Second-order optical nonlinearity and change in refractive index in silica glasses by a combination of thermal poling and x-ray irradiation

摘要

Second-order optical nonlinearity was found to be generated in high-purity silica glasses when they were exposed to x-ray radiation and then thermally poled. Two kinds of second-order optical nonlinearity, near-surface nonlinearity localized in a thin layer near the sample surface and bulk nonlinearity spreading throughout the whole sample, were observed. The maximum χ_(33)~((2)) values of near-surface and bulk nonlinearity were 0.20 and 0.43 pm/V, respectively. Further, a change in refractive index of Δn=4×10~(-3) was also observed in the sample. The χ_(33)~((2)) value of the bulk nonlinearity and the change in refractive index increased with the x-ray intensity. On the other hand, the χ_(33)~((2)) value of the near-surface nonlinearity had a tendency to saturate when the intensity was higher than 1.3 mW/cm~(2). Based on absorption spectra of the samples, both the bulk nonlinearity and the refractive index change were found to be associated with point defects such as the E′ center (≡Si·) and nonbridging oxygen ions (NBO~(-),≡Si-O~(-)).