Second harmonic generation in transparent surface crystallized glasses with stillwellite-type LaBGeO5

Transparent optical nonlinear crystallized glasses with the composition of 25La2O3 25B2O3 50GeO2, stoichiometric to ferroelectric stillwellite-type LaBGeO5 crystalline phase, have been prepared by a two-step heat-treatment (first heat treatment: T1=670 °C, t1=10 h, second heat-treatment: T2, t2), and their second harmonic (SH) intensities have been examined using the Maker fringe method. The samples obtained by heat treatments at T2=720∼725 °C for t2=3 h show only surface crystallization and exhibit clear and fine (narrow) fringe patterns. The samples heat treated at T2=740 and 750 °C exhibit relatively strong SH intensities, but the fringe patterns in such samples are broad. It is proposed that SH waves generated from surface LaBGeO5 crystalline layers scatter at LaBGeO5 crystals formed in the interior of glass, causing the disappearance of fine fringe patterns.