Ultrafast X-Ray Probes for THz-Scale Dynamics and Coupled Magnetic/Structural Fundamental Excitations

X-ray free-electron lasers and other accelerator-based sources of femtosecond-duration hard x-ray pulses allow new directions in ultrafast dynamics. A particularly exciting aspect of these probes is the opportunity to obtain information, simultaneously, about structural features and long-range order due to ferroelectricity and magnetism. In ferroelectric systems, impulsive excitation by above bandgap radiation can lead to the distortion of structural features such as the lattice parameter and octahedral rotation and to the oscillation of ferroelectric degrees of freedom. Free-electron laser experiments probing ferroelectric/dielectric superlattices reveal picosecond-scale oscillations after optical excitation that are linked to the impulsive perturbation of ferroelectric nanodomains due to photoinduced screening. Similarly, in magnetic garnets, optically induced impulsive strain couples to the magnetic moment and produces magnetic oscillations that can be tracked using x-ray magnetic diffraction. Future directions include exploring coupled excitations in the combination of THz frequency range, small sample size, and large wavevector in which existing optical or inelastic x-ray or neutron scattering techniques have not yet provided insight.