The development of ultrafast time-resolved techniques in the last decades allowed the direct observation of out-of-equilibrium transient states and dynamical processes up to then hidden in many different fields, ranging from chemistry to solid state physics and biology. Amongst them, Ultrafast Electron Diffraction (UED) allows the retrieval of direct information on the structural dynamics of solids, surfaces, nanostructures and molecular crystals with femtosecond (10^-15 s) and atomic (Ã…, 10^-10 m) temporal and spatial resolution.
The UED setup implemented at EPFL is characterised by 30 keV, 20 kHz pulses containing up to 105 electrons each and can work in both transmission and reflection geometry. A sub-500 fs time resolution in transmission geometry is achieved by means of a radio frequency compression cavity for electrons. This time resolution, combined with the high transverse coherence of our electrons, allowed the observation of order-disorder phenomena in 2D-supracrystals of functionalised gold nanoparticles. The insurgence of ligand-dependent photo- mechanical stiffness phenomena was observed and characterised in the aforementioned systems. In reflection geometry, the time resolution suffered severe limitations - especially when probing mm2-sized samples - due to the velocity mismatch between the pump and the probe beam. To overcome this limitation, an optimal optical front tilt scheme for the pump pulse was designed, implemented and characterised, thus disclosing a sub-500 fs temporal resolution also in reflection geometry.
The newly achieved time resolution allowed the direct visualisa- tion of ultrafast structural dynamics in two systems of current interest: graphite and magnetite. In graphite, the selective excitation via electron-phonon coupling of a subset of phonon modes called Strongly Coupled Optical Phonons - with a characteristic timescale of ~ 500 fs - was experimentally resolved. In magnetite, the photoinduction of the Verwey phase transition was achieved, unveiling phase segregation intermediate states during the transformation and disclosing structural differences between the photoinduced and the thermal phase transition mechanism.