High harmonic generation (HHG) is a nonlinear optical process, in which hundreds of photons, usually in the NIR spectral range, are combined together through interaction with a medium, resulting in frequency upconversion to produce coherent emission in the extreme-ultraviolet and soft x-ray regions. HHG depends strongly on the driving laser field; due to the phase matching conditions, the converted light has similar spatial coherence properties as the driving field. At the same time, the generated HHG pulses have durations ranging from femtoseconds to a few tens of attoseconds, making this technique promising for high-resolution time-resolved spectroscopy in the keV energy range. Since the first HHG experiment performed by Burnett et al. in 1977, significant progress has been made in this area towards increasing conversion efficiency and output photon energy. Modern HHG setups with optical parametric oscillators as a pump source and noble gases as a nonlinear medium provide a tunable table-top source of soft X-rays for wide-range practical applications. Nevertheless, the efficiency of the HHG process is still extremely low and efforts to increase photon flux and photon energy of the converted light still face a challenge.
Scientists from four research centers located in the US, Brazil and Germany reported on a high-flux soft X-ray HHG operating at kilohertz rate with photon energy up to 190 eV. To obtain this outstanding result, the authors designed a unique pump laser system and used this key element for HHG in a Ar/N2 gas cell. First, they developed an optical parametric chirped pulse amplification (OPCPA) laser system based on a kHz, 3-stage, multi-ten-milli-joule picosecond cryogenic Yb:YAG laser with chirped-pulse amplification design. Second, the energy of the OPCPA system was scaled up from 0.85 mJ to 2.6 mJ. Using numerical simulations of HHG in Ar, the authors confirmed the generation of soft X-ray attosecond pulse trains for the parameters of the experimental conditions. The researchers believe that further extension to the water-window region (~4.37–2.33 nm, where water is less absorbing than carbon) with high flux is possible using high-pressure Ne and He gas cells, and hope that this system will be a promising light source for soft X-ray attosecond science as well as bioimaging applications.