Engineering High Quality Quasi‐Bound States in the Continuum Through Controlled Symmetry Breaking in All‐Dielectric Metasurfaces

Abstract Symmetry breaking can transform symmetry‐protected (SP) bound states in the continuum (BICs) into quasi‐BICs with finite but high‐quality (Q) factors. However, how the Q‐factors of quasi‐BICs change with the asymmetry parameter under various types of symmetry breaking, remains largely unexplored. In this work, a comprehensive investigation is conducted into the engineering of Q‐factors in quasi‐BICs through strategic symmetry‐breaking configurations. Employing three distinct symmetry‐breaking approaches on an all‐dielectric metasurface of periodic silicon cuboids, SP‐BICs are transformed into quasi‐BICs with remarkable Q‐factors. The analysis reveals distinct Q‐factor responses to structural perturbations across configurations. For the metasurfaces with off‐center circular air holes, Q‐factors distinctly depend on spatial offset and air hole's radius. Subsequent investigation examines two additional geometrical transformations: U‐shaped and L‐shaped cross‐sectional modifications of the nanoparticle geometry. The observed diversity in Q‐factor scaling relationships with asymmetry parameters can be interpreted through eigenfield perturbation. To validate the theory, a series of silicon metasurfaces are fabricated and their scattering spectra via a home‐built cross‐polarization measurement system. Measured Q‐factors exceeded 10 000 in all symmetry‐breaking configurations, peaking at 30 270. This work establishes a generalized framework for achieving ultrahigh‐Q(>10⁴) resonances through symmetry engineering in dielectric metasurfaces, providing design guidelines for applications in nanophotonics.