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

Silica glass samples prepared by a sol-gel process, the mixture of silica s ol for gelation and colloidal silica particles, have been thermally poled i n vacuum by continuous high voltage (8-12 kV) at elevated temperature (280 degrees C). High second-order nonlinearities (>1 pm/V), located under the a nodic surface, have been measured and the values are higher than those obta ined in fused silica glass, poled under the same conditions. A model for th ermal poling is suggested which explains the experimental results in sol-ge l silica and the difference between thermal poling of sol-gel and fused sil ica. The granular structure and the boundaries of sol-gel silica seem to pl ay a major role in establishing the electrical properties of the depletion layer during and after poling. (C) 1999 American Institute of Physics. [S00 03-6951(99)02418-3].