Formation of Ba2TiGe2O8 phase in BaO–TiO2–GeO2 glasses and their optical non-linearities

Second-order optical non-linearities for the transparent Ba2TiGe2O8 (BTG) crystallized glasses with the composition of 40BaO · 20TiO2 · 40GeO2 (BTG40), 33.3BaO · 16.7TiO2 · 50GeO2 (BTG50) and 30BaO · 15TiO2 · 55GeO2 (BTG55) have been examined. A very large d33 value for a transparent BTG crystallized glass in BTG55 was achieved, i.e., d33 = 22 ± 3 pm/V, being comparable to that of LiNbO3 single crystal. The d33 for BTG50 and BTG40 were measured to be 12 ± 3 pm/V and 7 ± 2 pm/V, respectively. This result indicates that d33 decreases with GeO2 concentration, when approaching the glass composition of the stoichiometric BTG crystal. For a larger d33 value in BTG crystallized glasses, it is considered that the high homogeneity of the oriented surface Ba2TiGe2O8 crystalline layer on the crystallized BTG55 glass is accomplished by a suitable amount of benitoite phase of BaTiGe3O9, which acts a nucleation site for the BTG crystal.