ULTRAFAST NON-EQUILIBRIUM STUDIES OF COMPLEX MATERIALS THROUGH PUMP-PROBE AND STOCHASTIC SPECTROSCOPIES

Ultrafast spectroscopies established as the primary investigation tool for complex materials, i.e. systems in which the coupling between electronic, vibrational and magnetic degrees of freedom generates a multitude of competing phases. Indeed, the characterization of dynamic processes triggered by ultrashort light pulses allows for the disentanglement of the different microscopic contributions and provides unique information about the leading interactions in matter. In the first part of the thesis, we rely on the ultrafast technique of pump-probe spectroscopy to investigate the non-equilibrium response of two prototypical complex materials. At first, we present a dynamic study of the low-temperature ferrimagnet RbNiF3 where we unveil novel information hidden to the static response, like the presence of dd-phonon coupling or the discovery of a photo-induced metastable state. Secondly, we characterize the electronic excitations of the Bi2Sr2CaCu2O8−δ cuprate superconductor. Notably, the non trivial polarization dependence of the superconducting birefringence signal suggests a scenario where time-reversal symmetry is broken. The second part of the thesis approaches the problems of non-equilibrium physics from a different perspective which focuses on the development of new investigation tools. In particular, we present Femtosecond Covariance Spectroscopy (FCS), a novel spectroscopic technique which relies on multimode photonic correlations to unveil non-linear signals. We discuss two different applications of FCS: i) we test the capabilities of the technique in measuring magnetic excitations. ii) we perform a time-resolved measurement of electronic Raman scattering from Cooper pairs in cuprates. The finding of magnonic signatures in RbNiF3 and especially the discovery of a superconducting correlation signal above Tc in Bi2Sr2CaCu2O8−δ propose FCS as an ideal technique for the investigation of complex materials. In conclusion, we illustrate possible technological developments to overcome the major limitations of FCS (i.e. long acquisition times) and make it competitive with other spectroscopic techniques.