Ensemble effect on Rabi oscillations of excitons in quantum dots

We observed excitonic Rabi oscillations (ROs) in quantum dots (QDs) by four-wave mixing (FWM) and pump-probe techniques. When a large number of QDs are excited by a spatially nonuniform laser, the inhomogeneous distribution of the transition dipole moments of the QD excitons and the spa-tial distribution of the input electric field induce distribution of the Rabi frequencies. We investigated this ensemble effect on the ROs of the excitonic polarization and the excitonic population observed in the FWM and pump-probe signals. By calculating the FWM and pump-probe signals including the Gaussian distribution of the pulse area, we first showed that a part of the ensemble effect can be cancelled out only in case of the ROs of the excitonic polarization. This reflects the dif-ferences between the characteristics of excitonic polarization and population.