We estimated an optical second order nonlinearity of ferroelastic Ba2TiGe2O8 crystal in transparent crystallized glasses by means of Maker fringe technique for the first time. Transparent crystallized glasses were fabricated using a glass with the composition of 30BaO.15TiO2.55GeO2, similar to that of Ba2TiGe2O8. The crystalline layer consisting of highly oriented Ba2TiGe2O8 crystals was formed at the surface of about 9 µm thickness, and a clear second harmonic generation was observed. The optical second order nonlinearity of d33 for transparent Ba2TiGe2O8 crystallized glass was found to be ~10 pm/V, being comparable to d22 and d31 of LiNbO3, and demonstrating that Ba2TiGe2O8 crystal is an excellent candidate for nonlinear optical crystals.
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