Structural and optical properties of annealed proton-exchanged waveguides in z-cut LiTaO3

The structural properties of annealed proton-exchanged waveguides in z-cut LiTaO3 have been investigated. Observations made in a polarizing microscope together with infrared-absorption measurements indicate the formation of monoclinic Li(1−x)HxTaO3 after proton exchange in pure pyrophosphoric acid. This monoclinic phase disappears after annealing at 300 °C. High-resolution x-ray diffraction shows that the c lattice parameter is 0.57% larger in the exchanged layer than in the bulk. During annealing the c parameter relaxes and in addition there appears a second diffraction peak in the x-ray rocking curves. This peak is attributed to α-phase Li(1−x)HxTaO3. A model for the increase of the refractive index is suggested. It shows that the main reason for the difference in index increase between proton-exchanged waveguides in LiTaO3 and LiNbO3 is the stronger spontaneous polarization of LiNbO3. The model predicts an increase of the refractive index of LiTaO3 after annealing which is experimentally observed. A short-term postannealing index decrease is suggested to be related to a ferroelectric domain reversal.