Nonequilibrium Dynamics of Collective Excitations in Strongly Interacting and Correlated Quantum Systems

Revealing the emergence and the dynamics of collective excitations in complex matter is a subject of pivotal importance, as it provides insight into the strength and spatial distribution of interactions and correlations. At the same time, collectivity lies at the origin of several cooperative phenomena in many-body systems, which can lead to profound transformations, instabilities and, eventually, phase transitions. Mapping the interactions of the collective bosonic excitations with the fermionic particles and among themselves leads to the comprehension of the many-body problem. In this Thesis, we investigate the dynamics of collective excitations in strongly interacting and correlated systems by means of ultrafast broadband optical spectroscopy. Within this approach, a material is set out-of-equilibrium by an ultrashort laser pulse and the photoinduced changes in its optical properties are subsequently monitored with a delayed probe pulse, covering a broad spectral region in the visible or in the ultraviolet. Collective excitations can be unraveled either in the frequency domain as spectral features across the probed range or in the time domain as coherent modes triggered by the pump pulse. Studying the renormalization and temporal evolution of these collective excitations gives access to the hierarchy of low-energy phenomena occurring in the solid during its path towards the thermodynamic equilibrium. This framework is explored in a number of prototypical materials with an increasing degree of internal complexity beyond conventional band theory. Among the most remarkable results obtained in this work, we observe crosstalk phenomena between distinct electronic subsystems in MgB2, discover bound excitons coupled to the phonon bath in anatase TiO2, provide a selective and quantitative estimate of the electron-phonon coupling in La2CuO4, reveal precursor superconducting effects in NdBa2Cu3O(7-ÎŽ) and unravel a phonon-mediated mechanism behind the magnetic order melting in the multiferroic TbMnO3.