High-harmonic generation at 100 mhz repetition frequency using a femtosecond enhancement cavity

High-harmonic generation (HHG) [1],[2] provides a coherent source of vacuum ultraviolet (VUV) to soft X-ray radiation in a relatively compact system. Since its first observations [3],[4], HHG has relied on high-energy, low repetition rate laser systems to provide the peak intensities needed for ionization of the gas target. The small conversion efficiency of the process, combined with the low repetition rate of amplified laser systems, results in low average powers in the VUV generation. Experiments trying to utilize these sources therefore often suffer from poor signal-to-noise levels, resulting in long data-acquisition times. Furthermore, the use of these sources as precision spectroscopic tools remains limited in comparison with sources in the visible. Ramsey-type spectroscopy has been utilized to improve spectral resolution in the VUV [5],[6] but remains orders of magnitude away from the precision measurement capability available with fs laser based frequency combs in the optical and IR [7],[8]. This is due to the fact that the original frequency comb structure of the laser is lost in the HHG process from the reduction of the pulse train repetition rate required to actively amplify single pulses to the needed energies/intensities.