Pulsed luminescence is observed in time- and frequency-resolved measurements of the $Z_3$ exciton luminescence of CuCl single crystals under excitation at the highly absorptive region of the $Z_{12}$ exciton resonance. A high-repetition-rate tunable picosecond uv light source was used. The pulse traveling time coincides well with the time calculated from the corresponding polariton group velocity. It is found that the redistribution of polaritons due to intraband relaxation is not important to the temporal behavior of the free-exciton luminescence, but the polariton propagation effect is essential, in contrast to the discussions found in previous work.

• Received 11 May 1988

DOI:https://doi.org/10.1103/PhysRevLett.61.1226