Ultrafast Optical Spectroscopy of Large-Momentum Excitons in GaAs

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Abstract
Highly energetic excitons with wave vectors much larger than that of an absorbed photon are excited in thin GaAs films. We observe propagation beats between three polariton modes up to 300 meV above the absorption edge employing 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 dynamics is found to deviate strongly from the relaxation of free carriers. Theoretical simulations are in quantitative agreement with the data.