Direct Evidence of Ultrafast Energy Delocalization Between Optically Hybridized J‐Aggregates in a Strongly Coupled Microcavity

Abstract Strong coupling between light and matter in a microcavity can produce quasi‐particle states termed cavity‐polaritons. In cavity architectures containing more than one excitonic species, the photon mode can simultaneously couple to the different excitons, generating new ʻhybrid‐polaritonʼ states. It is demonstrated that such hybrid polariton states can energetically connect different molecular species, even when their intermolecular distance is much larger than the Förster transfer radius. Here, this mechanism is unveiled and observed in the time domain energy delocalization in a strongly coupled cavity containing two layers of donor and acceptor molecules, separated by an inert spacer layer of 2 µm thickness. 2D electronic spectroscopy is used, a technique that provides simultaneously high spectral and temporal resolution, to probe the dynamics of the energy flow processes following ultra‐fast excitation. It shows that energy is almost instantaneously delocalized among the polariton states, providing a direct connection between very highly separated donor and acceptor molecules. The results are of potential significance for light‐harvesting devices, optoelectronics, and bio‐photonic systems.