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We experimentally demonstrate an active terahertz (THz) resonance switch induced by the polarization conversion in a compound metasurface, which is a LC layer sandwiched by a metallic wire grating and resonance metamaterial (LCGM). Here, the liquid crystal (LC) plays the role of polarization conversion, which can induce the TE resonance. Moreover, there exists a localized resonance between metallic grating and metamaterial layers, and then the excited resonance will be greatly enhanced. The results show that the high extinction ratio of the resonance switch exceeds 30 dB at 0.82 THz. This work will bring new ideas for the research in developing THz phase, polarization, and switch devices with LC and metasurface.

High-index dielectric metasurfaces are rarely reported around 0.1-0.3 THz, as an extremely large etching depth is needed according to the millimeter-scale wavelength. In this work, we propose an easy solution to sub-THz wideband polarization control by utilizing 3D-printed low-index (n~1.5) metagratings. The metagrating with subwavelength lattice is shown as a very efficient half-wave plate (net polarization conversion of 87%) at 0.14 THz but showing noisy spectrum. The design with superwavelength lattice offers a smooth and wide bandwidth for linear polarization rotation. Study of the mechanism shows that the lattice size slightly above wavelength is a better choice for the low-index metadevice as it maintains high efficiency in the zero diffraction order and wide bandwidth due to the small mode dispersion. Such designs offer a feasible solution especially suitable for sub-THz polarization and phase control, complementary to the existing high-index dielectric and metallic metasurfaces.

Manipulation of terahertz (THz) wave plays an important role in THz imaging, communication, and detection. The difficulty in manipulating the THz wave includes single function, untunable, and inconvenient integration. Here, we present a mechanically tunable THz polarizer by using stretchable buckled carbon nanotube sheets on natural rubber substrate (BCNTS/rubber). The transmittance and degree of polarization of THz wave can be modulated by stretching the BCNTS/rubber. The experiments showed that the degree of polarization increased from 17% to 97%, and the modulation depth reached 365% in the range of 0.2-1.2 THz, as the BCNTS/rubber was stretched from 0% to 150% strain. These changes can be also used for high strain sensing up to 150% strain, with a maximum sensitivity of 2.5 M/S. A spatial modulation of THz imaging was also realized by stretching and rotating BCNTS/rubber. The theoretical analysis and numerical modeling further confirm the BCNTS/rubber changes from weak anisotropic to highly anisotropic structure, which play key roles in THz wave modulation. This approach for active THz wave manipulation can be widely used in polarization imaging, wearable material for security, and highly sensitive strain sensing.

Developing the broadband controllable or tunable terahertz (THz) polarization and phase devices are in an urgent need. In this paper, we demonstrate a broadband controllable THz quarter-wave plate (QWP) with double layers of graphene grating and a layer of liquid crystals. The double layer graphene gratings can achieve a switchable QWP to switch between linear-to-linear and linear-to-circular polarization states with over 0.35THz bandwidth in the ON or OFF state by applying biased electric field on the graphene grating or not. Moreover, this QWP based on the structure of periodic gradient grating can significantly enhance the phase difference between two orthogonally polarized components compared to that based on equal-periodic grating structure because of the additional phase distribution of the gradient structures. Furthermore, we incorporate liquid crystals into the graphene grating to form a tunable QWP, of which operating frequency can be continuously tuned in a wide frequency range by electrically controlling the molecular director of the liquid crystals. The results show that the graphene periodic gradient grating with LCs not only broadens the operating bandwidth, but also reduces the external electric field. This device offers a further step in the development of THz polarization and phase devices for potential applications in THz polarized imaging, spectroscopy, and communication.

We investigated THz nonreciprocal circularly polarized transmission in thin longitudinally magnetized InSb film, especially focusing on its non-eigen nonreciprocal transmission mechanism at room temperature. Then, based on this effect, we presented a THz isolator for linear polarized waves. The nonreciprocal transmission of the InSb film in this device is converted and enhanced by a pair of orthogonal artificial birefringence gratings. After the optimization, the isolation reaches 24 dB, and the insertion loss is <0.5  dB at room temperature and a low magnetic field.

In this work, the optically anisotropic property of dual-frequency liquid crystals (DFLC) in terahertz (THz) regime has been experimentally investigated, which indicates that the refractive index and birefringence of DFLC can be continuously modulated by both the alternating frequency and intensity of the alternating electric field. This tunability originates from the rotation of DFLC molecules induced by alternating electric fields. The results show that by modulating the alternating frequency from 1 kHz to 100 kHz under 30 kV/m electric field, the 600 μm thickness DFLC cell can play as a tunable quarter-wave plate above 0.68 THz, or a half-wave plate above 1.33 THz. Besides, it can be viewed as a tunable THz phase shifter from 0 to π. Therefore, due to its novel tuning mechanism, DFLC will be of great significance in dynamic manipulating on THz phase and polarization.

