Anti-crossing property of strong coupling system of silver nanoparticle dimers coated with thin dye molecular films analyzed by electromagnetism.

Abstract

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, experimentally tuning the plasmon resonance of such dimers for the observation is difficult. In this work, we calculate the anti-crossing property of dimers coated with thin dye films according to the classical electromagnetism. This property is quantitatively evaluated according to the coupled oscillator model composed of a plasmon and a molecular exciton representing the molecular multi-level system. 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 behavior 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 property; 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.