  While there have been numerous reports of long-range polariton transport at
room-temperature in organic cavities, the spatio-temporal evolution of the
propagation is scarcely reported, particularly in the initial coherent sub-ps
regime, where photon and exciton wavefunctions are inextricably mixed. Hence
the detailed process of coherent organic exciton-polariton transport and in
particular the role of dark states has remained poorly understood. Here, we use
femtosecond transient absorption microscopy to directly image coherent
polariton motion in microcavities of varying quality factor. We find the
transport to be well-described by a model of band-like propagation of an
initially Gaussian distribution of exciton-polaritons in real space. The
velocity of the polaritons reaches values of ~0.65x10^6 m s-1, substantially
lower than expected from the polariton dispersion. Further, we find that the
velocity is proportional to the quality factor of the microcavity. We suggest
this unexpected link between the quality-factor and polariton velocity and slow
coherent transport to be a result of varying admixing between delocalised dark
and polariton states.
