Tabletop nonlinear optics in the 100-eV spectral region

Nonlinear light-matter interactions in the extreme ultraviolet (XUV) are a prerequisite to perform XUV-pump/XUVprobe spectroscopy of core electrons.Such interactions are now routinely investigated at free-electron laser (FEL) facilities.Yet, electron dynamics are often too fast to be captured with the femtosecond resolution of state-of-theart FELs.Attosecond pulses from laser-driven XUV-sources offer the necessary temporal resolution.However, intense attosecond pulses supporting nonlinear processes have only been available for photon energy below 50 eV, precluding XUV-pump/XUV-probe investigation of typical inner-shell processes.Here, we surpass this limitation by demonstrating two-photon absorption from inner electronic shells of xenon at photon energies around 93 eV and 115 eV.This advance opens the door for attosecond real-time observation of nonlinear electron dynamics deep inside atoms.