  Quasi-bound states in the continuum (q-BICs) are resonant states of suitably
tailored nanostructures with long optical lifetimes controlled by
symmetry-breaking perturbations. While in planarized ultrathin devices the
resulting Fano resonance is limited to linear polarizations, we show here that
chiral perturbations extend q-BIC concepts to arbitrary elliptical
polarizations. Using geometric phase engineering, we realize metasurfaces with
ultrasharp Fano spectral features that can shape the impinging wavefront with
near-unity efficiency, while at the same time precisely filtering their
spectral content.
