Second-order optical nonlinearities, d33 values, of fresnoite-type crystals, Ba2TiSi2O8 (BTS), Sr2TiSi2O8 (STS), and Ba2TiGe2O8 (BTG), have been evaluated by measurements on transparent surface-crystallized glasses consisting of fresnoite-type crystals with the Maker fringe technique. It is found that all these crystals show large d33 values, i.e., d33=13±2 pm/V for BTS, d33=7.2±0.4 pm/V for STS, and d33=22±3 pm/V for BTG. In particular, it is emphasized that BTG has an extremely large d33 value, comparable to LiNbO3 single crystal. It is clarified that the d33 value of fresnoite-type crystals increases with increasing c/a ratio in the lattice parameters of their structures. It is suggested that the increase in the c/a ratio results in the elongation of TiO5 pyramidal units along the c axis, consequently enhancing spontaneous polarizations in the fresnoite-type structure. Transparent surface-crystallized glasses consisting of fresnoite-type crystals are candidates for nonlinear optical materials.
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Takahashi et al. (2004) studied this question.
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