Abstract
We studied the steady-state and time-resolved luminescence properties of CdTe and CdTe/CdS core/shell quantum dots by one-and two-photon excitation with a femtosecond laser of low intensity. The 800 nm excitation causes a blue shift of the emission peak compared with 400 nm laser excitation. Near-quadratic laser power dependence of the up-conversion intensity and biexponential decay kinetics are observed. It is found that upconversion luminescence is composed of a photoinduced trapping and a band edge excitonic state. The blue shift of the emission peak is caused by the relative change in luminescence intensity between excitonic and trapping states. Two-step two-photon absorption involving the surface as intermediate states has been proposed for the mechanism of up-conversion luminescence of CdTe/CdS quantum dots.
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