Experimental demonstration of a three-dimensional lithium niobate nonlinear photonic crystal

A nonlinear photonic crystal (NPC)(1) possesses space-dependent second-order nonlinear coefficients, which can effectively control nonlinear optical interactions through quasi-phase matching(2). Lithium niobate (LiNbO3) crystal is one of the most popular materials from which to fabricate NPC structures because of its excellent nonlinear optical properties(3-5). One- and two-dimensional LiNbO3 NPCs have been widely utilized in laser frequency conversion(6,7), spatial light modulation(8-12) and nonlinear optical imaging(13,14). However, limited by traditional poling methods, the experimental realization of three-dimensional (3D) NPCs remains one of the greatest challenges in the field of nonlinear optics(1,15). Here, we present an experimental demonstration of a 3D LiNbO3 NPC by using a femtosecond laser to selectively erase the nonlinear coefficients in a LiNbO3 crystal(16,17). The effective conversion efficiency is comparable to that of typical quasi-phase- matching processes. Such a 3D LiNbO3 NPC provides a promising platform for future nonlinear optical studies based on its unique ability to control nonlinear interacting waves in 3D configuration.