Quenching effects on crystallographic and optical properties of proton exchanged waveguides on Z-cut lithium niobate

The formation and decay of high temperature phases in H_xLi_1-xNbO₃ waveguide layers have been studied by means of waveguide mode spectroscopy and X-ray rocking curve measurement. The hydrogen doped layers with x∼0.47 (different x) in Z-cut LiNbO₃ were created by proton exchange reaction in melt benzoic acid. The high temperature phases were frozen by quenching from T=200°C. It has been shown that these phases possess ∼9% higher refractive index and ∼16% lower deformation in reference to H_xLi_1-xNbO₃ phase equilibrium at room conditions. The relaxation of the high temperature phase to equilibrium one has been traced at T=100°C. It is shown now that the phase transitions between the equilibrium and high temperature phases in H_xLi_1-xNbO₃ layers at high x, earlier detected by the refractive index variations, are also accompanied by drastic variations of crystal structure of the layer. The deformation change induced by the reversible transition is as high as ∼16% in reference to deformation created by doping. It may be supposed that the structural transformations are governed by some configuration redistribution of the protons within the possible crystal lattice positions.