Cerenkov-Type Second-Harmonic Generation in Two-Dimensional Nonlinear Photonic Structures

We study the Cerenkov-type second-harmonic generation in several different two-dimensional nonlinear photonic structures formed in birefringent crystals with the 3 m symmetry. Depending on the degree of birefringence, we observe either single or double Cerenkov-like second-harmonic rings. We discuss the properties of these parametrically generated rings and show that their sixfold azimuthal modulation is associated with the hexagonal symmetry of the individual ferroelectric domains.