# All-Optical Tuning of Fano Resonance for Quasi-BIC and Terahertz Sensing Applications
## 3 Results and Discussion
Firstly, the photosensitive silicon promotes the confinement of electromagnetic waves by the metal structure tightly bound to it when initially non-conductive, resulting in the enhancement of the underlying resonant structure. Secondly, when partially conducting, the upper and lower structures are gradually symmetrical, and a BIC-like state appears. Finally, the area of the metal structure is increased when it is approximated as a conductor, so that its electric field concentration effect mainly occurs in the square structure above with smaller area. Figure 5: **(a)** Simulated transmission spectra of the proposed structure with different values of conductivity. The dashed lines trace the evolution of the BIC-like to quasi-BIC states. **(b)** 2D scanning pattern of simulated transmission amplitude spectra by varying conductivity. The structure diagram is shown in the inset. Finally, we observe the sensitivity of the proposed THz sensing by varying the refractive index and the thickness of the analyte, respectively. As shown in Figure 7a, we analyze the effect of external media with different thicknesses (\(h_{f}\)) on the transmission spectrum. When the filling structure gap and the surrounding dielectric thickness increase from 15 \(\upmu\)m to 35 \(\upmu\)m, there is a clear red shift in the resonance peak position. When the refractive index is from 1.23 to 1.87 that is shown in Figure 7b, the resonant peak position also has an obvious red shift. However, the increase of the thickness and refractive index has little effect on the Q value of the resonance peak and the spectral contrast. Figure 6: Distribution of (**a**) electric field and (**b**) surface current in the plane when the conductivity is 0 S/m. Distribution of the (**c**) electric field and (**d**) surface current in the plane when the conductivity is 2000 S/m. Distribution of (**e**) electric field and (**f**) surface current in the \(x\)-\(y\) plane at a conductivity of 500,000 S/m. Figure 7: (**a**) Resonance frequency drift versus thickness \(h_{f}\) from 15 to 35 \(\upmu\)m with the refractive index of 1.23; top inset, the resonance spectrum drifts with thickness; bottom inset, cross-sectional schematic diagram of structure. (**b**) Resonance frequency drift versus refractive index with the thickness of 15 \(\upmu\)m; top inset, the resonance spectrum drifts with refractive index. In refractive index sensing, the sensitivity is:  
Figure 1:0  
where \(df\) and \(dn\) are the frequency offset and the refractive index difference or thickness difference. Quasi-BIC Fano resonance sensitivities were 3.62 GHz per refractive index unit (RIU) and 0.25 GHz per micrometer, respectively. Although the sensitivity is not dominant compared with other similar sensors, the biggest advantage of the Fano lineshape is the steepness of the slope of the resonance peak. Taking refractive index sensing as an example, when using light intensity for sensing measurements, the sensitivity can be calculated by the ratio of the change in light intensity amplitude to the change in refractive index [29]:  
Figure 1:1  
It can be seen from the formula that there are two parts multiplied together. The first part of the formula is the conventional sensitivity, which characterizes the sensitivity by the amount of frequency drift. It mainly related to the change of the refractive index, and has nothing to do with the lineshape. The second part is related to the resonance lineshape. The steep slope (\(dI/df\)) of the Fano resonance help to increase the sensitivity (\(dI/dp\)). As Figure 8 shows, it can be seen that the slope of Fano resonance will increase sharply at a conductivity of 500,000 S/m, so the sensing sensitivity can be greatly enhanced about 2.60 times higher than the aluminum filling. In this way, the sensitivity of the Fano resonant structure with photosensitive silicon can reach 9.41 GHz per refractive index unit (RIU) and 0.65 GHz per micron, respectively.