  We theoretically study how the peculiar properties of the vacuum state of an
ultra-strongly coupled system can affect basic light-matter interaction
processes. In this unconventional electromagnetic environment, an additional
emitter no longer couples to the bare cavity photons, but rather to the
polariton modes emerging from the ultra-strong coupling, and the effective
light-matter interaction strength is sensitive to the properties of the
distorted vacuum state. Different interpretations of our predictions in terms
of modified quantum fluctuations in the vacuum state and of radiative reaction
in classical electromagnetism are critically discussed. Whereas our discussion
is focused on the experimentally most relevant case of intersubband polaritons
in semiconductor devices, our framework is fully general and applies to generic
material systems.
