Ultrastrong Plasmon-Exciton Coupling by Dynamic Molecular Aggregation

Plasmon-exciton polaritons arise from the coherent coupling of the localized plasmon of metal nanoparticles and the exciton of nearby resonant nanoemitters. The behavior of such systems is strictly defined by the initial choice of the metallic and excitonic materials, with only weak control possibilities, essentially limited to polarization-related effects or photoswitchable molecules. Here we propose a new strategy to control the plasmon-exciton splitting, based on the number of excitonic dipoles involved in the interaction. By integrating plasmonic arrays in a microfluidic device and injecting a dilute near-infrared cyanine dye solution, we are able to probe in real time the emergence and evolution of the strong plasmon-exciton coupling regime. When dye molecules selectively aggregate on silver as a result of chemical affinity, we observe a continuous increase of the Rabi splitting up to an exciton energy fraction as high as 35%, compatible with an ultrastrong coupling regime.