Geometry-dependent anti-Stokes SERS radiation patterns from gold nanorod dimers

The geometry of individual plasmonic nanostructures and their collective arrangement plays a critical role in electromagnetic enhancement of surface-enhanced Raman scattering (SERS). Concomitantly, the same attributes also have a direct implication on plasmon resonance line-shapes that further affect the far-field radiation pattern of SERS in Rayleigh, Stokes and anti-Stokes spectral regimes. Here we numerically show how Fano-type plasmon resonance lineshapes in certain geometrical configurations of gold nanorod dimers can selectively influence the far-field radiation pattern in the anti-Stokes spectral region of SERS. We explored a variety of dimer angles (0° to 180°) between the gold nanorods, and compared their far-field radiation patterns in the anti-Stokes, Rayleigh and Stokes regions of their plasmon resonance. We found that angles 30° to 60° exhibited greater in-plane forward-to-backward intensity ratios in the anti-Stokes region, compared to other configurations. The results discussed herein highlight a three-way connection between the geometrical arrangement of plasmonic nanostructures, Fano-type lineshapes and far-field radiation patterns in SERS, especially in the anti-Stokes region. Our work will have implications not only in designing optical antennas for linear and nonlinear Raman scattering, but also in creating directional light sources based on nonlinear optical processes such as two-photon fluorescence and photon frequency upconversion.