Terahertz-Pump Experiments On Complex Solids At X-Ray Fels

We present recent progress in understanding nonlinear phonon dynamics driven by intense THz field strengths using ultrafast x-ray diffraction. This technique allows us to characterize the motion of the atomic lattice inside the sample on a sub-cycle timescale while the driving field is still present. In the example of strontium titanate (SrTiO3) we observe harmonic generation and phonon upconversion when the fundamental soft mode is driven into the nonlinear regime by a single-cycle THz pulse.