  Free-electron-based spectroscopies can reveal the nanoscale optical
properties of semiconductor materials and nanophotonic devices with a spatial
resolution far beyond the diffraction limit of light. However, the retrieved
spatial information is constrained to the excitation space defined by the
electron beam position, while information on the delocalization associated with
the spatial extension of the probed optical modes in the specimen has so far
been missing, despite its relevance in ruling the optical properties of
nanostructures. In this study, we demonstrate a cathodoluminescence method that
can access both excitation and emission spaces at the nanoscale, illustrating
the power of such simultaneous excitation and emission mapping technique by
revealing a sub-wavelength emission position modulation as well as by
visualizing electromagnetic energy transport in nanoplasmonic systems. Besides
the fundamental interest of these results, our technique grants us access into
previously inaccessible nanoscale optical properties.
