Nonlinear optical wave mixing is a widely used method to produce light with new frequencies. However, the scattering centers in the nonlinear crystals always decrease the conversion efficiency.

Theoretically, in second harmonic generation, one can recover the conversion efficiency caused by scattering defects by choosing the proper fundamental wavefront via wavefront shaping. In our experiments, BaMgF4 crystal with scattering centers was chosen as the nonlinear medium. Under birefringent phase matching condition, we recovered the SH conversion efficiency via feedback-based wavefront shaping. We found the conversion efficiency decreased to about 75.7% compared with the theoretical calculation because of the scattering loss. We used the stepwise sequential algorithm to do the optimization and realize the recovery with an efficiency of 86.3% (Fig. 1(a)). Similarly, under phase-mismatching condition, we also recovered the conversion efficiency to 83.7% (Fig. 1(b)). These results provide a way to recover the conversion efficiency in nonlinear wave-mixing processes with lossy media and a fine tuning method for PM adjustment.

This work was published in “Zhuo Wang, Yanqi Qiao, Shuo Yan, Haoying Wu, Yuanlin Zheng, and Xianfeng Chen, Recovery of wave-mixing conversion efficiency in weakly scattering nonlinear crystals, Optics Letters, Optics Letters, 43(15), 3734-3737(2018)”.