Energy-Filtered Ultrafast Electron Microscopy for Improving the Time Resolution

For the instrumentation in ultrafast electron microscopy (UEM), imaging the ultrafast phenomena at the nanoscale was challenging because the temporal resolution was insufficient to film the onset of the structural change associated with atomic motions. For imaging electron pulses, the duration is determined at the initial stages of photoemission, resulting from the mismatch of photon energy and the work function of the photocathode, inhomogeneities on the surface, and bandwidths of the photoemission-driving pulses, and during the propagation experiencing multiple beam crossovers. This energy spread of electron pulses develops a chirp, which is an energy (<i>E</i>)–time (<i>t</i>) correlation defined as a phase-space slope, resulting in the temporal broadening of the pulses because the leading electrons with higher energies accelerate and those with lower energies are retarded during propagation.<br/> <br/>In this presentation, we show the application of an energy filter to UEM to mitigate the temporal broadening of probe electron packets due to coulomb repulsion, and thus energy exchange, among the dense electrons. Energy-filtered TEM is advantageous for enhancing image contrast/resolution by mitigating chromatic aberration effects, which blur images. Likewise, a conventional energy filter gates the chirped photoelectron packets in UEM to select photoelectrons of narrow energy distribution and, therefore, short temporal duration.<br/><br/>With the requisite time resolution, we reveal the mechanism behind the ultrafast photoinduced phase transition of VO₂ and address its heterogeneous nature. For the optically induced phase transition of the polycrystalline VO₂film, the time-resolved electron imaging with gated photoelectrons revealed the enhancement of instrument response function from 2.8 ps to 700 fs. Utilizing the energy-filtered UEM, we show the heterogenous ultrafast phase transition of VO₂ nanoparticles through a transient low-symmetry metallic phase induced by local strains.