Generation of Second-Harmonic Bessel Beams by Transverse Phase-Matching in Annular Periodically Poled Structures

We demonstrate experimentally the generation of second-harmonic conical waves via parametric frequency doubling in a two-dimensional annular periodically poled nonlinear photonic structure fabricated in a stoichiometric lithium tantalate crystal. We apply the concept of nonlinear Bragg diffraction for explaining this effect, and show theoretically that in low-symmetry crystals this process results in generated second-harmonic waves being a superposition of the Bessel beams.