Quantum beats between trion and exciton transitions in modulation-doped CdTe quantum wells

We investigate the coherence properties of trions (charged excitons) and neutral excitons in modulation p-doped $\mathrm{CdTe}/\mathrm{Cd}\mathrm{Mg}\mathrm{ZnTe}$ quantum wells by means of femtosecond four-wave-mixing (FWM) experiments. Under simultaneous and coherent excitation of the exciton and trion resonances, we observe a modulation of the FWM traces due to quantum beats. This excludes a complete spatial separation of trions and neutral excitons. A minor contribution from polarization interferences is explained by the superposition of signals from localized trions and free excitons.