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Femtosecond differential absorption measurements of the quantum-confined transitions in CdSe microcrystallites are reported. Spectral hole burning is observed, which is accompanied by an induced absorption feature on the high-energy side. The spectral position of the burned hole depends on the excitation wavelength. For excitation on the low-energy side of the lowest quantum-confined transition, a slight shift of the hole towards the line center is observed. The hole width increases with pump intensity and the magnitude of the induced transparency saturates at the highest excitation level. The results are consistently explained by bleaching of one-pair states and induced absorption caused by the photoexcited two electron-hole pair states. It is concluded that the presence of one electron in the excited state prevents further absorption of photons at the pair-transition energy and accounts for the major portion of the bleaching of the transition.>
Peyghambarian et al. (Sun,) studied this question.