Plasmon-Exciton Coupling between Metallic Nanoparticles and Dye Monomers

We investigate electromagnetic coupling between localized surface plasmons and excitons in a Ag nanoparticle (NP) and monomer dye conjugate. Strong coupling is observed as Rabi splitting in the optical scattering response of the conjugate. We measure the strength of coupling as a function of NP-dye separation distance using bifunctional molecules that link the dye and NP with nanoscale control. Coupling strength follows a single-exponential decay with a decay length of 13.7 +/- 5 nm, indicating that the conjugates can be used as a plasmon ruler. In addition, we find that at separation distances below 2 nm the coupling strength is strongly decreased by quantum effects such as electron tunneling which interfere with plasmon-exciton hybridization. Using single NP spectroscopy to investigate conjugate coupling energy, we are able to tune separation distance to achieve coupling energies as high as 600 meV, among the highest energies reported for Ag NPs.