Nonequilibrium Green's function theory of scanning tunneling microscope-induced light emission from molecule covered metal surfaces: Effects of coupling between exciton and plasmon modes

Abstract

Light emission from interface plasmons and molecular excitons induced by the tunneling current of a scanning tunneling microscope is theoretically investigated using the nonequilibrium Green's function method. Luminescence property of the system is found to be well interpreted in terms of superposition of two coupled modes that form due to coupling between a molecular excitonic mode and an interface plasmon mode. The coupled mode whose energy is closer to the energy of the molecular excitonic mode (the interface plasmon mode) leads to a sharp (broad) peak structure in luminescence spectra of interface plasmon. Interference between these coupled modes gives rise to suppression or enhancement of luminescence intensity depending on the energy region. Especially, in the energy region near the molecular excitonic mode, the interference leads to strong suppression of luminescence from interface plasmons. Thus novel aspects of interpretation of the luminescence spectra of the system are obtained by using the analytical approach presented here.