  The manipulation of molecular excited state processes through strong coupling
has attracted significant interest for its potential to provide precise control
of photochemical phenomena. However, the key limiting factor for achieving this
control has been the dark state problem, in which photoexcitation populates
long-lived reservoir states with similar energies and dynamics to bare
excitons. Here, we use a sensitive ultrafast transient reflection method with
momentum and spectral resolution to achieve the selective excitation of organic
exciton-polaritons in open photonic cavities. We show that the energy
dispersions of these systems allow us to avoid the parasitic effect of
reservoir states. Under phase-matching conditions, we observe the direct
population and decay of polaritons on time scales of less than 100 fs and find
that momentum scattering processes occur on even faster timescales. We
establish that it is possible to overcome the dark state problem through
careful design of strongly coupled systems.
