Or new paper on THz driven dynamics is now out
M. E. Kozina et al, Terahertz-driven phonon upconversion in SrTiO3, Nature Physics 15, 387 (2019)
Here we excite the soft mode SrTiO3 with an intense THz pulse and probe the structural response of the lattice with hard x-rays at the XPP endstation at LCLS. We observe that higher frequency phonon modes at 5 and 7.8 THz are excited if the soft mode is driven so strongly into the nonlinear regime that new frequency content is generatted.