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 $20\mathrm{J}/{\mathrm{cm}}^2$, show evidence for a nonmonotonic EUV transmission trend. A decreasing transmission up to about $5–10\mathrm{J}/{\mathrm{cm}}^2$ 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.

Figure 1

Sketch of the experimental setup including optics, diagnostics, and detectors for the transfer and control of the FEL pulses generated by the Fermi@Elettra FEL-1 source (right, not shown). From the right to the left (main figure) FEL pulses pass through ionization chambers (measuring the intensity $I_0$), a gas cell absorber (fluence reduction), a plane mirror, and selected filters. On the left side we show the setup inside the UHV TIMEX chamber: the motorized manipulator holding sample and pulse diagnostics, the detector (photodiode measuring $I_1$), the focusing mirror, and the telemicroscope. (a) The FEL focal spot at sample position; (b) an image of the Al ultrathin sample surface after FEL pulse exposures; (c) the multisample assembly for this experiment.

Figure 2

Single-shot transmission data of FEL-1 pulses across an ultrathin 100 nm Al foil as a function of the estimated fluence $F$. Data refer to a FEL photon energy of 23.7 eV (red circles), 33.3 eV (green squares), and 37.0 eV (blue circles). Lines depicted for each photon energy enclose bands of estimated uncertainty (standard deviation) for the single-shot data points within successive $F$ intervals.

Figure 3

Experimental EUV transmission data compared with different calculations (see text). Curve $A$ includes only optical saturation phenomena without accounting for temperature effects; curve $B$ includes electron heating but neglects saturation phenomena; curve $C$ takes into account both electron heating and saturation effects. The vertical bars refer to the average and standard deviations of single-shot data points within successive $F$ intervals.

Figure 4

Estimated electron temperature in the 100 nm thick Al thin film as a function of incoming fluence at different FEL photon energies (curves enclose maximal and minimal temperatures through the film for given fluence). Electron temperatures in excess of 1 eV are obtained for fluence regimes above $\sim 3$ $\mathrm{J}/{\mathrm{cm}}^2$. Saturable absorption is found to limit the deposited energy above 10 $\mathrm{J}/{\mathrm{cm}}^2$ providing uniform bulk heating in the high-fluence regime (see the inset on a linear scale).