Excitation of two-photon photoemission where epsilon is near zero on Ag(111)

Because silver has epsilon-near-zero (ENZ) in the near-UV spectral region, its optical response is expected to be dominantly plasmonic and nonperturbatively nonlinear. The ENZ properties of Ag have made it the material of choice for investigation of plasmonic optical effects in physics, chemistry, optics, and nanotechnology. We investigate the nonlinear angle-resolved two-photon photoemission (2PP) spectroscopy of Ag(111) surface in the ENZ region. In addition to the well-understood and documented spectroscopic features of Einsteinian photoemission involving dipole excitations of the single-particle occupied and unoccupied surface states, 2PP spectra possess other distinct features where the optical fields excite or are modified by the collective virtual and real plasmon excitations.