Nonlinear absorption and nonlinear refraction of colloidal CdSe quantum dots (QDs) of two sizes were investigated in a wide spectral range with the Z-scan technique using a tunable femtosecond laser system. The nonlinear absorption was found to be the strongest close to twice the wavelength of the second exciton absorption band of the QDs. Based on nonlinear optical parameters the exciton binding energy has been determined. The current results are compared to the nonlinear properties of CdSe QDs presented in the literature. The features of this system relevant for multiphoton fluorescence microscopy applications are discussed.

Wide wavelength range (550–1600 nm) measurements of nonlinear optical properties of ca. 5 nm CdS quantum dots (QDs) colloidal solution were performed using the Z-scan technique with femtosecond laser pulses at 1 kHz. Parameters characterizing nonlinear absorption and nonlinear refraction were determined. Nonlinear absorption has the character of a two-, three- and four-photon process depending on the wavelength range, with the values of the relevant coefficients peaking at 750 nm, 1200 nm and 1500 nm, respectively. The order of the nonlinear processes was confirmed by measurements of the integral luminescence intensity vs. input excitation intensity. Maxima of the multi-photon absorption bands correspond to the second absorption band of the QDs. The maximum value of the two-photon absorption cross-section σ₂ was estimated to be 7.2 × 10⁻⁴⁷ cm⁴ s per QD at 750 nm.