Attosecond and few-femtosecond light pulses provide the time resolution required to track and control the very first instants of electron dynamics in molecules and condensed matter. Since its first demonstration, High-order Harmonic Generation (HHG) sources have been at the core of experimental schemes aiming at unravelling ultrafast electron motion. This thesis reports advancements in high-order harmonic sources and their applications to both solid-state spectroscopy and Free-Electron Laser (FEL) pulse characterization. First, it demonstrates and rationalizes efficient soft X-ray HHG in the so-called overdriven regime enabled by a custom-designed differentially pumped glass chip. This development directly addresses some of the technical challenges of conventional high-photon-energy HHG sources, providing a more accessible approach. Second, this work presents results on solidstate High-order Harmonic Generation (sHHG) spectroscopy in crystalline, amorphous, and semi-periodic TiO2 samples. The experiment showcases the potential of HHG not only as a source, but also as a spectroscopic probe of the valence electrons governing materials functional properties. Finally, this thesis reports the experimental validation of Double-Blind Holography (DBH) of ultrashort FEL pulses. This is a fully optical platform for single-shot temporal haracterization, phase retrieval, and few-femtosecond resolution delay tagging that is based on the interference with an unknown external field, in this case a standard HHG source. Together, these developments extend the capabilities of high-order harmonic sources, opening new scenarios for investigating and controlling ultrafast electron dynamics in atoms, molecules and condensed matter, both with table-top schemes and at FELs.
Agata Azzolin (Thu,) studied this question.