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 matching². 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.