The phase coherence and orientational relaxation of excitons resonantly excited in optically thin GaAs layers by a picosecond light pulse is explored by means of time-resolved degenerate-four-wave mixing. At low excitation intensities where contributions due to exciton-exciton scattering can be neglected, a rapid loss of the phase coherence as well as of the optical alignment of the excitonic dipoles within 7 ps is observed. This implies a weak coherent coupling between excitons and photons and raises the question about the range of validity of the commonly employed polariton concept for GaAs.

• Received 27 January 1986

DOI:https://doi.org/10.1103/PhysRevLett.57.1797