# All-Optical Tuning of Fano Resonance for Quasi-BIC and Terahertz Sensing Applications
## 3 Results and Discussion  
Next, we investigated the effect of different structural parameters on the transmission spectrum to optimize the sensing structure. In this structure, photosensitive silicon is not \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Material**} & **Relative** & **Conductivity** & **Relative** & **Refractive** \\  & **Permitivity** & **(S/m)** & **Permeability** & **Index** \\ \hline PolyTetraFluoroEthylene (PTFE) & 2.08 & 0 & 1 & 1.4 \\ Aluminum & 1 & 3.8 \(\times\) 10\({}^{7}\) & 1.000021 & 1 \\ Photosensitive Silicon & 11.9 & 0–5 \(\times\) 10\({}^{5}\) & 1 & 3.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Material properties of the metasurface used in the model [24; 25]. Figure 1: (**a**) 3D optical schematic of the metasurfaces. (**b**) Schematic diagram of the metasurfaces of the quasi-BIC. introduced firstly, and the asymmetry of the upper and lower structures is mainly realized by filling the aluminum structure in the lower frame. The asymmetry along the \(y\)-direction is represented by the asymmetry parameter \(\Delta l\). Specifically, \(\Delta l=0\) means that the upper and lower holes are equal in size, both \(360\times 170\)\(\upmu\)m. When a rectangular frame structure with equal width is introduced into the lower hole, the asymmetric parameter \(\Delta l=(l_{1}-l_{3})/2\). As shown in Figure 1b, when the size of the lower hole is \(280\times 90\)\(\upmu\)m, \(\Delta l=40\)\(\upmu\)m. This means that symmetry breaking elements are introduced. In the wave system, the incompatibility of resonant states for coupling with energy radiating channels leads to the formation of the bound states. In general, when the antisymmetric modes persist in the in-plane symmetry structures and cannot couple to the symmetric plane waves propagating as the radiation channel, which is also known as symmetry-protected BIC (SP-BIC) [15]. The energy in the resonator is trapped without leakage to the continuum. While when the structure is no longer symmetrical, the radiation channels are established between non-radiated bound states and the free space continuum, it results in more incident light radiating to free space, which manifests as resonance peaks in the transmittance spectra. In this paper, the simulation results of the asymmetric structure are shown in Figure 2a. It can be seen that when \(\Delta l=0\), the structure is symmetrical and the system state is in the BIC state, which means that no energy leaks from the bound state to the free space continuum. With the increase of \(\Delta l\), the asymmetry also gradually increases, and the transmission intensity also reaches the maximum at \(40\)\(\upmu\)m. At this time, our structure breaks the symmetry to the greatest extent and reaches the quasi-resonant state. The interference between the discrete bound states supported by the microstructure and the continuous radiation in free space forms the Fano resonance lineshape with almost constant resonance positions [17; 28]. It can be seen from the fitting that the Q value reaches a maximum of \(1130\). Figure 2: Effect of structural parameters on resonance. Transmittance spectra of the metasurface with different (**a**) asymmetry of aluminum inner frame \(\Delta l\); (**b**) meta-structure size \(l_{0}\); (**c**) aluminum structure thickness \(h_{1}\); and (**d**) bottom plate thickness \(h_{2}\). The different structures are shown in the inset, respectively. In addition, structural parameters such as height and shape also affect resonance properties. The asymmetric parameter \(\Delta l=40\)\(\upmu\)m is set to continue the research of subsequent numerical simulation. As shown in Figure 2b, increasing the size \(l\) of the array of metal aluminum deposited on the silicon substrate from 400 to 560 \(\upmu\)m, the resonance frequency gradually decreases, and the intensity of the Fano resonance peak also decreases gradually.