Nonequilibrium exciton physics in hybrid lead-halide perovskites and anatase titanium dioxide

The strength of the electron-hole interaction in bulk semiconductors is not only determined by the dielectric environment, but also depends on the presence of other quasiparticles - free charge carriers or phonons - that populate the system. In the former case, a high density of charge carriers is expected to screen the Coulomb attractive force, eventually driving the transition from an insulating exciton gas to a conductive electron-hole plasma. In the case of lattice vibrations, the coupling between the exciton and the phonon field can give rise to significant modulations of the exciton amplitude and binding energy. Understanding how excitons react to these perturbations is of pivotal importance, as the strength of the Coulomb interaction affects the way light is absorbed and emitted, and determines how energy is converted and transported in several optoelectronic technologies. The aim of this thesis is to investigate the nonequilibrium exciton physics in highly-excited semiconductors. For this scope, we will combine different steady-state and ultrafast optical techniques, spectro-temporal analysis, and advanced theory calculations, with the aim of investigating the fundamental processes that lead to the renormalization of the excitonic states in hybrid lead-halide perovskites and titanium dioxide. In our experiments, we use an ultrashort laser pulse to excite the system above the fundamental energy gap; a broadband pulse then monitors the optical properties in the exciton spectral region at different time delays upon photoexcitation. The temporal evolution of the exciton parameters are retrieved via quantitative lineshape analysis, which is achieved by modelling the steady-state optical quantities and combining them with the time-resolved spectra. Thanks to this approach, we are able to elucidate the dynamics of exciton renormalization in hybrid perovskites and titanium dioxide in presence of elevated carrier densities and coherent strain pulses, and disentangle the many-body effects that lye at the origin of the observed optical nonlinearities