New transparent crystallized glasses with optical nonlinear LiBGeO₄ crystals

Crystallization behaviors of the glass with a composition of 25Li₂O·25B₂O₃·50GeO₂ corresponding to lithium borogermanate LiBGeO₄ have been examined. It has been confirmed that the LiBGeO₄ crystalline phase is formed at the surface of heat-treated glasses. The second harmonic (SH) generation is found from transparent surface crystallized glasses, demonstrating for the first time that the LiBGeO₄ phase shows optical nonlinearity. The SH intensity of LiBGeO₄ crystallites (powdered state) prepared through crystallization is about ten times as large as that of pulverized α-quartz. The SH intensity of transparent crystallized glasses (bulk state) with crystalline layers of 3-4.5 μm thickness increases with increasing heat treatment temperature (540-560°C) and time (1-6 h), and the maximum SH intensity among the samples studied is in the order of 1/10 in comparison with that of α-quartz single crystal. The transparent crystallized glass obtained by heat treatment at 550°C for 3 h exhibits a clear and fine Maker fringe pattern, indicating a highly orientation of LiBGeO₄ crystals at the surface.