Kinetics of the Domain Structure in Uniform Electric Field in LiNbO₃ with Surface Layer Modified by Soft Proton Exchange

The creation of periodic domain structures in lithium niobate crystals makes it possible to realize highly efficient nonlinear optical interactions in optical waveguides. The influence of near-surface H+ ion concentration gradient on the domain kinetics in uniform electric field in lithium niobate crystals modified by soft proton exchange is studied. The spatial distribution of the H+ ions concentration in the surface layer is obtained by confocal Raman microscopy. The observed anomalous evolution of the domain structure at the surface represents the formation of stripe domains and their anisotropic growth along three directions. The domain imaging in the bulk allows revealing a comb-like domain shape. The obtained strong influence of the proton exchange duration on the threshold field is attributed to the action of the concentration gradient in the near-surface layer, which is considered as an analog of the internal bias electric field. It is shown that the internal bias field is proportional to the increase in the concentration gradient in the surface layer. The mechanism of the comb-like domain formation is proposed. The obtained results can be used for development of domain engineering methods in optical waveguides.