Ultrafast optical nonlinearities of CdSe quantum dots

Femtosecond differential absorption measurements have been performed on the quantum-confined transitions in CdSe microcrystallites. Spectral hole burning accompanied by increased absorption on the high-energy side is observed. The spectral positions of the induced features depend on the excitation wavelength. For excitation on the low-energy side of the lowest quantum confined transition, a shift of the burned hole with respect to the pump frequency is observed. It is explained in the framework of an inhomogeneous absorption line composed of broad homogeneous lines. A 25 ± 10-fs dephasing time is obtained from the measurements. The results are explained by bleaching of one-pair states and induced absorption caused by the photoexcited two-electron-hole-pair states. It is concluded that Coulomb effects are important even in quantum dots. They give rise to a ground state two-pair resonance on the low-energy side of the one-pair state and to higher-energy two-pair resonances which are spectrally located between the one-pair states.