# Fast-Tunable Terahertz Metamaterial Absorber Based on Polymer Network Liquid Crystal
## 3 Experimental Results and Comparative Analysis
incident light source, in this case, the light source could not pass through the polarizers perpendicular to each other. Therefore, the response speed of the absorption peak is reflected by detecting the intensity of the transmitted light during power-on and power-off. It should be noticed that we measured the response speed here at visible frequencies instead of THz frequencies. Because the polarization state of transmitted light, which is affected by birefringence effect of liquid crystal at visible frequencies, is determined by molecular rotation of LC. However, the LC molecular rotation is also responsible for the change of permittivity of LC layer in terahertz band. As the rotation speed of LC molecules is mainly determined by driving voltages, it is independent on testing frequencies. Taking the nematic S200-based absorber as an example, the experimental results show that the absorption peak of the sample is at 411.0 GHz when no voltage is applied. Correspondingly, the detected light intensity is the strongest. When a saturated voltage is applied, the absorption peak of the sample is at 393.5 GHz and the detected light intensity is the weakest. Therefore, the time required for the relative light intensity of the transmitted light from 90% to 10% is defined as the adjustment time of the absorber. And the time consumed for the relative light intensity from 10% to 90% is defined as the recovery time of the absorber when the power is off [23]. The adjustment time and recovery time comprehensively reflect the response speed of the absorber to the driving voltage. The measured results of response time of the LCand the PNLC-based MAs are illustrated in Figure 8.  
From Figure 8a, it is clear that the adjustment time and recovery time of the nematic S200 LC-based MA are 47.5 ms and 1466.0 ms, respectively. However, as shown in Figure 8b, after the UV photopolymerization of the PNLC for 30 min, the response time drastically reduces to 7.5 ms and 206.9 ms by an order of magnitude. This is due to the strong anchoring of the polymer network, corresponding to a larger splay elasticity, as the free relaxation time of the liquid crystal molecules becomes much shorter. Furthermore, Table 1 lists the response time of various samples. By increasing the UV exposure time from 30 min to 90 min, the recovery time of the MA can be further decreased from 207.0 ms to 84.0 ms. This is due to the fact that for a shorter exposure time, the formed polymer network is sparser, and the less liquid crystals are limited by the network, corresponding to a smaller elastic coefficient. With the increase of the exposure time, the polymer network will be denser in the sample, so the anchoring effect of the network on the liquid crystal will also increase, which corresponds to a larger elastic coefficient. However, there is no obviously change of the response time when the UV exposure time increases from 60 min to 90 min, which indicates that the photopolymerization precursor of the polymer network has been thoroughly reacted by the UV light. Figure 7: Experimental platform for measuring the response time of the MAs.