Physics & Astronomy Catherine Kealhofer - Williams College

"Generating and Controlling Ultrafast Electron Pulses for Time-resolved Electron Diffraction"

Ultrafast electron diffraction (UED) is a powerful technique that lets us measure the positions of atoms in a crystal as they evolve during chemical and physical transformations. These measurements can give insight into how materials work on a microscopic level as well as guide the design of devices that respond on fast timescales. For some time, the best-available time resolution in UED has hovered around 100 fs (1 fs =10^-15 s), just shy of the 10 fs resolution required to see the fastest atomic motions. In my talk, I’ll explain how ultrafast electron diffraction works and why it’s challenging to break the 100-fs resolution barrier. I’ll show experimental results on a new technique that uses terahertz electromagnetic fields to compress electron pulses in time. This technique has the potential to improve the resolution of UED below 100 fs and maybe even below ~1 fs, into the regime of electronic dynamics. The talk will also introduce ultrafast electron sources based on laser-triggered electron emission from nanometrically sharp metal tips. Not only is the physics of ultrafast electron emission in this system very rich, but the extraordinarily small source size in comparison with typical ultrafast flat photocathode sources can dramatically improve the beam quality, leading to smaller probe sizes, higher quality diffraction patterns, and enabling new techniques like ultrafast electron holography.