Title : Ultrafast Electron Microscopy: Femtosecond-Resolved Imaging Matter at Nanoscales


Date / Time: 31 May, 2022 / 5:00 P.M


Speaker : Oh-Hoon Kwon(UNIST)



Abstract : Recently, our group has achieved the sub-nanometer spatial resolution in ultrafast electron microscopy (UEM) imaging vibrating individual gold nanorods by integrating a direct electron detection camera for the first time, but with limited time resolution of several picoseconds. As an energy filter in transmission electron microscopy has enabled the elemental mapping at the atomic scale and improved the precision of structural determination by gating inelastic and elastic imaging electrons, respectively, the introduction of the energy filter to UEM can advance the time resolution to the domain of atomic motion. Imaging transient structures with femtosecond temporal precision was made possible by gating imaging electrons of narrow energy distribution from dense chirped photoelectron packets of broad longitudinal (and transverse) momentum distribution, thus, typically posing picosecond duration. Presented are the demonstration of the energy-filtered UEM achieving the temporal resolution reaching the femtosecond-duration of an optical excitation pulse, filming ultrafast insulator-to-metal phase transition of individual vanadium dioxide nanodomains. Our approach leads the access of electron microscopy to the timescale of elementary nuclear motions visualizing the onset of structural dynamics of matter at nanoscales. Also, we uniquely combine cathodoluminescence (CL) with UEM. A synergistic use of the two methodologies is essential because CL and UEM are the best approaches with required spectral and spatiotemporal sensitivity, respectively. For nitrogen-related color centers in nanodiamonds, we demonstrate the measurement of CL lifetime with the local sensitivity of 50 nm and the time resolution of 100 ps. It is revealed that the emitting state of the H3 color center (N2V) can be populated by hole transfer from the NV center excited by free carrier transfer diffusing across diamond lattices upon electron beam excitation. The technical advance achieved in this study will deliver new concepts for specific control over energy conversion relevant to quantum dots and single photon sources.