The hybrid light-matter character of exciton-polaritons enables strikingly long-range energy transport in organic materials. This is typically determined through steady-state spatial mapping, with little investigation of the transport dynamics. In particular, organic polariton propagation in has not been probed in the initial coherent regime, where photon and exciton wavefunctions are inextricably mixed. Hence key mechanistic questions remain poorly understood: What structural properties determine transport velocity and range? What is the role of intracavity dark states? Here, we use femtosecond transient absorption microscopy to directly image coherent polariton motion in microcavities of varying quality factor, an approach that allows us to circumvent the challenges of disentangling polariton signatures from optical artefacts. 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 ~0.8 × 10<sup>6</sup> m s<sup>-1</sup>, substantially lower than expected from the polariton dispersion. Surprisingly, we find that the velocity is proportional to the quality factor of the microcavity. We suggest the unexpectedly slow coherent transport and this link between quality-factor and polariton velocity to reflect an ultrafast interplay between delocalised dark states and the polaritons. Our results demonstrate the important role that intracavity dark states play even within the coherent regime and reveal a new lever to control polariton transport.

Tuning the coherent propagation of organic exciton-polaritons through dark state delocalization

ACS Spring 2022

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