Probing Raman enhancements for a colloidal metasurface with optical gap distances in the quantum regime†

Abstract

While quantum plasmonic behaviors benefit many applications in quantum optics and nanophotonics, they can have detrimental effects in optical processes such as surface-enhanced Raman spectroscopy (SERS). Here, we measure the SERS intensity for a colloidal metasurface composed of Ag nanocubes coupled to an ultra-flat Au backplane to characterize the classical, crossover, and quantum regimes of the metasurface as a function of gap distance. Gap distance is controlled via chemical modification of the nanocube and backplane surface with self-assembled monolayers composed of alkanethiols with varying chain lengths. Electrodynamic simulations employing a quantum-corrected model are used to characterize the hybrid plasmon modes and charge transfer plasmon modes of the metasurface with respect to gap distance. These results indicate the importance of charge transfer effects in dictating SERS intensities for even relatively large optical gap distances due to the presence of molecular analytes.