Polariton Wave Packet Propagation in the Exciton Resonance of a Semiconductor

This paper reports the first experimental observation of "slow" pulse propagation in the exciton-polariton resonance of a direct-gap semiconductor. The propagation delay of short light pulses transmitted through a 3.7-\ensuremath{\mu}m-thick GaAs crystal was measured directly with a cross-correlation technique. Polariton group velocities from $\frac{{c}_{\mathrm{vac}}}{3.6}$ down to $\frac{{c}_{\mathrm{vac}}}{2000}$ were found when the wave-packet center frequency was tuned through the $n=1$ exciton resonance.