Dependence of Nonlinear Optical Response of Anatase TiO_2 on Shape and Excitation Intensity

Nonlinear optical (NLO) properties of anatase TiO_2 with nanostructures of nanoparticle (NP), nanowire (NW) and annealed nanowire (NWA) are studied by open-aperture and closed-aperture Z-scan techniques with a femtosecond pulsed laser at wavelengths of 532 nm and 780 nm simultaneously. At 532 nm, when increasing excitation intensity, NLO absorption of TiO_2 NPs transforms from saturable absorption to reverse-saturable absorption. However, NWs and NWAs exhibit the opposite change. At 780 nm, all samples show reverse-saturable absorption, but have different sensitivities to excitation intensity. Due to the larger surface-to-volume ratio of NPs and less defects of NWAs by annealing, nonlinear optical absorption coefficients follow the order NPs\geNWs\geNWAs. The results also show that these shape and annealing effects are dominant at low excitation intensity, but do not exhibit at the high excitation intensity. The NLO refractive index of NPs shows a positive linear relationship with the excitation intensity, whereas NW and NWAs exhibit a negative linear relationship. The results could provide some foundational guidance to applications of anatase TiO_2 in optoelectronic devices or other aspects.