In recent years, bound states in the continuum (BIC), have attracted much attention because of their infinite photon lifetime and quality factor, and their applications in enhancing light-matter interactions. However, optical BIC or quasi-BIC structures are generally machined by metamaterial/ metasurface/ photonic crystal of metal or dielectric materials. Once the structural parameters are determined, it is difficult to realize the active regulation of the device, which greatly limits its functionality and application scenarios. The combination of tunable materials with metamaterials or metasurfaces provides an effective way to realize tunable terahertz (THz) quasi-BIC devices. In particular, liquid crystal (LC) has large birefringence and low absorption loss in the THz band, and its orientation can be actively controlled by external electric, magnetic or light fields, which is a very effective THz active material. At present, metasurfaces based on liquid crystal integration have attracted much attention due to their advantages in the tunable control of electromagnetic waves.

The research group of Shitong Xu, associate Professor of Qufu Normal University, and Longqing Cong, associate Professor of Southern University of Science and Technology, proposed a liquid crystal integrated metasurface, and experimentally demonstrated the polarization-induced THz quasi-BIC active modulation response. Under the action of external field, the quasi-BIC resonance of the device can be controlled from OFF state to ON state by adjusting the orientation of the liquid crystal. Based on this metasurface and liquid crystal integrated heterogeneous device, this work also validates its asymmetric transmission capability. This work provides a new paradigm for actively tunable quasi-BIC devices and has important applications in terahertz communication, switching and sensing systems. The relevant research results were published in Photonics Research, Volume 12, Issue 10, 2024. [Shi-Tong Xu, Junxing Fan, Zhanqiang Xue, Tong Sun, Guoming Li, Jiandi Li, Dan Lu, Longqing Cong, "Active control of terahertz quasi-BIC and asymmetric transmission in a liquid-crystal-integrated metasurface," Photonics Res. 12, 2207 (2024)]

Figure 1 (a) shows the proposed liquid crystal integrated continuous domain bound device, which consists of a metal resonant ring with an upper double opening, a graphite electrode layer, an intermediate liquid crystal layer and a silica substrate. A double-opening metal resonant ring is used to provide a quasi-BIC mode. The orientation of liquid crystal is controlled by the graphite electrode layer and the external magnetic field. As shown in Fig. 1 (b), under the combined orientation of electric field and magnetic field, the liquid crystal molecule can be regulated in the z-u plane. The basic principle of the device is as follows: the polarization state of the incident THz is changed by the orientation change of the liquid crystal molecule, and then the non-resonant mode is converted to the quasi-BIC mode, as shown in Fig. 1 (c).

Fig. 1. (a) Schematic diagram of liquid crystal (LC) integrated quasi-BIC metasurface. Here LC is filled into the cell between two graphite electrodes, and a static magnetic field pre-anchors LC molecules along the u-axis. (b) Schematic diagram of LC orientation evolution with actuation between low and high voltages. (c) Active modulation of quasi-BIC from OFF- to ON-states due to polarization rotation enabled by LC layer.

Figure 2 introduces the resonance of metal double-gap split ring resonator (DSRR), and the schematic diagram of the unit structure is shown in Fig. 2 (a). This work can realize the transformation of BIC mode to quasi-BIC mode by changing the symmetry breaking angle of DSRR. Moreover, the transmittance spectra at different breaking angles are analyzed, and it can be found that when the breaking angle is 0, only a resonance valley appears at 0.8 THz in the transmission spectrum, and a new resonance is generated at the low frequency position with the increase of the angle. The experimental results are in good agreement with the simulation, as shown in Fig. 2(b) and 2(c). The change of quality factor with the breaking angle is shown in Fig. 2(d), and the current distribution of the simulation is shown in Fig. 2(e)-(f).

Fig. 2. (a) Schematic diagram of DSRR as a building block of a metasurface array. (b) Experimental and (c) simulated transmission amplitude spectra of DSRR with different rotation angles of 0°, 15°, 30°, and 45° under y-polarization excitations. (d) Q-factors of quasi-BIC with α. Surface current distributions of (e) DSRR with α =0° at 0.8 THz, DSRR with α = 30° located at (f) 0.56 THz, and (g) 0.87 THz, and the arrows indicate the direction of the surface current.

