The Many Facets of Ultrafast Electron Diffraction and Microscopy—The Development and Applications

In the past two decades, the rapid pace in the development of ultrafast electron diffraction and microscopy instruments has ushered in a new area of investigating structural dynamics and related material property responses[1]. In this presentation, I will review the multifaceted development that now reaches temporal resolution of 100 fs-level or below with electron density as high as 10⁵ particles per pulse for structure dynamics involving nanometer-thin or micro-structured material systems. Prototypical examples for instrument design, data collections, and specialized protocols for structure analysis relevant for photoinduced phase transitions will be outlined in some detail with the hope that they can be reproduced by interested audience in their own studies[2]. The need to simultaneously obtain information about the electronic states, functionalities, as well as structure dynamics motivates the development of ultrafast electron microscopy in which diffraction, imaging, and spectroscopy are united to enable the multimodality observations at the ultrafast timescales for a broad range of material science problems[3]. Examples will be given to illustrate the technologies on two prototypical examples: (1) Scale-dependent self-organization in vanadium dioxide thermally induced and photoinduced phase transitions. (2) Nonequilibrium universal dynamics in the condensation of 2D charge-density waves in 1T-TaS₂ and 1T-TaSe₂.<br/><br/>The work is funded by the U.S. Department of Energy, Grant DE-FG0206ER46309.<br/><br/>[1] C.-Y. Ruan, in: K. Amini, A. Rouzée, M.J.J. Vrakking (Eds.) Structural Dynamics with X-ray and Electron Scattering, The Royal Society of Chemistry2023.<br/>[2] X. Sun, S. Sun, C.-Y. Ruan, Toward nonthermal control of excited quantum materials: framework and investigations by ultrafast electron scattering and imaging, Comptes Rendus. Physique, 22 (2021) 15-73.<br/>[3] X. Sun, J. Williams, S. Sharma, S. Kunjir, D. Morris, S. Zhao, C.-Y. Ruan, Precision-controlled ultrafast electron microscope platforms. A case study: Multiple-order coherent phonon dynamics in 1T-TaSe2 probed at 50 fs–10 fm scales, Structural Dynamics, 11 (2024) 024305.