Nanosized structural transformation and nonlinear optical properties of lithium niobium germanate glasses

Glass formation in Li2O–Nb2O5–GeO2 (LNG) system, the structure and crystallization behavior of glasses that have compositions near the ratio Li2O/Nb2O5 ∼ 1 corresponding to stoichiometry of ferroelectric phase LiNbO3 were examined by differential thermal analysis, X-ray diffraction, small-angle neutron scattering and second harmonic generation (SHG). LNG glasses were subjected to heat treatments at temperatures in the range between Tg and temperature of the first exothermic peak in order to initiate nonlinear optical activity by nanoheterogeneity formation. Transparent nanostructured glasses with second-order optical nonlinearity were obtained for compositions characterized by the Li2O/Nb2O5 molar ratio ranging from 0.83 to 1.2 and GeO2 40–45 mol%. As prolonged heat treatments of nanostructured glasses result in crystallization of ferroelectric LiNbO3 the origin of SHG in transparent LNG glasses is supposed to be connected predominantly with polarity of nanoheterogeneities formed at the initial stage of phase separation.