A high-repetition-rate ultrafast electron diffraction setup with direct detection for solid-state samples

This thesis presents an in-depth description and analysis of an ultrafast electron diffraction (UED) instrument, with a focus on the optical configuration and the components that generate, propagate, and detect the electron beam. We have successfully produced a stable, high-quality electron beam with sufficient transverse coherence length, low emittance, and suitable beam diameters at both the sample and detector stages, facilitating high-resolution reciprocal-space measurements with standard samples. We employ direct electron detection with a high repetition rate laser system to achieve the same electron flux (10⁶ electrons/second) as compared to existing UED setups, but with significantly lower bunch charges, while detecting weak electron signals with a much higher signal-to-noise ratio (SNR) than traditionally employed UED detection systems. Furthermore, we have demonstrated the capabilities of direct electron detection, in which each pixel operates independently, even when some become saturated. The combined use of this detector with advanced correction algorithms enables real-time compensation for intensity and pointing fluctuations, significantly improving data quality. These advancements provide a robust foundation for performing time-resolved UED measurements of atomic-scale structural dynamics. To evaluate the temporal resolution of our instrument, we employed laser-induced plasma lensing techniques with a temporal resolution of one-to-a-few picoseconds. Our fluence-dependent experiments have demonstrated the ability to track transient field dynamics at high repetition rates by using a narrow, convergent electron beam at the interaction site, resulting in positive signal intensity variations as the beam focuses in response to the transient field. Additionally, experiments with varying bunch charges highlight the low-noise performance of our direct electron detection system. Ultrafast dynamic phenomena with temporal resolution on the order of a few hundred femtoseconds are demonstrated through non-thermal laser-induced heating experiments, which are commonly used to elucidate transient lattice distortions in simple metallic systems. Particularly, we measured the differential diffraction signal from a photoexcited aluminum thin film. Further analysis of the mean-squared displacement (MSD) at lower diffraction orders suggests the presence of multiple long-range phonon-mediated energy-dissipation mechanisms, associated with shorter thermalization times than those observed in higher-order diffraction planes. Crucially, differentiating the power spectral density (PSD) components across diffraction planes leads us to infer that specific phonon branches interact differently at various diffraction orders, which is consistent with the Non-Thermal Lattice Model as discussed in the literature. Our study delineates the spatial range of dynamics explicitly by examining diffraction from each interplanar distance individually, with high sensitivity and temporal resolution without temporal compression. Finally, we performed a preliminary characterization of a radio-frequency (RF) electron-pulse compression system. The RF synchronization jitter is optimized to a sub-20 fs RMS level, which is favorable for the estimated IRF (184 fs). Additionally, the RF compression is indirectly confirmed through measurements of the change in the electron beam transverse diameter as a function of the RF cavity power.