Second-harmonic flat-top beam shaping via a three-dimensional nonlinear photonic crystal.

We experimentally extend the nonlinear Gaussian to flat-top beam shaping from one to two dimensions through a three-dimensional nonlinear photonic crystal. Employing a near-infrared femtosecond laser, we induce a modification inside lithium niobate to achieve a second-order nonlinear optical coefficient modulation in three dimensions. The flat-topped truncation of wavefront has been adjusted in a mutual perpendicular coordinate separately. Among the generated flat-topped beams, the optimal flatness is 97.1%, and the nonlinear conversion efficiency is 10 at the peak power of 37 kW with the interaction length of 630 µm. By adding an extra dimension, our work simultaneously enables full-wavefront flat-top distribution and nonlinear frequency conversion.