Electromagnetic coupling between subradiant plasmons and dye molecular excitons analyzed by spectral changes in ultrafast surface-enhanced fluorescence

Electromagnetic (EM) coupling between molecular exciton and plasmon has been studied using Rayleigh scattering or extinction spectroscopy. However, evaluating EM coupling involving a subradiant plasmon is challenging because this resonance does not manifest clearly in far-field spectra. In this study, we developed a method to evaluate such coupling using EM enhancement factors (FR) derived from ultrafast surface-enhanced fluorescence (ultrafast SEF). This SEF, which appears as a broad background in surface-enhanced resonant Raman scattering (SERRS) spectra, was measured using silver nanoparticle dimers containing dye molecules within their nanogaps. Our results show that the spectral peaks of FR for subradiant resonances appear near the dips in Rayleigh scattering spectra. Furthermore, these FR peaks exhibit blue shifts during the quenching processes of both ultrafast SEF and SERRS. We examined these static and temporal spectral properties using a coupled oscillator model composed of radiant plasmons, subradiant plasmons, and molecular excitons. The static properties were reproduced by increasing the linewidths of the radiant plasmon resonance, while the temporal properties were captured by decreasing the EM coupling energies between the exciton and both plasmon oscillators. These findings indicate that this methodology is a powerful tool for evaluating EM coupling between subradiant plasmons and molecular excitons.