The features of nonlinear absorption of two-photon excited excitons in CdSe/ZnS quantum dots

The nonlinear absorption features have been discovered in the case of two-photon resonant excitation of excitons in colloidal CdSe/ZnS quantum dots by powerful picosecond laser pulses. Two utilized methods — measurement of the traces of luminescence (the dependence of luminescence intensity upon distance) and nonlinear transmission measurement — have allowed us to determine the coefficients of two-photon absorption and to reveal the deviation from two-photon absorption and from the dependence of two-photon excited photoluminescence intensity upon distance at high excitation. The decreasing of two-photon absorption at high excitation was explained by accompanying processes of state-filling and Stark shift of exciton transition. The extinction of luminescence at high excitation was attributed to dominating nonradiative Auger recombination and to state-filling and Stark shift of exciton transition provoking the decreasing of two-photon absorption probability.