  Precise control of light-matter interactions at the nanoscale lies at the
heart of nanophotonics. Experimental examination at this length scale is
challenging, however, since the corresponding electromagnetic near-field is
often confined within volumes below the resolution of conventional optical
microscopy. In semiconductor nanophotonics electromagnetic fields are further
restricted within the confines of individual subwavelength resonators, limiting
access to critical light-matter interactions in these structures. In this work,
we demonstrate that photoelectron emission microscopy (PEEM) can be used for
polarization resolved near-field spectroscopy and imaging of electromagnetic
resonances supported by broken-symmetry silicon metasurfaces. We find that the
photoemission results, enabled through an in-situ potassium surface layer, are
consistent with full-wave simulations and far-field reflectance measurements
across visible and near-infrared wavelengths. In addition, we uncover a
polarization dependent evolution of collective resonances near the metasurface
array edge taking advantage of the far-field excitation and full-field imaging
of PEEM. Here, we deduce that coupling between eight resonators or more
establishes the collective excitations of this metasurface. All told, we
demonstrate that the high-spatial resolution hyperspectral imaging and
far-field illumination of PEEM can be leveraged for the metrology of
collective, non-local, optical resonances in semiconductor nanophotonic
structures.
