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

Abstract

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.

How Dark Are Radial Breathing Modes in Plasmonic Nanodisks?

“…Increasing the nanodisk diameter from 150 to 500 nm this ratio increases by roughly 1 order of magnitude, in agreement with recent theoretical work on nanodisk RBMs. 21 This clearly demonstrates that RBMs that are dark in small nanodisks can efficiently couple to light for larger disk sizes.…”

“…However, increasing size gives rise to retardation, thus making the RBM bright. 20 , 21 We find that with increasing size (150–500 nm) the EELS signal does not change significantly, but the CL signal increases by about 1 order of magnitude, corresponding to increased scattering 22 and thus radiation damping. To quantify the light coupling properties of RBMs, we introduce EELS/CL ratios that we obtain from different disk sizes and support the experimental results with numerical simulations.…”

Plasmonic breathing modes in C60 molecules: A quantum hydrodynamic approach

“…The plasmonic monopole (or breathing) mode is more difficult to excite due to its spherical symmetry. It is also harder to measure as it does not emit any electromagnetic radiation -it is a dark mode 12,13 . Nevertheless, monopole modes have been observed in silver nanodisks using electron energy loss spectroscopy (EELS) 14 or, more recently, optical spectroscopy 11 .…”

“…12 1 σ 2 + 195R10 1 σ 4 + 2145R8 1 σ 6 + 19305R 6 1 σ 8 + 135135R 4 1 σ 10 +675675R 2 1 σ 12 + 2027025σ 14 , (B4) F 2 (R 2 , σ) = R 14 2 + 15R 12 2 σ 2 + 195R 10 2 σ 4 + 2145R 8 2 σ 6 + 19305R 6 2 σ 8 + 135135R 4 2 σ 10 +375375R 2 2 σ 12 + 2027025σ 14 .…”

“…12 1 σ 2 + 80R10 1σ 2 + 1810R 8 1 σ 4 + 28020R 6 1 σ 6 + 305130R 4 1 σ 8 +2231880R 2 1 σ 10 + 675675(16 − R 1 + R 2 ) σ 12 , (B13) K 2 (R 1 , R 2 , σ) = 2R 12 2 σ 2 + 80R10 2 σ 2 + 1810R 8 2 σ 4 + 28020R 6 2 σ 6 + 305130R 4 2 σ 8 +2231880R 2 2 σ 10 + 675675(16 − R 1 + R 2 ) σ 12 , (B14)…”