High-Brightness Photocathodes through Ultrathin Surface Layers on Metals

We report how ultrathin MgO films on Ag(001) surfaces can be used to control the emittance properties of photocathodes. In addition to substantially reducing the work function of the metal surface, the MgO layers also favorably influence the shape of the surface bands resulting in the generation of high-brightness electron beams. As the number of MgO surface layers varies from 0 to 3, the emitted electron beam becomes gradually brighter, reducing its transverse emittance to $0.06\mathrm{mm}\mathrm{mrad}$. We suggest the use of such photocathodes for the development of free-electron x-ray lasers and energy-recovery linac x-ray sources.

Figure 1

The structure of the model system for 2 ML MgO(001) on the upper and lower surfaces of 4 ML Ag(001), denoted here as $\mathrm{MgO}(2\mathrm{ML})/\mathrm{Ag}(001)(4\mathrm{ML})/\mathrm{MgO}(2\mathrm{ML})$. Interface Ag atoms (gray, center) are only attached to O atoms (red), and not to Mg (green).

Figure 2

Surface bands (left-hand panels) and the band structure (right-hand panels) of $\mathrm{MgO}(n_{\mathrm{MgO}}\mathrm{ML})/\mathrm{Ag}(001)(4\mathrm{ML})/\mathrm{MgO}(n_{\mathrm{MgO}}\mathrm{ML})$ systems in the Brillouin zone for $n_{\mathrm{MgO}}=0–4$. Dark blue spots denote $k$-space regions with occupied electrons; otherwise, coloring indicates band height above $E_F$, the Fermi energy. Only surface bands with the highest energy occupied crystal orbitals in the center of the Brillouin zone are shown for each value of $n_{\mathrm{MgO}}$.

Figure 3

Schematic of a photocathode based on a multilayer (red rectangle) topped Ag-nanorod array embedded in a UV-transparent MgO substrate. Typical dimensions of the nanorods are 50–200 nm width and several $\mu \mathrm{m}$ length; the space between the nanorods is commensurate with the width of the nanorods.