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Volume 194, 2016
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Four-wave-mixing experiments with seeded free electron lasers

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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.

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Publication details

The article was received on 22 Apr 2016, accepted on 23 May 2016 and first published on 23 May 2016

Article type: Paper
DOI: 10.1039/C6FD00089D
Faraday Discuss., 2016,194, 283-303

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    Four-wave-mixing experiments with seeded free electron lasers

    F. Bencivenga, A. Calvi, F. Capotondi, R. Cucini, R. Mincigrucci, A. Simoncig, M. Manfredda, E. Pedersoli, E. Principi, F. Dallari, R. A. Duncan, M. G. Izzo, G. Knopp, A. A. Maznev, G. Monaco, S. Di Mitri, A. Gessini, L. Giannessi, N. Mahne, I. P. Nikolov, R. Passuello, L. Raimondi, M. Zangrando and C. Masciovecchio, Faraday Discuss., 2016, 194, 283
    DOI: 10.1039/C6FD00089D

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