  Room temperature cavity quantum electrodynamics with molecular materials in
optical cavities offers exciting prospects for controlling electronic, nuclear
and photonic degrees of freedom for applications in physics, chemistry and
materials science. However, achieving strong coupling with molecular ensembles
typically requires high molecular densities and substantial electromagnetic
field confinement. These conditions usually involve a significant degree of
molecular disorder and a highly structured photonic density of states. It
remains unclear to what extent these additional complexities modify the usual
physical picture of strong coupling developed for atoms and inorganic
semiconductors. Using a microscopic quantum description of molecular ensembles
in realistic multimode optical resonators, we show that the emergence of a
vacuum Rabi splitting in linear spectroscopy is a necessary but not sufficient
metric of coherent admixing between light and matter. In low finesse multi-mode
situations we find that molecular dipoles can be partially hybridised with
photonic dissipation channels associated with off-resonant cavity modes. These
vacuum-induced dissipative processes ultimately limit the extent of
light-matter coherence that the system can sustain.
