Nanoscale photoemission from a focused propagating surface plasmon

Propagating surface plasmons (PSPs) provide an important platform for designing various photoelectric devices such as nanometer-confined ultrafast electron sources. Here, we investigate the nanoscale photoemission from a focused PSP hot spot using time-of-flight photoemission electron microscopy. It is found that the induced photoelectrons from the focused PSP exhibit an obvious feature of the above threshold photoemission process, while the photoelectron spectra from a local surface plasmon (LSP) under the same illumination condition exhibit a characteristic of thermally assisted multiphoton photoemission. In addition, results show that the measured photoelectron yield from the focused PSP region is nearly one order of magnitude higher than that from the LSP region, and we found this is contrary to the calculation results that the yield from the LSP is much higher than that from the focused PSP when the local field intensity and plasmon lifetime of the two plasmonic modes are included. The large discrepancy in the photoelectron energy spectra, as well as in the measured and calculated photoelectron intensity between the focused PSP and LSP, can be attributed to the existence of different decay channels between the two plasmonic modes. This paper provides a deep understanding of the photoemission mechanism induced by plasmon effects, and it shows that the focused PSP is promising as a robust nanoscale electron source which can supply energetic and higher flux electron emission.

Figure 1

Schematics of the experimental setup.

Figure 2

(a) PEEM image of a plasmon focusing lens illuminated by 800-nm $p$-polarized femtosecond light. (b) The photoemission intensity profile excited by focused PSP along the pink dashed line shown in Fig. 2; the red line is obtained by Gaussian fitting. (c) PEEM image of a PSP focusing lens illuminated by 800-nm right circularly polarized femtosecond light. (d) The photoemission intensity profile excited by focused PSP along the pink dashed line shown in Fig. 2; the red line is obtained by Gaussian fitting.

Figure 3

(a) The measured photoelectron spectra at a focus spot denoted by the red circle in Fig. 2; the upper left inset is the measured photoelectron yield at 0.4 and 1.9 eV as a function of incident power, respectively. The lower right inset is the one-order-of-magnitude linear photoemission UV-PEEM image of the sample illuminated by Hg lamp. (b) The measured photoelectron spectra on the Au plane without the effect of PSP at varied incident powers.

Figure 4

(a) The photoelectron spectra from the LSP with different incident powers. The inset shows the measured photoelectron yield at 0.4 and 1.9 eV as a function of incident power, respectively. (b) The photoelectron spectrum from the Si substrate with 170-mW femtosecond laser illumination.

Figure 5

(a) The extraction regions of the yield induced by LSP (red circles) and PSP (green circles) in the experiment. (b) The simulated $|E_z|$ of near field distribution illuminated by right circularly polarized light by FDTD. (c) $|E_z|$ as a function of time for LSP and PSP, respectively.