  Nanoscale phase-control is one of the most powerful approaches to
specifically tailor electrical fields in modern nanophotonics. Especially the
precise sub-wavelength assembly of many individual nano-building-blocks has
given rise to exciting new materials as diverse as metamaterials, for
miniaturizing optics, or 3D assembled plasmonic structures for biosensing
applications. Despite its fundamental importance, the phase-response of
individual nanostructures is experimentally extremely challenging to visualize.
Here, we address this shortcoming and measure the quantitative scattering phase
of different nanomaterials such as gold nanorods and spheres as well as
dielectric nanoparticles. Beyond reporting spectrally resolved responses, with
phase-changes close to pi when passing the particles' plasmon resonance, we
devise a simple method for distinguishing different plasmonic and dielectric
particles purely based on their phase behavior. Finally, we integrate this
novel approach in a single-shot two-color scheme, capable of directly
identifying different types of nanoparticles on one sample, from a single
widefield image.
