Ahlfeldt, H., Webjörn, J., Thomas, P.A. and Teat, S.J.
Structural and optical properties of annealed proton-exchanged waveguides in z-cut LiTaO3
Journal of Applied Physics, 77, . (doi:10.1063/1.359477).
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The structural properties of annealed protonexchanged waveguides in z-cut LiTaO3 have been investigated. Observations made in a polarizing microscope together with infrared absorption measurements indicate the formation of monoclinic phase Li(1-x)H(x)TaO3 after proton exchange in pure pyrophosphoric acid. This monoclinic phase disappears after weak annealing. Characterization using high-resolution X-ray diffraction shows that the c lattice parameter is 0.57% larger in the exchanged layer than in the bulk. On 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 alpha-phase Li(1-x)H(x)TaO3. A model for the increase of the refractive index is suggested where the main reason for the difference in index increase between proton exchanged waveguides in LiTaO3 and LiNbO3 is the larger value of the spontaneous polarization of LiNbO3. The model predicts an increase of the refractive index on annealing of LiTaO3 which is of the same order of magnitude as that experimentally observed. The short-term post-annealing index decrease observed in a previous study is further examined and suggested to be closely related to the ferroelectric domain reversal.
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