Direct low-harmonic generation in gas at MHz repetition rate

High harmonic generation (HHG) at multi-megahertz pulse repetition rates is challenging, since pulse energy is several orders of magnitudes lower for such lasers than for those conventionally used for the HHG: micro-joules instead of milli-joules. However we have observed the 5th and 7th harmonic (till approx. 17 eV) with 5 mJ pulses at 50 MHz repetition rate with frequency doubled Yb:YAG Innoslab amplifier. The single-pass HHG with the Innoslab amplifier at 20.8 MHz repetition rate was demonstrated [1]. For these experiments the nonlinear pulse compression was implemented.