For well over a century x rays have been a powerful tool for probing atomic-scale structure due to their short wavelength and relatively weak interaction with matter. As sources have become ever more brilliant, scientists have been able to probe the microscopic world with more and more exquisite detail. In the past couple of decades free-electron lasers have provided the most intense laboratory source of x rays with femtosecond pulse durations---short enough to capture the fastest vibrations in solids, and the making and breaking of chemical bonds. In this colloquium, I'll present a few examples of how we utilize these remarkable light sources to gain new insight into material properties. I’ll present a novel method for studying non-equilibrium lattice dynamics in the time domain[1] which we’ve used to identify a novel lattice instability in photoexcited SnSe[2], as well as identify the changes in interatomic forces that drive it [3]. The high brightness further allows us to isolate valence electron density within the atomic bonds[4]. I’ll show how we’ve been able to view the local nonlinear response to sub-bandgap excitation in the prototypical semiconductor silicon[5]. These results advance our goals of developing a mechanistic understanding, and novel methods of controlling, the remarkable properties of materials on their fundamental length and time scales.

[1] M. Trigo, et al., Fourier-transform inelastic x-ray scattering from time- and momentum-dependent phonon-phonon correlations. Nat. Physics, 9(12):790–794, 2013.
[2] Y. Huang, et al., Observation of a novel lattice instability in ultrafast photoexcited SnSe. Phys. Rev. X, 12(1):011029, 2022.
[3] Y. Huang, et al., Nonthermal bonding origin of a novel photoexcited lattice instability in SnSe. Phys. Rev. Lett. 131:156902, 2023
[4] T. E. Glover, et al., X-ray and optical wave mixing. Nature, 488(7413):603–608, 08 2012.
[5] C. Ornelas-Skarin, et. al., Second-order microscopic nonlinear optical susceptibility in a centrosymmetric material: Application to imaging valence electron motion. Phys. Rev. X, 16:011006, 2026.