There is growing interest in the use of ultrafast light pulses to generate and control novel states of matter with properties that are inaccessible in near thermal equilibrium conditions. New methods are required to characterize these states and build an understanding of the microscopic mechanisms that govern their behavior. Here we describe recent measurements of time- and momentum-resolved non-equilibrium lattice dynamics of ultrafast laser-excited materials using femtosecond x-ray pulses from free-electron lasers. We will focus on the prototypical Peierls distorted material bismuth, where we demonstrate the first channel-resolved measurements of three-body anharmonic phonon decay and the renormalization of the interatomic forces following a partial photo-excited reversal of the Peierls distortion. Time permitting, we will discuss other recent examples of how ultrafast lattice dynamics measurements provide key insight into materials behaviors both near and far from equilibrium.
This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences through the Division of Materials Sciences and Engineering under Contract No. DE-AC02-76SF00515. Measurements were carried out at the Linac Coherent Light Source, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences.