Electric field induced structural modification and second order optical nonlinearity in potassium niobium silicate glass

Second harmonic generation properties have been studied in 23 K2O · 27Nb2O5 · 50 SiO2 glass subjected to thermal poling. The poling-induced optical nonlinearity, with χ(2) = 3.8 pm/V, has been related to structural modifications within a surface layer of a few microns on the anode side, as evidenced by means of confocal micro-Raman mapping along the sample thickness. The data indicate that the structural changes result from a charge transport process that causes network modifications in an alkali depleted layer whose thickness is comparable with that of the non-linear region. The Raman data also indicate that in the alkali depleted layer the network polymerization degree increases as a consequence of ion migration. The origin of the nonlinearity and the mechanisms activated by poling are discussed. The mechanism of non-bridging-oxygen to bridging-oxygen bond switching is proposed to explain ion migration and the subsequent structural changes in the glass.