  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.
