Increasing of Q Factor of Quasi-Bound States in the Continuum With Increased Asymmetry Parameter in a Silicon...

Quasi-bound states in the continuum (qBICs) can support resonances with high quality $\left(Q\right)$ factor. However, the $Q$ factor of qBICs decreases rapidly as the asymmetry parameter $\alpha$ increases. Traditionally, maintaining a high $Q$ factor requires a small $\alpha$ , which brings significant challenges for nanofabrication. In contrast, our study finds an increase in the $Q$ factor with increasing of $\alpha$ in a metasurface array composed of Si blocks with circular notch. For small asymmetry parameter $\left(\alpha \le 0.14\right)$ , the $Q$ factor of the qBIC follows the inverse square law. However, when $\alpha >0.14$ , the $Q$ factor increases gradually. Through detailed analysis of the coupling strength and electromagnetic field distributions, we attribute the increase of the $Q$ factor to the coupling between the qBIC and the lattice surface mode (LSM), which arises from the alignment between the diffraction wavevector and the reciprocal lattice vector. In this case, the $Q$ factor under large $\alpha$ can reach nearly $10^5$ and is maintained over a wide range of $\alpha$ . Our work offers a new pathway for the design of high $Q$ qBIC resonances at large asymmetry and contributes to other research field related to the high- $Q$ nature of qBIC in photonics.