Accurate measurement of Rayleigh scattering is crucially important for fundamental understanding of the plasmonic properties of meltimeric (≥ 3) nanoparticles that can be served as efficient SERS sensing platforms and nanophotonic materials. Thus, using the laser-scanning assisted dark-field microscopy that enabled to precisely collect far-field (Rayleigh) scattering from the centers of individual trimeric nanoparticles, we monitored spectral redistributions of oscillating coupled plasmonic modes as a function of trimer symmetry. As a consequence of the precise measurement of the polarization-resolved Rayleigh scattering spectra obtained from triangular trimers to linear trimers via elongated triangular trimers, the in-phase horizontally oscillating plasmonic mode with the largest dipole moment is found to be greatly increased by 20-folds, whereas the axially oscillating plasmonic mode with the second-largest dipole moment is dramatically decreased by 70-folds. Consequently, the overall quantity of the far-field scattering, the total sum of the individual coupled plasmonic modes, was gradually increased by 2-folds. The precise polarization-resolved Rayleigh scattering measurement also visualizes directly the directions of the radiation fields of individual oscillating coupled plasmonic modes, which would be valuable information in systematic controlling the polarization direction of the scattered light from the trimers. Overall, we showed an exemplary quantitative and extensive study of the coupled plasmonic modes from nanoparticles, giving a simple but clear insight.
Investigating the characteristics of the electromagnetic field generated inside plasmonically coupled metallic
nanostructures with a small nanogap <1 nm is significantly important for the rational deign of plasmonic
nanostructures with enormously enhanced electric field. Especially, plasmonic dimeric nanostructures have been
heavily studied, mainly because of relatively easier structural reproducibility among the coupled multimeric
nanostructures. However, controlling the geometrical structure with ~sub nm accuracy and the corresponding change
in the magnitude of the electric field in a single dimeric nanostructure is still highly challenging, such that it is
difficult to obtain reliable and reproducible surface-enhanced Raman scattering (SERS) signal essentially originating
from the enhanced electric field inside the nanogap. This is indeed a critical issue because the SERS enhancement
factors (EFs) exhibit a broad distribution (>106) with a long population tail even within a single SERS hot-spot,
which could be largely attributable to subtle change in the plasmonic nanostructures and the random orientation and
position of an analyte molecule within the plasmonic hot spot. Therefore, it is of paramount importance to
systematically investigate a relationship between the geometry of nanostructure and the optical signals at the singlemolecule
and single-particle levels.