High-quality beams are often produced using photoemission. While copper photocathodes can produce superb, albeit low-current, beams, higher currents call for semiconductor photocathodes.  Until recently, however, the rough surface of semiconductor materials can reduce the beam’s quality by giving it unwanted transverse energy spread.

Using CBB’s interdisciplinary approach, grad student Pallavi Saha (Arizona State U.), then postdoc Oksana Chubenko (Arizona State U.), and others grew ultra-smooth semiconductor photocathodes, expected to produce beams with very low transverse energy. In a parallel effort, Cornell grad students Chris Parzyck, Kevin Nangoi and Will DeBenedetti with then postdoc Alice Galdi combined their expertise in epitaxy, ab initio calculations and surface chemistry to grow the first single-crystal semiconductor photocathode, which opens a path to even lower transverse energy by capitalizing on the cathode’s band structure.

With careful optimization of the beamline, the benefits of a good photocathode can be felt at the target.  With that in mind, SLAC and CBB are collaborating on the photocathode and injector for the planned upgrade of the SLAC Xray FEL, LCLS-II-High Energy. Besides XFELs, applications of CBB photocathodes include colliders, ultrafast electron diffraction, and even electron microscopes.

References:

 Karkare,  Adhikari, Schroeder, Nangoi, Arias, Maxson, Padmore, "Ultracold electrons via Near-Threshold Photoemission from Single-Crystal Cu(100)", Phys. Rev. Lett. 125, 054801(2020) [Online] Available: https://arxiv.org/abs/2002.11579

Bae, Bazarov, Musumeci, Karkare, Padmore, Maxson, "Brightness of femtosecond nonequilibrium photoemission in metallic photocathodes at wavelengths near the photoemission threshold", Appl. Phys. 124, 244903, (2018)  [Online] Available: https://aip.scitation.org/doi/10.1063/1.5053082

Saha, Chubenko, Gevorkyan, Kachwala, Knill,  Sarabia-Cardenas, Montgomery, Poddar, Paul, Hennig, Padmore, and Karkare, "Physically and chemically smooth cesium-antimonide photocathodes on single crystal strontium titanate substrates",  Appl. Phys. Lett. 120, 194102 (2022) [Online] Available: https://aip.scitation.org/doi/full/10.1063/5.0088306

Parzyck, Galdi, Nangoi, DeBenedetti, Balajka, Faeth, Paik, Hu, Arias, Hines, Schlom, Shen, and Maxson, " Single-Crystal Alkali Antimonide Photocathodes: High Efficiency in the Ultrathin Limit", Phys. Rev. Lett. 128, 114801 (2022) [Online] Available: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.114801

Pierce, Andorf, Lu, Gordon, Kim, Gulliford, Bazarov, Maxson, Norvell,  Dunham, Raubenheimer, "The Role of Low Intrinsic Emittance in Modern Photoinjector Brightness", Phys. Rev. Accel. Beams 23, 070101 (2020) [Online] Available: https://arxiv.org/abs/2004.08034