A process for reducing the poling field of congruent LiNbO.sub.3 and LiTaO.sub.3 crystal-based nonlinear optical devices and for the production of domain structures with precise reproducibility of the main parameters by depositing a thin layer of a stoichiometric LiNbO.sub.3 film on the Z-face of a congruent LiNbO.sub.3 or LiTaO.sub.3 wafer. A new domain nucleation, evolution and stabilization process is provided to afford a uniform, short period domain superstructure for the conversion of near infra-red light to near ultraviolet, blue and green light.