"Ultrafast and Nonlinear X-Ray Scattering in Solids."

"There has been remarkable progress on using the combination of short pulse, high spatial coherence and large flux from x-ray free electron lasers (FEL) for nonperiodic imaging and nanocrystallography. The possibility of atomic-scale imaging remains a primary motivation for the construction of FELSs. Their unprecedented properties also motivate the study the dynamics of atomic-scale fluctuations directly in the time-domain. However, an important open question is under what conditions the textbook picture of linear, x-ray–matter interactions holds. In this colloquium, I will describe evidence for coherent nonlinear interactions at LCLS and SACLA FELs as well as the first direct time-domain measurements of the dispersion relation for lattice vibrations in photo-excited materials."