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

摘要

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