  The evidence of strong coupling between plasmons and molecular excitons for
plasmonic nanoparticle (NP) dimers exhibiting ultra-sensitive surface enhanced
resonant Raman scattering is the observation of anti-crossing in the coupled
resonance. However, it is not easy to experimentally tune plasmon resonance of
such dimers for the observation. In this work, we theoretically investigate the
anti-crossing properties of the dimers coated by the thin dye films with
thicknesses greater than 0.1 nm and gap distances larger than 1.2 nm according
to the principles of classical electromagnetism. The plasmon resonance spectra
of these dimers are strongly affected by their coupling with the exciton
resonance of dye molecules. A comparison of the film thickness dependences of
dimer spectral changes with those of silver ellipsoidal NPs indicates that the
dipole plasmons localized in the dimer gap are coupled with molecular excitons
of the film much stronger than the dipole plasmons of ellipsoidal NPs.
Furthermore, the anti-crossing of coupled resonances is investigated while
tuning plasmon resonance by changing the morphology and refractive index of the
surrounding medium. The spectral changes observed for ellipsoidal NPs clearly
exhibit anti-crossing properties; however, the anti-crossing behavior of dimers
is more complex due to the strong coupling of dipoles and higher order plasmons
with multiple molecular excitons. We find that the anti-crossing for dimers is
clearly confirmed by the refractive index dependence of coupled resonance.
