Symmetry-broken quasi-bound states in the continuum dielectric metasurfaces

Bound states in the continuum (BICs) have attracted significant attention in recent years as a route to realizing optical resonances with exceptionally high quality factors in compact photonic platforms. In dielectric metasurfaces, these states arise from the suppression of radiation leakage despite residing within the continuum of radiating modes. Deliberate breaking of the symmetries that protect ideal BICs transforms them into quasi-bound states in the continuum (quasi-BICs), allowing controlled coupling to free-space radiation while preserving strong field confinement and enhancement. This balance between accessibility and confinement has made symmetry-broken quasi-BIC metasurfaces an active area of research in nanophotonics. In this review, we discuss the physical origins of quasi-BIC formation and examine the role of rotational, chiral, and parity-time (PT) symmetry breaking in tailoring their optical properties. We highlight recent developments linking quasi-BICs with polarization singularities, topological phenomena, multipolar radiation engineering, and non-Hermitian photonics. We then survey emerging applications in lasing, emission engineering, strong light-matter interactions, nonlinear optics, sensing, and dynamically reconfigurable photonic devices. Finally, we discuss future opportunities involving topological robustness, active material integration, and quantum photonics, which are expected to further expand the functionality and impact of symmetry-broken quasi-BIC dielectric metasurfaces.