Controlling the crystallinity and nonlinear optical properties of transparent TiO₂-PMMA nanohybrids

Titania-polymer nanohybrid thin films represent a new class of potential materials for optoelectronic applications. While most such nanohybrid thin films lack control in crystallinity, we report in this paper transparent nanohybrids of titania-polymethyl methacrylate (TiO₂-PMMA) thin films having a remarkably enhanced nanocrystallinity. Post-treatments with water vapor at relatively low temperatures were applied on these thin films, following in situ sol-gel polymerization. They promoted rearrangement of flexible Ti-O-Ti bonds leading to enhanced crystallization of the TiO₂ phase. The degree of TiO₂ crystallinity in the resulting nanohybrid films was studied by using XRD, FTIR, UV-Vis spectroscopies and HRTEM. Both linear and nonlinear optical responses increase with the enhancement of TiO₂ crystallinity in the nanohybrids. The highest two-photon absorption coefficient (β) and nonlinear refractive index (n₂) were observed for the nanohybrid thin films with highest TiO₂ crystallinity, as confirmed by open and closed aperture Z-scan techniques using 250 fs laser pulses at 800 nm, having a value of 2260 cm GW^-1 and 6.2 × 10^-1 cm² GW^-1, respectively.