Advances in Chiral Quasi-Bound States in the Continuum: From Fundamentals to Functional Metasurfaces

Chirality and resonance lie at the heart of modern nanophotonics, governing the interaction between light and structured matter. Bound states in the continuum (BICs), originally regarded as mathematical curiosities, have evolved into powerful physical platforms for achieving ultrahighquality optical resonances. When symmetry breaking introduces controlled radiation loss, quasi-BICs emerge, offering a unique bridge between ideal confinement and radiative coupling. Recent advances in chiral quasi-BICs (qBICs) reveal that such systems can host exceptionally strong chiral responses, enabling phenomena ranging from nonlinear enhancement and directional emission to vortex generation and superchiral sensing. Moreover, the exploration of local-nonlocal transitions provides new insight into how energy, topology, and radiation channels intertwine in structured media. This Review traces the evolution from BICs to chiral qBICs, emphasizing the underlying physical principles and their translation into functional metasurfaces for compact photonic devices, chiral lasers, and molecular detection.