Review of recent progress on advanced photocathodes for SC RF guns

As well known, the quality of the photocathodes is essential for the stability and the reliability of photo injector operation. Especially for the superconducting ratio frequency photo injectors (SRF guns), the photocathode represents one of the most critical parts. Benefit from the fast de-veloping photocathode technology in last years, several SRF guns were successfully operated or tested for the beam generation at kHz - MHz repetition rate. In this paper, we will review the achievements as well as the open questions in the applications of the photocathodes for SRF gun operation. Furthermore, we will discuss the possible improvement from cathodes side for the future CW electron sources.