  Plasmonic nanostructures and devices are rapidly transforming light
manipulation technology by allowing to modify and enhance optical fields on
sub-wavelength scales. Advances in this field rely heavily on the development
of new characterization methods for the fundamental nanoscale interactions.
However, the direct and quantitative mapping of transient electric and magnetic
fields characterizing the plasmonic coupling has been proven elusive to date.
Here we demonstrate how to directly measure the inelastic momentum transfer of
surface plasmon modes via the energy-loss filtered deflection of a focused
electron beam in a transmission electron microscope. By scanning the beam over
the sample we obtain a spatially and spectrally resolved deflection map and we
further show how this deflection is related quantitatively to the spectral
component of the induced electric and magnetic fields pertaining to the mode.
In some regards this technique is an extension to the established differential
phase contrast into the dynamic regime.
