Near-field Imaging of Optical Resonances in Silicon Metasurfaces Using Photoelectron Microscopy

Precise control of light-matter interactions at the nanoscale lies at theheart of nanophotonics. Experimental examination at this length scale ischallenging, however, since the corresponding electromagnetic near-field isoften confined within volumes below the resolution of conventional opticalmicroscopy. In semiconductor nanophotonics electromagnetic fields are furtherrestricted within the confines of individual subwavelength resonators, limitingaccess to critical light-matter interactions in these structures. In this work,we demonstrate that photoelectron emission microscopy (PEEM) can be used forpolarization resolved near-field spectroscopy and imaging of electromagneticresonances supported by broken-symmetry silicon metasurfaces. We find that thephotoemission results, enabled through an in-situ potassium surface layer, areconsistent with full-wave simulations and far-field reflectance measurementsacross visible and near-infrared wavelengths. In addition, we uncover apolarization dependent evolution of collective resonances near the metasurfacearray edge taking advantage of the far-field excitation and full-field imagingof PEEM. Here, we deduce that coupling between eight resonators or moreestablishes the collective excitations of this metasurface. All told, wedemonstrate that the high-spatial resolution hyperspectral imaging andfar-field illumination of PEEM can be leveraged for the metrology ofcollective, non-local, optical resonances in semiconductor nanophotonicstructures.