Characteristics of photoemission from radiative and sub-radiative localized surface plasmons in metal nanostructures.

Revealing the mechanism of photoemission from plasmonic nanostructures that supports different localized surface plasmon modes is crucial for designing new ultrafast photoelectric cathodes, as well as for enhancing localized surface plasmon (LSP)-based photocatalysis, energy harvest, and photoluminescence, but the investigation on this topic is still lacking. In this paper, we directly investigated the photoemission yield and photoemission mechanism from a sub-radiative Fano mode in an asymmetric nanorod dimer, a radiative dipole mode in an isolated nanorod, and a radiative coupled dipole mode in a symmetric nanorod dimer, respectively, using time-of-flight photoemission electron microscopy. We found that the photoemission yield from the sub-radiative Fano mode is almost equal to that of the radiative dipole mode but more than four times higher than that of the radiative coupled dipole mode case. We reproduced the physical process using a two temperature model and Fowler-Dubridge theory and demonstrated that the thermal effects of the electron gas play an influential role in photoemission from LSP. Interestingly, it is found that the sub-radiative nanorod dimer, although exhibiting a much lower electron temperature, maintains a similar photoemission yield to that of the radiative dipole mode, and this feature of the sub-radiative nanorod dimer makes it potentially a high-brightness electron source with strong robustness. The demonstrated results in this work help to understand the LSP-assisted photoemission process in plasmonic nanostructures, which lays the foundation for designing new ultrafast photoelectric cathodes and many other applications.