The four wave mixing (FWM) approach is exploited in the optical domain in a large array of scientific and technological applications. A relevant step forward, theoretically conceived more than one decade ago , is represented by the exploitation of extreme ultraviolet (XUV) and soft x-ray range radiation in FWM processes. However, this advancement was not experimentally pursued because of the absence of XUV photon sources with enough brigthness and coherent.
This situation has changed with the advent of free electron lasers (FELs), in particular those based on seeding processes. In this context, the XUV pulses delivered by the FERMI FEL have been used to experimentally demonstrate the FWM response stimulated by XUV transient gratings . We hereby report on the demonstration of FEL-based XUV FWM and on our efforts for the realization of user-dedicated experimental infrastructures for XUV-optical and "all-XUV/soft x-ray" FWM experiments . We also report on recent results, which includes the exploitation of the unique multi-pulse/multi-colour seeded FEL emission available at FERMI . These achievements can provide the basis over which one may build up the sophisticated experiments envisioned by theoreticians .
The exploitation of XUV/x-ray FWM can provide access to the ultrafast dynamics of high-energy excitations, such as valence-band excitons, with elemental selectivity and nano to atomic spatial resolution. On the other hand, the the large wavevector of EUV/soft x-ray photons (as compared to optical ones) may allow probing the “mesoscopic” (0.1-1 nm-1) wavevector range, which is of the highest relecance for the study of dynamics in disordered systems and nanostructures . Such capabilities can be exploited in contexts as diverse as charge transfer processes in molecules, vibrational dynamics in non-periodic solids or thermal transport processes in nanoelectronic devices.
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