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 = 10h, 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.