Towards the Ultimate Spatiotemporal Resolution in Low-Repetition Rate, Laser-Based Ultrafast Electron Microscopy: Theoretical and Experimental
2022-09
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Towards the Ultimate Spatiotemporal Resolution in Low-Repetition Rate, Laser-Based Ultrafast Electron Microscopy: Theoretical and Experimental
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2022-09
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Since its inception, ultrafast electron microscopy (UEM) has proved to be a versatile tool for probing sub-nanosecond dynamics in materials. Combining the high resolutions of traditional TEM with ultrafast pump-probe techniques, UEM allows scientists to observe optically induced dynamics in both real and reciprocal space. However, deleterious electron-electron interactions lead to both spatial and energetic broadening, reducing the ability to image subtle dynamics. An approach to overcoming these interactions has been to generate images using single electron packets, which limits packet degradation due to the absence of space-charge. In this regime, a careful balance must be struck between laboratory stability and repetition rate, as long acquisitions must be used to form a coherent image. However, innate instrument parameters and architecture also play a key role in the shaping of the single-electron beam quality. Presented in this work, is a set of systematic simulations of the thermionic electron gun in a minimally modified system. Using particle tracing simulations, we have explored the intricate link between photoemission properties, the internal electrostatic architecture of the TEM and their effects on collection efficiency, temporal resolution, and chromatic aberrations. We have found that careful consideration of electron emission and the diameter of the Wehnelt aperture must be taken in order to assume utmost beam quality. This portion of work ultimately serves as the foundation for a large, multi-faceted effort to map the entire parameter space for minimally modified UEM. In addition to these simulations, the first steps towards low repetition rate, high resolution UEM are performed. Using an un-floated TEM column, we are able to demonstrate near-instrument limited resolution using a thermionic electron beam. In addition, we perform a direct comparison between the imaging capabilities of pulsed and thermionic electron beams. In an effort to push the limits of specimen relaxation, we show that nanometer scale resolution is achievable for long acquisition times (80-120s), dose rates as low as 0.014 e- Å-2 s-1, and probe repetition rates as low as 10 kHz. We also demonstrate at similarly low repetition rates we are able to continuously resolve nanometer scale features over multiple hours, the duration of an ultrafast experiment while under specimen illumination. These results provide the fundamental proof-of-ability for direct observation of real-time, nanometer scale materials dynamics in UEM and lay the groundwork and methodology for the realization of low repetition rate Å-fs spatiotemporal resolution. In tandem with efforts to improve the resolving capabilities of ultrafast pulsed electron beams, this work also provides the design and implementation of a custom in situ laser beam profiler. Upon the final development of this specialty holder, unparalleled access to the specimen excitation profiles will be possible. In total, the work presented here demonstrates a developmental effort to achieve the highest possible resolutions in low repetition rate, laser based UEM using both experimental and theoretical methodology.
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University of Minnesota Ph.D. dissertation. September 2022. Major: Chemical Engineering. Advisors: David Flannigan, Bharat Jalan. 1 computer file (PDF); xi, 213 pages.
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Curtis, Wyatt. (2022). Towards the Ultimate Spatiotemporal Resolution in Low-Repetition Rate, Laser-Based Ultrafast Electron Microscopy: Theoretical and Experimental. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/250058.
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