  Strong coupling between plasmons and excitons gives rise to new hybrid
polariton states with various fields of potential applications. Despite a
plethora of research on plasmon--exciton systems, their transient behaviour is
not yet fully understood. Besides Rabi oscillations in the first femtoseconds
after an optical excitation, coupled systems show interesting non-linear
features on the picosecond time scale. Here, we conclusively show that the
source of these features is heat that is generated inside the particles. Until
now, this hypothesis was only based on phenomenological arguments. We
investigate the role of heat by recording transient spectra of plasmon--exciton
core--shell nanoparticles with excitation off the polariton resonance. We
present analytical simulations that precisely recreate the measurements solely
by assuming an initial temperature rise of the electron gas inside the
particles. The simulations combine established strategies for describing
uncoupled plasmonic particles with a recently published model for static
spectra. The simulations are consistent for various excitation powers
confirming that indeed heating of the particles is the root of the changes in
the transient signals.

Optics;Mesoscale and Nanoscale Physics;Materials Science