Spatial, Spectral, and Coherence Mapping of Single-Molecule SERS Active Hot Spots via the Discrete-Dipole Approximation

The electromagnetic scattering properties of Ag nanoparticle aggregates known to be antennas for single-molecule surface-enhanced Raman scattering are investigated from a continuum electrodynamics perspective. High-resolution mappings of the spatial, spectral, and polarization dependence of the volumes of the aggregate's electromagnetic hot spots reveal multiple active regions for enhanced Raman scattering activity by molecular chromophores. Further analysis of these regions using maps of polarization surface-charge density shows that some hot spots are due to the collective and phase-coherent excitation of localized surface-plasmon resonances, whereas others derive from interfering plasmonic excitations resulting from scattering from gaps and surfaces. The latter are still capable of generating intense local fields at certain excitation energies, whereas the former tend to provide the most spatially delocalized regions of high electromagnetic-field strength.