High second-order optical nonlinearities in thermally poled sol-gel silica

Silica glass samples prepared by a sol-gel process, the mixture of silica sol for gelation and colloidal silica particles, have been thermally poled in vacuum by continuous high voltage (8–12 kV) at elevated temperature (280 °C). High second-order nonlinearities (>1 pm/V), located under the anodic surface, have been measured and the values are higher than those obtained in fused silica glass, poled under the same conditions. A model for thermal poling is suggested which explains the experimental results in sol-gel silica and the difference between thermal poling of sol-gel and fused silica. The granular structure and the boundaries of sol-gel silica seem to play a major role in establishing the electrical properties of the depletion layer during and after poling.