Dimitris N. Chorafas Foundation Award 2017 – Edoardo Baldini

"For his contributions to the study of fundamental excitations in solids, delivering unprecedented microscopic details about non-equilibrium phenomena on the femtosecond to picosecond timescales, and opening the route to a coherent manipulation of quantum particles such as phonons, excitons and plasmons."

Revealing the dynamics of collective excitations in complex materials 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. 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.