High Performance High Current CW polarized photocathodes for Electron Ion Colliders

"Photocathodes with improved performance for the production of spin
polarized electrons for delivering beams with average currents up to several mA with operational
lifetimes >24 hours, longitudinal spin polarization greater than 90% and an initial quantum
efficiency >5% at ~ 780nm are required for nuclear physics and other basic sciences application.
Photocathodes delivering high bunch charges, > 100 pC/bunch, are required for the storage ring at
the Electron Ion Collider (EIC) facilities. Spin polarized electron sources are
currently based on strained III-V material (GaAs/GaAsP) layer-based photocathodes. The
photocathodes Quantum Efficiency (QE) (i.e. the number of electrons produced per photon)
depends on many parameters; including the defect density and is also limited by the weak photon
absorption of the absorbing medium near the band edge. The Electron Spin Polarization efficiency
(ESP) of the photocathode layer depends on the energy separation of the heavy hole and light hole
bands, which is enhanced by the mechanical strain of the absorption layers. To simultaneously
increase QE and ESP, a III-V material structure that can effectively trap the light in thinner
absorption layers will be attempted to increase quantum efficiency and spin polarization. A gallium arsenide phosphide-based material structured will be modelled to
achieve high performance electron spin polarized photocathode. Metalorganic chemical vapor
deposition (MOCVD) techniques will be used to grow highly efficient structures to demonstrate
highly efficient photocathodes MOCVD has generally been proven to be faster and more effective
in producing high-quality device layers than other techniques. The anticipated public benefits of this work
include technical, economic, social, and other benefits to the public as a whole and advancement
in science by advancing the capabilities to understand phenomenon at atomic scale resolution, new
discoveries. This project will advance the nuclear physics sector with the development of highenergy
electron accelerators, the possibility to study nuclei, quarks, and gauge bosons using the
polarized electron sources presented here. Furthermore, the technology helps in enabling basic
energy and medical sciences, understanding of universe and also US to lead on cutting edge
technology in the world."