# Liquid Crystal-Tuned Planar Optics in Terahertz Range
## 3 LC-Integrated Plasmonic Metadevices
Figure 4c shows that the response of the modulator can be significantly improved by integrating the in-plane-switching dual-frequency LC into a plasmonic metasurface [78]. As shown in Figure 4d, in 2018, [PERSON] et al. proposed an integrated polymer network LC-based absorber, which achieved an electrically driven peak shift in 10 ms [79]. For LC-only THz devices, the required thickness of the LC layer is on the submillimeter scale, inducing a high driving voltage and slow response. The integration of metamaterials with LCs can significantly improve the response while maintaining a large phase shift. As shown in Figure 5a, [PERSON] et al. achieved an amplitude modulation of 20% and a phase modulation of 40\({}^{\circ}\) by hybridizing 12 um thick LCs with metasurfaces [80]. In 2019, [PERSON] et al. designed a polarization converter using an LC/metal grid configuration, reducing the LC thickness by two orders of magnitude compared to pure LC-based devices [81]. In Figure 5b, benefiting from the wide-band characteristics of the metal gratings, broadband linear polarization generation can be achieved by mixing LCs as a refractive index variable environmental medium to actively tune the Fabry-Perot-like resonance in two orthogonally arranged metal gratings [82]. Figure 5c shows that the combination of LCs and chiral metamaterials enabled spin conversion and electronically manipulated optical chirality [83]. With the rapid development of THz technology, many other LC-integrated THz metadevices have been designed and demonstrated, such as the THz demultiplexer [84] and the spin controllable THz source [85], and so on. Figure 4: (**a**) Schematic diagram of an LC THz-resonant switch [72]. (**b**) Illustration of an optically tunable and thermally erasable THz modulator with LC doped by methyl red [76]. (**c**) Fast-response THz modulator fabricated by in-plane switching of dual-frequency LCs (**left**) and measured time-dependent transmission (**right**) [78]. (**d**) Fast-tunable THz absorber fabricated by polymer network LCs (**left**) and the measured absorptions (**right**) [79]. Dynamic THz beam steering has great potential in high-speed wireless communication, high-resolution imaging, and radar. Traditional radar relies on mechanical scanning for continuous beam steering to capture the target [86]. Phased arrays are considered good candidates for beam steering in the microwave band [87]. However, in the THz band, the high loss of the semiconductor switch restricts the phase shift. In 2014, [PERSON] et al. proposed a programmable metasurface with the phase of each pixel unit switchable between 0 and \(\pi\). Therefore, digitized dynamic manipulation of electromagnetic waves was realized by programming the coding sequences [88]. Illustrated in Figure 6a, in 2020, [PERSON] et al. designed a programmable LC-integrated metasurface by coding the \"0/1\" phase to dynamically control beam steering, and the maximum deflection angle reached 32\({}^{\circ}\)[89]. In Figure 6b, [PERSON] et al. adopted the LC sandwiched by S-type metamaterials to demonstrate an efficient and ultrathin spatial phase modulator [90]. By applying a gradient voltage, continuous phase retardation and high-resolution spatial phase control were achieved. To extend the range of phase modulation, [PERSON] et al. used two layers of asymmetric metamaterials to spatially manipulate the transmitted THz beam, and demonstrated double-layer beam steering with a maximum deflection angle of 30\({}^{\circ}\), as shown in Figure 6c [91]. The above designed programmable metasurfaces are all based on binary phase coding. To suppress unnecessary high-order diffraction, [PERSON] et al. proposed a multibit coding scheme based on resonant switching and achieved maximum single-beam scanning of \(\pm\)21\({}^{\circ}\) with significantly improved efficiency, as illustrated in Figure 6d [92]. STMs can spatially address THz amplitudes by binary coding each pixel as well. In 2014, [PERSON] et al. designed a STM with 8 \(\times\) 8 pixels using metamaterial absorbers.