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Nonlinear optical (NLO) properties of anatase TiO2 with nanostructures of nanoparticle (NP),nanowire (NW)and annealed nanowire (NWA) are studied by open-aperture and dosed-aperture Z-scan techniques with a femtosecond pulsed laser at wavelengths of 532 nm and 780 nm simultaneously.At 532 am,when increasing excitation intensity,NLO absorption of TiO2 NPs transforms from saturable absorption to reverse-saturable absorption.However,NWs and NWAs exhibit the opposite change.At 780hm,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≥NWs≥NWAs.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 TiO2 in optoelectronic devices or other aspects.