Realizing Ultrahigh‐Q Resonances Through Harnessing Symmetry‐Protected Bound States in the Continuum

Abstract Harnessing the power of symmetry‐protected bound states in the continuum (SP BICs) has become a focal point in scientific exploration, promising many interesting applications in nanophotonics. However, the practical realization of ultrahigh quality (Q) factor quasi‐BICs (QBICs) is hindered by the fabrication imperfections. In this work, an easy approach is proposed to achieve ultrahigh‐Q resonances by strategically breaking symmetry. By introducing precise perturbations within the zero eigenfield region, QBICs with consistently ultrahigh‐Q factors, beyond conventional limitations are achieved. Intriguingly, intentionally disrupting symmetry in the maximum eigenfield region leads to a rapid decline in QBIC's Q‐factors as the asymmetry parameter increases. Leveraging this design strategy, ultrahigh‐Q modes with a high Q‐factor of 36,694 in a silicon photonic crystal slab are experimentally realized . The findings establish a robust and straightforward pathway toward unlocking the full potential of SP BICs, enhancing light‐matter interactions across diverse applications.