A dielectric metasurface with line-square compound lattice structure has been fabricated and demonstrated in the terahertz (THz) regime by the THz time-domain spectroscopy and numerical simulation. A polarization dependent electromagnetically induced transparency (EIT) effect is achieved in this metasurface due to the mode coupling and interference between the resonance modes in line and square subunits of the metasurface. Accompany with the EIT effect, a large artificial birefringence effect between two orthogonal polarization states is also observed in this compound metasurface, of which birefringence is over 0.6. Furthermore, the liquid crystals are filled on the surface of this dielectric metasurface to fabricate an electrically tunable THz LC phase shifter. The experimental results show that its tunable phase shift under the biased electric field reaches 0.33π, 1.8 times higher than the bare silicon, which confirms the enhancement role of THz microstructure on the LC phase shift in the THz regime. The large birefringence phase shift of this compound metasurface and its LC tunable phase shifter will be of great significance for potential applications in THz polarization and phase devices.

In this work, we investigate the nonreciprocal circular dichroism for terahertz (THz) waves in magnetized InSb by the theoretical calculation and numerical simulation, which indicates that longitudinally magnetized InSb can be applied to the circular polarizer and nonreciprocal one-way transmission for the circular polarization THz waves. Furthermore, we propose a double-layer magnetoplasmonics based on the longitudinally magnetized InSb, and find two MO enhancement mechanisms in this device: the magneto surface plasmon resonance on the InSb-metal surface and Fabry-Pérot resonances between two orthogonal metallic gratings. These two resonance mechanisms enlarge the MO polarization rotation and greatly reduce the external magnetic field below 0.1T. The one-way transmission and perfect linear polarization conversion can be realized over 70dB, of which the transmittance can be modulated from 0 to 80% when the weak magnetic field changes from 0 to 0.1T under the low temperature around 200K. This magnetoplasmonic device has broad potential as a THz isolator, modulator, polarization convertor, and filter in the THz application systems.

The state conversion and terahertz (THz) wave modulation based on a plasmonic device composed of silicon column arrays with vanadium dioxide (VO2) coating were experimentally demonstrated. For double 45° tilted optical pumping, a state conversion from dielectric photonic crystal (PC) to metallic PC was demonstrated due to the insulator-metal transition (IMT) of VO2 with the pump power increasing. In this process, a broadband intensity modulation with 70% modulation depth was achieved. Furthermore, for normally incident optical pumping, another state conversion from dielectric PC to plasmonic device was also demonstrated due to the partial IMT of VO2, and the out of plane PC resonance gradually changed to be plasmonic resonances. This device and its modulation scheme will be of great significance for potential THz applications.

A tunable metal/magneto-optic plasmonic lens for terahertz isolator is demonstrated. Based on the magneto-optical effect of the semiconductor material and non-symmetrical structure, this plasmonic lens has not only the focusing feature but also nonreciprocal transmission property. Moreover, a transmission enhancement through this device greatly larger than that of the ordinary metallic slit arrays is contributed by the extraordinary optical transmission effect of the magneto surface plasmon polaritons. The results show that the proposed isolator has an isolation bandwidth of larger than 0.4THz and the maximum isolation of higher than 110dB, and its operating frequency also can be broadly tuned by changing the external magnetic field or temperature. This low-loss, high isolation, broadband tunable nonreciprocal terahertz transmission mechanism has a great potential for terahertz application systems.

A terahertz modulator based on the insulator-metal transition (IMT) in a photonic crystal waveguide (PCW) coated by vanadium dioxide (VO2) film is proposed. The numerical simulations show that a dielectric state and a metallic state with quite different photonic band structures and transmission properties in the proposed PCW are reciprocally converted because of the IMT of VO2, and the pass-bands of this PCW are greatly shifted from 0.68 to 0.8 and 1.02 to 1.25 THz to 0.8-1.45 THz. This PCW significantly enhances the modulation depth and sensitivity compared with bare VO2 film. Extensive investigation demonstrates that the thickness of VO2 film greatly affects the IMT process in the PCW, and limits the ultimate modulation depth of the device. The proposed modulation scheme will be of great significance for potential THz applications.