Scattering has usually been considered detrimental for optical focusing or imaging. Recently, more and more research has shown that strongly scattering materials can be utilized to focus coherent light by controlling or shaping the incident light, leading to imaging through turbid media. However, it is noticeable that the turbid media applied in previous studies are usually linear materials, which means no frequency conversion occurred directly during the scattering. We show a flexible manipulation of both disordered linear and nonlinear scattering signals by a nonlinear turbid medium, indicating more controllable degrees of freedom for the description of turbid media. This technique also provides a possible way to an efficient transmission of nonlinear signal at a desired location in designed patterns, which is simple and flexible compared to hybrid structured fundamental wave and domain design methods.

Here, LN nanocrystal powder was used as the second-order nonlinear scattering medium. We achieved an SH focal spot (only 380 nm in size) in the backward direction via shaping the fundamental wavefront via an SLM and a genetic algorithm. Besides, two SH foci and a focal line were also achieved after controlling the shape and location of the feedback signals, indicating the accuracy and flexibility of SH focusing.

This research was published in “Yanqi Qiao, Yajun Peng, Yuanlin Zheng, Fangwei Ye, and Xianfeng Chen, Second-harmonic focusing by a nonlinear turbid medium via feedback-based wavefront shaping, Optics Letters, 42(10), 1895-1898 (2017)”.