Stable Photoemission from the Wehnelt Aperture Surface in 4D Ultrafast Electron Microscopy*

Laser-based femtosecond (fs) transmission electron microscopy (TEM), dubbed 4D ultrafast electron microscopy (UEM), consists of coupling a fs pulsed laser with a TEM to extend temporal resolution to fs scales. A longstanding challenge has been inherent source instability coinciding with impracticalities of switching between pulsed and conventional beams. Here we show that stable and robust photoemission  can be generated from the anode-facing surface of the Wehnelt aperture in a TEG-based UEM. Further, we show that the Wehnelt-aperture photoelectron beam quality is at least as good as that for the oft-used LaB₆ cathode in both the photoelectron and conventional thermionic modes. Finally, we emphasize that such an arrangement is readily amenable to efficient switching between pulsed- and conventional-beam modes.