  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).
