Nanostructured GaAs Photocathodes for Light Trapping and Ion Damage Tolerance

Photocathodes provide laser-triggered pulses with high peak currents. High average currents require high QE to avoid laser-heating damage. However, QE in DC guns decays with extracted charge due to back-bombardment from gas ionized by the emitted electron beam. Thus higher current increases back-bombardment and shortens life proportionally to extracted charge. This proposal addresses the charge-limit, following advancements at JLab, by patterning a Mie array of nanopillars on GaAs. Cathode lifetime will increase significantly: non-normal nanopillar surfaces receive low ion bombardment, but will contribute most of the QE by appropriately tuning the dipole Mie resonance. Second, nanostructures will improve QE via resonant optical properties, because nearly 50% more photons can be absorbed versus flat cathodes. A higher surface-to-absorption-volume ratio also means photoexcitation at favorably shallow depth for polarization. We anticipate nanostructured GaAs will exhibit high QE and good spin polarization while exceeding 5,000 Coulomb charge lifetime. In Phase I, simulations predicting QE enhancement will be performed using existing GaAs Monte Carlo models. A series of GaAs and strained superlattice cathodes grown in existing dedicated MBE facilities will be patterned by Euclid with nanopillar arrays using e-beam lithography. Euclid and Jlab personnel will test the initial cathodes at JLab for optical reflectance, quantum efficiency, polarization ratio, and tolerance for ion damage. At the end of Phase I, we will quantitatively conclude which nanostructures promise optimized QE enhancement and lifetime extension to be optimized for production in Phase II of the project. The potential market for high QE, ion-damage-tolerant GaAs photocathodes is significant. Spin-polarized photocathodes are essential at proposed electron-ion collider facilities such as JLEIC and eRHIC, and advancements would also affect existing facilities such as JLab's CEBAF. In addition, high QE and long-lived photocathodes are of broader interest including SRF facilities such as SLAC's LCLS-II, Fermilab's IARC, and RF linac based FELs for industrial applications. Spin-polarized transmission electron microscopy will also benefit from advanced spin-polarized photocathodes.