![]() The grating induced in the material takes the form of periodic regions of varying temperature, carrier density, or magnetic moment 4. Absorption of the optical energy from the interference pattern excites a response in the sample with a commensurate spatial profile. TG techniques employ a spatially structured intensity pattern, generated via optical interference, with a sinusoidal spatial dependence. Optical TG has been widely applied in studying nonequilibrium material dynamics. Here, we apply high spatial resolution and direct lattice sensitivity to the transient grating (TG) technique by employing a focused X-ray probe to resolve the evolution of the excited structural grating in real space. Several characterization techniques provide either spatial or temporal resolution, but, as we demonstrate, unique insight can be gained by having both spatial and temporal resolution simultaneously. Simultaneously measuring the ultrafast, nanoscale dynamics of materials promises to advance such fields. The nanoscale, nonequilibrium behavior of materials underpins a host of phenomena, from ultrafast optical activation of functional electronic and magnetic properties 1, to laser-based additive manufacturing 2, to solid–solid phase transformations relevant to neuromorphic computing 3. TGXD successfully characterizes mesoscopic energy transport in functional materials without relying on a specific transport model. The focused X-ray probe provides spatial resolution within the engineered optical excitation profile, resolving the spatiotemporal flow of heat through FeRh locally heated above the phase transition temperature. The strain profile of the structural grating in FeRh, in comparison, deviates from the sinusoidal excitation and exhibits both higher-order spatial frequencies and a location-dependent relaxation. In BiFeO 3, structural relaxation is location independent, and the strain persists on the order of microseconds, consistent with the optical excitation of long-lived charge carriers. We demonstrate TGXD using two thin-film samples: epitaxial BiFeO 3, which exhibits a photoinduced strain (structural grating) with an amplitude proportional to the optical fluence, and FeRh, which undergoes a magnetostructural phase transformation. This method adds spatial resolution and direct structural sensitivity to the established utility of a sinusoidal transient-grating excitation. Here, we introduce an optical transient grating pump and focused X-ray diffraction probe technique (TGXD) to examine the structural evolution of materials excited by modulated light with a precisely controlled spatial profile. A fundamental understanding of materials’ structural dynamics, with fine spatial and temporal control, underpins future developments in electronic and quantum materials.
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