Figure 3 shows the transmission spectra of liquid crystal birefringence and quasi-BIC under specific symmetry breaking. Through the time domain spectrum and the frequency spectrum of Fourier transform, it can be found that the single liquid crystal layer exhibits large birefringence under the external field modulation. The polarization-dependent spectral response of the DSRR is shown in Figs. 3(e) and 3(f), using a sample with a 30° breaking angle as an example.

Fig. 3. (a) Schematic diagram of LC birefringence measurement under electric (z-axis) and magnetic fields (y-axis). (b) Experimental time-domain signals when LC orient along the y-axis and z-axis, and double quartz layer without LC as the reference. (c) Experimental LC refractive indexes and phase difference. (d) Microscopic image of quasi-BIC structure with an angle α = 30°. Experimental (e) and simulated (f) transmission amplitude spectra of quasi-BIC structure at x-pol and y-pol excitations.

Figure 4 shows that the device achieves active tunability in quasi-BIC mode by adjusting the orientation of the liquid crystal. Fig. 4 (a) is the test diagram of the experiment. In order to illustrate the polarization conversion effect of the device, a polarizer is placed behind the sample, and the unconverted and converted linear polarization components are detected by rotating the polarizer. Fig. 4 (b, c) shows the device polarization evolution and the corresponding transmittance spectra of the liquid crystal under two absolute orientations. The experimental polarization components under forward incidence and backward incidence are shown in Fig. 4 (d-g). The results show that the device can achieve quasi-BIC active modulation under different external fields, and asymmetric transmission can also be realized based on the difference between forward incidence and backward incidence.

Fig. 4. (a) Schematic diagram of measurement setup for LC-integrated metasurface. The first polarizer (P1) ensures the incident light is linear polarization along the x-axis, and the transmitted parallel (T_xx) and crossed (T_yx) polarization components are detected by rotating the second polarizer (P2). Schematic diagram of polarization state conversion and transmission spectra when LC along the (b) z-axis and (c) u-axis. Experimental measured (d) T_xx and (e) T_yx polarization components under different voltages at forward incidence (+z direction). Experimental measured (f) T_xx and (g) T_yx polarization components at backward incidence (-z direction).

Figure 5 shows the results and physical mechanism of asymmetric transmission of the device. By comparing Fig. 5 (a) and 5 (b), it can be found that the THz waves emitted from DSRR exhibit two different transmission effects due to the different polarization responses of the device at the front and back incidences. Fig. 5 (c-f) shows the transmittance spectra for forward and backward incidence at different voltages. At zero voltage, the polarization conversion ability of the liquid crystal layer is the strongest, and the isolation degree of 9.8 dB is achieved at 0.56 THz. With the increase of voltage, the liquid crystal layer gradually loses the polarization conversion ability, the asymmetric transmission effect is weakened, when the voltage is 150 V, the isolation degree is only 1.9 dB. Therefore, this work realizes the active control of asymmetric transmission by adjusting the orientation of liquid crystal.

Fig. 5. Analysis of asymmetric transmission of LC integrated metasurface. Polarization conversion and transmittance spectra at (a) forward incidence and (b) backward incidence. Amplitude transmission (dB) at forward and backward incidence under the voltage of (c) 0 V, (d) 70 V, (e) 110 V, and (f) 150 V.

Associate Professor Cong Longqing said: "Bound states in the continuum (BIC) research provides a new platform for enhancing the interaction between light and matter and designing ultra-high quality factor devices, and has shown important application value in sensing and nonlinear optics. To realize the active regulation of BIC is an important direction to explore its application in terahertz technology. This paper provides a feasible scheme for the realization of active modulator by integrating liquid crystal modulator with quasi-BIC, and lays a theoretical and device foundation for the subsequent expansion of spatial light modulator, imaging and communication applications."