FIG. 6.

Four wave mixing (FWM) experiments with EUV transient gratings. (a) Sketch of the FEL-based FWM experiment: 2 EUV pulses (27.6 nm) and an optical pulse (392.8 nm) are crossed at well-defined angles. The two FEL beams generate the EUV dynamic grating on the sample. The time evolution of the TG is probed by the scattering of the optical pulse. A CCD sensor is placed in the expected propagation direction of the FWM signal beam, which is determined by the phase matching conditions of the three pulses. Reproduced with permission from Bencivenga et al., Nature 520, 205 (2015). Copyright 2015 Macmillan Publishers Limited.