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 T_g 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.