# Fast-Tunable Terahertz Metamaterial Absorber Based on Polymer Network Liquid Crystal
## 3 Experimental Results and Comparative Analysis  
Figure 4 shows the fabricated prototype, with the glass substrate size of 2 cm \(\times\) 2 cm and 60 \(\times\) 50 unit cells with the dimensions of a = 300 \(\upmu\)m, b = 15 \(\upmu\)m, and r = 140 \(\upmu\)m. A total of five sets of experiments were carried out, in which the nematic LC (S200)-based MA was used as the contrast experiment, representing the common LC-based THz devices. In addition, the PNLC-based MAs were used as exploratory experiments with different exposure time. The absorptivity of MAs can be determined by A = 1 - \(|s_{11}|^{2}\) - \(|s_{21}|^{2}\), where A, \(s_{11}\), and \(s_{21}\) denote the absorptivity, reflection coefficient, and transmission coefficient, respectively. Since the transmission coefficient is equal to zero due to the existence of the ITO plate, the absorptivity can be simplified as A = \(1-|s_{11}|^{2}\). The absorption spectra of the S200 LC-based MA is shown in Figure 5. It was observed that the peak resonant frequency occurs at 411.0 GHz without the bias voltage (0 V). With the increase of the driving voltage, the absorption peak frequency shows a red shift until it reaches a saturated bias voltage of 5 V. The resonance peak frequency under fully biased voltage is 393.5 GHz  
Figure 4: Fabricated metamaterial absorber. Figure 5: Measured absorption spectra of the S200 LC-based MA. Then, in Figure 6a-d, the tunabilities of the PNLC-based MAs in the case of UV exposure times of 0 min, 30 min, 60 min and 90 min, were experimentally demonstrated, and the resonant frequencies were found to be 406.5 GHz, 416.5 GHz, 416.5 GHz, and 418.5 GHz, respectively, with a bias voltage of 0 V. For each case, by increasing the bias voltage, the absorption peaks continuously shift to lower frequencies until the LC molecules were fully rotated. As a result, the resonant frequencies were located at 391.5 GHz, 402.5 GHz, 405.0 GHz, and 407.5 GHz, respectively, under the saturated bias voltage. From Figure 6, it can be seen that for PNLC-based MA, the saturated bias voltage (40 V) after UV exposure is much higher than that (5 V) of the initial state. This may be attributed to the strong anchoring of the polymer network after UV exposure as the LC molecules in the polymer network require a greater voltage to spin. Moreover, with the increase of UV exposure time, the peak absorption frequency in both the unbiased state and fully biased state shows a slightly blue shift. Hence, increasing UV exposure time leads to a decrease of the effective permittivity of the PNLC. The measured results have proved the tunability of the absorption peak of the designed MAs. In practical applications, it is required that the absorption peak of the MAs can be quickly converted under the driving voltage. As can be seen from the foregoing, the LC is treated as the tunable electromagnetic material. Under the action of driving voltage, the molecular orientation changes, leading to the change of dielectric constant so that the MAs are capable of tunable resonant frequency in THz band. According to the electro-optic effect of the LC, under the driving of voltage, the reorientation of LC molecules causes the optical axis to rotate so that the transmission intensity correspondingly changes. So, we set up the experimental platform as shown in the Figure 7. The sample was placed between polarizers whose polarization direction was perpendicular to each other. At this time, the angle between the transmission axis of the sample and the transmission axis of the two polarizers was 45\({}^{\circ}\), and the light source could pass through. When the driving voltage was applied to the sample, the LC molecules rotated and kept parallel to the electric field. The sample could not polarize the  
Figure 6: Measured absorption spectra of the PNLC based MA: (**a**) Without UV exposure; (**b**) with 30 min UV exposure; (**c**) with 60 min UV exposure; and (**d**) with 90 min UV exposure.