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

The development of photoemission electron sources to specifically address the competing and increasingly stringent requirements of advanced light sources such as x-ray free-electron lasers (XFELs) motivates a comprehensive material-centric approach that integrates predictive computational physics models, advanced nanosynthesis methods, and sophisticated surface-science characterization with in situ correlated study of photoemission performance and properties. Related efforts in material science are adopting various forms of nanostructure (such as compositionally graded stoichiometry in heterostructured architectures, and quantum features) allowing for tailored electronic structure to control and enhance optoelectronic properties. These methods influence the mechanisms of photoemission (absorption, transport, and emission) but have not, as yet, been systematically considered for use in photocathode applications. Recent results and near-term opportunities are described to exploit controlled functionality of nanomaterials for photoemission. An overview of the requirements and status is also provided.