Enhancing the Quality Factors of Guided Resonances via Rational Symmetry Breaking

ABSTRACT Optical resonators supporting quasi‐bound states in the continuum (quasi‐BICs) and quasi‐guided modes (quasi‐GMs) induced by structural symmetry‐breaking perturbations can confine light with exceptionally high quality ( Q ) factors. However, their Q factors are highly sensitive to the wavevector and structural asymmetry parameters. Merging multiple BICs and Brillouin zone folding can substantially enhance the Q factor robustness of quasi‐BICs and quasi‐GMs against symmetry‐breaking perturbations, but is limited by the specific mode types and modest asymmetry levels. Here, we demonstrate that the Q factors of these modes can be simultaneously enhanced even under significant levels of asymmetry. By rationally engineering the symmetry breaking, we achieve an order‐of‐magnitude improvement in the Q factors compared to previously reported Brillouin‐zone‐folding‐induced guided resonances. Group symmetry analysis and coupling amplitude calculations reveal that the enhancement of the Q factor originates from the suppression of radiative losses by selectively breaking certain symmetries while preserving others. We further experimentally validated that the Q factors of quasi‐GMs exhibit substantial enhancement, surpassing those of conventional designs. Our strategy provides an effective approach for realizing high‐ Q resonances under significant symmetry‐breaking conditions, thus relaxing the stringent requirements on nanofabrication tolerance.