Relaxation phenomena in second-order nonlinearity of thermally and optically poled Nb2O5-TeO2 glasses

We report on relaxation of second-harmonic intensity for optically poled 15Nb 2 O 5 ·85TeO 2 bulk glass as well as thermally/electrically poled 15Nb 2 O 5 ·85TeO 2 bulk glass doped with Na + . The second-order nonlinearity induced by the thermal poling is rather stable. The decay of second-harmonic intensity is barely observed at 50{}C at least 5 h after ceasing of thermal poling. The activation energy for relaxation of second-harmonic intensity obtained from temperature dependence of relaxation time is 77 kJ mol -1 , which is somewhat larger than the activation energy for 20WO 3 ·80TeO 2 glass doped with Na + reported previously by the present authors. For optical poling of the 15Nb 2 O 5 ·85TeO 2 bulk glass, fundamental and second-harmonic waves from a nanosecond pulsed Nd : YAG laser were utilized to induce second-order nonlinearity in the 15Nb 2 O 5 ·85TeO 2 glass. The magnitude of second-order nonlinearity is saturated after optical poling for about 400 s. The decay of second-harmonic intensity is very rapid compared with the case of thermal poling. We speculate that some charged defects formed by laser irradiation are responsible for the second-order nonlinearity induced by optical poling.