Metastable phases in HxLi1-xTaO3 waveguide layers and pure LiTaO3

The formation of high-temperature phases in low-doped H:LiTaO3 waveguide la yers in Z-cut LiTaO3 has been observed both by refractive-index and IR-spec tra measurements. This permits us to correlate the index jumps to the chang es of the OH bonds in the crystal lattice. Reversible phase transitions wer e detected in the temperature interval T = 50-200 degreesC over a wide rang e of hydrogen content including as-grown LiTaO3. The high-temperature phase s are metastable close to room temperature. This was demonstrated by tracin g the time evolution of the refractive-index change. It was shown that the high-temperature phases are responsible for the long-term refractive-index instabilities in both H:LiTaO3 waveguides and virgin LiTaO3