The thickness-dependent saturable absorption behaviour of atomically thin bismuth selenide films has been optimized using 515 nm and 1030 nm laser excitation wavelengths. The studies have been performed by the Z-scan technique employing 400 fs laser pulses emitted by a fiber laser at a repetition rate of 100 Hz. The obtained results allowed the determination of the nonlinear optical parameters as a function of the film thickness. Additional studies have been carried out in order to determine the third-order optical nonlinearities as a function of the annealing temperature. The samples have also been studied by spectrophotometry, electron microscopy and X-ray diffraction, which allowed a deeper knowledge of the optical properties, as well as the crystalline structure of the Bi2Se3 material. •Giant ultrafast nonlinear optical responses have been recorded and compared with the literature.•Precise optimization of both the 2D material and excitation parameters for maximizing the optical nonlinearities.•The 2D materials are nonlinear optically active in the visible (515 nm) and infrared (1030 nm) parts of the spectrum.•The topological insulators were characterized by X-ray diffraction, scanning electron microscopy and spectrophotometry.