Observation of propagation of exciton-polaritons along stacking fault plane in layered crystal BiI 3

Abstract

Fast and ballistic spreading of excitons excited at the stacking fault interface in BiI 3 crystals was observed in the temporal behavior of exciton luminescence with space- and time-resolved measurements. The spreading velocity reaches ∼7 × 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 × 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.