Résumé : Colloidal PbSe nanocrystals (Q-PbSe) have a bandgap tunable around 1.3 and 1.55textmum, providing excellent candidates for NIR optoelectronic applications, such as quantum dot LED?s or lasers (due to a high luminescence), or Mach-Zehnder interferometers (due to a high nonlinear refractive index). Here we present a detailed study on the nonlinear optical properties of Q-PbSe with the Z-scan technique. Different sizes between 3 and 6nm were used. The nonlinear refractive index n2 and nonlinear absorption have been measured as a function of wavelength (1.2-1.75textmum), optical intensity and nanocrystal concentration, both in suspension[1], and in a close-packed thin film. Our results demonstrate that n2 shows negative resonances near the optical transitions in Q-PbSe suspensions, with maximal values of (-10^-11cm^2/W) for a Q-PbSe concentration of 1textmuM. These resonances showed saturation behaviour. We attributed the high n2 in suspension to bi-exciton creation within the Q-PbSe. Measurements on close-packed films showed a positive n2 of the order of (10^-9cm^2/W). This value is 4 orders of magnitude larger than values for Si of AlGaAs at telecom wavelengths. At the first exciton transition, the films showed absorption saturation. These results, combined with the flexible processing of colloidal nanocrystals, suggests Q-PbSe thin films might be a promising material for all optical signal processing based on a high n2 or absorption saturation.[1]Moreels et al. Appl.Phys.Lett. 89, 193106 (2006)