SPDC in Thin-Film LN: From Tunable Polarization Entanglement to Doubly-Resonant Metasurfaces

We experimentally show a new mechanism for directly generating and tuning polarization entanglement for spontaneous parametric down-conversion (SPDC) in thin-film lithium niobate (TFLN), which relies on its three-fold rotational crystal symmetry. Furthermore, we demonstrate a nonlocal LN metasurface that supports frequency-tunable, degenerate and non-degenerate SPDC with doubly-resonant enhancement.