| This study examines the physics underlying the formation of a second-order nonlinearity through thermal poling in bulk fused silica glass. The technique of thermal poling involves applying a large voltage across the glass at an elevated temperature, and then allowing the glass to cool with voltage applied. Nominally, a nonlinearity is created from a high-field space-charge region that forms within ;In situ experiments are reported that monitor second-harmonic generation and electrical conductivity as the nonlinearity is formed, and as the applied field's polarity is reversed as well as switched on and off. Complex dynamic behavior was observed.;Two microscopic probe techniques are presented that indicate the extent of the nonlinearity in the glass. Hydroflouric acid etching was used to etch the cross-section of thin samples. Secondary ion mass spectrometry (SIMS) was used to probe impurity distribution levels under the two electrode surface of poled samples.;A two-carrier conductivity model is developed, based on very slow ion-exchange from the surface as well as fast movement of a host alkali ion. This model provides the first explanation of experimental observations such as different response times, an incubation period on field reversal or initial poling, and a second harmonic signal at the cathode. These more complex behaviors were not predicted by the earlier single-carrier model. |
