Femtosecond laser writing is a powerful three dimensional (3D) engineering technique for materials processing. It has been used to modify the refractive index of dielectric materials (e.g., glass, crystals) to construct optical waveguides for diverse applications. Lithium niobate (LiNbO3) is a multifunctional crystal with excellent nonlinear optical properties. Nonlinear photonic crystal (NPC) could control nonlinear optical interactions through quasi-phase matching (QPM) due to the periodical nonlinear coefficients. Although the 1D and 2D NPCs have been fabricated by electric-field poling, the implementation of 3D NPCs remains as a major challenge in nonlinear optics because of the limitation of traditional poling methods.
Wei et al. †[1] demonstrated a new technique to fabricate 3D NPC in LiNbO3 crystal by using femtosecond laser engineering of domain structures. Instead of the traditional domain inversion, the authors utilized femtosecond laser pulses to selectively erase the nonlinear coefficients in LiNbO3 crystal, by which the domain structures of the whole crystal were modulated in a periodic way. This design was considered to satisfy the QPM mechanism as well. By using a Čerenkov-type second-harmonic (SH) confocal microscopy, the effective conversion efficiency based on laser-engineered LiNbO3 was comparable to that of typical QPM processes. This work demonstrates the advantage of femtosecond laser 3D writing for materials processing, and paves a way to construct intriguing nonlinear optical platforms for controlling nonlinear interacting waves in 3D configuration.