Coherent Polariton Dynamics in Quantum Wells

It is demonstrated that the exciton–photon coupling, or equivalently the polariton nature of the exciton, may lead to qualitative changes in the lineshape of time‐resolved four‐wave mixing. For small oscillator strength and hence small longitudinal–transverse splitting this coupling acts very similar to the recently discovered coherent exciton–exciton interaction, i.e. as an exponential rise of the time‐integrated signal for negative time delay. Here the exciton–photon coupling can be interpreted as a mediated exciton–exciton interaction. For larger coupling a distinct nonperiodic modulation evolves that reflects the oscillation between the two branches of the polariton dispersion. Spectrally and time‐resolved four‐wave mixing is shown to give direct access to the dynamics on the two branches. Corresponding femtosecond experiments on two‐dimensional magneto‐excitons are compared with numerical calculations.