Splitting and delaying fully coherent FELs pulses for advancing ultrafast x-ray science: the AC/DC optical device at the FERMI FEL

Free-electron lasers (FELs) are currently the most advanced light sources operating worldwide, thanks to their capability to lase coherent ultrashort pulses, marked by photon energies bridging the gap between the Extreme-Ultraviolet (EUV) and the Soft (SXR) and Hard (HXR) X-Rays, alongside with unique high-brightness and temporal duration lying in the femtosecond (fs) timescale. FELs can exploit, in a time-resolved approach, spectroscopies daily employed at synchrotron light sources, mostly combining EUV, SXR and HXR pulses with optical ones. Nonetheless, the next advances in ultrafast x-Ray science are strongly linked to the extension of these time-resolved schemes to perform experiments engaging exclusively EUV, SXR and HXR pulses, so triggering (and probing) matter at its (or nearby) electronic resonance(s), to reveal the microscopic mechanisms hiding behind matter phases of primary interest for broadband applications. Indeed, designing the next generation of quantum devices, as well as tailoring a new classes of biomolecules for pharmacological applications, are just two examples that can be strongly boosted by means of this optical approach. To do this, is mandatory to split and delay (in time) FELs pulses, without impacting on both the radiation coherence properties and on the photon transport. At the seeded FERMI FEL (Trieste, Italy) this goal is committed by the optical device known as AC/DC, which stands for the Auto Correlator/Delay Creator, designed to split the incoming EUV and/or SXR pulse, introducing a tunable delay between these two pulses, marked by an intrinsic resolution in the sub-fs, and aided by an opto-numerical pointing feedback system.