## 3 Results and Discussion
### Active Modulation of Metamaterial EIT-like Window  
In the previous section, the modulation effect of parameter variation on transparent window was investigated under the passive modulation mode. However, under this mode, the parameters of the metamaterials were fixed, leading to a relatively high cost and limitations for practical uses. In this section, an active modulation method is proposed by introducing extra optical pumping to modulate transparent EIT-like window. As the conductivity of photosensitive silicon (Si) can be changed by different incident light intensity, therefore, by introducing photosensitive Si at different locations of the metamaterial (between the bright and dark modes or between the two USRs), EIT-like windows can be modulated by simply varying the light intensity of the incident THz wave, which can be an effective and cheaper way of enhancing the tunability properties of the designed metamaterials. As shown in Figure 6a,c, by burying photosensitive Si between bright and dark modes, an active modulation of the EIT-like effect could be achieved by changing the coupling effect between the eigenmodes. When there was no pumping, the conductivity of the photosensitive Si was very low, and the light and dark modes were electrically isolated. In this case, the induced current mainly flowed around the two USRs, as a result, the current in the CW could be completely suppressed, forming an EIT-like window at a certain frequency. As the intensity of the pumping light increased, the conductivity of Si grew, and the bright and dark modes gradually became conductive. However, as the current direction of the bright and dark modes was antiparallel, they suppressed each other. As the conductivity  
Figure 5: The influence of the asymmetric structural parameters on the transmission characteristics. (**a**) The transmission spectra with various coupling distances \(\delta_{x}\) between CW and USRs, the coupling distance \(\delta x\) is smaller than that of the symmetric structure; (**b**) The transmission spectra with various coupling distances \(\delta_{y}\) between USR pairs, similar to the symmetric structure. increased to a certain level, i.e., \(\sigma\) = 2000 S/m for instance, the coupling between bright and dark modes was almost completely suppressed. At this point, the current was limited around the CW area, while the LSP resonance as well as the EIT-like window disappeared, leaving only the LC resonance. In addition, by burying photosensitive Si between the USR pairs, a similar modulation effect was found, see Figure 6b,d. In this structure, the two USRs were electrically isolated without introducing the pumping light, and the induced current flow around two separate USRs with the current in CW suppressed, forming an EIT-like window. The increasing of pumping intensity enhanced the conductivity of the Si, and therefore the conductivity  
Figure 6: The effect of adding photosensitive Si into the metamaterial structure on the transmission characteristics. (**a**) Structure of buried photosensitive Si with different conductivity between CW and USRs; (**b**) Structure of buried photosensitive Si with different conductivity between USR pairs; (**c**) The transmission spectra buried with different conductivity photosensitive Si between CW and USRs. As the conductivity of Si battery increases, the LSP resonance disappears, so only LC resonance trough remains, while the EIT-like window disappears completely; (**d**) The transmission spectra of photosensitive Si buried with different conductivity between USR pairs. When the conductivity of the Si battery increases, the current will be limited around the CW, which cause the EIT-like window to completely disappear. of the whole device. The currents in the USR structure tended to cancel each other as the direction of each was opposite. When the conductivity of Si became large enough (\(\sigma\) = 22,500 S/m), the current in USRs was suppressed and all the current was confined in the CW area, and in this case, no transparent window was found in the spectrum.