Interplay of electron heating and saturable absorption in ultrafast extreme ultraviolet transmission of condensed matter

High intensity pulses obtained by modern extreme ultraviolet (EUV) and x-ray photon sources allows the observation of peculiar phenomena in condensed matter. Experiments performed at the Fermi Elettra FEL-1 free-electron-laser source at 23.7, 33.5, and 37.5 eV on Al thin films, for an intermediate-fluence range up to about 20 J/cm super(2), show evidence for a nonmonotonic EUV transmission trend. A decreasing transmission up to about 5-10 J/cm super(2) is followed by an increase at higher fluence, associated with saturable absorption effects. The present findings are interpreted within a simplified three-channel model, showing that an account of the interplay between ultrafast electron heating and saturation effects is required to explain the observed transmission trend.