  We theoretically investigate electron energy loss spectroscopy (EELS) of
metallic nanoparticles in the optical frequency domain. Using a quasistatic
approximation scheme together with a plasmon eigenmode expansion, we show that
EELS can be rephrased in terms of a tomography problem. For selected single and
coupled nanoparticles we extract the three-dimensional plasmon fields from a
collection of rotated EELS maps. Our results pave the way for a fully
three-dimensional plasmon-field tomography and establish EELS as a quantitative
measurement device for plasmonics.
