Electron-beam excited photon emission from monopole modes of a plasmonic nano-disc.

Plasmonic dark modes are not easy to be observed in the far field due to their weak photon emission. By contrast, it has been shown that a dark mode can be excited effectively by a near-field source such as an electron beam. In this Letter, we show theoretically that the photon emission from the monopole-like dark mode supported on a plasmonic nano-disc could be unexpectedly strong when excited by an electron beam through its hole. Even though this monopole mode is considered to be dark, it is found that the emission can be even "brighter" than the dipolar bright modes when the electron speed is higher than 0.6c. Due to the high conversion efficiency from electron energy loss to photon energy, the results could also suggest an optical method for the detection of high-energy electrons passing through the hole with negligible changes in electron speed.