Effects of Interference between Energy Absorption Processes of Molecule and Surface Plasmons on Light Emission Induced by Scanning Tunneling Microscopy

TL;DR

This study investigates how interference between molecular absorption and surface plasmon re-absorption affects light emission in STM, revealing new excitation channels and mechanisms for upconverted luminescence.

Contribution

It introduces a nonequilibrium Green's function approach to analyze interference effects and identifies lower-energy excitation channels induced by exciton-plasmon coupling.

Findings

Interference causes enhancement and suppression of energy-absorption processes.

New excitation channels lower than the first electronic excitation energy are identified.

Vibrational excitations facilitate upconverted photon emission.

Abstract

Effects of coupling between an molecular exciton and a surface plasmon (exciton-plasmon coupling) on the luminescence properties of the molecule and the surface plasmons are investigated using nonequilibrium Green's function method. Molecular absorption and enhancement by molecular electronic and vibrational modes (molecular modes) lead to dip and peak structures in the luminescence spectra of the surface plasmons. The re-absorption by the surface plasmons leads to a dent structure in their luminescence spectra. It is found that the processes of the molecular absorption and the re-absorption by the surface plasmons interfere with each other. Enhancement and suppression of these energy-absorption processes are due to the interference. Moreover, due to the exciton-plasmon coupling, excitation channels of the molecule arise in the energy range lower than the first electronic excitation energy of the molecule. It is found that the electron transitions via these excitation channels give rise to the molecular luminescence and the vibrational excitations at the bias voltage lower than the ratio of the first electronic excitation energy of the molecule to the elementary charge. The results also indicate that the vibrational excitations assist the emission of photons, whose energy exceeds the product of the elementary charge and the bias voltage (upconverted luminescence).