Developing Rugged, High Quantum Efficiency, Graphene Photocathodes

Photocathodes have been the focus of intense experimental and theoretical development for many decades, and their use in RF photoinjectors has been indispensible for the evolution of light sources such as free-electron lasers. At present there is a significant science and technology gap for photocathodes in the context of the development of high-current electron injectors for next generation light sources. Photocathodes that have high quantum efficiency (QE), that is to say number of electrons per photon, have very short lifetimes, whereas cathodes with long lifetimes have poor QE. At this point the use of high-QE photocathodes for the coming generation of high-brightness, high-current electron injectors seems to be a technological imperative. Therefore, the challenge before us is the development of new high-QE photocathodes that are hardened against their environment for long service life. In this phase-one SBIR, we propose to investigate the plausibility of high-QE, graphene-based photocathodes. Graphene is a nanomaterial with remarkable chemical, physical, and electrical properties, and has experienced a dramatic rise to prominence over the past decade. The most important properties of graphene in the context of this application are its impermeability to residual gases, the ease with which it can be chemically functionalized, with hydrogen in particular, and the apparent ability to intercalate alkali metals beneath it. By leveraging these three properties we hope to produce a novel, rugged photocathode with high QE. Commercial Applications and Other Benefits: The success development of the proposed technology will have a significant impact on the performance of electron injectors at existing light source facilities, and is of particular relevance to the design of the next generation of free-electron lasers. More specifically, a long life, high- QE photocathode has been identified by the National Academies as a critical enabling technology for the development of high-power FEL systems. Because of the promise of being able to use robust, affordable drive lasers with long times between cathode replacement, one can anticipate the migration of this technology into some commercial systems that presently use thermionic cathodes, thereby finally opening up large commercial markets for photoinjector systems.