“…One of the most fundamental and technologically relevant phenomena in light-matter interaction is the demonstration of the photoelectric effect, where photons have been used to emit electrons in free space. Efficient electron emission from various types of materials has enabled technologies for many cutting-edge applications such as free-electron lasers, image intensifiers for night imaging, sources for next-generation electron-beam lithography, electron microscopy, and photomultipliers; it has also led to the elucidation of novel materials physics probed through the investigation of the spin, momentum, and energy of the emitted electron [14][15][16][17] . State-of-the-art electron sources (or photocathodes) are classified into three families depending on the active material type: metals (Cu, Mg, Pb), alkali antimonide and tellurides (Cs 2 Te, K 2 CsSb, Cs 3 Sb), and III-V semiconductors (GaAs, GaN, tertiary alloys of III-V materials) 18 .…”