Speaker
Description
The ultrafast X-ray science community is looking for new spectroscopical layouts capable of disclosing how out of equilibrium states of matter rise and relax, as proof of principle schemes to achieve new insight on the mechanisms responsible for driving novel phases of matter.
Specifically, the advent of free-electron lasers (FELs) paves the way to the implementation of XUV-XUV time-resolved spectroscopies by virtue of their unique capability to lase high-brightness pulses marked by wavelengths spanning the extreme-ultraviolet, the soft and hard X-ray spectral domains, as well as by temporal lengths lying in the femtosecond (fs) timescale. In specific circumstances, as in the case of the FERMI FEL (Italy), it is possible to lase within a multi-color mode and to offer full polarization control of light, too. Such an approach allows the possibility to trigger, via XUV pulses, specific transition at (or nearby) core resonances, by measuring in real-time its response by means of an XUV probe.
At FERMI this goal is committed to the optical layout known as AC/DC (the auto correlator/delay creator), designed to split the FEL pulse into a couple acting, once focused on the sample under study, as pump and probe radiation. This is done by splitting the incoming XUV beam or by properly separating the XUV multi-color emission (in the FERMI multi-color configuration) along two different grazing-incidence optical paths, paying attention to preserve the unique spectral and temporal pulse properties. The AC/DC core optical layout takes advantage of a dedicated laser-based feedback system implemented to compensate in real-time for any mismatch affecting the beams optical trajectory. These are ascribable to both mechanical instabilities, imperfections and paraxial errors rising during a time delay scan and may particularly affect the beams pointing, particularly when the incoming pulses are focused on the samples under study in the few microns focal spot regimes.
To date, the AC/DC has been used for several proof of principle time-resolved XUV-XUV experiments within photon energies spanning between the extreme ultraviolet regime and the third harmonic emission of FERMI (in the FEL2 configuration), i.e. resonant nearby the transition metal L-edges. Future upgrades are currently under study.