Coherent nonlinear spectroscopy and coherent control are powerful techniques that are readily used at visible wavelengths. These methods are, however, extremely challenging to implement in the XUV regime due to the required interferometric stability and high sensitivity. At the seeded Free Electron Laser (FEL) FERMI, we have recently established a phase cycling scheme, where timing and phase of XUV pulses are independently controlled to high precision. The high stability and background-free detection of this approach allowed us to resolve the coherent evolution and dephasing of electronic wave packets exhibiting oscillation periods of only 145 attoseconds. Our technique sets the basis for a wide range of nonlinear spectroscopy and coherent control schemes that may now come within reach at seeded FELs and tabletop high harmonic sources.
A. Wituschek and L. Bruder et al., arXiv:1906.07112 (2019)