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.