Intra-Cavity Femtosecond Standing Waves for Multi-Photon Pump-Probe Experiments at 100MHz

Femtosecond enhancement cavities for frequency combs are a powerful tool to study nonlinear processes at high repetition rates. The transfer of the coherence to shorter wavelengths by high-harmonic generation (HHG) enabled the development of frequency combs in the extreme ultraviolet (XUV), with promising prospects in precision spectroscopy [1]. However, HHG is not the only nonlinear process which can be investigated. Other phenomena, such as multi-photon ionization (MPI), above-threshold ionization (ATI), or the excitation of metastable states benefit from the high intensities reached at a high repetition rate. However, the operation of a cavity with HHG demands specific requirements, e. g., a dense gas target and a technique to couple the XUV light out of the cavity that might hinder other studies.