Fig. 1. Simulated mode profiles of the quasi-TM-polarized mode (a) at 812 nm for the downconverted photons and (b) at 406 nm for the pump. The scale bar is 2 µm.

Fig. 2. (a) Experimental setup for femtosecond laser direct writing ferroelectric domain patterns in the Ti-indiffused LN channel waveguide. HWP, half-wave plate; PBS, polarizing beam splitter; DM, dichroic mirror; IF, interference filter. (b) Optical microscopic image of the 2D optically poled domain pattern with the period of 2.74 µm in the x direction and 1.15 µm in the y direction. The inverted domains are visible as small circles. (c) 3D profiles of the inverted domains obtained by Cerenkov second-harmonic microscopy.

Fig. 3. Measured output intensity distribution of (a) the fundamental and (b) the second-harmonic waves in the far field; spectrum for (c) the fundamental and (d) the second-harmonic waves; (e) normalized output power of the second harmonic versus the input polarization of the fundamental wave at temperature 30°C; (f) normalized conversion efficiency of second harmonic versus the quasi-phase-matching temperature.

Fig. 4. (a) Schematic of the experimental setup used for photon-pair measurements. SMF, single-mode fiber; FC, fiber coupler; SPD, single-photon detector; M, mirror. (b) The coincidence of photon pairs varies with the average pump power.