Transparent TiO₂-PMMA nanohybrids of high nanocrystallinity and enhanced nonlinear optical properties

Nanohybrid thin films of titania-polymethyl methacrylate (TiO ₂-PMMA) with varying degrees of nanocrystallinity have been successfully synthesized via an in-situ sol gel-polymerization route, assisted by subsequent thermal and water vapor treatments. Posthydrothermal treatment by water vapor at relatively low temperatures led to a higher degree of crystallinity for TiO₂ nanoparticles than the conventional thermal annealing. The degree of TiO₂ crystallinity in the resulting nanohybrid films was studied by using XRD, FTIR, UV-Vis spectroscopies and HRTEM. The resulting nanohybrid thin films are highly transparent in the visible region, with an estimated band gap energies, E_g, close to that of anatase TiO₂ (∼ 3.20eV). The nanocrystallinity level of TiO ₂ phase strongly affects both linear and nonlinear optical properties of the nanohybrids. A significant enhancement in linear refractive index, n_o, up to 1.780 and a third-order nonlinear optical susceptibility, χ⁽³⁾ as high as 5.27 × 10^-9 esu, were demonstrated with the nanohybrid exhibiting the enhanced TiO₂ crystallinity and well-preserved PMMA matrix.