FIG. 7.

Time evolution of the EUV FWM signal of a vitreous SiO₂ sample with the surface oriented orthogonally to the bisector of the FEL beams (Figure 4). (a) Black circles show the time dependence of the FWM signal, scaled to the intensity of the input beams. The blue and red lines are cross-correlation and the expected signal modulation (expanded for better visibility) due to acoustic modes, respectively. (b) Black circles connected by lines are the FWM signal after subtraction of the cross-correlation peak. The red line is the modulation due to oscillations of optical phonons at frequencies 1.15 THz and 4.1 THz. Reproduced with permission from Bencivenga et al., Nature 520, 205 (2015). Copyright 2015 Macmillan Publishers Limited.