Nonlinear properties and structural rearrangements in thermally poled niobium germanate glasses

Second order nonlinear optical properties and structural rearrangements in GeO2-Na2O-Nb2O5 glasses were achieved by thermal poling. The effects of applied voltage as well as sodium and niobium contents on nonlinear optical (NLO)-active layer were investigated. Structural rearrangements in the anodic microlayer were investigated and occur due to sodium depletion promoting variation in bridging/non-bridging oxygen ratio and formation of a more polymerized network. Quantitative analysis of second harmonic generation signals confirm the electrooptical origin of the nonlinear optical response described by the electric-field-induced second harmonic model. chi(2) susceptibility values range from 0.42 to 0.76 pm/V depending on the niobium content. Lastly, the charge compensation mechanism with increasing applied voltage was described in detail. A progressive decrease in chi(2) for higher voltages was observed due to a greater poled thickness than expected by classical electrostatic models. In this case, the compensation mechanism occurs due to structural rearrangement, redox reactions, and motion of negative charges.