  In this work, we present a novel technique to directly measure the phase
shift of the optical signal scattered by single plasmonic nanoparticles in a
diffraction-limited laser focus. We accomplish this by equipping an inverted
confocal microscope with a Michelson interferometer and scanning single
nanoparticles through the focal volume while recording interferograms of the
scattered and a reference wave for each pixel. For the experiments,
lithographically prepared gold nanorods where used, since their plasmon
resonances can be controlled via their aspect ratio. We have developed a
theoretical model for image formation in confocal scattering microscopy for
nanoparticles considerably smaller than the diffraction limited focus We show
that the phase shift observed for particles with different longitudinal
particle plasmon resonances can be well explained by the harmonic oscillator
model. The direct measurement of the phase shift can further improve the
understanding of the elastic scattering of individual gold nanoparticles with
respect to their plasmonic properties.
