Model of charge migration during thermal poling in silica glasses: Evidence of a voltage threshold for the onset of a second-order nonlinearity

The creation of a second-order nonlinear susceptibility χ⁽²⁾ in thermally poled silica glasses is known to be related to positive charge migration. As opposed to currently used models, we herein propose a model that takes into account charge dissociation and charge recombination occurring during the poling process. This model, known as the Proctor and Sutton model, was used to determine the space-charge distribution within silica plates submitted to an electric field. In this paper, we perform theoretical calculations in order to adapt this model to the high values of the applied electric field during the poling process. Moreover, we prove that there is a voltage threshold below which no χ⁽²⁾ can be induced. We also point out the existence of a nonzero electric field within the entire sample. To test the validity of this model, we poled 1 mm thick Infrasil™ silica slabs using voltages ranging from 0 to 4 kV. Maker fringe patterns have been recorded in order to estimate the magnitude of the induced nonlinear χ⁽²⁾ coefficient. We report experimental evidence of a poling voltage threshold of 900 V in these samples.