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.
Bibliographical noteFunding Information:
Funding. Project Transregio TR18, Deutsche Forschungsgemeinschaft (DFG); Munich Centre for Advanced Photonics (MAP); Euratom research and training program 2014-2018, FP7 Fusion Energy Research (EURATOM-FUSION) within the framework of the EUROfusion Consortium (633053); H2020 European Research Council (ERC); Laserlab-Europe (MPQ002119).
© 2018 Optical Society of America.