Soft x-ray high harmonic generation using an infrared light source and its application to the measurements of x-ray absorption near-edge structure at the oxygen K edge

We demonstrate high harmonic generation beyond the water window using an intense infrared light source operating at 2000 nm in lithium niobate-based optical parametric chirped-pulse amplification by using an Yb:YAG thin-disk laser as a pump. The output pulses from the parametric amplifier (pulse energy: 1.68 mJ, wavelength: 1750-2500 nm, repetition rate: 5 kHz) are temporally compressed with an efficiency of 79% (1.32-mJ compressed pulse) down to 19.5 fs, which corresponds to the 2.9-cycle duration. The generated soft x-ray harmonics have been employed to measure x-ray absorption near-edge structure around the K edges of carbon, nitrogen, and oxygen, with acquisition times short enough to enable practical applications in time-resolved soft x-ray spectroscopy.