Nonlinear optical characterization of Ag₂Se and PbSe colloidal quantum dots using 1030 nm femtosecond pulses

Nonlinear optical characteristics of chemically prepared silver selenide (Ag₂Se, 3.7 nm) and lead selenide (PbSe, 6.3 nm) colloidal quantum dots (QDs) are investigated using the 1030 nm femtosecond pulses. Ag₂Se QDs demonstrated two-photon absorption followed by saturable absorption. PbSe QDs initially showed the saturable absorption at small intensities of 1030 nm probe pulses, followed by reverse saturable absorption at stronger excitation. The difference in the positive nonlinear optical absorption mechanisms in these two QD colloids is attributed to their different bandgaps. The studied QD colloids exhibited the Kerr-induced positive nonlinear refraction. Transient absorption studies of the QDs showed that photoexcitation creates a hot exciton that forms band-edge exciton within 1.2 ps after excitation. Most of the electron-hole pairs further recombine through band-to-band recombination with a time constant of 1490 ps. Furthermore, the results from the transient absorption studies indicate the contribution of trap-assisted recombination occurs on a time scale exceeding that of the experimental time window.