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Abstract

Cavity-enhanced frequency combs are a powerful tool for studying highly nonlinear light-matter interactions, such as multiphoton ionization (MPI) and high-harmonic generation (HHG), with promising prospects for precision spectroscopy beyond the optical spectral range. In this work, a metrology-capable extreme ultraviolet (XUV) frequency comb is produced by transferring a near-infrared comb at 1039 nm to the XUV, using intra-cavity HHG. Intensities of ∼10¹⁴ W cm-² are reached in the cavity focus, producing XUV radiation up to 42 eV (30 nm) and tens of microwatts of outcoupled power. A high-pressure closed-loop noble gas recycling and compression system enables long-term measurements. Additionally, a novel polarization-insensitive cavity with an integrated velocity-map imaging spectrometer was developed. 3D photoelectron angular distributions from xenon MPI are tomographically reconstructed, revealing resonant Rydberg states during ionization. Furthermore, polarization-shaped pulse pairs with a variable time delay are provided for pump-probe experiments. Intense femtosecond standing waves, produced by counter-propagating pulses colliding at the focus, are probed at the nanometer scale using photoemission from a nano-tip. The coherence of the frequency comb is imprinted on the photoelectrons, allowing future precision measurements with coherent matter waves. This work broadens the scope of cavity-enhanced frequency combs and enables strong-field studies at 100 MHz repetition rate.