Ultrafast X-Rays

Non-equilibrium states in light-driven quantum materials arise from microscopic processes that require interrogating finite-momentum dynamics on ultrafast timescales. To address this challenge, our group employs a suite of ultrafast X-ray scattering techniques, including time-resolved X-ray diffraction (tr-XRD), resonant soft X-ray scattering (tr-RSXS), X-ray absorption spectroscopy (tr-XAS), and resonant inelastic X-ray scattering (tr-RIXS). tr-XRD and tr-RSXS provide incisive insights into structural and electronic ordering, capturing lattice distortions induced by optical phonon excitation and the dynamics of ordered electronic and spin states. tr-XAS measures the unoccupied density of states in light-driven quantum materials via direct X-ray photon absorption. tr-RIXS, an emerging spectroscopic technique, probes valence-band excitations—such as phonons, spin waves, orbital transitions, and plasmons—through momentum- and energy-resolved detection of scattered photons. Utilizing femtosecond X-ray pulses from free-electron lasers, this combination of techniques offers unparalleled access to the microscopic physics of nonequilibrium photoexcited states. Our group routinely performs experiments at leading X-ray free-electron laser facilities worldwide— including the LCLS in the US, the European XFEL in Germany, SwissFEL in Switzerland, and PAL-XFEL in South Korea.