Prof. Xijie Wang
Control and Characterization of Non-equilibrium Materials using MeV Ultrafast Electron Diffraction
Modern ultrafast laser techniques are capable of bringing materials into states far from equilibrium, ultrafast electron probe is one of the most powerful tools to investigate the structure dynamics of the materials far from equilibrium. To achieve atomic spatial and temporal resolution simultaneously, MeV high-brightness beam generated by a photocathode RF gun was proposed for ultrafast electron diffraction (UED) and microscopy (UEM) applications . Recent development in MeV-UED  has enabled broad scientific opportunities in ultrafast material science, warm dense matter physics, and chemical dynamics. The ripples of monolayer MoS2  and light-induced structural reorganizations in the hybrid perovskites  were captured for the first time by the MeV-UED. MeV UED has been successfully explored for diffuse scattering  and warm dense matter physics studies . With single-shot MeV-UED, we first observe heterogeneous melting on time scales of 100 ps to 1000 ps, transitioning to homogeneous melting that occurs catastrophically within 10-20 ps at higher energy densities. We recently use a single THz pulse to trigger a structural phase transition from the orthorhombic to a different metastable phase of 2-D material WTe2 in a reversible manner, and probe the switching using MeV-UED. These findings serve as the first direct evidence of a THz field induced structural transition in a two-dimensional material, and offer a new promising way to optically control the topological properties of solids.
Bio: Dr. Xijie Wang is the principal investigator of the Ultrafast Electron Diffraction & Microscopy (UED/UEM) initiative at SLAC National Accelerator Laboratory. Xijie Wang obtained his undergraduate degree from Shaanxi Normal University, Xian, China. Xijie joined Brookhaven National Laboratory (BNL) after earning a PhD in physics from UCLA in 1992. Xijie Wang was awarded a tenure position at BNL in 2001 based on his leadership in developing the BNL Accelerator Test Facility (ATF) and seminal work on ultra-short electron generation and characterization. Xijie Wang moved to SLAC National Accelerator Laboratory in December 2013 to lead SLAC UED/UEM initiative. Dr. Wang’s research covers a wide range of topics in accelerator physics and ultrafast science & technology. Dr. Xijie Wang has made major contributions to the science and technologies enabling the X-FEL and ultrafast electron diffraction. The MeV-UED pioneered by Xijie has demonstrated atomic spatial and temporal resolutions required to make atomic and molecular movies, and made it possible the first visualization of the heterogeneous to homogeneous melting transition. He is an inaugural recipient of SLAC Director Award.