Extrinsic-chiral flatband quasi-bound states in the continuum as versatile sources of quantum-entangled photons [Invited]

This is the authors' abstract. We don't add key points for this paper.

High-quality factor photonic eigenstates stemming from bound states in the continuum (BICs) significantly enhance the nonlinear-optical response, enabling many peculiar scenarios of light–light interaction in dielectric metasurfaces. We consider a quasi-BIC metasurface built of dimers of AlGaAs rods and model spontaneous parametric down-conversion, yielding pairs of quantum-entangled photons. We show that specific properties of the quasi-BIC—maximum extrinsic chirality along straight flatbands in the reciprocal space—allow for the emission of pairs of photons outgoing from different metasurface sides in slightly oblique directions and carrying maximum quantum entanglement. As the same quasi-BIC is linearly polarized for normal incidence, it can also generate co-polarized photons in unentangled product states. We simulate different regimes of generation of photon pairs, having a continuously variable degree of polarization entanglement, with and without frequency entanglement, all controlled by subtle changes in the pump and detection wavelengths.