Hot Electron Photoemission from Plasmonic Nanostructures: The Role of Surface Photoemission and Transition Absorption

We study mechanisms of photoemission of hot electrons from plasmonic nanoparticles. We analyze the contribution of "transition absorption", i.e., loss of energy of electrons passing through the boundary between different materials, to the surface mechanism of photoemission. We calculate photoemission rate and transition absorption for nanoparticles surrounded by various media with a broad range of permittivities and show that photoemission rate and transition absorption follow the same dependence on the permittivity. Thus, we conclude that transition absorption is responsible for the enhancement of photoemission in the surface scenario. We calculate the ratio of photoemission cross-section for a gold nanosphere embedded in different materials such as silicon, zinc oxide, and titanium dioxide. For the calculations, we include both surface and bulk mechanisms of photoemission, using quantum calculations for the former one and a three-step phenomenological approach for the latter one. By comparison of both mechanisms, we show that the role of surface mechanism in the total photoemission cannot be neglected, as it dominates in the near-infrared wavelength range. We also show that in order to increase the photoemission rate, one benefits from placing nanoparticles in materials with lower permittivity. Finally, we apply our results to the case of nanowires partially embedded in a semiconductor substrate, which is a practically relevant design for narrow-band photodetection. Summarizing these results, we show that the reported narrow-band photoemission increase can at least partially be attributed to the surface mechanism.