Strong Local Field Enhancement of Raman Scattering Observed in Metal-Dielectric Gratings due to Vertical Fabry-Perot Modes of Surface Plasmon Polaritons

The height dependence of Raman light scattering on organic molecules deposited onto thick onedimensional metal-dielectric gratings is investigated theoretically and experimentally. We observe oscillations of the intensity of Raman light scattering as a function of the height of the metastructure's strips and demonstrate that these oscillations are explicitly associated with a type of Fabry-Perot effect. We show that the intensity of surface-enhanced Raman scattering can be additionally increased by local field enhancement (by an order of magnitude) at resonances of both pump and Stokes as well as anti-Stokes frequency, due to coupling of the surface plasmon polaritons on the top and bottom metal parts of the gratings via the vertical Fabry-Perot resonances over their middle dielectric part. A semianalytical one-mode model to describe the effect qualitatively is proposed. A significant deviation of the dispersion of FabryPerot-coupled plasmon polaritons from the surface-plasmon-polariton dispersion just folded into the first Brillouin zone is demonstrated.