Perspectives on Designer Photocathodes for X-ray Free-Electron Lasers: Influencing Emission Properties with Heterostructures and Nanoengineered Electronic States

Design methodologies for high-performance photocathodes become increasingly important, driven by the needs of advanced x-ray light sources and ultrafast electron diffraction and microscopy. Cathode design has been hampered by interdependent performance metrics, where improving one compromises another. Rapid developments in modeling and nanomaterial fabrication allow tailoring of electronic structure in ways that influence the fundamental mechanisms of photoemission: absorption, transport, and emission. This survey of recent efforts and results in each of these areas proposes research priorities that are likely to yield significant gains in cathode design.