Next-Generation Ultrafast Transmission Electron Microscopy – Development and Applications
von Armin Feist
Datum der mündl. Prüfung:2018-06-05
Erschienen:2018-08-22
Betreuer:Prof. Dr. Claus Ropers
Gutachter:Prof. Dr. Claus Ropers
Gutachter:Prof. Dr. Tim Salditt
Gutachter:PD Dr. Klaus Sokolowski-Tinten
Dateien
Name:eDiss-Feist_Armin.pdf
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Zusammenfassung
Englisch
The present cumulative thesis covers the development and applications of a novel type of ultrafast transmission electron microscope (UTEM) employing high-coherence electron pulses from a nanoscale photocathode. Specifically, a commercial Schottky field-emission TEM is modified to allow for ultrashort electron pulse generation by photoemission from a nanoscopic field emission tip. At the sample position, electron focal spot sizes down to 9 Å, an electron pulse duration of 200 fs (full-width at half-maximum) and a spectral bandwidth of 0.6 eV are demonstrated. The instrumental capabilities for ultrafast electron diffraction, imaging and spectroscopy are illustrated. A first detailed experiment harnesses the coherent quantum state manipulation of free-electron wavefunctions. The kinetic energy distribution of high-energy electrons traversing an intense optical near-field evolves into a comb of spectral sidebands spaced by the photon energy. Multilevel Rabi oscillations are observed in the optical-field dependent sideband populations, experimentally demonstrating the preparation of a coherent longitudinal momentum superposition state. Numerical simulations verify the formation of an attosecond electron pulse train after dispersive propagation. In a second study, the optically-induced ultrafast structural dynamics close to the edge of a single crystalline graphite membrane are investigated with a 28-nm/700-fs spatio-temporal resolution. Ultrafast convergent beam electron diffraction (U-CBED) is established as a quantitative technique to access the local lattice deformations on the femtosecond timescale. The complex acoustic distortions are disentangled by reconstruction of the relevant deformation gradient tensor components. Lateral scanning of the electron probe tracks the excitation, propagation and dissipation of the optically induced expansion and shear deformations.
Keywords: ultrafast transmission electron microscopy; UTEM; nanoscale photoemitters; coherent ultrashort electron pulses; nanotip; nanoscale structural dynamics; graphite; ultrafast dynamics; ultrafast convergent beam electron diffraction; U-CBED; nanostructures; free-electron quantum state; optical phase modulation; optical near-field; Rabi oscillations; coherent quantum state superposition; attosecond electron pulse train; ultrafast solid-state physics