Controlling the nonlinear absorption characteristics of TiO2/carbon nanocomposites on films

Abstract

In this study, nonlinear and saturable absorption behaviors of nanocomposite TiO2/carbon based materials in PMMA were investigated. Transparent thin films of TiO2, HC/TiO2, CNT/TiO2, CNT/TiO2-Pt nanocomposites were synthesized by spin coating. Linear absorption measurements indicate blueshift in energy due to a quantum confinement effect. Nonlinear optical absorption characteristics of the composite films were investigated using open aperture Z-scan technique with at the same fluence values for nanosecond and femtosecond pulse durations. To derive the transmission in open aperture Z-scan data, a theoretical model incorporating one photon, two photon, and free carrier absorptions and their saturations were considered. Nonlinear absorption coefficients and saturation intensity thresholds were extracted from the fitting of the experimental results for both pulse durations used for the experiments. Saturation intensity threshold values in femtosecond regime are significantly bigger than the values in nanosecond regime while nonlinear absorption coefficients values in nanosecond regime are significantly bigger than the values in femtosecond regime. These behaviors are attributed to localized defect states in the energy gap due to the loss of long-range order and defects such as dangling bonds and impurities. With thermal annealing processes at 200 °C and 450 °C nonlinear absorption coefficients and saturation intensity thresholds decrease due to reduction of localized defect states.