  Plasmonic nano structures such as wire waveguides or antennas are key
building blocks for novel highly integrated photonics. A quantitative
understanding of the optical material properties of individual structures on
the nanoscale is thus indispensable for predicting and designing the
functionality of complex composite elements. In this letter we study
propagating surface plasmon polaritons in single silver nanowires isolated from
its environment by levitation in a linear Paul trap. Symmetry-breaking effects,
e.g., from supporting substrates are completely eliminated in this way.
Illuminated with white light from a supercontinuum source, Fabry-P\`erot-like
resonances are observed in the scattering spectra obtained from the ends of the
nanowires. The plasmonic nature of the signal is verified by local excitation
and photon collection corresponding to a clean transmission measurement through
a Fabry-P\`erot resonator. A numerical simulation is used to compute the
complex effective refractive indices of the nanowires as input parameter for a
simple Fabry-P\`erot model, which nicely reproduces the measured spectra
despite the highly dispersive nature of the system. Our studies pave the way
for quantitative characterization of nearly any trappable plasmonic nano
object, even of fragile ones such as droplets of liquids or molten metal and of
nearly any nanoresonator based on a finite waveguide with implications for
modeling of complex hybrid structures featuring strong coupling or lasing.
Moreover, the configuration has the potential to be complemented by gas sensors
to study the impact of hot electrons on catalytic reactions nearby plasmonic
particles.
