Fig. 1. (a) Experimental setup. (b) and (c) The FWM changing process of the experimental phenomenon when rotating the nonlinear crystal on the experiment stage.

Fig. 2. (a) Three-dimensional phase-matching geometry of the FWM, difference frequency, and sum frequency assisted by the scattered light. (b) The perfect phase-matching geometry of FWM in the horizontal plane.

Fig. 3. (a) Light intensity of fundamental frequency wave varying with the radiated angle of FWM and the incident angle of fundamental frequency wave. (b) Light intensity of FWM varying with the central wavelength of FWM and the incident angle of fundamental frequency wave. (c) Theoretically calculated and experimentally measured emergence angles of FWM at different incidence angles of fundamental waves. (d) Theoretically calculated and experimentally measured central wavelengths of FWM at different incidence angles of fundamental waves.

Fig. 4. (a) Photographs of FWM signals with different incidence angles of fundamental waves. (b) Spectra of FWM signals with different incidence angles.