  We report strong coupling of a monolayer of J-aggregated dye molecules to the
whispering gallery modes of a dielectric microsphere at room temperature. We
systematically studied the evolution of strong coupling as the number of layers
of dye molecules was increased, we found the Rabi splitting to rise from 56 meV
for a single layer to 94 meV for four layers of dye molecules. We compare our
experimental results with 2D numerical simulations and a simple coupled
oscillator model, finding good agreement. We anticipate that these results will
act as a stepping stone for integrating molecule-cavity strong coupling in a
microfluidic environment since microspheres can be easily trapped and
manipulated in such an environment, and provide open access cavities.
