Chiral Quasi-Bound States in the Continuum

Quasi-bound states in the continuum (QBICs) are Fano resonant with long optical lifetimes controlled by symmetry-breaking perturbations. While conventional responses limited to linear polarizations and do not support tailored phase control, here we introduce QBICs born of chiral perturbations that encode arbitrary elliptical polarization enable geometric engineering. We thereby design metasurfaces ultrasharp spectral features shape impinging wave front near-unity efficiency. Our findings extend resonances beyond their limits, opening opportunities for nanophotonics, classical quantum optics, acoustics.