Formation of fresnoite-type Ba₂TiGe₂O₈ crystallites in BaO-TiO₂-GeO₂ glass thin film and its optical nonlinearity

We report on fabrication of the BaO-TiO₂-GeO₂ glass thin films on a fused silica substrate by RF sputtering, and investigation of the crystallization behavior, surface texture and the optical non-linearity in the glass thin film by means of AFM, XRD, Raman and Maker fringe techniques. Although the Ba₂TiGe₂O₈, hereafter abbreviated to BTG, phase with second-order optical nonlinearity was mainly crystallized, the precipitation of the sub-phase of BaGe₄O₉ was also confirmed in the heat-treated glass thin films. In the glass thin film heat-treated at heat-treatment temperature (7_HT) = 730-750°C, the (211) preferential orientation of BTG phase and an upheaval about 300-400 nm were confirmed at the surface. These transparent crystallized glass thin films indicated a clearly Maker fringe patterns, demonstrating a second-order optical nonlinearity, and the effective second-order nonlinear optical constant, d _eff, was estimated to be ∼0.8 pm/V.