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 spin-conserving 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.