Quasi-bound states in the continuum (quasi-BICs) offer an excellent platform for the flexible and efficient control of light-matter interactions by breaking the structural symmetry. The active quasi-BIC device has great application potential in fields such as optical sensing, nonlinear optics, and filters. Herein, we experimentally demonstrate an active terahertz (THz) quasi-BIC device induced by the polarization conversion in a liquid crystal (LC)-integrated metasurface, which consists of a symmetrically broken double-gap split ring resonator (DSRR), an LC layer, and double graphite electrodes. In the process of LC orientation control under the external field, the device realizes the active control from the OFF state to the ON state. In the OFF state, the LC has no polarization conversion effect, and the device behaves in a non-resonant state; but for the ON state, the device exhibits obvious quasi-BIC resonance. Furthermore, we achieve asymmetric transmission based on polarization-induced quasi-BIC modulation precisely at the quasi-BIC resonance position, and its isolation can be controlled by the external field. The study on dynamic quasi-BIC by the LC-integrated metasurface introduces a very promising route for active THz devices, which guarantees potential applications for THz communications, switching, and sensing systems.

A compound metasurface for terahertz (THz) wave polarization mode conversion has been experimentally investigated, which is integrated with an H-shaped metallic metamaterial and 45° arranged subwavelength dielectric grating on the two surfaces of a Si substrate. The polarization mode conversion from a TM to a TE resonance mode is achieved at 1.3 THz for forward transmission and 0.63 THz for backward transmission. Based on this property, a unidirectional transmission is obtained with the highest extinction of 23 dB at 0.63 THz. Moreover, due to the multiple reflections and subwavelength integration, a localized resonance mechanism in this metasurface greatly enhances the polarization conversion rate, reduces the insertion loss, and expands the operating bandwidth from 0.3 to 1.6 THz, not merely at the π phase matching point, which are quite different with the properties of the discrete metallic metamaterial and dielectric grating. This work provides an efficient way towards practical applications in THz broadband polarization conversion, polarization resonance mode manipulation, and unidirectional transmission.

We experimentally investigate the linear polarization conversion for terahertz (THz) waves in liquid crystal (LC) integrated metamaterials, which consist of an LC layer sandwiched by two orthogonally arranged sub-wavelength metal gratings. A Fabry–Perot-like cavity is well constructed by the front and rear gratings, and it shows a strong local resonance mechanism, which greatly enhances the polarization conversion efficiency. Most importantly, the Fabry–Perot-like resonance can be actively tuned by modulating the refractive index of the middle LC layer under the external field. As a result, the integrated metamaterial achieves multi-band tunable linear polarization conversion.