title: Four-wave-mixing experiments with seeded free electron lasers.
abstract: The development of free electron laser (FEL) sources has provided an unprecedented bridge between the scientific communities working with ultrafast lasers and extreme ultraviolet (XUV) and X-ray radiation. Indeed, in recent years an increasing number of FEL-based applications have exploited methods and concepts typical of advanced optical approaches. In this context, we recently used a seeded FEL to demonstrate a four-wave-mixing (FWM) process stimulated by coherent XUV radiation, namely the XUV transient grating (X-TG). We hereby report on X-TG measurements carried out on a sample of silicon nitride (Si3N4). The recorded data bears evidence for two distinct signal decay mechanisms: one occurring on a sub-ps timescale and one following slower dynamics extending throughout and beyond the probed timescale range (100 ps). The latter is compatible with a slower relaxation (time decay > ns), that may be interpreted as the signature of thermal diffusion modes. From the peak intensity of the X-TG signal we could estimate a value of the effective third-order susceptibility which is substantially larger than that found in SiO2, so far the only sample with available X-TG data. Furthermore, the intensity of the time-coincidence peak shows a linear dependence on the intensity of the three input beams, indicating that the measurements were performed in the weak field regime. However, the timescale of the ultrafast relaxation exhibits a dependence on the intensity of the XUV radiation. We interpreted the observed behaviour as the generation of a population grating of free-electrons and holes that, on the sub-ps timescale, relaxes to generate lattice excitations. The background free detection inherent to the X-TG approach allowed the determination of FEL-induced electron dynamics with a sensitivity largely exceeding that of transient reflectivity and transmissivity measurements, usually employed for this purpose.
authors.affiliation: Elettra-Sincrotrone Trieste S.C.p.A., S.S. 14 km 163.5 in AREA Science Park, 34149 Basovizza, Italy. filippo.bencivenga@elettra.eu.
authors.forename: F
authors.initials: F
authors.lastname: Bencivenga
authors.affiliation: Department of Physics, University of Trieste, Via A.Valerio 2, 34127 Trieste, Italy.
authors.forename: A
authors.initials: A
authors.lastname: Calvi
authors.affiliation: Elettra-Sincrotrone Trieste S.C.p.A., S.S. 14 km 163.5 in AREA Science Park, 34149 Basovizza, Italy. filippo.bencivenga@elettra.eu.
authors.forename: F
authors.initials: F
authors.lastname: Capotondi
authors.affiliation: IOM-CNR, Strada Statale 14-km 163.5, 34149 Basovizza, Trieste, Italy.
authors.forename: R
authors.initials: R
authors.lastname: Cucini
authors.affiliation: Elettra-Sincrotrone Trieste S.C.p.A., S.S. 14 km 163.5 in AREA Science Park, 34149 Basovizza, Italy. filippo.bencivenga@elettra.eu.
authors.forename: R
authors.initials: R
authors.lastname: Mincigrucci
authors.affiliation: Elettra-Sincrotrone Trieste S.C.p.A., S.S. 14 km 163.5 in AREA Science Park, 34149 Basovizza, Italy. filippo.bencivenga@elettra.eu.
authors.forename: A
authors.initials: A
authors.lastname: Simoncig
authors.affiliation: Elettra-Sincrotrone Trieste S.C.p.A., S.S. 14 km 163.5 in AREA Science Park, 34149 Basovizza, Italy. filippo.bencivenga@elettra.eu.
authors.forename: M
authors.initials: M
authors.lastname: Manfredda
authors.affiliation: Elettra-Sincrotrone Trieste S.C.p.A., S.S. 14 km 163.5 in AREA Science Park, 34149 Basovizza, Italy. filippo.bencivenga@elettra.eu.
authors.forename: E
authors.initials: E
authors.lastname: Pedersoli
authors.affiliation: Elettra-Sincrotrone Trieste S.C.p.A., S.S. 14 km 163.5 in AREA Science Park, 34149 Basovizza, Italy. filippo.bencivenga@elettra.eu.
