# Electrically Tunable Hydrogen-Bonded Liquid Crystal Phase Control Device
## 3 Results and Discussions  
Figure 5 shows the transmitted terahertz intensity of LC1 as a function of the applied voltage. The data were obtained by using an FIR CW laser as shown in Figure 3. We placed the LC device between the polarizers of closed Nicols. The direction of the polarizers and the director of the LC device were \(\Psi_{P}=45^{\circ}\), \(\Psi_{A}=-45^{\circ}\), and \(\Psi_{i}=0^{\circ}\), respectively. The transmitted 2.5 THz wave was measured by varying the voltage applied to the LC device. Figure 5a shows the result from LC1, and the result of E44 is shown in Figure 5b for comparison. We successfully observed the transmittance oscillation by increasing the applied voltage. The threshold voltage was high in both cases of LC1 and E44, which was due to the influence of the PEDOT/PSS electrodes. We believe that the threshold voltage will be lower once the fabrication process of the PEDOT/PSS electrodes has been optimized. The oscillation of transmittance shown in Figure 5 arose from the variation in polarization after passing through the LC device. In the case of E44, the transmittance oscillated with decreasing intensity, as shown in Figure 5b. This is due to the difference in the absorption coefficient between the extraordinary and ordinary direction. In the case of E44, the extraordinary absorption coefficient was smaller than that of the ordinary directions [33]. The electric field in the \(x\) direction felt the extraordinary absorption, and the electric field in the \(y\) direction felt the ordinary direction of absorption at 0 V. However, upon increasing the voltage, the LC molecules rose up, and the electric fields in both the \(x\) and \(y\) directions gradually felt the ordinary direction of absorption. Since the ordinary absorption coefficient in E44 was larger than the extraordinary absorption coefficient, the total transmittance decreased. Conversely, the transmittance oscillation doses did not decrease in the case of LC1, as shown  
Figure 4: The Jones matrix calculation for the homogeneous alignment of the LC device in the terahertz region. Figure 5: Transmitted terahertz intensity of (**a**) a hydrogen-bonded liquid crystal (LC1) and (**b**) a standard nematic LC (E44) as a function of the applied voltage. in Figure 5a. These results indicate that LC1 had the same value of ordinary and extraordinary absorption coefficients. We previously reported that the birefringence of E44 is 0.2 [33]. The thickness of the LC layer was 800 \(\upmu\)m in this case, and the retardation \(R\) of the E44 phase shifter was 160 \(\upmu\)m. The frequency of the incident wave was 2.5 THz, which corresponded to a wavelength of 117.7 \(\upmu\)m. This value means that two minima appeared in the transmittance versus applied voltage plot, which correlated with the result shown in Figure 5b. Since there were also two minima in Figure 5a, the birefringence of LC1 might have been about 0.2 at 2.5 THz. To estimate the values of the birefringent and absorption coefficients of LC1, we investigated the dependence of transmittance on the director angle \(\Psi_{i}\). Figure 6 shows the experimental and calculated terahertz transmittance of LC1 at 2.5 THz. Here, we placed the sample between the polarizers of a parallel Nicols, and then we rotated the sample. The graph shows the transmittance as a function of LC director angle \(\Psi_{i}\). Since we set the direction of the polarizers \(\Psi_{P}=\Psi_{A}=0^{\circ}\), the data for \(\Psi_{i}=0^{\circ}\), 180\({}^{\circ}\), and 360\({}^{\circ}\) corresponded to the results of the electric field of the terahertz wave that was parallel to the LC director. On the other hand, the data when \(\Psi_{i}=90^{\circ}\) and 270\({}^{\circ}\) showed the results of the terahertz wave that was perpendicular to the LC director. In Figure 6, the transmittances of these two cases were almost the same. This result indicates that the extraordinary and ordinal absorption coefficients of LC1 had the same value.