OBSERVATION OF PROPAGATION OF EXCITON-POLARITONS ALONG STACKING-FAULT PLANE IN LAYERED CRYSTAL BII3

Fast and ballistic spreading of excitons excited at the stacking fault interface in BiI3 crystals was observed in the temporal behavior of exciton luminescence with space- and time-resolved measurements. The spreading velocity reaches approximately 7 x 10(8) cm/s, indicating rapid energy transfer by an exciton-polariton along the layer plane. Time-of-flight measurements were also performed in this system, in the direction parallel to the crystal layer containing the stacking fault. Wavelength-sensitive time delay was detected in the picosecond time domain in the stacking fault exciton transition region. A group velocity of 6 x 10(8) cm/s is obtained, corresponding to the fast exciton propagation observed on the luminescence. An apparent anomaly in the times-of-flight of different excitons is shown to be related to the polarization dependence of the absorption spectra.