If one turns down the temperature of a standard, zirconiated tungsten, Schottky cathode, it will still emit electrons when photo-activated. By arranging the polarization of the photon beam, one maintains the usual tight, high-brightness emission characteristic of Schottky TFE, albeit at considerably reduced current.
A group at Göttingen has adapted a JEOL JEM-2100F for this purpose. In an informative preprint, they give a quite complete description of the setup and its performance. In a project like this one expects a few difficulties, and these they find: limited beam current and stability issues, to name two. Given the reduced tip temperature, the zirconium does not migrate adequately to maintain the steady state population at the apex. Apparently the tip can be regenerated.
While column setup can be performed with the tip at normal TFE temperature, my experience has always been that the beam position changes depending on tip temperature. I look forward to seeing how this works out.
Reference: A Feist, et al., “Ultrafast transmission electron microscopy using a laser-driven field emitter: femtosecond resolution with a high coherence electron beam”, arXiv:1611.05022.