# Liquid Crystal-Tuned Planar Optics in Terahertz Range
## 4 Tunable Dielectric Metasurfaces Based on LCs
As shown in Figure 10e,\(f\), the deflection angle of about 0.9-1.1 THz changes its sign when the magnetic field is applied [107]. The tunable PB phase of LCs gives the metasurface tunable phase compensation and promotes advances in LC-integrated metadevices. Figure 8: (**a**) SEM image of the Si dielectric metasurface. (**b**) Structural diagram of the LC-Si metasurface at different applied voltages. (**c**) Illustration of LC orientation in the LC-Si metasurface when the biased electric field \(E\) = 0 V/mm. The LC is initially anchored along the \(y\)-axis (\(\theta\) = 0\({}^{\circ}\), \(\gamma\) = 0\({}^{\circ}\)) and the \(x\)-axis (\(\theta\) = 90\({}^{\circ}\), \(\gamma\) = 0\({}^{\circ}\)). When \(E\) = 30 V/mm, the LC is turned to the \(z\)-axis (\(\gamma\) = 90\({}^{\circ}\)), and the long axis of the Si column is fixed along the y-axis. The experimental THz time-domain signals for \(x-\)LP and \(y-\)LP components with a biased electric field in the range of 0 to 30 V/mm when the LC is initially along (**d**) the \(x\)-axis and (**e**) the \(y\)-axis. (**f**) The experimental phase shift of the device under different initial orientations and biased electric fields [105]. Figure 7: (**a**) The LC orientation in the bias-saturated state. (**b**) Simulated spectra of reflectance, transmittance, and absorbance in the bias-saturated state. (**c**) The LC orientation at the bias OFF state. (**d**) Simulated spectra of reflectance, transmittance, and absorbance in the bias OFF state [100]. Figure 10: Schematic diagrams of the LC-PB metadevice in different cases: (**a,b**) LCP incidence; (**c,d**) RCP incidence. (**a,c**) LC molecules along the \(y\)-axis without an EMF; (**b,d**) LC molecules along the \(z\)-axis with an EMF. The simulated electric field distributions of the spin states at 1.0 THz are located below the corresponding diagram. The simulated diffraction efficiency map of the LC-PB metadevice varies with frequency and deflection angle when the RCP is incident: (**e**) LC molecules are along the \(y\)-axis and (**f**) \(z\)-axis [107]. Figure 9: (**a**) The broadband achromatic focusing state without bias and the dispersive focusing state with a saturated bias. The superstrate depicts the dielectric metasurface, while the blue ellipsoids between the superstrate and substrate denote LCs. (**b**) Simulated THz fields in the \(xz\) plane and (**c**) measured THz fields in the \(xz\) and \(xy\) planes (z = 12.0 mm) at 0.9, 1.1, and 1.4 THz when no bias is applied. (**d**) Simulated and (**e**) measured THz fields in the \(xz\) and \(xy\) planes (z = 13.0 mm) of the same sample with a saturated bias [106]. However, the modulation speed has been rarely mentioned in these works on dielectric-LCs metadevices. The combination of LCs and dielectric metasurfaces supplies two distinguished ways to formdynamic functions: one case is that LCs are used to provide extra PB phase modulation, the typical thickness of which is hundreds of micrometers to meet the half-wave condition, and the modulation time is in second scale; the other is adopting gradient voltage to tune the birefringence in order to generate a gradual refractive index change with a millisecond scale response.