Grating-enhanced transverse broadening for direct measurement of electron pulse duration in an ultrafast electron microscope

To address the limitations in temporal resolution of an ultrafast electron microscope (UEM) and the shortcomings of existing pulsed-electron characterization methods, we introduce a fast, energy-filter-free technique for measuring electron pulse duration that exploits the interaction between free electrons and the phase-matched near-field of a periodic grating. The interaction induces a measurable transverse broadening of the electron beam, which directly encodes the temporal profile of the electron pulse. Implementing this method on a Wehnelt-controlled UEM, we observe an asymmetric temporal distribution with a long trailing edge at high bias voltage. Finite-element simulations of electron trajectories replicate the experimental trends and identify the temporal aberration of the electrostatic lens as the primary cause of asymmetric broadening. This approach enables rapid, high-precision diagnostics of electron-pulse characteristics without requiring an energy-filter spectrometer, and the findings provide valuable insights into the factors that presently limit UEM temporal resolution.