Ultrafast control of emergent quantum matter probed by electron microscopy

In the quest for controlling materials' properties, light as an external stimulus has a special place as it can create new states of matter and enable their ultrafast manipulation. In particular, spintronics, an exciting emergent field relying on the electron spin property, has tremendous potential as it breaks ground for high-density, high-speed, and low-power-consuming memory devices. Skyrmion, a specific magnetic texture of whirling spins, has attracted broad interest due to its unique topological properties. Skyrmion-based devices stand out as they combine ultra-efficient control and robustness on nanometer scales. More fundamentally, skyrmions offer an exclusive tabletop playground to study emergent fields, topological phase transitions, cosmology, and black holes, to mention a few. Optical manipulation of skyrmions is hence crucial as it paves the way for efficient ultrafast control and provides a direct approach to studying low-energy collective modes and topological fluctuations in real space.In this thesis, I show the manipulation of quantum materials through three groundbreaking experiments. First, I used in-situ Lorentz transmission electron microscopy, a magnetic imaging technique, and showed the laser-induced skyrmion formation in the multiferroic Cu2OSeO3 compound. I, therefore, provide the first demonstration of the magnetic free energy landscape manipulation leading to a topological phase transition. Notably, a new recipe is revealed that allows the control of the magnetic phase diagram on a picosecond timescale by transiently modifying the material's inherent magnetic interactions. This study has profound consequences on out-of-equilibrium topological phase transition investigation and technological applications as it marks an important milestone for efficient ultrafast spintronic devices.Furthermore, I established a new protocol using femtosecond light to coherently control a skyrmion crystal eight orders of magnitude faster than previously achieved by exploiting the collective nature of the skyrmion lattice. Remarkably, as the process relies on a collective periodic mode, it can be coherently manipulated by adjusting the time delay between laser pulse sequences. Consequently, the skyrmion orientation can be deterministically chosen. In other words, I present the manipulation in real space of a few spins orientation in an ultrafast and energy-efficient way, vital for next-generation devices. In addition, our observations demonstrate emergent properties of the skyrmion interactions at the mesoscale, opening exciting perspectives for investigating the collective skyrmion dynamics at a large scale which might be relevant for unconventional superconductors where magnetic vortices similar to skyrmions exist at similar length scales.Last but not least, I show the importance of the different electronic excitations in Magnetite (Fe3O4), the prototypical metal-insulator system, and the oldest known magnetic material. The structural response along the specific crystallographic [110] axis is investigated using ultrafast electron diffraction, providing sub pm/ps spatio-temporal resolution. By tuning the photon energy, thus triggering different electronic transitions, two distinct lattice responses are unveiled, unattainable thermodynamically. This work paves the way to establishing novel hidden phases in quantum materials via specific electronic excitations in a strongly correlated environment.