  Using femtosecond pump-probe spectroscopy, we identify excitation induced
dephasing as a major mechanism responsible for the breaking of the
strong-coupling between excitons and photons in a semiconductor microcavity.
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 Fermion
exchange and photon assisted exchange processes and the contribution to
collisional broadening.
