  Strong coupling of plasmonic excitations and dipolar emitters, such as
organic molecules, have been studied extensively in the last years. The
questions whether strong coupling can be achieved with a single molecule only
and how this is unambiguously proven are still under debate. A critical issue
of plasmonic in contrast to photonic systems is additional excitonic line
broadening, which is often neglected when modeling such systems. This has led
to too optimistic design predictions or incorrect interpretation of ambiguous
experimental data, for example in models relying on Maxwell solvers without
self-consistent incorporation of line broadening effects. In this paper, we
present a heuristic modeling approach for strongly coupled systems based on
plasmonic nanoparticles and dipolar emitters that accounts for such broadening
and elucidates on recent experiments with single emitters. We explicitly focus
on a clear and intuitive classical description that utilizes well-established
methods, easy to use within typical Maxwell solvers. The heuristic model (i)
provides experimentally relevant numbers like emitter densities and spectra
(ii) allows to discriminate systems, which can reach the strong coupling regime
from those, which can not (iii) allows to identify optimization routes and (iv)
nicely matches with experimental findings. In particular, we employ an approach
related to quasi normal modes and extinction simulations where the excitonic
system is represented by a frequency dependent permittivity. As examples, we
investigate two configurations with many, but also single emitters, which have
been studied in recent experiments.
