Ultra-Clean Graphene Coating for Robust Operation of High Brightness Photocathodes

The next generation of free electron lasers FELs) and the small-scale electrons accelerators designed to perform ultra-fast microscopy and diffraction experiment rely on the production of new electron guns with ultrahigh brightness cathodes. High-efficiency semiconductor e.g., GaN-based) photocathodes have been studied for this application, and there have been some advances in reducing the mean transverse energy of their emitted electrons, which ultimately limits the performances of these electron guns. However, no significant progress has been reported in solving another major problem with semiconductor photocathodes, which is their high sensitivity to surface contaminations, and hence the requirement for operation in ultrahigh vacuum UHV) conditions. In general, UHV conditions are too restrictive or canbe incompatible with respect to construction and maintenance of high-intensity electron guns. As a result, for a wide range of photon energies, photocathodes with both high quantum efficiency and long-life operation under practical conditions are not currently available. In this SBIR project, Qrona Technologies will collaborate with both high energy physics HEP) and photocathode expert groups at Cornell University and UC Berkeley on 1) fabrication of In)GaN photocathodes with very low thermal emittance, by tuning the bandgap energy for operation near threshold, and 2) development of a UHV encapsulation process for high-performance photocathodes using a protective graphene layer. This will enable both long-life and high-efficiency operation of fast In)GaN-based photocathodes in the non-ideal vacuum conditions inside high-brightness electron guns, for DOE projects as well as other scientific, industrial, and defense applications. In addition to DOE applications in high energy and nuclear physics experiments, the high-performance, long-life photocathodes proposed in this work can provide a springboard to commercialization of two main products - an efficient electron emitter and a sensitive photodetector. These products have a great commercialization potential for many applications, such as ultra-violet imaging, spectroscopy, maskless electron lithography, and thin-film metrology.