Terahertz polarization conversion and sensing with double-layer chiral metasurface

The terahertz(THz)resonance,chirality,and polarization conversion properties of a double-layer chiral metasurface have been experimentally investigated by THz time domain spectroscopy system and polarization detection method.The special symmetric geometry of each unit cell with its adjacent cells makes a strong chiral electromagnetic response in this metasurface,which leads to a strong polarization conversion effect.Moreover,compared with the traditional THz transmission resonance sensing for film thickness,the polarization sensing characterized by polarization elliptical angle(PEA)and polarization rotation angle(PRA)shows a better Q factor and figure of merit(FoM).The results show that the Q factors of the PEA and PRA reach 43.8 and 49.1 when the interval film is 20µm,while the Q factor of THz resonance sensing is only 10.6.And these PEA and PRA can play a complementary role to obtain a double-parameter sensing method with a higher FoM,over 4 times than that of resonance sensing.This chiral metasurface and its polarization sensing method provide new ideas for the development of high-efficiency THz polarization manipulation,and open a window to the high sensitive sensing by using THz polarization spectroscopy.

Continuous terahertz omnidirectional beam steering by dual diffraction of metagratings

Dynamic beam steering with unlimited angular range and fast speed remains a challenge in the terahertz gap,which is urgently needed for next-generation target tracking,wireless communications,and imaging applications.Different from metasurface phased arrays with element-level phase control,here we steer the beam by globally engineering the diffraction of two cascaded metagratings during in-plane rotation.Benefiting from large-angle diffraction and flexible on/off control of the diffraction channels,a pair of metagratings with optimized supercells and proper orientation successfully directs the incoming beam towards any arbitrary direction over the transmission half space,with the steering speed improved more than twice that of the small-angle diffractive designs.Single-beam and dual-beam steering within the solid angle of 1.56πand elevation angle of±77°has been demonstrated with average throughput efficiency of 41.4%at 0.14 THz,which can be generalized to multiple-beam cases.The dual diffraction engineering scheme offers a clear physical picture for beamforming and greatly simplifies the device structure,with additional merits of large aperture and low power consumption.

Dynamic terahertz anisotropy and chirality enhancement in liquid-crystal anisotropic dielectric metasurfaces

To enhance and actively control terahertz(THz) anisotropy and chirality, we have designed and fabricated a THz composite device with a liquid crystal(LC) layer and Si anisotropic metasurface. By initial anchoring and electrically rotating the spatial orientation of the LC optical axis, the different symmetry relationships are obtained in this hybrid device. When the optical axis of LC is parallel or perpendicular to the optical axis of the Si metasurface, the anisotropy of the device will be enhanced or offset, which leads to a tunable phase-shift range of more than 180°. When there is an angle between the two optical axes, due to the destruction of the mirror symmetry in the LC-Si anisotropic medium, the highest circular dichroism of the device reaches 30 dB in the middle orientation state of the LC optical axis, and the active modulation can be realized by changing the bias electric field on the LC layer. This composite device demonstrates rich characteristics for the feasible manipulation of THz polarization conversion and chiral transmission, which can be applied in THz polarization imaging and chiral spectroscopy.

Active terahertz beam manipulation with photonic spin conversion based on a liquid crystal Pancharatnam–Berry metadevice

Active terahertz (THz) beam manipulation is urgently needed for applications in wireless communication, radar detection, and remote sensing. In this work, we demonstrate a liquid crystal (LC) integrated Pancharatnam–Berry(PB) metadevice for active THz beam manipulation. Through theoretical analysis and simulation design, the geometric phase of the PB metasurface is engineered to match the tunable anisotropic phase shift of LCs under an external magnetic field, and dynamic beam deflection accompanied by spin conversion is obtained. The experimental results show that the device realizes a dynamic modulation depth of >94% and maximum efficiency of over 50% for the different spin states. Moreover, due to the broadband operating characteristics of devices at 0.7–1.3 THz, the deflection angles are frequency dependent with a scanning range of over ±20° to ±32.5°.Moreover, the two conjugate spin states are always spatially separated in different deflection directions with an isolation degree of over 10 dB. Therefore, this metadevice provides a scheme of active THz beam deflection and spin state conversion, and it also achieves both controllable wavelength division multiplexing and spin division multiplexing, which have important potential in large-capacity THz wireless communication.