Dark excitons and hot electrons modulate exciton-photon strong coupling in metal-organic optical microcavities

Polaritons, formed as a result of strong hybridization of matter with light, are promising for important applications including organic solar cells, optical logic gates, and qubits. Owing to large binding energies of Frenkel excitons (matter), strong matter-light coupling phenomena are possible at room temperature, high exciton densities, and even with low-quality-factor microcavities. In such cases, due to polaritons' high degree of delocalization, simultaneous effects from dark excitons and hot electrons may affect performance of potential devices. Their understanding, therefore, is of paramount importance, but their disentanglement in optical spectroscopy, however, thus far remained unattainable. Here, we overcome this challenge by careful and systematic analysis of transient polaritonic spectra, supported by analytical models. In doing so, we conclude that dark excitons affect the strength of exciton-photon coupling and manifest themselves as Fano-like polaritonic gain-loss spectra. Free electrons add additional loss component to and imprint a two-temperature dynamics on the polaritonic response. The developed general methodology can be applied to a variety of other microcavity structures. Our findings are significant for distinguishing polaritons and other excitations in studies of polariton-electron and plasmon-electron coupling phenomena as well as photonic control over photophysical and photochemical processes.