Structural phase diagram of proton-exchange HxLi1-xNbO3 waveguides in lithium niobate crystals

It is shown that strained single-crystalline HxLi1-xNbO3 layers of a lithiu m niobate crystal formed due to proton exchange and used as optical wavegui des are characterized by an informative structural phase diagram. Under var ious conditions of proton exchange and post-exchange annealing, up to seven HxLi1-xNbO3 phases can arise. All of them have the crystal structure of th e lithium niobate-type. The dependence of the structure parameters on refra ctive indices of the ordinary and extraordinary beams was studied for each of newly identified phases.