Ultrafast optical spectroscopy of large-momentum excitons in GaAs

Highly energetic excitons with wave vectors much larger than that of an abs orbed photon are excited in thin GaAs films. We observe propagation beats b etween three polariton modes up to 300 meV above the absorption edge employ ing femtosecond transmission spectroscopy. The dispersion relations of the coherent excitations are measured. Ultrafast exciton damping via scattering with nonequilibrium carriers and with phonons is investigated. The dynamic s is found to deviate strongly from the relaxation of free carriers. Theore tical simulations are in quantitative agreement with the data.