Dynamical behavior of the exciton polariton in CuCl: Coherent propagation and momentum relaxation

Abstract

Exciton-polariton luminescence spectra in CuCl single crystals of high purity are investigated at 2 K using picosecond time-resolved spectroscopy with high temporal and spectral resolutions. The excitation density is kept in the weak-excitation regime. Experimental results are analyzed in the ‘‘polariton-picture’’ framework for the exciton-photon system. Under excitation well above the lowest exciton energy, the intraband relaxation and the bottleneck effect of the polaritons are clearly manifested in the 2-LO-phonon replica of the exciton luminescence. On the other hand, pulsed responses are found in the polariton resonant luminescence near the transverse-exciton energy. The coherent propagation of polaritons and their radiative escape from the crystal surface produce the pulsed responses in the luminescence while they propagate to and fro between the front and the rear surfaces of the slab sample. They propagate for a distance of more than 20 μm. The momentum-relaxation probability per unit time is evaluated as a function of the energy from the intensity ratio of successive pulsed responses of the resonant luminescence. It varies from ${10}^9$ ${\mathrm{s}}^{\mathrm{-}1}$ near the transverse-exciton energy up to ${10}^{10}$ ${\mathrm{s}}^{\mathrm{-}1}$ near the longitudinal-exciton energy. The sample dependence of the polariton luminescence spectra is also elucidated by the competitive relation between the lifetime, propagation time, and the intraband relaxation time of the polaritons. The discrepancy between the transmission spectrum calculated from the momentum-relaxation probability and that obtained experimentally is discussed. This discrepancy is associated with the strong effect that very fine surface roughness may have on the transmission spectrum.