Water window soft x-ray source enabled by a 25 W few-cycle 2.2 µm OPCPA at 100 kHz

Abstract

Coherent soft x-ray (SXR) sources enable fundamental studies in the important water window spectral region. Until now, such sources have been limited to repetition rates of 1 kHz or less, which restricts count rates and signal-to-noise ratio for a variety of experiments. SXR generation at high repetition rate has remained challenging because of the missing high-power infrared laser sources to drive the high-harmonic generation (HHG) process. Here we present an optical parametric chirped pulse amplifier (OPCPA) centered at a wavelength of 2.2 µm and generating 16.5 fs pulses (2.2 oscillation cycles of the carrier wave) with 25 W of average power and a peak power exceeding 14 GW at 100 kHz pulse repetition rate. This corresponds to the highest reported peak power for high-repetition-rate long-wavelength laser systems. The output of this 2.2 µm OPCPA system was used to generate a coherent SXR radiation extending beyond 0.6 keV through HHG in a high-pressure gas cell.