Fig. 1: Layer-poled lithium niobate (LPLN) nanophotonic waveguide for efficient photon-pair generation.

a Schematic of LPLN waveguide cross-section in x-cut TFLN. Dark and light pinks indicate inverse domain polarities. b Mode profiles (E_z component) of TE₀₀ mode at 1550 nm and TE₀₁ mode at 775 nm for MPM. c Theoretical comparison of normalized SHG conversion efficiency among different nonlinear TFLN waveguide schemes, including LPLN (red), PPLN (yellow), MPM between TE₀₀ at FH and TE₂₀ at SH (purple), and MPM between TE₀₀ at FH and TE₀₁ at SH without poling (green). The blue line is normalized SHG efficiency versus nonlinear coupling parameter, a measure of mode overlap considering χ⁽²⁾ polarity distribution, with MPM condition. d A false-colored scanning electron micrograph of a LPLN waveguide cross-section, showing the electrical poling induced inverse polarities. e A top-view optical micrograph of a fabricated LPLN waveguide. f Top-view laser-scanning SHG imaging of a LPLN waveguide, where the unpoled waveguide is bright but the poled waveguide becomes dark due to the destructive interference of SH signals from the inversely polarized LN layers. g Schematic of cascaded SHG-SPDC processes for photon-pair generation. h Coincidence spectrum measured from 1486 nm to 1625 nm, covering telecom S, C, and L bands. i, Joint spectral intensity of the photon pairs. The dark cross is due to the residual pump noise