Towards Ultra-Smooth Alkali Antimonide Photocathode Epitaxy

Photocathodes lead in brightness among electron emitters, but transverse momenta are unavoidably nonzero. Ultra-low transverse emittance would enable brighter, higher energy x-ray free-electron lasers (FEL), improved colliders, and more coherent, detailed ultrafast electron diffraction/microscopy (UED/UEM). Although high quantum efficiency (QE) is desired to avoid laser-induced nonlinearities, the state-of-the-art is 100 pC bunches from copper, 0.4 mm-mrad emittance. Advances towards 0.1 mm-mrad require ultra-low emittance, high QE, cryo-compatible materials. We report efforts towards epitaxial growth of cesium antimonide on lattice matched substrates. DFT calculations were performed to downselect from a list of candidate lattice matches. Co-evaporations achieving >3% QE at 532 nm followed by atomic force and Kelvin probe microscopy (AFM and KPFM) show ultra-low 313 pm rms (root mean square) physical and 2.65 mV rms chemical roughness. We simulate roughness-induced mean transverse energy (MTE) to predict 1 meV from roughness effects at 10 MV/m in as-grown optically thick cathodes, promising low emittance via epitaxial growth.
Proceedings of the 12th International Particle Accelerator Conference, IPAC2021, Campinas, SP, Brazil