Structuring light beams via nonlinear diffraction in 3D nonlinear photonic crystal.

• We have designed and fabricated a new type 3D nonlinear photonic crystal containing two-layer fork-shaped χ(2) gratings. Different from the periodic fork structures in Nat. Commun. 10 , 4193 (2019), these fork gratings are aligned in an antiparallel configuration. In this way, we are able to study the interference of these layers in the context of nonlinear wave front shaping, instead of pure energy flow in Nat. Commun. 10 , 4193 (2019). • We show 3D nonlinear photonic crystals have advantages over the one- and two-dimensional peers because the multiple χ(2) layers can lead to their own nonlinear diffracted second harmonics and their subsequent interference offers more possibilities to generate structured beams at the harmonic frequency. According to the best of our knowledge, no similar study has been conducted prior to this work. We show that nonlinear diffraction in nonlinear photonic crystal offers a convenient way to generate structured light at new frequencies. To this end we fabricate a 3D nonlinear photonic crystal containing two layers of fork-shaped ferroelectric domain gratings. Illumination of the structure by a fundamental Gaussian beam leads to the generation of two co-propagating second harmonic vortices with opposite topological charges, whose subsequent interference forms Hermite–Gaussian (HG) beams in far field. [ABSTRACT FROM AUTHOR]