Crystallization and optical nonlinearity in GeO₂-SiO₂ glass poled with ArF excimer-laser irradiation

We report on crystallization and second- and third-order optical nonlinearities of GeO₂-SiO₂ glass poled with ArF laser irradiation. With laser power and 100 mJ/cm²/pulse, the treatment generates crystallites in the glass, provided that the poling field strength is greater than 0.5 × 10⁵ V/cm. Its crystal structure is possibly the cristobalite phase of the Ge-Si-O system. A large coefficient of second-harmonic generation (SHG), d₃₃, is induced in the glass with the crystallite generation, and its value is comparable to d₂₂ of LiNbO₃. Moreover, the third-order optical susceptibility χ⁽³⁾ increases by a factor of approximately 15 compared to that of glass without the crystallites. Even after the d₃₃ coefficient decays out, the crystallites and χ⁽³⁾ are retained. The result suggests that the large SHG in the glass is not induced from inert second-order nonlinearity of the crystallites, but the main origin of the induction is the associated effective second-order nonlinearity with the formation of the space-charge field.