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 16262$\pm$850. 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.