Strong Plasmonic Enhancement of Photoelectric Quantum Efficiency of Nb using In Nano islands

The development of superconducting photocathodes is presented which explores thin film coatings to enhance the quantum efficiency (QE) of superconducting Nb above its bulk value of < 10^-6. Deposition of a 10nm layer of Mg (work function = 3.6 eV) onto Nb after UHV anneal increases QE by a factor of 10. Deposition of ultra-thin islands of In (4 nm) on top of Nb/Nb oxide or Nb/Mg/Mg oxide leads to overall enhancements of QE by up to 400 times. We attribute this latter enhancement to plasmonic effects where the stored EM fields in the In islands couple to Nb electrons