Observation of Two-photon Photoemission from Cesium Telluride Photocathodes Excited by a Near-infrared Laser

We explore nonlinear photoemission in cesium telluride (Cs${}_2$Te) photocathodes where an ultrashort ($\sim 100$ fs full width at half max) 800-nm infrared laser is used as the drive-laser in lieu of the typical $\sim 266$-nm ultraviolet laser. An important figure of merit for photocathodes, the quantum efficiency, we define here for nonlinear photoemission processes in order to compare with linear photoemission. The charge against drive-laser (infrared) energy is studied for different laser energy and intensity values and cross-compared with previously performed similar studies on copper [P. Musumeci et al., \emph{Phys. Rev. Lett.}, $\mathbf{\text{104}}$, 084801 (2010)], a metallic photocathode. We particularly observe two-photon photoemission in Cs${}_2$Te using the infrared laser in contrast to the anticipated three-photon process as observed for metallic photocathodes.