Strong coupling of localized surface plasmons and ensembles of dye molecules.

We have studied strong exciton-plasmon coupling in the films of Ag nanoislands as well as in the layer-by-layer (LBL) deposited films of Au nanoparticles (NPs) coated with highly concentrated rhodamine 6G (R6G) dye. Their absorbance and the reflectance spectra featured the peaks or dips, which were not characteristic of dye or NPs/nanoislands taken separately. The positions of the spectral maxima (or minima) in the dye-doped films, plotted against those in pristine Ag nanoislands films, resulted in the dispersion curves comprised of three branches. They could be described by the analytical model based on the Hamiltonian accounting for the unperturbed energies of the surface plasmon (SP) resonance, the two bands composing the absorption spectrum of R6G dye, and the exciton-plasmon coupling energy Δ. Its value was larger in Ag nanoislands films deposited on hyperbolic metamaterials (0.221 eV) than on glass (0.165 eV). The minimal gap between the upper and the lower branches was equal to ≈3Δ. The dispersion curves in the Au NPs LBL films could be described with the Hamiltonian equation at relatively small dye concentrations. At larger concentrations of R6G molecules, the spectral peaks shifted and became more pronounced. The corresponding dispersion curve could not be described in terms of the existing model, indicating the need for further theoretical studies.