  There is growing interest in using strongly coupled organic microcavities to
tune molecular dynamics, including the electronic and vibrational properties of
molecules. However, very little attention has been paid to the utility of
cavity polaritons as sensors for out-of-equilibrium phenomena, including
thermal excitations. Here, we demonstrate that non-resonant infrared excitation
of an organic microcavity system induces a transient response in the visible
spectral range near the cavity polariton resonances. We show how these optical
response can be understood in terms of ultrafast heating of electrons in the
metal cavity mirror, which modifies the effective refractive index and
subsequently the strong coupling conditions. The temporal dynamics of the
microcavity are strictly determined by carriers in the metal, including the
cooling of electrons via electron-phonon coupling and excitation of propagating
coherent acoustic modes in the lattice. We rule out multiphoton excitation
processes and verify that no real polariton population exists despite their
strong transient features. These results suggest the promise of cavity
polaritons as sensitive probes of non-equilibrium phenomena.
