Nanoscale Ultrafast Lattice Modulation with Hard X-ray Free Electron Laser

Abstract Understanding and controlling microscopic dynamics across spatial and temporal scales has driven major progress in science and technology over the past several decades. While ultrafast laser-based techniques have enabled probing nanoscale dynamics at their intrinsic temporal scales down to femto- and attoseconds, the long wavelengths of optical lasers have prevented the interrogation and manipulation of such dynamics with nanoscale spatial specificity. With advances in hard X-ray free electron lasers (FELs) 1–4, significant progress has been made developing X-ray transient grating (XTG) spectroscopy5, 6, aiming at the coherent control of elementary excitations with nanoscale X-ray standing waves. So far, XTGs have been probed only at optical wavelengths 5, 6, thus intrinsically limiting the achievable periodicities to several hundreds of nanometers. By achieving sub-femtosecond synchronization of two hard X-ray pulses at a controlled crossing angle, we demonstrate the generation of an XTG with spatial periods of ∼ 10 nm. The XTG excitation drives a thermal grating that drives coherent monochromatic longitudinal acoustic phonons in the cubic perovskite, SrTiO3 (STO). With a third X-ray pulse with the same photon energy, time-and-momentum resolved measurement of the XTG-induced scattering intensity modulation provides evidence of ballistic thermal transport at nanometer scale in STO. These results highlight the great potential of XTG for studying high-wave-vector excitations and nanoscale transport in condensed matter, and establish XTG as a powerful platform for the coherent control and study of nanoscale dynamics.