Efficient generation of broadband photon pairs in shallow-etched lithium niobate nanowaveguide

We design and fabricate shallow-etched periodically poled lithium niobate waveguide to realize highly-efficient broadband spontaneous parametric down-conversion (SPDC) on nanophotonic chip. The shallow-etched waveguide is capable to tolerate the non-uniformities of waveguide width induced by fabrication imperfections, enabling generation of photon pairs with high count rate and bandwidth. We demonstrate photon-pair generation with a high brightness of 11.7 GHz/mW and bandwidth of 22 THz, in a 5.7-mm-long PPLN waveguide. The generated photon pairs exhibit strong temporal correlation with a coincidence-to-accidental ratio up to 16262pm850. Our results confirm the feasibility of shallow etching in fabrication of efficient SPDC device on platform of lithium niobate on insulator, and benefit quantum information processing with broadband photon source.