Browsing by Subject "ultrafast"
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Item Investigations on the dynamics of excess electrons in pyrrolidinium bistriflimide and diacyanoamide RTILs(2015-05) Molins i Domenech, FrancescItem Real-Space Imaging of Picosecond Structural Dynamics in Metals with Ultrafast Transmission Electron Microscopy(2021-01) Gnabasik, RyanProbing the first picoseconds of structural response after photoexcitation in metal systems allows us to construct a timeline of how the absorbed energy evolves in space and time. To do this, we need a technique that is sensitive to structural changes at nanometer-picosecond spatiotemporal resolutions. Often techniques that fit these requirements operate in reciprocal space (e.g., ultrafast electron and x-ray diffraction) which are spot-size limited resulting in ensemble averaging over interesting defect and microstructure interactions. Here we use ultrafast transmission electron microscopy (UEM) to image the structural dynamics of two metal materials systems, the iron pnictide LaFeAsO and plasmonic gold nanorods, after ultrafast photoexcitation. First, plasmonic gold nanorods offer a viable route for coupling light into structures smaller than its wavelength. However, there is a convolution of optical and structural effects that prevent a complete understanding of how plasmonic structures behave, specifically in assemblies. The goal of this work is to use imaging in UEM to elucidate the structural response of a non-trivial assembly of gold nanorods to serve as complimentary structural information that when combined with optical models can form a complete picture. Investigations reveal early incoherent-to-coherent structural dynamics from an increase in thermal diffuse scattering followed by a roughly 10 ps delay believed to be phonon-phonon coupling ending with the launch and propagation of strain waves at the speed of sound. Because we are imaging, the response from individual nanorods in the assembly can be resolved as well as the potential effect on neighboring rods. This work has shown that we can construct a timeline for the first 100 picoseconds after ultrafast excitation from the electron-phonon coupling to the whole nanorod acoustic modes. Second, the iron pnictide class of materials display unconventional superconductivity when doped. Mechanistically different than the cuprates, the iron pnictide superconductivity has yet to be fully understood. It is believed that the nearly simultaneous structural and magnetic phase transitions of the parent compound offer clues to understanding the superconductivity mechanism. We use imaging in UEM to explore the structural dynamics of the parent compound. After photoexcitation, hypersonic Lamb modes emanate from the crystal-vacuum interface towards the bulk then interact with microscale crystal boundaries (e.g. step-edges, interfaces) that lead to reflection and wave interference. In addition, we present some initial exploration of probing the structural phase transition; showcasing the differences between the tetragonal and orthorhombic phases while demonstrating the capability of UEM to detect changes in elastic constants. The observations from these experiments show the strength of UEM to capture real-space, structural dynamics not easily accessible with reciprocal space techniques to provide complimentary information to the materials science and physics community.Item Towards the Ultimate Spatiotemporal Resolution in Low-Repetition Rate, Laser-Based Ultrafast Electron Microscopy: Theoretical and Experimental(2022-09) Curtis, WyattSince 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.