Observation of Surface-Enhanced Raman Scattering from Strongly Coupled Molecules and Localized Surface Plasmons in Resonance

The photoexcitation characteristics of materials have been investigated using surface enhanced Raman scattering (SERS) [1]. Signal enhancement by< 10⁴ times of magnitude is caused by resonances that are induced by optical absorption. Previously documented studies have found that the absorption spectra of chromophores that were adsorbed to Au or Ag plasmonic nanoparticles can be strongly perturbed. This perturbation of the electronic transition should affect the SERS profiles in ways that differ from those predicted based on simple enhancement of the local electromagnetic field. Previously documented studies discussed about the SERS effect as involving transition dipole coupling between the chromophore and the metal, and also theoretical simulation including resonance coupling between the molecular electronic transition and the optical transition of the nanoparticles. Despite these attempts to research the Raman characteristics when the system achieved hybridized states, the Raman features in the strong coupling regime when using LSPR have not been clarified yet.

In this study, polarized Raman scattering measurement was carried out using a hybridized system of Ag nano-dimer structures and organic dye molecules. Tuning of the localized surface plasmon resonance energy leads to modulation of the hybridized polariton energy. The anti-crossing behavior of the polariton energy implies a strong coupling regime with maximum Rabi splitting energy of 385 meV. The observation proves the effective Raman enhancement via the excitation of the upper and the lower blanches of the hybridized states at the gap of the metal dimer. Maximum Raman enhancement was obtained at an optimized resonant energy between the hybrid states and Raman excitation.

Reference

[1] M. Takase, F. Nagasawa, H. Nabika, K. Murakoshi, “Single Molecule Surface-Enhanced Raman Scattering as a Probe for Adsorption Dynamics on Metal Surfaces", in Frontiers of Surface-Enhanced Raman Scattering: Single-Nanoparticles and Single Cells, Y. Ozaki, K. Kneipp, and R. P. Aroca, John Wiley & Sons, New York, (2014).