Direct low-harmonic generation in gas at MHz repetition rate

摘要

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 μJ 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. The pulses of about 35 fs duration generated about 1 nW at approx. 18 eV. In our experiment laser system consists only of Yb:doped oscillator (Amplitude Systems), Yb:YAG Innoslab amplifier [2] and a frequency doubling stage. It is worth mentioning, that chirped pulse amplification and a nonlinear pulse compression were not integrated in the setup. Nevertheless after the frequency doubling stage, at about 515 nm, we got about 240 W average power on the target of a noble gas jet (Xe or Kr) for the HHG experiments. The pulse duration was about 600 fs. For single-pass HHG, the laser beam was tightly focused into an effusive gas target realizing an intensity of 2 × 1013 W/cm2. The generated photon fluxes in extreme ultraviolet spectral range are shown in Figure 1. The photon fluxes of the HHG depicted on the y-axis are the estimated values from the measured data if taking into account the losses and the efficiency of the detection system.