  Strong light-matter coupling to form exciton- and vibropolaritons is
increasingly touted as a powerful tool to alter the fundamental properties of
organic materials. It is proposed that these states and their facile tunability
can be used to rewrite molecular potential energy landscapes and redirect
photophysical pathways, with applications from catalysis to electronic devices.
Crucial to their photophysical properties is the exchange of energy between
coherent, bright polaritons and incoherent dark states. One of the most potent
tools to explore this interplay is transient absorption/reflectance
spectroscopy. Previous studies have revealed unexpectedly long lifetimes of the
coherent polariton states, for which there is no theoretical explanation.
Applying these transient methods to a series of strong-coupled organic
microcavities, we recover similar long-lived spectral effects. Based on
transfer-matrix modelling of the transient experiment, we find that virtually
the entire photoresponse results from photoexcitation effects other than the
generation of polariton states. Our results suggest that the complex optical
properties of polaritonic systems make them especially prone to misleading
optical signatures, and that more challenging high-time-resolution measurements
on high-quality microcavities are necessary to uniquely distinguish the
coherent polariton dynamics.
