Periodical poling of lithium niobate crystals with Li-enriched surface layer

We present two approaches for production of periodically poled lithium niobate crystals by surface modification of congruent crystals: first - by Li enrichment during annealing and second - by deposition a thin layer of stoichiometric lithium niobate film on the +Z face of a crystal. Both methods have allowed to create on the surface of congruent sample a layer with composition close to stoichiometric and to reduce a poling electric field without changing of the composition of crystal volume and thereby to keep all advantages of congruent crystal.