Ultrahigh-Q Quasi-BICs via Precision-Controlled Asymmetry in Dielectric Metasurfaces.

Achieving ultrahigh quality factor optical resonances is crucial for advancing low-threshold lasers, high-sensitivity sensors, and nonlinear photonics. While dielectric metasurfaces supporting quasi-bound states in the continuum (qBICs) show great potential, their experimental realization has been challenging due to the difficulty of precisely controlling symmetry-breaking at the nanoscale. Here, we introduce a precision-controlled symmetry-protected qBIC method using angular perturbations to precisely tune asymmetry, ensuring both high precision and reproducibility of Q-factors. In contrast to traditional SP-qBIC excitation, which relies on uncontrolled asymmetry, our method offers more accurate and consistent control by precisely tuning angular perturbations instead of structural variations. Additionally, our approach defines the lower limit of achievable Q-factors, providing a reliable lower bound. The experimental demonstration of SP-qBICs in composite nanoslit metasurfaces achieves a record-breaking Q-factor of 1.1 × 10-the highest reported for SP-qBICs to date. These findings offer a promising platform for designing ultrahigh-Q resonators for next-generation photonic-applications.