A naturally grown three-dimensional nonlinear photonic crystal

Nonlinear frequency conversion via three-dimensional(3D) quasi-phase matching (QPM) process is experimentally realized based on a Ba0.77Ca0.23TiO3 (BCT) crystal. The ferroelectric domains in BCT crystal are observed, and the results reveal that the antiparallel domains distribute in three dimensions and can provide 3D reciprocal lattice vectors for QPM processes. Broadband petal-like second-harmonic patterns are achieved, which are well consistent with the theoretical quasi-cubic model of 3Dnonlinearphotonic crystals. Our work not only promotes the development of QPM technique but also builds a platform for 3Dnonlinear optics and quantum optics.