Structure of sol-gel-derived PbO-TiO2 amorphous monoliths with high third-order optical nonlinearity

The PbO-TiO2 amorphous monoliths with TiO2 content from 60 to 80 mol% were prepared by the sol-gel method. The amorphous monoliths obtained were somewhat porous, but had a high refractive index of more than 2.0. The third-order optical nonlinear susceptibility, (Chi) (3), of them evaluated from third-harmonic generation (THG) was 5.7 X 10-13 esu for the transparent monolith containing 60 mol% TiO2, and increased with increasing TiO2 content. The largest (Chi) (3) was 1.4 X 10-12 esu, for the monolith with 80 mol% TiO2. Ti-K edge x-ray absorption (XANES and EXAFS) analyses were carried out for gels and heat-treated monoliths. XANES spectra of Ti were very similar to that of crystalline PbTiO3 in which Ti is coordinated with six oxygen atoms. Similarly, EXAFS analysis on the basis of Fourier transformation and double-shell fitting method revealed sixfold Ti atoms both in the heated products and even in gels.