High-power few-cycle laser systems enable table-top coherent XUV and soft X-ray sources via high harmonic generation (HHG) and, consequently, subsequent applications in advanced spectroscopy and imaging [1, 2]. As a result of the application-driven demand for bright, laser-like soft X-ray sources with spectral coverage up to the water window (280 – 530 eV), the community is developing ultrafast lasers at carrier wavelengths beyond the emission bands of Yb- or Ti:Sa-based gain media. The reason for this wavelength shift is the fundamental upscaling of the HHG-driving laser’s ponderomotive force and with it, the photon energy cut-off. However, the increased cut-off comes at the cost of conversion efficiency, which is why the long-wavelength driving lasers must deliver pulses as short as possible to allow for optimizing the phase-matched peak intensity (and therefore the efficiency ). Additionally, high repetition rates are aspired to increase the overall yield.
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