Alexei Maznev1,Filippo Bencivenga2,Riccardo Mincigrucci2,Ettore Paltanin2,Wonseok Lee3,Scott Cushing3,Nicolas Jaouen4,Fabian Kammerbauer5,Vincent Polewczyk6,Cyril Léveillé4,Laura Foglia2,Danny Fainozzi2,Nupur Khatu2,7,Dario De Angelis2,Christian Gutt8,Dmitriy Ksenzov8
Massachusetts Institute of Technology1,Elettra Sincrotrone Trieste2,California Institute of Technology3,Synchrotron SOLEIL4,University of Mainz5,Istituto Officina dei Materiali6,University of Venice7,University of Siegen8
Alexei Maznev1,Filippo Bencivenga2,Riccardo Mincigrucci2,Ettore Paltanin2,Wonseok Lee3,Scott Cushing3,Nicolas Jaouen4,Fabian Kammerbauer5,Vincent Polewczyk6,Cyril Léveillé4,Laura Foglia2,Danny Fainozzi2,Nupur Khatu2,7,Dario De Angelis2,Christian Gutt8,Dmitriy Ksenzov8
Massachusetts Institute of Technology1,Elettra Sincrotrone Trieste2,California Institute of Technology3,Synchrotron SOLEIL4,University of Mainz5,Istituto Officina dei Materiali6,University of Venice7,University of Siegen8
The advent of free electron lasers (FELs) enabled the expansion of nonlinear optical spectroscopy methods into extreme ultraviolet (EUV) and x-ray ranges. In particular, four-wave mixing techniques with EUV and EUV/optical fields are being actively developed [1-4]. Self-diffraction is the simplest non-collinear four-wave mixing process, in which the interference of two beams crossed in the sample results in a spatially periodic excitation acting as a diffraction grating for the same beams. Self-diffraction is being widely used in optics but has not hitherto been observed in the EUV or x-ray ranges. Generally, the self-diffraction signal from femtosecond EUV FEL pulses is expected to be low, as the polarizability in the EUV range is mainly determined by the average electron density, which does not have time to change significantly within the femtosecond pulse duration. In this report, we show that the EUV self-diffraction efficiency is greatly enhanced and very bright signals are observed when the EUV photon energy is tuned to an absorption edge of the sample. In an experiment conducted at the TIMER beamline at the FERMI FEL, we crossed two 70 fs EUV pulses in a 20 nm –thick cobalt film and scanned the photon energy between 52 – 74 eV. At the M<sub>2,3</sub>-edge of Co (59 eV) we observed a sharp peak in the self-diffraction efficiency, with the signal level reaching ~4000 photons/shot. DFT/Bethe-Salpeter equation based simulations involving a model assumption that the excitation is equivalent to an increase of the electronic temperature are found to reproduce the peak at the M-edge but not the fine structure observed above the edge. We believe that the results open the prospect for a new element-specific nonlinear EUV spectroscopy technique.<br/><br/>[1] F. Bencivenga et al., Nature 520, 205 (2015), [2] L. Foglia et al., Phys. Rev. Lett. 120, 263901 (2018), [3] F. Bencivenga et al., Sci Adv. 5, eaaw5805 (2019), [3] H. Rottke et al., Sci Adv. 8, eabn5127 (2022).