Plasmonic decay into hot electrons in silver

Abstract

Light at optical frequencies interacting with a metal surface can excite interband quantum transitions, or intraband currents at frequencies approaching the PHz range. Momentum conservation enables the interband excitation to occur in first order as a dipole transition, while intraband excitations involve second-order momentum scattering processes. The free electron response to optical fields can also be collective, causing the optical field to be screened by the multipole plasmon response. We describe the exitation of single crystal silver surfaces in the region where the dielectric response transits from negative to positive passing through the epsilon near zero (ENZ) condition. There, electrons can no longer screen the optical field, so that it penetrates as a collective charge density wave of the free electron plasma, in other words, as a bulk transverse or longitudinal plasmon field. We examine two-photon photoemission (2PP) signals from Ag(1 1 1), (1 0 0) and (1 1 0) surfaces through the ENZ region under conditions where intraband, and interband single particle, and bulk plasmon collective responses dominate. We are specifically interested in the bulk plasmon decay into plasmonic photoemission. Plasmonic decay into excitation of electrons from the Fermi level, which we observe as a nonlinear 2PP process, has been established for the free electron and noble metals, but its significance to transduction of optical-to-electronic energy has not penetrated the plasmonics community. 2PP spectra show evidence for intraband hot electron generation, interband surface and bulk band excitation, and nonlinear bulk plasmon driven plasmonic single particle excitation. Because the intraband and plasmonic decay into hot electron distributions have been extensively considered in the literature, without reference to explicit experimental measurements, we discuss such processes in light of the directional anisotropy of the electronic structure of single crystalline silver. We note that projected band gaps in silver exclude large regions of the unoccupied state density from hot electron generation, such that it predominantly occurs in the (1 1 0) direction. Moreover, the excited hot electron distributions do not follow expectations from the joint density of the occupied and unoccupied states of a free electron metal, as assumed in majority of research on hot electron processes. We describe the strongly anisotropic hot electron distributions recorded by 2PP spectroscopy of Ag surfaces, and the plasmonic photoemission process that occurs on all surfaces irrespective of the momentum-dependent single particle band structure of silver. Plasmonic photoemission, or its linear analog that excites hot electrons at energies below the work function of Ag, is an important process for harvesting hot electron energy in photocatalytic and electronic device applications because the plasmon energy is not distributed between an electron and hole. This plasmonic decay channel is robust, but many aspects raise further questions. The accompanying publication by Gumhalter and Novko discusses the plasmonic photoemission from a theoretical point of view and its extension to Floquet engineering, as an exploration of novel plasmonic excitation processes in metals.