Strong-Field Extreme-Ultraviolet Dressing of Atomic Double Excitation

We report on the experimental observation of a strong-field dressing of an autoionizing two-electronstate in helium with intense extreme-ultraviolet laser pulses from a free-electron laser. The asymmetricFano line shape of this transition is spectrally resolved, and we observe modifications of the resonanceasymmetry 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, drivenby classical extreme-ultraviolet (XUV) electric fields, evidences strong-field-induced energy and phaseshifts of the doubly excited state, which are extracted from the Fano line-shape asymmetry. Theexperimental results obtained at the Free-Electron Laser in Hamburg (FLASH) thus correspond to transientenergy shifts on the order of a few meV, induced by strong XUV fields. These results open up a new way ofperforming nonperturbative XUV nonlinear optics for the light-matter interaction of resonant electronictransitions in atoms at short wavelengths