Observation of two-photon photoemission from cesium telluride photocathodes excited by a near-infrared laser

Abstract

We explore nonlinear photoemission in cesium telluride (Cs2Te) photocathodes where an ultrashort (∼ 100 fs full width at half max) 800-nm infrared laser is used as the drive-laser in lieu of the typical ∼ 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., Phys. Rev. Lett., 104, 084801 (2010)], a metallic photocathode. We particularly observe two-photon photoemission in Cs2Te using the infrared laser in contrast to the anticipated three-photon process as observed for metallic photocathodes.Photocathodes are widely used to generate bright electron bunches with durations comparable to the emissiontriggering laser [1]. Semiconductor photocathodes are uniquely attractive due to their high quantum efficiencies (QEs) defined as the number of electrons emitted per unit photon of the drive-laser. Cesium telluride (Cs 2 Te) cathodes have substantial QEs (up to 20 %) [2,3], long lifetimes, and picosecond response times [1]. The photoemission photon energy requirement of Cs 2 Te dwells around ∼ 4 eV corresponding to an excitation-laser wavelength in the ultraviolet (UV) region. The photoemission band extends from UV to higher wavelengths [4] but any reasonable amount of charge extraction has so far been accomplished using UV laser pulses. Since most of the high-gain lasers use solid state media lasing in the infrared (IR), the typical production of a UV pulse relies on frequency up-conversion from IR to the UV. For laser systems based on the titanium sapphire (Ti:Sapph) medium (wavelength ∼ 800 nm), the UV pulses needed for photoemission are obtained from frequency tripling of the IR pulses using a two-stage process consisting of a second harmonic generation (SHG) stage followed by a sum frequency generation (SFG) stage. In order to preserve the short pulse duration during the up-conversion process, both stages generally use thin barium borate (BBO) crystals which limits the IR-to-UV conversion efficiency to typically < 10%.Operation of UV drive-lasers is technically challenging for high-average-current photoinjectors, while finding cathodes at the longer wavelengths available from solidstate lasers has proven challenging to date [5]. It was pointed out that nonlinear photoemission from metallic photocathodes could be advantageous [6,8] prompting us to explore possible multi-photon photoemission from Cs 2 Te using a Ti:Sapph laser.If an IR laser were to be used for photoemission from Cs 2 Te, then a nonlinear photoemission processspecifically three-photon photoemission (simultaneous absorption of three photons)-would have to take place if we assume the charge emission is strictly from photoemission and the photoemission threshold is unaltered; this is because, in the current context,...

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“…25 Cesium telluride (Cs 2 Te) [26][27][28][29][30][31] is one of the most widely used semiconductor materials in photoinjectors. 7,[32][33][34][35][36][37][38][39][40][41] Several calculations of emission from Cs 2 Te have been published, 15,42,43 and also experiments have been performed to determine the time response of this material 27,28,44 as well as of others. 45,46 The experiments employed a streak camera, energy spectrum analysis, and an RF-deflector to resolve the temporal distribution of electron bunches extracted from semiconductor cathodes.…”