Periodic Lensing from a One-Component Plasma Imaged by Ultrafast Transmission Electron Microscopy

Transmission electron microscopy (TEM) has become a powerful technique to study the structure of materials at nanoscale. The temporal resolution of TEM, however, is typically limited by the maximum frame rate of the detector to the ms regime. Ultrafast TEM combines the high temporal resolution of ultrafast laser spectroscopy with the excellent spatial resolution of electron microscopy. The structural and electronic changes in the material are initiated by fs laser pulses followed by similarly short photoelectron pulses to prob the dynamics by imaging, diffraction, or electron spectroscopy. Here, we will present our ultrafast TEM at UIUC, and demonstrate its first application in observation of periodic plasma lensing. We generated a one-component electron plasma (OCEP) by two-photon photoemission from a Cu grid. We directly imaged the subsequent evolution of the OCEP under the influence of an external uniform magnetic field inside the TEM on the ps time scale. The cyclotron oscillations give rise to a periodic lensing effect which maximizes when the OCEP is refocused along the magnetic field. We analytically described the dynamics of OCEP and its time-dependent focal length to extract the number of electrons and their velocity spread. We verified the model by N-body simulations.