authors.forename: E
authors.initials: E
authors.lastname: Principi
authors.affiliation: Department of Physics, University of Trento, Via Sommarive 14, Povo, TN, Italy.
authors.forename: F
authors.initials: F
authors.lastname: Dallari
authors.affiliation: Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA, USA.
authors.forename: R A
authors.initials: RA
authors.lastname: Duncan
authors.affiliation: Elettra-Sincrotrone Trieste S.C.p.A., S.S. 14 km 163.5 in AREA Science Park, 34149 Basovizza, Italy. filippo.bencivenga@elettra.eu.
authors.forename: M G
authors.initials: MG
authors.lastname: Izzo
authors.affiliation: Paul Scherrer Institute, Villigen 5232, Switzerland.
authors.forename: G
authors.initials: G
authors.lastname: Knopp
authors.affiliation: Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA, USA.
authors.forename: A A
authors.initials: AA
authors.lastname: Maznev
authors.affiliation: Department of Physics, University of Trento, Via Sommarive 14, Povo, TN, Italy.
authors.forename: G
authors.initials: G
authors.lastname: Monaco
authors.affiliation: Elettra-Sincrotrone Trieste S.C.p.A., S.S. 14 km 163.5 in AREA Science Park, 34149 Basovizza, Italy. filippo.bencivenga@elettra.eu.
authors.forename: S
authors.initials: S
authors.lastname: Di Mitri
authors.affiliation: Elettra-Sincrotrone Trieste S.C.p.A., S.S. 14 km 163.5 in AREA Science Park, 34149 Basovizza, Italy. filippo.bencivenga@elettra.eu.
authors.forename: A
authors.initials: A
authors.lastname: Gessini
authors.affiliation: Elettra-Sincrotrone Trieste S.C.p.A., S.S. 14 km 163.5 in AREA Science Park, 34149 Basovizza, Italy. filippo.bencivenga@elettra.eu and ENEA CR Frascati, Via E. Fermi 45, 00044 Frascati, Rome, Italy.
authors.forename: L
authors.initials: L
authors.lastname: Giannessi
authors.affiliation: Elettra-Sincrotrone Trieste S.C.p.A., S.S. 14 km 163.5 in AREA Science Park, 34149 Basovizza, Italy. filippo.bencivenga@elettra.eu.
authors.forename: N
authors.initials: N
authors.lastname: Mahne
authors.affiliation: Elettra-Sincrotrone Trieste S.C.p.A., S.S. 14 km 163.5 in AREA Science Park, 34149 Basovizza, Italy. filippo.bencivenga@elettra.eu.
authors.forename: I P
authors.initials: IP
authors.lastname: Nikolov
authors.affiliation: Elettra-Sincrotrone Trieste S.C.p.A., S.S. 14 km 163.5 in AREA Science Park, 34149 Basovizza, Italy. filippo.bencivenga@elettra.eu.
authors.forename: R
authors.initials: R
authors.lastname: Passuello
authors.affiliation: Elettra-Sincrotrone Trieste S.C.p.A., S.S. 14 km 163.5 in AREA Science Park, 34149 Basovizza, Italy. filippo.bencivenga@elettra.eu.
authors.forename: L
authors.initials: L
authors.lastname: Raimondi
authors.affiliation: Elettra-Sincrotrone Trieste S.C.p.A., S.S. 14 km 163.5 in AREA Science Park, 34149 Basovizza, Italy. filippo.bencivenga@elettra.eu and IOM-CNR, Strada Statale 14-km 163.5, 34149 Basovizza, Trieste, Italy.
authors.forename: M
authors.initials: M
authors.lastname: Zangrando
authors.affiliation: Elettra-Sincrotrone Trieste S.C.p.A., S.S. 14 km 163.5 in AREA Science Park, 34149 Basovizza, Italy. filippo.bencivenga@elettra.eu.
authors.forename: C
authors.initials: C
authors.lastname: Masciovecchio