Enhanced soft-X-ray high-harmonic generation driven by mid-IR pulses mixed with their third harmonic

This work is a systematic study of the enhancement of soft-X-ray high-harmonic generation (HHG) driven by two-color (ω + 3ω) waveforms in argon at different pressures. The observed enhancement due to the two-color driver reaches factors over one order of magnitude when compared to single-color driven HHG. The driver pulses were provided by a kHz, multi-mJ optical parametric chirped pulse amplifier system (OPCPA) at the Massachusetts Institute of Technology. The setup was carefully characterized to provide accurate parameters for simulations. Theexperimental setup and data acquisition were automated to ensure the necessary level of precision and improve measurement speed. Clear modulation of the high harmonic yield with the relative phase of the driving waveform indicates that sub-cycle control over the ionization and electron trajectories on the single-atom level was achieved. The results of the two-color experiments are compared to single-color driven high harmonic spectra at optimized conditions as well as to full 3D simulations. To the best of our knowledge, this is the first report ontwo-color driven HHG using such long fundamental driving wavelengths and thus provides valuable data adding to the very few publications on ω + 3ω mixing in HHG so far