ULTRAFAST ENERGY RELAXATION IN QUANTUM DOTS

By use of femtosecond differential absorption spectroscopy the electro n-hole pair relaxation times have been determined in CdSe quantum dots with excited state energies larger than the LO-phonon energy. The fas t energy relaxation within 500 fs is independent of the relation betwe en level spacing and LO-phonon energy and shows no bottleneck effect. The population dynamics is mainly governed by the hole relaxation proc ess. With increasing intensity, the formation of two-pair states (biex citons) of various energies dominates the population dynamics due to t he strong influence of Coulomb interaction in quantum dots even in the strong confinement. At high pair densities, the created two-pair stat es mediate a new relaxation path due to their stimulated decay into on e photon and one electron-hole pair with lower energy.