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

Sai Duan, Guangjun Tian, Yongfei Ji, Jiushu Shao, Yi Luo

Published 2014-06-14, updated 2014-10-09Version 3

Under the local plasmonic excitation, the Raman images of a single molecule can now reach sub-nm resolution. We report here a description for 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 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.