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Ryabov, Andrey (2017): Electron microscopy of electromagnetic waveforms. Dissertation, LMU München: Fakultät für Physik



Quickly oscillating electric and magnetic fields are the foundation of any information processing device or light-matter interaction. An electron microscope exceeds the diffraction limit of optical microscopes and is therefore a valuable device for condensed-matter structure and nanoscale objects investigations. While the electron microscope easily provides structural information, other methods are usually necessary to reveal the electromagnetic phenomena. Moreover, for ultrafast devices, in which charge-carrier dynamics occurs on femtosecond to picosecond time scales, the temporal resolution has to reach such values in order to successfully access the sample's electromagnetic response. Here, we introduce and demonstrate a concept for electron microscopy of electromagnetic waveforms. We achieve sub-optical-cycle and sub-wavelength resolutions in time and space. The technique can be applied to a transmission electron microscope, which expands its capabilities to the regime of electromagnetic phenomena. The approach thus may give researchers access to additional important information on the object under investigation. We let a short electron pulse pass through a sample, which is excited by an electromagnetic pulse, and record the time-dependent deflection. If the electron pulse, the key element of the technique, has a sub-cycle duration with respect to excitation radiation, the electrons are deflected by a time-frozen Lorentz force in a quasi-classical way and therefore directly reveal the sample's dynamics. By using an all-optical terahertz compression approach, we succeeded to shorten a single-electron pulse of 930 fs duration down to 75 fs, which is 15 times shorter than the period of excited in the sample dynamics. To characterize such short electron pulse, streaking with THz fields in a sub-wavelength structure was applied, which provided sub-20-femtosecond resolution. The reconstruction of electromagnetic fields from the electron deflection is not a trivial problem. We solve it by recording the electron density evolution after the interaction with a sample in a pump-probe experiment and employ the Gauss-Newton algorithm for an iterative fitting analysis. As a result, we acquire a time delay sequence containing two-dimensional spatial distributions of the field vector dynamics with a sub-cycle resolution in time. Further analysis of the evaluated data can provide frequency and material response information together with mode structures and their temporal dynamics. If the new technique is combined with a transmission electron microscope, it will be possible to study the fastest and smallest electrodynamic processes in light-matter interactions and devices.