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

TL;DR

This study visualizes the initial coherent transport of organic exciton-polaritons, revealing a link between microcavity quality and polariton velocity, influenced by dark state delocalization.

Contribution

It provides direct femtosecond imaging of coherent polariton motion and uncovers the role of dark states in modulating transport velocity in organic microcavities.

Findings

Polariton transport is band-like and Gaussian in real space.

Polariton velocity is proportional to the microcavity quality factor.

Dark state delocalization influences the coherent transport velocity.

Abstract

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.