Development of off-axis photoemission for 4-dimensional ultrafast electron microscopy

Abstract

Ultrafast Electron Microscopy (UEM) is a novel technique for imaging which features generation of ultrashort electron probes to achieve on the order of femtosecond-nanometer spatiotemporal resolutions. In typical TEG based UEM architectures, photoemission is carried out from an on-axis LaB6 conical filament large enough to mitigate the effects of laser pointing stability changes. Such designs also have the advantage of versatility: the LaB6 can be aligned and used for both thermionic emission and photoemission entirely in-situ. However, LaB6, due to low work function and surface chemical instability, has long been plagued by current stability issues, and the overall quality of the photoemitted beam is invariably deleteriously affected by using the LaB6 for thermionic emission. These issues have partially motivated an exodus of the ultrafast electron imaging community from laser-based photoemission using traditional thermionic sources.Here, we demonstrate a new method for achieving high quality photoelectron beams by photoemitting directly from the nickel Wehnelt aperture. This architecture greatly improves photoemission current stability in practical implementation after thermionic emission from the LaB6. Around 15 minutes after thermionic emission, the LaB6 filament still shows biexponential photocurrent decays for a measured 5 hours, while the Wehnelt aperture photocurrent is linearly stable within 1% current for an hour. We demonstrate that the minimum probe size achievable using the Wehnelt aperture is around ~19 nm for the Flannigan UEM instrument, identical to the LaB6 photobeam and LaB6 thermionic beam. We perform basic imaging using the off-axis emitted Wehnelt photobeam in parallel beam and convergent beam conditions to demonstrate usability of the photobeam for basic UEM experiments. All of these demonstrated photoelectron beam properties are achieved in situ and without sacrificing the viability of using a LaB6 filament optimized for thermionic imaging for standard high resolution TEM. Using General Particle Tracer, a physics modeling engine for particle dynamics, we comprehensively study the effects of varying geometric parameters of Wehnelt photoemission upon the photoelectron beam. We find that the Wehnelt is optimized for photoelectron current at a Wehnelt diameter of 0.7 mm, a radial photoemission position of 80 µm away from the inner edge, and a setback distance of 350 µm relative to the photoemitting surface of the LaB6 filament. We also establish general trends for collection efficiency, time of arrival, and packet temporal dispersion for simple geometric factors that can be easily modified or implemented in a real UEM instrument. These studies pave the way for sophisticated design and implementation of off-axis photoemission microscopes. We have thoroughly quantified and demonstrated the experimental usability of off-axis photoemission and studied a range of additional geometries through simulation. We believe that this work demonstrates the versatility of laser-based UEM instruments, and enables development of UEM gun architectures that can operate both as thermionic and photoelectric sources with high spatiotemporal resolution.