Lateral motion of confined excitonic polaritons

Abstract

Transient-grating measurements on high-quality ${\mathrm{Z}\mathrm{n}\mathrm{S}\mathrm{e}/\mathrm{Z}\mathrm{n}\mathrm{S}}_{x}{\mathrm{Se}}_{1\ensuremath{-}x}$ heterostructures reveal a strong increase of the lateral polariton-transport coefficient with the ZnSe thickness $L$ in a range from 5 to 30 excitonic Bohr radii. In this weak-confinement regime a strict quantization of the polariton--wave-vector component ${k}_{z}$ normal to the layer occurs, as is experimentally verified for our samples. On this basis, it is shown by use of polariton theory that the observed increase of the lateral transport coefficient can be explained by an intrinsic drastic increase of the lateral polariton group velocity. This is caused by a transition from quasiexcitonic to polaritonic transport when the first quantized mode with ${k}_{z}^{1}=\ensuremath{\pi}/L$ passes through the bottleneck region of the lower polariton branch for increasing $L$.