In 1885, Heinrich Hertz first demonstrated the concept of using a photocathode to detect visible light photons. Albert Einstein won the 1921 Nobel Prize in Physics for explaining the photoelectric effect, in terms of quantization of photon energy. Since then, photocathodes have become an essential tool in the areas of radiation detection and electron sources.
Bi-alkali photocathodes such as potassium cesium antimonide (K2CsSb) and cesium antimonide (Cs3Sb) are very popular for their relatively good quantum efficiency of 20-30% (peak response) in the visible wavelength and fast temporal response. They are the photocathodes of choice in photomultiplier tubes (PMTs) and also as bright electron sources in particle accelerators.
Despite many decades of research and development, serious challenges persist in reliably manufacturing photocathodes over large areas, improving the quantum efficiency or reducing the surface emittance (necessary for accelerators). The traditional K2CsSb manufacturing process, involves the sequential evaporation of the three elements(sometimes up to 20 steps), is fundamentally limiting to any major performance enhancements in photocathodes.
RMD made a major breakthrough in 2016 by growing alkali antimonide photocathodes with an alternate but a mature thin film growth technique – Sputtering. RMD has developed this method in collaboration with Brookhaven National Laboratory and University of Chicago. The sputtering process simplifies the growth process into a two-step process and improves the manufacturing reliability since 100s of cathodes can be grown from a single sputtering target. In a short 2-year span, sputtered K2CsSb cathodes have successfully demonstrated peak QE of 20-25% and green QE of 3-5%, and most importantly ultra-smooth morphology with sub-nm surface roughness- a critical condition required to meet the low-emittance electron emission for particle accelerators.
K2CsSb sputter-target measuring 2” diameter