High-brightness photocathodes through ultrathin surface layers on metals

Realizing Ultrafast Electron Pulse Self-Compression by Femtosecond Pulse Shaping Technique

Uncorrelated position and velocity distribution of the electron bunch at the photocathode from the residual energy greatly limit the transverse coherent length and the recompression ability. Here we first propose a femtosecond pulse-shaping method to realize the electron pulse self-compression in ultrafast electron diffraction system based on a point-to-point space-charge model. The positively chirped femtosecond laser pulse can correspondingly create the positively chirped electron bunch at the photocathode (such as metal-insulator heterojunction), and such a shaped electron pulse can realize the self-compression in the subsequent propagation process. The greatest advantage for our proposed scheme is that no additional components are introduced into the ultrafast electron diffraction system, which therefore does not affect the electron bunch shape. More importantly, this scheme can break the limitation that the electron pulse via postphotocathode static compression schemes is not shorter than the excitation laser pulse due to the uncorrelated position and velocity distribution of the initial electron bunch.

Ultrabright and ultrafast III-V semiconductor photocathodes

Crucial photoemission properties of layered III-V semiconductor cathodes are predicted using Monte Carlo simulations. Using this modeling, a layered GaAs structure is designed to reduce simultaneously the transverse energy and response time of the emitted electrons. This structure, grown by molecular beam epitaxy and activated to negative electron affinity, is characterized. The measured values of quantum efficiency and transverse energy are found to agree well with the simulations. Such advanced layered structures will allow generation of short electron bunches from photoinjectors with superior beam brightness.

Metal-insulator photocathode heterojunction for directed electron emission

We use angle-resolved photoemission under ultraviolet laser excitation to demonstrate that the electron emission properties of Ag(001) can be markedly enhanced and redirected along the surface normal by the deposition of a few monolayers of epitaxial MgO. We observe new low-binding energy states with small spreads in their surface parallel momenta as a result of MgO/Ag(001) interface formation. Under 4.66 eV laser excitation, the quantum efficiency of MgO/Ag(001) is a factor of 7 greater than that of clean Ag(001), revealing the utility of such heterojunctions as advanced photocathodes.

Probing the properties of metal-oxide interfaces: silica films on Mo and Ru supports

The influence of metal-oxide interactions on the workfunction and band alignment in thin oxide films is investigated for silica mono- and bilayers grown on Mo(112) and Ru(0001) supports. By analyzing the position of field-emission resonances and the Kelvin-probe signal deduced from conductance and force spectroscopy, we have identified a substantial lowering of the workfunction in the monolayer films, with the oxide bands shifting accordingly. We explain this observation with a stronger coupling and a shorter binding length of the silica monolayer to the metal substrate, which removes the effect of electron spill-out, produces a positive interface dipole and reduces the workfunction of the system. In contrast, the van der Waals bound bilayer film interacts only weakly with the Ru support, conserving the effect of electron spill-out and keeping the workfunction high. Direct evidence for the relevance of interface interactions comes from experiments on buckled silica films, for which regular workfunction modulations are revealed that follow the topographic height of the film above the metal surface.