INTERFEROMETRIC OBSERVATION OF POLARITON PULSE-PROPAGATION THROUGH GAAS

We report amplitude and phase resolved measurements of polariton propa gation excited with 45 fs laser pulses in a pure bulk gallium arsenide sample of thickness 4.2 mu m at 1.8 It. The time development of the e nvelope E(tau) and the phase phi(tau) of the transmitted pulses was me asured with inter ferometric cross correlation. The amplitude shows ap eriodic beating behavior that is well described with polariton propaga tion ill the framework of a local dielectric response. However, the in stantaneous frequency of the transmitted pulse is not constant as pred icted by the model but gradually approaches the exciton resonance with in roughly 10 ps from 2 meV below.