  The way molecules absorb, transfer, and emit light can be dramatically
modified by coupling them to optical cavities. The extent of the modification
is often defined by the cavity-molecule coupling strength. Evaluating this
coupling strength for different types of modes supported by a cavity is crucial
in designing cavities for molecule-cavity coupling. Here we probe a unique
multimode cavity, a dielectric microsphere, also called a soft-cavity, which
supports two distinct types of mode, dark-field scattering (DFS) modes and
whispering gallery modes (WGM). Though seemingly similar, these modes show
different characteristics such as spatial electric field profile, resonance
line-width etc. We investigated coupling of a mono-layer of J-aggregated dye
molecules and a dielectric plastic microsphere using two techniques, far-field
excitation and evanescent excitation to generate DFS modes and WGMs
respectively. We found that using WGMs we observe a clear signature of strong
coupling, whereas with DFS modes we do not. We compared our experimental data
to a simple coupled oscillator model and performed finite-element method based
numerical simulations to provide a clearer understanding of our experimental
findings.
