# All-Optical Tuning of Fano Resonance for Quasi-BIC and Terahertz Sensing Applications
## 3 Results and Discussion
As shown in Figure 2c, when the height \(h_{1}\) of the aluminum structure is gradually decreased, the Fano resonance peak is gradually enhanced, but the peak position is almost unchanged. Similarly,, with the decrease of the substrate thickness \(h_{2}\), the resonance peak is weakly enhanced, and the resonance frequency gradually increases, showing a weak blue shift in Figure 2d. It can be seen that the change in the structure height has the similar effect on the resonance than the change caused by the base thickness. According to the above simulation results, the final selection parameters are as follows: the upper inner hole size is \(l_{1}\times l_{2}=360\times 170\)\(\upmu\)m, the lower inner hole size is \(l_{3}\times l_{4}=280\times 90\)\(\upmu\)m, and the asymmetric parameters \(\Delta l=40\)\(\upmu\)m, \(l_{0}=400\)\(\upmu\)m, \(h_{1}=30\)\(\upmu\)m, \(h_{2}=100\)\(\upmu\)m.  
As can be seen from Figure 2, there are two types of methods to improve sensitivity. One is to increase the asymmetry by increasing the width of the aluminum inner hole, and the other is to reduce the volume of the metal structure, such as reducing the area or reducing the thickness. The mechanism is speculated that the ability to confine the electric field is weakened as the volume of the aluminum structure decreases, and the binding effect on energy is weakened, which leads to resonance leakage and enhances the resonance strength. This can be attributed to the enhancement of asymmetry and the weakening of the binding effect on energy. The transition from BIC to quasi-BIC characteristic response is seen in the simulations in Figure 3a, and the trajectories given are simulation results for different \(\Delta l\). As \(\Delta l\) gradually decreases, the overall spectrum exhibits the red-shifted. However, there is only a slight red shift in the Fano resonance peak position, the spectral linewidth is gradually narrowed, and the transmission intensity is gradually reduced, which reflects the decrease in the leakage rate of the quasi-BIC. With \(\Delta l\) decreases to zero, the resonance linewidth and the transmission disappears until the upper and lower structures are completely symmetrical. As shown in Figure 3b, the SP-BIC state increases with increasing Q-factor. Thus, it can be seen as a significant method for designing and tuning the resonance Q-factor and linewidth at quasi-BIC mode. It is worth noting that although the Q-factor is maximized when infinitely close to the BIC state, and slight perturbations result in infinitely low leakage of the BIC, resulting in weak transmission intensity. Due to the limitation of the resolution of the equipment and the signal-to-noise ratio, \(\Delta l=40\)\(\upmu\)m with obvious transmission is still selected for research. Figure 3: Influence of asymmetric parameters \(\Delta l\) on resonances. (**a**) Transmission spectrum with different \(\Delta l\). (**b**) 2D scanning pattern of transmission spectrum as a function of different symmetry values \(\Delta l\). After that, the resonance phenomenon was investigated under the variation of electric field and current, as shown in Figure 4. To observe the localization of the energy more clearly, the color and size of the current arrows represent the amplitude distribution on the surface. Figure 4a shows that the resonance is suppressed when the structure is symmetric. As shown in Figure 4b, the flow of the current is mainly along the \(y\)-axis and presents a symmetrical distribution. When the symmetry is broken, as shown in Figure 4c, localization also occurs on both sides of the structure along the \(x\)-axis. At the same time, the local enhancement is particularly evident at the outer edge of \(l_{1}\) above the aluminum structure. It can be seen that the enhancement effect can be attributed to two reasons. Firstly, the asymmetry of the structure leads to the generation of oscillating currents, which leads to the accumulation of charges mainly in the smaller aluminum frame.