# Liquid Crystal-Tuned Planar Optics in Terahertz Range
## 4 Tunable Dielectric Metasurfaces Based on LCs
combined large birefringent NJU-LDn-4 NLCs in the THz band, with a circular silicon column array metasurface to fabricate a THz absorber, as shown in Figure 7a,c, with an adjustable power absorption rate and resonant peak position and which has a 47% modulation depth at 0.79 THz. Figure 7b shows that the absorption rate can reach approximately 80% at 0.8 THz when an electric field is applied. [100] However, there is no high absorption at the same frequency when bias is OFF. In 2020, [PERSON] et al. made use of the controllable anisotropy of DFLCs. They combined DFLCs with a silicon column array metasurface to fabricate a wide-band THz modulator that can manually control its achromaticity and birefringence. This device can be turned from OFF state (no polarization conversion effect) to a controllable waveplate, exhibiting over 97% polarization conversion ratio (PCR) in 0.97-1.3 THz as a half-wave plate, or over 96% PCR in 0.67-1.3 THz as a quarter-wave plate [101]. In addition, they used polymer-dispersed liquid crystals (PDLCs) as the birefringent medium in another work. Driven by an electric field, the orientations of LCs rotate in the droplets of PDLCs, which in turn causes anisotropy variation and a reduction in dispersion in a wider bandwidth compared to the original silicon gradient grating metasurface [102]. In the same year, [PERSON] et al. designed a linear polarization-multiplexing dielectric bifocal metalens, in which two orthogonal linear polarization incident lights have two different foci. They introduced LCs to select the outgoing light of different polarizations to achieve the bifocal function [103]. In 2021, [PERSON] et al. used LCs as the medium of silicon metalens to control the environmental refractive index, thus changing the focal length of the metalens between 8.3 mm and 10.5 mm [104]. In 2022, [PERSON] et al. fabricated a silicon-based elliptical-cylinder-array metasurface whose structures are shown in Figure 8a,b. As shown in Figure 8c, the purpose of manually controlling the anisotropy of THz devices was achieved through the dynamic coupling between the anisotropy caused by this metasurface in the THz band and the anisotropy caused by tunable LC orientations under different electric fields. With different initial LCs orientations, the difference between the \(E_{x}\). and \(E_{y}\) changes differently when applying the external electric fields, which is shown in Figure 8d,e. The highest circular dichroism reached 30 dB when comparing the power of two orthogonal electric fields [105]. Thus, using LCs as a tunable birefringence medium, one can change the working states of a dielectric metasurface. The other way is to adopt LCs to supply an extra PB phase modulation. Moreover, the phase modulation can be temporarily erased by an external field. This gives the dielectric THz metasurface another method for tunability. In 2020, [PERSON] et al. combined the PB phase of LC with the resonant phase of the silicon metalens (shown in Figure 9a), enabling a switch between an achromatic lens (Figure 9b,c) and a large chromatic lens (Figure 9d,e). This introduced a new approach for the combination of LCs and metasurfaces in the THz band, which has great potential in THz imaging and communications [106]. By orienting the LCsinto a PB phase grating pattern, an electrically adjustable focus separation between incident LCP waves and RCP waves for an off-axis bifocal THz metalens was achieved [103]. In 2022, [PERSON] et al. fabricated a silicon-based PB phase metasurface and deflected incident light with different chirality in different directions, which realizes a dynamic modulation depth of \(>\)94% and maximum efficiency of over 50% for the different spin states. At the same time, the PB phase of the LCs is used to change the deflection direction of light under the same chirality incident light (as shown in Figure 10a-d), and a magnetic field is introduced to control the orientation of the LCs, making subsequent communication applications possible.