  We report on the resonant coupling between localized surface plasmon
resonances (LSPRs) in nanostructured Ag films, and an adsorbed monolayer of
Rhodamine 6G dye. Hybridization of the plasmons and molecular excitons creates
new coupled polaritonic modes, which have been tuned by varying the LSPR
wavelength. The resulting polariton dispersion curve shows an anticrossing
behavior which is very well fit by a simple coupled-oscillator Hamiltonian,
giving a giant Rabi-splitting energy of ~400 meV. The strength of this coupling
is shown to be proportional to the square root of the molecular density. The
Raman spectra of R6G on these films show an enhancement of many orders of
magnitude due to surface enhanced scattering mechanisms; we find a maximum
signal when a polariton mode lies in the middle of the Stokes shifted emission
band.
