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

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.