# Liquid Crystal-Tuned Planar Optics in Terahertz Range
## 2 Properties of LCs and LC-Based THz Devices
By injecting E7 into the groove of a silicon grating, as seen in Figure 2b, the intensity ratio between the 0 th order and 1 st order was magnetically tuned from 4:1 to 1:2 at 0.3 THz [48]. The twisted nematic LC cell accomplished the linear polarization conversion of the incident THz beam, and applying an electric field further adjusted the polarization [49]. Polarization-dependent imaging was achieved by inserting a uniformly oriented LC polymer film with different orientations compared to the orthogonal polarizers. The output intensity was varied by adjusting the orientations, which can be used as attenuators or switches [50]. In previous works, the LC alignments were either controlled by a strong magnetic/electric field or by mechanical rubbing. Both of them were restricted to homogeneous alignment. The appearance of photoalignment [51] makes the micropatterning of LC achievable. Our group used the photoalignment technique to encode the geometric phase into LC cells. By \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Freq** & **\(n_{\text{o}}\)** & **\(n_{\text{e}}\)** & **\(\Delta n\)** & **Temp (K)** \\ \hline
5 CB [28] & 0.3–1.4 & 1.59–1.83 & 1.74–2.04 & 0.13–0.21 & 298 \\ E7 [29] & 0.2–2.0 & 1.557–1.581 & 1.690–1.704 & 0.130–0.148 & 299 \\
1825 [30] & 1.5 & 1.574 & 1.951 & 0.377 & Room temperature \\ BL037 [34] & 0.2–2.0 & 1.58–1.64 & 1.78–1.80 & 0.17–0.20 & 294 \\ RDP-97304 [34] & 0.2–2.0 & 1.55–1.61 & 1.77–1.79 & 0.18–0.22 & 294 \\ NJU-LDn-4 [32] & 0.4–1.6 & 1.5–1.51 & 1.80–1.82 & \(\sim\)0.31 & Room temperature \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Birefringence of LCs in the THz band. this means, q-plates [52] and bifocal lenses [53] (shown in Figure 2c) were demonstrated. Similar procedures were carried out in LC polymers as well; thus, lenses, deflectors and axicons were realized, as illustrated in Figure 2d [54]. These devices eliminate the substrates of traditional THz LC devices, making them more compact and energy efficient. The Brewster angle was introduced by the refractive index difference between the silicon and the LC. In the range of 0.2-1.6 THz, a 99.6% modulation depth was achieved based on a double-switch electrical modulation. Linear to circular polarization conversion was achieved in 0.4-1.8 THz, as seen in Figure 2e. This design modulates both the intensity and the phase of the THz wave simultaneously, which is a milestone in THz wave manipulation [55]. Figure 1: (**a**) Illustration of the stop-band filter and (**b**) corresponding transmission at the ON state [37]. (**c**) Schematic of the electrically driven phase shifter [41]. (**d**) Configuration of the broadband LC THz wave plate [44]. (**e**) The dependency of the phase difference on the frequency of the DFLC THz wave plate [46]. Figure 2: Schematic diagrams of (**a**) the wedge cell [47] and (**b**) the LC phase grating [48]. (**c**) Structure and micrograph of a bifocal lens. Scale bar: 1 mm. [53] (**d**) Photopatterned LC polymer-based planar THz optics [54]. (**e**) The magnitude ratio (**top-left**) and phase difference (**bottom-left**) between the p and s components and polarization output at the OFF (**top-right**) and ON (**bottom-right**) states [55].