Strong-Field Extreme-Ultraviolet Dressing of Atomic Double Excitation

• Published in: Physical review letters 2019-10, Vol.123 (16), p.1
• Main Authors: Ott, Christian, Aufleger, Lennart, Ding, Thomas, Rebholz, Marc, Magunia, Alexander, Hartmann, Maximilian, Stooß, Veit, Wachs, David, Birk, Paul, Borisova, Gergana D, Meyer, Kristina, Rupprecht, Patrick, Castanheira, Carina da Costa, Moshammer, Robert, Attar, Andrew R, Gaumnitz, Thomas, Loh, Zhi-Heng, Düsterer, Stefan, Treusch, Rolf, Ullrich, Joachim, Jiang, Yuhai, Meyer, Michael, Lambropoulos, Peter, Pfeifer, Thomas
• Format: Article
• Language: English
• Subjects: Asymmetry; Dipoles; Electric fields; Electron states; Electron transitions; Free electron lasers; Lasers; Line shape; Nonlinear optics; Time dependence; Ultraviolet lasers
• ISSN: 0031-9007; 1079-7114
• DOI: 10.1103/PhysRevLett.123.163201

We report on the experimental observation of a strong-field dressing of an autoionizing two-electron state in helium with intense extreme-ultraviolet laser pulses from a free-electron laser. The asymmetric Fano line shape of this transition is spectrally resolved, and we observe modifications of the resonance asymmetry structure for increasing free-electron-laser pulse energy on the order of few tens of Microjoules. A quantum-mechanical calculation of the time-dependent dipole response of this autoionizing state, driven by classical extreme-ultraviolet (XUV) electric fields, evidences strong-field-induced energy and phase shifts of the doubly excited state, which are extracted from the Fano line-shape asymmetry. The experimental results obtained at the Free-Electron Laser in Hamburg (FLASH) thus correspond to transient energy shifts on the order of a few meV, induced by strong XUV fields. These results open up a new way of performing nonperturbative XUV nonlinear optics for the light-matter interaction of resonant electronic transitions in atoms at short wavelengths.