Nanophotonic chip-space interfaces for multidimensional nonlinear optics.

Reconfigurable interfaces between confined optical modes in integrated photonic chips and structured light in free space would benefit fundamental optical science and photonic technologies. Here we exploit the anisotropic nonlinear susceptibility tensors associated with thin-film lithium niobate to construct nanophotonic chip-space interfaces capable of generating and multidimensionally engineering structured light via injections of photons to on-chip waveguides. Harnessing the nonlinear Čerenkov radiation in integrated nonlinear microring resonators, we tailor the spatial profile, polarization state, emission wavelength, topological charge and temporal wave packet of structured optical vortices, exhibiting reconfigurability and tunability. To showcase the capabilities of our platform, we use continuous-wave excitation to generate tunable optical skyrmions via the spin-orbit coupling and multistate integrated vortex microcombs in the short near-infrared range via synergistic χ(2) and χ(3) nonlinear optical processes. Our work bridges the research fields of structured light and integrated nonlinear optics, providing opportunities for spatiotemporal light generation and on-chip multidimensional nonlinear optics.