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^-2 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.