  Devices possessing the ability to sense both electrically and optically
molecular targets are of fundamental and technological interest. Towards this
end, it has been shown that covering the ends of gapped gold-nanorod-dimer
nanoantennas can improve the enhancement factor (EF) that quantifies the
nanoantenna efficiency for surface-enhancement Raman spectroscopy (SERS) for an
incident wave coming from the top of the sample. Here, as the covering breaks
the top-bottom symmetry, we investigate the behaviour of the EF for excitation
coming from the bottom of the sample. This is relevant in presence of a
reflecting substrate or due to the placement of the device in a cavity field.
We also study the case of a superposition of waves coming from both directions
in the limit cases in which a node or an antinode of the total incident field
lies at the center of the gold nanorods. In all these situations we find that
the EF of the covered device can continue to be higher than for the uncovered
case when the geometrical parameters are tuned to the peak values of the
calculated enhancement factor.
