Evaluating Conditions for Strong Coupling between Nanoparticle Plasmons and Organic Dyes Using Scattering and Absorption Spectroscopy

Interactions between surface plasmons in metal nano particles and electronic excitations in organic chromophores have resulted in many notable findings, including single-molecule Raman scattering, nanoscale lasing, and enhanced fluorescence. Recently, plasmon-exciton interactions have been shown to reach the strong coupling limit, a nonperturbative regime in which a coupled plasmon-exciton system should be treated as a unified hybrid. Strong coupling effects could open up exciting possibilities for manipulating nano particle plasmons via molecular degrees of freedom, or vice versa. Optical properties of such hybrid systems can differ drastically from those of noninteracting components. Specifically, optical spectra of a strongly coupled system are expected to exhibit mode splitting due to Rabi oscillations of excitation energy between the system components. However, the interpretation of optical spectra in terms of strong coupling is not a straightforward matter. Here we clarify the nature of plasmon-exciton coupling for the case of rhodamine-6G (R6G) interacting with localized surface plasmons in silver nanodisks using scattering and absorption spectroscopy. We show that this system is only marginally able to reach the strong coupling limit, even for very high molecular concentrations and despite the appearance of obvious mode splitting in scattering. For lower molecular concentrations, the mode splitting we observe should be interpreted as being due to surface-enhanced absorption rather than strong coupling. These results allow us to evaluate the critical concentration necessary for reaching the strong coupling limit and propose conditions for observing strong coupling between single-particle plasmons and organic dyes, such as R6G.