Raman Images of a Single Molecule in a Highly Confined Plasmonic Field

TL;DR

This paper presents a theoretical framework explaining how highly confined plasmonic fields enable sub-nanometer resolution in Raman imaging of single molecules, revealing their electronic transition densities.

Contribution

It introduces a new theoretical model describing molecule-plasmon interactions that depend on spatial distribution, advancing understanding of high-resolution Raman imaging.

Findings

Resonant Raman images reflect electronic transition densities.

Simulated images match experimental results for porphyrin derivatives.

The theory provides a basis for analyzing molecular responses in confined plasmonic fields.

Abstract

Under the local plasmonic excitation, the Raman images of a single molecule can now reach sub-nanometer resolution. We report here a theoretical description of the interaction between a molecule and a highly confined plasmonic field. It is shown that when the spatial distribution of the plasmonic field is comparable with the size of the molecule, the optical transition matrix of the molecule becomes to be dependent on the position and the spatial distribution of the plasmonic field, resulting in spatially resolved Raman image of a molecule. It is found that the resonant Raman image reflects the electronic transition density of the molecule. In combination with the first principles calculations, the simulated Raman image of a porphyrin derivative adsorbed on the silver surface nicely reproduces its experimental counterpart. The present theory provides the basic framework for describing linear and nonlinear responses of molecules under the highly confined plasmonic field.