# All-Optical Tuning of Fano Resonance for Quasi-BIC and Terahertz Sensing Applications
## 3 Results and Discussion
Due to the edge effect, the charge will accumulate at the position of \(l_{1}\) above the aluminum resonant ring, as well as the left and right sides, as shown in Figure 4d. Secondly, the increase in the size of the conductive structure weakens the confinement of electromagnetic waves, resulting in a corresponding weakening of resonance. Therefore, by comparing Figure 4b,d, it can be found that when the size of the metal frame is increased, the enhancement of the current is no longer obvious. As shown in Figure 1b, when the size of the structure is optimized, the aluminum inner frame is changed to photosensitive silicon, and the electrical conductivity can be controlled by light intensity, realizing the continuous modulate of the BIC processes. Interestingly, when the conductivity is 0, although the size of the conductive structure does not change, a strong enhancement effect of the Fano lineshape appears on the photosensitive Si-metal subsurface, as shown in Figure 5a. However, with the gradually increase of the conductivity and the substantial increase of its free electron concentration, the conductive  
Figure 4: The distribution of (**a**) electric field and (**b**) surface current in the x-y plane when the structure is completely symmetrical; and the distribution of (**c**) electric field and (**d**) surface current in the plane when the structure is asymmetric (\(\Delta l=40\) μm). properties of photosensitive silicon will change from semiconductor-like to metal-like. When the conductivity is 1000-2000 S/m, the BIC-like state is approximately reached, and the enhancement effect is suppressed. It is easy to understand as the conductivity is further enhanced. When the electron concentration is high, the conductive properties of photosensitive silicon are similar to those of metals, and the interaction strength of photosensitive silicon with THz waves is comparable to that of metal with THz waves. At this point, the presence of photosensitive silicon can be regarded as a metal. This greatly enhances the asymmetry of the device, resulting in progressive enhancement of the Fano lineshape. In order to further explore the mechanism of optical modulation of the structure by photosensitive silicon, three types of structures with conductivity of 0 S/m, 2000 S/m and 500,000 S/m were selected, and the distribution of electric field and surface current in the \(xy\) plane was observed, as shown in Figure 6. In Figure 6a,b, when there is no light irradiation, the conductivity of the photosensitive silicon structure is 0, so the silicon structure can be regarded as an insulator, and metal aluminum is the main resonant structure. However, the discontinuous boundary between the photosensitive silicon and the metal makes the current in the metal aluminum asymmetric in the \(x\)-direction, which greatly enhances the oscillating current of the underlying metal structure, and the enhancement effect of the current at the lower edge of the metal frame becomes more and more obvious. As the conductivity increases to 2000 S/m, the discontinuity between photosensitive silicon and metal becomes smaller in Figure 6c,d. The entire resonant structure is made symmetrical along the \(x\)-direction, and then the current along the \(x\)-direction is suppressed in Figure 6d, resulting in a BIC-like state. As shown in Figure 6e,f, when the conductivity is further increased up to 500,000 S/m, the photosensitive silicon can be regarded as a metal, gradually. The discontinuity between it and the metal basically disappears, making the whole resonant structure and the surface current asymmetric again in the \(x\)-direction, which leads to the reappearance of the quasi-BIC state. This is similar to Figure 4d wherein the current concentration effect mainly occurs in the upper metal frame with smaller structure size. It can be seen that when the structure is controlled by photosensitive silicon, and will go through three stages.