Excitonic Bloch Equations
Using femtosecond pump-probe spectroscopy, we have identified excitation induced dephasing as a major mechanism responsible for the breaking of the strong-coupling between excitons and photons. The effects of dephasing are observed on the transmitted probe pulse spectrum as a density dependent broadening of the exciton-polariton resonances and the emergence of a third resonance at high excitation density. A striking asymmetry in the energy shift between the upper and the lower polaritons is also evidenced. Using the excitonic Bloch equations, we quantify the respective contributions to the energy shift of many-body effects associated with Coulomb fermion exchange and photon assisted exchange processes and the contribution to collisional broadening.