  We launch surface plasmons from one end of a silver nanowire by asymmetric
illumination with white light and use plasmon-to-light scattering at the
nanowire ends to probe spectroscopically the plasmonic Fabry-Perot wire modes.
The spectral positions of the maxima and minima in the scattered intensity from
both nanowire ends are found to be either in phase or out-of phase, depending
on the nanowire length and the spectral range. This behavior can be explained
by a generalized Fabry-Perot model. The turnover-point between the two regimes
is sensitive to the surface plasmon round trip losses and thus opens a new
possibility for detecting changes of the optical absorption in the nanowire
environment.
