Large second-order optical nonlinearity in 30BaO-15TiO₂- 55GeO₂ surface crystallized glass with strong orientation

The correlation between the orientation degree of surface crystalline phase and the second-order optical susceptibility, d coefficient for second-harmonic generation, in 30BaO-15TiO₂-55GeO₂ (BTG55) crystallized glass was investigated using x-ray diffraction measurements and a Maker fringe technique. The d coefficient in BTG55 crystallized glass was confirmed to depend on the orientation degree of the surface crystalline phase. An oriented fresnoite (Ba₂TiGe₂O₈) phase was observed in transparent BTG55 crystallized glass which had been heat treated at 720 °C for 3 h. The orientation degree along the polar c axis in the fresnoite phase increased with increasing removal thickness by polishing from the surface up to several microns. A d coefficient in transparent BTG55 crystallized glass of 24±3 pm/V has been obtained, which is the largest value in crystallized glass ever reported.