Role of Coulomb correlations for femtosecond pump-probe signals obtained from a single quantum dot

We present a theoretical analysis of femtosecond pump-probe experiments performed on a single negatively charged quantum dot. The influence of Coulomb-correlation effects as well as carrier relaxation on transient transmission change signals is investigated. Our model describes ultrafast disappearance of the fundamental trion absorption due to instantaneous Coulomb renormalizations and a delayed onset of gain at the same frequency, as found in the measurements. Going beyond previous experimental information, we predict that after spinconserving carrier relaxation, new optical transitions exhibiting either gain or absorption should emerge that build up on a picosecond timescale. The time dependence of these new transitions provides insight into details of the carrier relaxation processes.