Optical nonlinearities and ultrafast carrier dynamics in semiconductor nanocrystals

Femtosecond transient absorption in the visible and infrared spectral range s has been applied to study carrier dynamics and mechanisms for resonant op tical nonlinearities in CdSe nanocrystals (NCs) with a variety of surface p assivations. Sequential filling of the 1S, 1P, and 1D atomic-like electron orbitals, governed by Fermi statistics, is clearly observed in the NC bleac hing spectra recorded at progressively higher pump intensities. We observe that electron-hole (e-h) spatial separation strongly affects electron intra band dynamics. Such dependence indicates a nonphonon energy relaxation mech anism involving e-h interactions. A strong difference in electron and hole relaxation behavior in the stage following initial intraband relaxation is observed. In contrast to electron relaxation, which is sensitive to the qua lity of surface passivation (i.e., is affected by trapping at surface defec ts), depopulation dynamics of the initially-excited hole states are identic al in NCs with different surface properties, suggesting that these dynamics are due to relaxation into intrinsic NC states. In the regime of multipart icle excitation, a quantization of relaxation rates corresponding to differ ent multiple e-h pair states is observed. This effect is explained in terms of quantum-confined Auger recombination.