High Performance Photocathodes with 50% Quantum Efficiency

Multialkali photocathodes are used from 200 to 850 nm, but high transparency at longer wavelengths reduces the quantum efficiencies. Theoretically, routes to enhance absorption could achieve high efficiency over most of this spectral range. Realisations of some of the concepts have been successfully attempted leading to quantum efficiencies of ~50%, with examples from 200 to 750 nm. Improvement factors of up to 50 times occur by ~900 nm, giving an extension of the useful wavelength operating range.