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Abstract

Striving for precision spectroscopy on highly charged ions (HCI), we employ high harmonic generation (HHG) to transfer a near-infrared (NIR) frequency comb to the extreme ultraviolet (XUV). Aiming to increase the harmonic yield, we spectrally broaden the NIR pulses for temporal compression.

Within a Herriott-type multi-pass cell (MPC), pulses are focused into a nonlinear crystal, where they experience self-phase modulation. Multiple passes incrementally broaden the spectrum, while the material dispersion is partially compensated using chirped mirrors. To characterise the compressed pulses and optimise them, a set-up for Frequency Resolved Optical Gating (FROG) is designed and implemented. The initial pulses exhibit a duration of 225 fs featuring satellite pulses; their spectral phase is cubic. Further, we reduce the empty MPC’s power losses by 38%, reaching a total transmission of 92%. Yttrium aluminium garnet broadens the spectrum by a factor of two, surpassing our previous results with fused silica by 15%. Notably, the cell output power remains stable. A first FROG trace reveals that the pulses become spatially chirped in or after the MPC, requiring further investigation. Looking ahead, the spectral broadening is promising for a future post-compression with pulse durations well below the previously obtained 90 fs. Additionally, a broader spectrum enables access to a wider range of HCI transitions.