MECHANISM OF 2ND-HARMONIC GENERATION FOR THERMAL POLED SILICA GLASSES

The mechanism of second-harmonic generation (SHG) was investigated for the thermal poled fused quartz and synthesized silica glasses. The cu rrent was measured for silica glasses during the thermal poling proces s. As a result, fused quartz with a relatively higher current generate d SHG. We discussed the relationship between crystallization from glas s to crystal and second-order nonlinearity, based on a continuous-heat ing transformation (C-H-T) diagram and the sample temperature, which w as calculated from the Joule's heat of the poling process. It was obse rved that grain size of the alpha-quartz and alpha-cristobalite crysta l was about 20 nm at the depth at 5 mu m from the anodic surface in th e poled fused quartz (553 K, 4 kV, 10(3.56) s), by a field emission sc anning electron microscope (FE-SEM) and X-ray diffraction (XRD) analys is. The distribution of intensity of the second-harmonic (SH) signal w as measured in a layer of SHG near the anodic surface. As a result the mechanism of SHG was deemed due to crystallization from silica glass to the orientated alpha-quartz possessing second-order nonlinearity un der a high electric field and Joule's heat in the thermal poling condi tion. It was considered that the intensity of SH signal was higher wit h increasing quantity of orientated nano-crystals, but was lower with decreasing orientation of the nano-crystals, but was lower with decrea sing orientation of the nano-crystals, along with decreasing electric field.