Ultrafast Dynamics of Organic Exciton-Polaritons.

Exciton-polaritons are increasingly proposed as a route to control molecular excited-state dynamics through hybridization with light, with potential implications for nanophotonic and quantum photochemical applications. However, directly characterizing the dynamics and excited-state signatures of exciton-polaritons remains a major challenge. In ultrafast transient absorption experiments, photoexcitation of polariton states typically results in a host of overlapping spectral responses that overwhelm any signals from the polariton population, leading to conflicting accounts of the photophysical pathways under strong coupling. In this study, we provide a framework to systematically identify the signatures of photoexcited polaritons through simultaneous control of optical cavity structure and excitation conditions. Under phase-matched excitation, we directly track coherent polariton relaxation dynamics on sub-100 fs time scales, revealing an ultrafast decay pathway that circumvents the intracavity dark states. We establish that the critical design parameter to achieve purely polaritonic dynamics is the degree of spectral overlap between the lower polariton and the tail of the molecular absorption. Our results provide a route to engineer coherent light-matter interactions at the nanoscale and call into question the prevailing framework of polariton photophysics, based on entropy-driven scattering into intracavity dark states.