Enhanced quantum efficiency from optical interference in alkaliantimonide photocathodes: modeling and experimental results

We present measurements of enhanced quantum efficiency (QE) in thin film alkali antimonide photocathodes fromoptical interference in the cathode-substrate multilayer. Modulations in the spectral response are observed over a rangeof visible wavelengths, and are shown to increase the QE by more than a factor of two at specific wavelengths. Wepresent a model describing the QE modulations based on the three step photoemission process incorporating cases ofboth constant density of states and density functional theory (DFT)-derived density of states and show that the calculatedresults are in good agreement with the measurements. Model predictions demonstrate that QE can be enhanced by morethan a factor of 5 by optimization of cathode and substrate layer thicknesses. Additionally, these calculations revealthat optical interference can yield higher quantum efficiencies in thin films compared to thick, optically dense films.We model the QE versus excitation wavelength of multiple alkali antimonide compounds at different thicknesses. Wethen discuss the advantages of this interference effect for electron accelerators.