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

Second harmonic generation properties have been studied in 23K2O·27Nb2O5·50 SiO2 glass subjected to thermal poling. The poling-induced optical nonlinearity, with χ(2)=3.8pm/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.