Non-phase-matched spontaneous parametric down-conversion from lithium niobate thin films

Spontaneous parametric down-conversion (SPDC) is one of the most efficient sources for quantum emitters, which holds promise for numerous applications in quantum key distribution [1] , quantum computing [2] , and quantum networks [3] . Despite its vast potential yet simple measurement setup, the applicability of the SPDC sources is still limited due to their modest conversion efficiency. Therefore, many attempts have been made to improve the efficiency of the process, but mainly focused on phase-matched regime which relied upon the use of macroscopic birefringent crystals or material engineering. Here, we study the SPDC process at the telecom wavelength range in 200 nm thick thin-film lithium niobate (TFLN). This study not only provides an insight into the two-photon light generation process in the new non-phase-matched regime [4] , [5] also paves the way for the monolithic integration of available SPDC sources in chip-based devices [6] and metasurfaces [7] from nonlinear materials such as barium titanate, and gallium arsenide.