Surface-Plasmon Resonance-Enhanced Multiphoton Emission of High-Brightness Electron Beams from a Nanostructured Copper Cathode

We experimentally investigate surface-plasmon assisted photoemission to enhance the efficiency of metallic photocathodes for high-brightness electron sources. A nanohole array-based copper surface was designed to exhibit a plasmonic response at 800 nm, fabricated using the focused ion beam milling technique, optically characterized and tested as a photocathode in a high power radio frequency photoinjector. Because of the larger absorption and localization of the optical field intensity, the charge yield observed under ultrashort laser pulse illumination is increased by more than 100 times compared to a flat surface. We also present the first beam characterization results (intrinsic emittance and bunch length) from a nanostructured photocathode.

Figure 1

(a) Scanning electron microscopy images of the nanohole array. Inset: Zoomed-in cut view of the nanoholes. (b) Nanohole array under an optical microscopy with off-resonance visible light. (c) A nanopattern consisting of 6-by-6 $25\mu \mathrm{m}$ square patches illuminated at resonant laser wavelength.

Figure 2

(a) Simulated reflectivity at normal incidence of two arrays of Gaussian (solid curve) and square (dashed curve) holes with the same $p$, $h$, and $w$ values. The shaded areas show the results of changing $h$ and $w$ by $\pm 5\%$. (b) Measured reflectivity of two nanopatterns fabricated on single crystal substrates with 2 nm FWHM bandwidth lasers. The two patterns have the same hole width and depth and are only different in spacing.

Figure 3

(a) Charge yield map of the nanopatterned cathode. The black square indicates the nanopatterned area. The rms laser spot size is illustrated. (b) The charge density $\sigma$ versus the absorbed laser intensity. $\sigma$ is obtained by averaging the measured beam charge over the full pattern area. $\sigma$ can be expressed as $\sigma =J\tau$, where we choose $\tau$ to be equal to the pulse duration of the drive IR laser of 150 fs FWHM and $J$ is an equivalent current density. Fitting of the low charge density part yields a slope of $3.05\pm 0.07$.

Figure 4

(a) Intensity $I$ distribution on the nanohole surface. The lineouts of $I$ along the metal-vacuum boundaries in the $x=0$ plane (curve $b$, gray line) and $z=-14\mathrm{nm}$ plane (curve $c$, white line) are shown in (b) and (c), respectively, both normalized by the average intensity over the entire surface.

Figure 5

Measurement of intrinsic emittance of the nanostructured cathode using the grid technique (square) and the solenoid scan method (dot), compared with GPT simulation results.