Detailed correlations between SERS enhancement and plasmon resonances in subwavelength closely spaced Au nanorod arrays

Depending on the experimental conditions and plasmonic systems, the correlations between near-field surface enhanced Raman scattering (SERS) behaviors and far-field localized surface plasmon resonance (LSPR) responses have sometimes been accepted directly or argued or explored. In this work, we focus on the attractive subwavelength closely spaced metallic nanorod arrays and investigate in detail the complex relationship between their SERS behaviors and plasmon resonances. This is achieved utilizing a combination of array fabrication, conventional LSPR spectra, SERS measurements, electron microscopy and numerical modeling. Three key factors that may impact the correlations have been comprehensively analyzed: the intrinsic near-field to far-field red-shift is found to be rather small in the lattice; the surface roughness has actually little impact on the spectral alignment of the near- and far-field responses; the continuous dependence of individual SERS peak heights on the Stokes Raman shift has been visualized and further clarified. By 3D finite element method (FEM) plasmon mapping, the physical origin of the collective resonances in the lattice is verified directly to be the Fabry-Perot-like cavity mode. The strong near-field enhancement results from the coupling of surface plasmon polaritons (SPPs) propagating at the two sidewalls of neighbouring nanorods forming the resonant cavity. The physical principles demonstrated here benefit significantly the optimization of nano-optic devices based on closely spaced metallic nanorod arrays, as well as the fundamental understanding of the near- and far-field relationship.