Enhancement of second harmonic intensity in thermally poled ferroelectric nanocrystallized glasses in the BaO-TiO₂-SiO₂ system

The transparent nanocrystallized (heat-treatment: 750 ^{{ring operator}}C, 1 h) glasses consisting of ferroelectric Ba₂TiSi₂O₈ nanocrystals (size: 100-200 nm) have been prepared in 40BaO-20TiO₂-40SiO₂ glass, and the effect of a thermal poling (DC electric voltage: 8.8 kV/cm, temperature: 110-300 ^{{ring operator}}C, time: 1 h) on the second harmonic (SH) intensity has been examined. It is found that the formation behavior of nanocrystals at the surface differs from that in the interior of glass, giving a new insight into the well-known concept for nanocrystallization or homogeneous nucleation in glass. The Maker fringe patterns with fine structures are observed, indicating a high orientation of the polarization axes of Ba₂TiSi₂O₈ crystals formed at the surface. The prominent enhancement in the SH intensity is observed due to thermal poling, demonstrating that thermal poling is an effective method in enhancing anisotropic polarization of ferroelectric Ba₂TiSi₂O₈ nanocrystals in crystallized glasses. The present study proposes that the Maker fringe pattern for SH intensity of nanocrystallized glasses is very sensitive to the anisotropic polarization of nanocrystals at the surface, indicating the importance of the Maker fringe technique for the characterization of nanocrystals in materials.