Femtosecond enhancement cavity with kilowatt average power**Project supported by the National Natural Science Foundation of China (Grant Nos. 11674356 and 11527807) and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB21010400).

Abstract

Femtosecond enhancement cavity (fsEC) has been proved to be a powerful tool in a diverse range of applications. Here, we report the recent progresses in building an fsEC on kilowatt level average power, with the aim of realization of intracavity high harmonic generation (HHG) and extension of the wavelength of femtosecond optical frequency comb from infrared (IR) to extreme ultraviolet (XUV). Upon mode-matching optimization and cavity length locking, an intracavity average power of 6.08 kW is reached and the corresponding buildup is 225. After introducing noble gas of Xe into the focus region, clear sign of plasma has been observed. The generated HHG is also coupled out by a sapphire plate placed at Brewsterʼs angle for the fundamental laser. Our work paves the way for the realization of an XUV comb.