Fig. 1. (a) Scheme of SFG phase-matching exploiting reflected FW and incident SHG. (b) Scheme of SFG phase-matching exploiting incident FW and reflected SHG.

Fig. 2. (a) Schematic of the KDP sample. (b) Schematic of the main experimental set-up.

Fig. 3. Diagram of KDP interior, indicating the interaction range of reflected FW and incident SHG.

Fig. 4. (a) Photograph of the screen in the SFG experiment, where the lower two small round pictures show the process from phase-mismatching to phase-matching. (b) The phase-matching diagram of the process in (a), showing the scheme of scattering-assisted conical SFG as well as the reflection-assisted SFG.

Fig. 5. (a) Diagram of comprehensive, simplified reflection and refraction processes at boundaries of KDP, and the triangular phase-matching of SFG. (b) The prediction curves of the SFG critical angles at the phase-matching point for the FW wavelength from 800 to 1500 nm. The red line shows the results at the wavelength we use.

Fig. 6. Regular variation of output power with the rotation of the half-wave plate.

Fig. 7. Output power $P_3$ in the SFG experiment as a function of the external incident angle ${\theta }_1$ (${\theta }_2$). This group of data is measured when the incident average power (sum of FW and SHG powers before shooting into KDP) is 4.38 mW.

Fig. 8. Output power $P_3$ in the SFG experiment as a function of the product of FW and SHG powers.