Overview and development of metallic photocathodes

Abstract

Research and development on photocathodes materials has become an important task for X-ray free electron laser and new generation of particles accelerator. The choice of the optimum cathode type and its further improvement is a fundamental issue for the progress in RF photoinjectors [1]. Photocathode materials can be subdivided in two main classes, namely metallic and semiconductor photocathodes. Metallic photocathodes offer several advantages over the semiconductor ones, e.g. long lifetime and prompt response time on the photoemission. Nevertheless they are characterized by lower quantum efficiency (QE) mainly due to their high reflectivity, to the high work function and to the electron-electron scattering in the conduction band. This work reviews the requirements and the current status of metallic photocathodes. Particular attention is given to Yttrium and Magnesium which have many advantages respect to Copper based photocathode which is generally used in RF photoinjectors [2]. Among the different techniques used to prepare photocathodes based on thin metallic films, pulsed laser ablation deposition has been proposed as a promising technique for realizing photocathodes based on Magnesium and Yttrium thin films of good quality in terms of high adhesion and chemical composition [3–5]. The presence of droplets in the deposited films is the main drawback of pulsed laser deposition technique. It will be shown that the optimization of laser parameters and, in particular, of the laser fluence can help to reduce the droplets formation [6]. The present investigations on the optimization of the deposition process of thin films are useful in preparing metallic photocathodes which have been tested in a photodiode cell. Even in ultra high vacuum (few 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−9</sup> mbar), QE is modified by the adsorption of the residual gases resulting in a short lifetime of the cathode itself [7]. It was found that the laser cleaning treatment is mandatory in order to remove the contamination layers from the cathode surface improving the photoemission performance.