FEL-based four-wave-mixing @ FERMI

Free Electron Laser technology is nowadays developing extremely fast and the possibility to have fully coherent pulses in the Extreme UltraViolet (EUV) and soft x-ray is becoming a reality. FERMI is the proof of principle that table-top laser experiments can now be carried out at much shorter wavelengths making possible to probe dynamical processes occurring in molecular and nanostructured materials with an unprecedented time-space (femtosecond-nanometer) resolution. Indeed, the use of high energy photons allows stimulating and probing electronic transitions from core levels thus providing chemical selectivity.
In particular, the development of experimental methods based on coherent non-linear interactions, such as four-wave-mixing (FWM) processes, in the optical spectral range has represented a fundamental milestone for experimental physics, chemistry and biology. The FWM approach can provide ultrafast time resolution as well as energy and wavevector selectivity, other than allowing probing dynamics inaccessible by linear methods. The possible extension of FWM at wavelengths shorter than the optical ones was proposed in the early 2000’s. It has been theoretically showed how the multi-wave nature of FWM in combination with soft X-ray resonances might represent a unique experimental tool, able to probe, e.g., the real-time dynamics of electronic excitations between selected atoms with femtosecond-nanometer time-space resolution [1]. FWM at sub-optical wavelengths may also allow detecting elementary excitations (phonons, polarons, etc.) in an energy-wavevector range inaccessible by optical FWM [2].

Though in the last few years FEL sources have been employed to take fundamental steps into X-ray non-linear interactions, FEL-based FWM has not been put on practice. In July 2014 we used a purposely designed compact experimental set-up (Fig. 1a) to demonstrate how the coherent FEL pulses delivered by the FERMI seeded FEL facility (Trieste, Italy) can generate transient gratings (TGs) in the extreme ultraviolet (EUV) range and how such TGs, when illuminated by an optical laser pulse, can stimulate an appreciable FWM response [3]. The latter has the form of a well-defined beam (Fig. 1b) that propagates downstream the sample along the “phase matched” direction (k_out; Fig. 1c). In our experiment we also observed a time-evolution of the FEL-stimulated FWM signal, extended throughout the whole probe timescale (≈ 0-100 ps) and featured by modulations compatible with those expected by ultrafast molecular vibrations and slower acoustic modes. This result is a fundamental milestone for more advanced EUV/soft X-ray FWM applications, which we are going to develop at FERMI in the next future.

Figure 1. Sketch of the FEL-based FWM experiment: k_FEL1, k_FEL2, k_opt and k_out are the wavevectors of the two crossed FEL excitation pulses, the optical probing pulse and the FWM signal. (a) Picture of the setup (close to the sample area) inside the DiProI end-station, the arrows are k_FEL1, k_FEL2 k_opt and k_out. (b) Image of the FWM signal beam propagating along the phase-matched direction (kout), defined by the diagram shown in (c).

References
[1] S. Tanaka and S. Mukamel, Phys. Rev. Lett. 89, 043001 (2002).
[2] F. Bencivenga and C. Masciovecchio, NIMA 606, 785-789 (2009).
[3] F. Bencivenga et al., Nature 520, 205-208 (2015).

This research was conducted by the following research team:

Filippo Bencivenga, Riccardo Cucini, Flavio Capotondi, Andrea Battistoni, Riccardo Mincigrucci, Erika Giangrisostomi, Alessandro Gessini, Michele Manfredda, Ivaylo P. Nikolov, Emanuele Pedersoli, Emiliano Principi, Cristian Svetina, Pietro Parisse, Francesco Casolari, Miltcho B. Danailov, Maya Kiskinova and Claudio Masciovecchio.

• Elettra Sincrotrone Trieste, Trieste, Italy.

Contact persons:

Filippo Bencivenga:
Claudio Masciovecchio: