  Live cell imaging using metallic nanoparticles as tags is an emerging
technique to visualize long and highly dynamic processes due to the lack of
photobleaching and high photon rate. However, the lack of excited states as
compared to fluorescent dyes prevents the use of resonance energy transfer and
recently developed super resolution methods to measure distances between
objects closer that the resolution limit. In this work, we experimentally
demonstrate a technique to determine subdiffraction distances based on the near
field coupling of metallic nanoparticles. Due to the symmetry breaking in the
scattering cross section, not only distances but also relative orientations can
be measured. Gold nanoparticles were prepared on glass in such way that a small
fraction of dimers was present. The sample was sequentially illuminated with
two wavelengths to separate background from nanoparticle scattering based on
their spectral properties. A novel total internal reflection illumination
scheme in which the polarization can be rotated was used to further minimize
background contributions. In this way, radii, distance and orientation were
measured for each individual dimer and their statistical distributions were
found to be in agreement with the expected ones. We envision that this
technique will allow fast and long term tracking of relative distance and
orientation in biological processes.
