## 3 Results and Discussion
### Active Modulation of Metamaterial EIT-like Window
It was found that the EIT-like transparent window could be actively modulated by changing the conductivity of the buried photosensitive Si structure. With the help of a proper pumping light intensity, the application of terahertz switch could be realized. It can also be noticed that, the size of introduced Si structure can also play an important role of influencing the transparent window. Figure 7 shows the results of modulation effects by varying the effective length of photosensitive Si, \(L_{\textit{eff}}\), and the effective distance between two USRs, \(W_{\textit{eff}}\). In Figure 7a, the photosensitive Si is filled between the bright and dark modes and the conductivity is set to be 1000 S/m, it can be found that the larger effective length \(L_{\textit{eff}}\) results in a stronger conductivity of the photosensitive Si, which shows significant influence on the EIT-like window of the metamaterial structure. Similarly, as shown in Figure 7b, with a setting conductivity of \(\sigma\) = 8000 S/m, the EIT-like window could be affected by varying the effective width of \(W_{\textit{eff}}\). As a result, by optimizing the structure size of the buried Si, the active modulation effect of the pumping light could be enhanced. In order to understand the active modulation effect of the photosensitive Si on the asymmetric structure, two types of Si positions, between the CW and USR as well as between the USRs were investigated. As shown in Figure 8a,c, the lengths of the two photosensitive Si structures were both set to be 30 \(\upmu\)m. The widths were set as same as the coupling distance \(\delta_{xup}\) and \(\delta_{xdown}\), which were 13 \(\upmu\)m and 4 \(\upmu\)m, respectively. In this case, the active modulation was due to the change of the conductivity of the photosensitive Si, leading to a change in the coupling effect between the bright and dark modes. The trend is consistent with that of the symmetric structure. However, as the conductivity of photosen  
Figure 7: The influence of size parameters of photosensitive Si on transparent window. (**a**) The modulation effect of different effective length \(L_{\textit{eff}}\) of photosensitive Si. As the \(L_{\textit{eff}}\) is larger, the more obvious influence on the EIT-like window of the metamaterials; (**b**) The modulation effect of photosensitive Si with different effective width \(W_{\textit{eff}}\). With the increase of \(W_{\textit{eff}}\), the influence on the EIT-like window is greater. sitive Si increased to 1000 S/m, the EIT-like window disappeared completely, which was only half of that required in the unilateral symmetric structure case (\(\sigma\) = 2000 S/m). The modulation effect of the buried Si between the asymmetric USR pairs on the transmission window is shown in Figure 8b,d. The width of photosensitive Si was set as 21 \(\upmu\)m and the length was the same as the coupling distance \(\delta_{y}\), which was 9 \(\upmu\)m. It can be found that the trend is also similar as the symmetric structure. However, as the conductivity of Si increased to 15,000 S/m, the current in the two USRs was completely suppressed, causing the EIT-like window to disappear completely. Compared with the unilateral symmetric structure in the same situation, the light intensity needed by the EIT-like window to disappear was smaller. Figure 8: The effect of burying photosensitive Si between asymmetric metamaterials on the transparent window. (**a**) Structure of buried photosensitive Si with different conductivity between CW and USRs; (**b**) Structure of buried photosensitive Si between USR pairs; (**c**) The transmission spectra buried with different conductivity photosensitive Si between CW and USRs; (**d**) The transmission spectra of photosensitive Si buried between USR pairs.