization conversions simultaneously. Specifically, the designed device behaves like a half-wave plate, where a linear polarization conversion ratio exceeds 96.5% at a frequency of 9.17 THz. Meanwhile, it acts as a quarter-wave plate which can convert the linear polarization light into left-handed and right-handed circularly polarized lights with high efficiencies at frequencies of 9.04 and 9.3 THz, respectively. Moreover, the performance of the designed structure can be actively controlled by adjusting the geometrical parameters and Fermi energy of graphene. This work provides a new avenue in developing multi-functional terahertz metamaterial devices.

Mechanical modulation of multifunctional responses in three-dimensional terahertz metamaterials

Reconfigurable metamaterials have attracted a surge of attention for their formidable capability to dynamically manipulate the electromagnetic wave. Among the multifarious modulation methods, mechanical deformation is widely adopted to tune the electromagnetic response of the stereotype metamaterial owing to its straightforward and continuous controllability on the metamaterial structure. However, previous morphologic reconfigurations of metamaterials are typically confined in planar deformation that renders limited tunable functionalities. Here we have proposed a novel concept of out-of-plane deformation to broaden the functionalities of mechanically reconfigurable metamaterials via introducing a cross-shaped metamaterial. Our results show that the out-of-plane mechanical modulation dramatically enhances the magnetic response of the pristine metamaterial. Furthermore, by uncrossing the bars of cross-shaped meta-atoms, a L-shaped metamaterial is proposed to verify the effectiveness of such a mechanical method on the handedness switching via changing mechanical loading-paths. More importantly, the differential transmission for circularly polarized incidences can be continuously modulated from -0.45 to 0.45, and the polarization states of the transmission wave can be dynamically manipulated under the linearly polarized illumination. Our proposed mechanical modulation principle might open a novel avenue toward the three-dimensional reconfigurable metamaterials and shows their ample applications in the areas of chiroptical control, tunable polarization rotator and converter.

High-Performance Asymmetric Optical Transmission Based on a Dielectric-Metal Metasurface

Asymmetric optical transmission plays a key role in many optical systems. In this work, we propose and numerically demonstrate a dielectric-metal metasurface that can achieve high-performance asymmetric transmission for linearly polarized light in the near-infrared region. Most notably, it supports a forward transmittance peak (with a transmittance of 0.70) and a backward transmittance dip (with a transmittance of 0.07) at the same wavelength of 922 nm, which significantly enhances operation bandwidth and the contrast ratio between forward and backward transmittances. Mechanism analyses reveal that the forward transmittance peak is caused by the unidirectional excitation of surface plasmon polaritons and the first Kerker condition, whereas the backward transmittance dip is due to reflection from the metal film and a strong toroidal dipole response. Our work provides an alternative and simple way to obtain high-performance asymmetric transmission devices.

Asymmetric transmission for dual-circularly and linearly polarized waves based on a chiral metasurface

We propose a chiral metasurface (CMS) that exhibits asymmetric transmission (AT) of double circularly and linearly polarized waves at the same frequency band. In order to realize the manipulation of electromagnetic (EM) waves in the whole space, the unit cell of CMS consists of three layers of dielectric substrate and four layers of metal patches. The Z-shaped chiral micro-structure and a grating-like micro-structure are proposed and designed to achieve AT. The simulated results show that the x-polarized wave that is incident along one direction can be transmitted into the right-hand circularly polarized (RHCP) wave and the left-hand circularly polarized (LHCP) wave that is incident along the opposite direction can be reflected as the LHCP wave in the frequency band of 4.69GHz-5.84 GHz. The maximum chirality response can be reflected by AT and circular dichroism (CD) and they can reach up to 0.38 and 0.75, respectively. In addition, we also produced the sample of CMS, and the experimental results are in good agreement with the simulated results.

Dynamically reversible and strong circular dichroism based on Babinet-invertible chiral metasurfaces

We propose a Babinet-invertible chiral metasurface for achieving dynamically reversible and strong circular dichroism (CD). The proposed metasurface is composed of a VO₂-metal hybrid structure, and when VO₂ transits between the dielectric state and the metallic state, the metasurface unit cell switches between complementary structures that are designed according to Babinet's principle. This leads to a large and reversible CD tuning range between ±0.5 at 0.97 THz, which is larger than the one found in the literature. We attribute the CD effect to extrinsic chirality of the proposed metasurface. We envision that the Babinet-invertible chiral metasurface proposed here will advance the engineering of active and tunable chiro-optical devices and promote their applications.

Thermally switching between perfect absorber and asymmetric transmission in vanadium dioxide-assisted metamaterials

In this paper, we propose a switchable bi-functional metamaterial device based on a hybrid gold-vanadium dioxide (VO₂) nanostructure. Utilizing the property of a metal-to-insulator transition in VO₂, perfect absorption and asymmetric transmission (AT) can be thermally switched for circularly polarized light in the near-infrared region. When VO₂ is in the metallic state, the designed metamaterial device behaves as a chiral-selective plasmonic perfect absorber, which can result in an optical circular dichroism (CD) response with a maximum value ∼ 0.7. When VO₂ is in the insulating state, the proposed metamaterial device exhibits a dual-band AT effect. The combined hybridization model and electromagnetic field distributions are presented to explain the physical mechanisms of chiral-selective perfect absorption and AT effect, respectively. The influences of structure parameters on CD response and AT effect are also discussed. Moreover, the proposed switchable bi-functional device is robust against the incident angle for obtaining perfect absorption and strong CD response as well as the AT effect. Our work may provide a promising path for the development of multifunctional optoelectronic devices, such as thermal emitters, optical modulators, CD spectroscopy, optical isolator, etc.