Dynamics of Strongly Coupled Modes between Surface Plasmon Polaritons and Photoactive Molecules: The Effect of the Stokes Shift
We have investigated the dynamics of strongly coupled modes of surface plasmon polaritons (SPPs) and fluorescent molecules by analyzing their scattered emission polarization. While the scattered emission of SPPs is purely transverse magnetic (TM) polarized, the strong coupling with molecules induces transverse electric (TE) polarized emission via the partial molecular nature of the formed SPP–molecule polariton mode. We observe that the TM/TE ratio of the polariton emission follows the contribution of the molecular excited states in this hybrid mode. By using several types of molecules, we observe that, in addition to the coupling strength, which determines the contribution of the molecular excited states, also the Stokes shift of the molecule fluorescence influences the polarization of the emission: the larger the shift, the lower the TE-polarized emission. We argue that due to random orientation of the molecules, the emission of a fully coherent SPP–molecule polariton should be purely TM-polarized, like SPP. However, as a result of the unique microenvironments of the molecules in combination with thermal motion, this symmetry may break for individual excitations, providing a route to TE emission. The experimental results agree qualitatively with this model, including the symmetry breaking. Furthermore, the relaxation rate of the polariton correlates with the Stokes shift, so that TE emission can occur only if the Stokes shift is small and consequently the lifetime is long. Our results suggest that taking into account microscopic details of the molecules in SPP–molecule polaritons is important for a thorough understanding of the molecular dynamics of molecules under strong coupling with light modes. Theoretical models that include these details will be essential to systematically exploit strong coupling for plasmonics or even controlling chemical reactions. Graphic Abstract ACS Electronic Supporting Info
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