An ultrafast transmission electron microscopy with integration of transient optical spectroscopy

Ultrafast condensed-matter science seeks to unravel how photoexcited electrons exchange energy and momentum with the lattice, setting the causal pathways for relaxation, coherent phonon generation, and light-driven changes of structure and function. Establishing such causality requires simultaneous access to electronic response and structural dynamics under truly identical excitation conditions. Here, we report the development of an ultrafast transmission electron microscopy (UTEM) with an integrated transient optical spectroscopy, enabling co-registered transient optical reflectance, and ultrafast electron diffraction/imaging/spectroscopy measurements from the same microscopic region while sharing the same pump condition. As a first application, we demonstrate its unique capability in deciphering coupled electron-lattice dynamics in layered MoS 2 and GaTe semiconductors. The MoS₂ illustrates a three-stage energy-flow scenario captured by joint optical and diffraction transients, whereas the GaTe shows coherent phonons that imprint on both dielectric and structural observables, with overlapping and mode-selective oscillatory components across the two signals. This multimodal instrument provides a powerful route to causality-resolved, quantitatively constrained studies of nonequilibrium energy flow and structural dynamics with high spatiotemporal resolution, bridging electronic and lattice degrees of freedom across diverse quantum and functional materials.