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

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. Ultrahigh‐Q resonances are realized by strategically breaking the structural symmetry of all dielectric metasurfaces. The operation principle is to excite quasi‐bound states in the continuum through introducing precise perturbation within the minimum eigenfield region. Experimental results show that the maximum measured Q‐factor is up to 36,694 for silicon photonic crystal slab on SiO2 substrate by leveraging this design strategy.