Excitation induced phase dependence of polariton-pulse propagation in GaAs

We report on amplitude and phase precision measurements of 40 fs pulses pro pagating through an ultrapure, 3.8 mu m thick GaAs platelet at 2 K. Additio nally to the propagation beats in the transmitted intensity, see Frohlich e t al., we found a characteristic behavior of the phase of the transmitted f ield. While for low excitation densities (< 10(13) pairs/cm(3)) phase shift s ascend at the individual beat minima. with increasing excitation the jump s of the phase shifts flip from +pi to -pi. A simple one-oscillator model f or the dielectric function fails to describe this effect, however, our more sophisticated analysis of the exciton line shape based on the solution of the Bethe-Salpeter equation for the susceptibility shows, that the flipping is caused by an asymmetry of the 1s-exciton line caused by scattering of e xcited carriers with each other and with the laser-induced polarization.