MONITORING PHONON DYNAMICS IN QUANTUM MATERIALS BY TIME-RESOLVED SPONTANEOUS RAMAN SCATTERING

This thesis collects the results of my work as doctoral student of the Ph.D. School in Physics, Astrophysics and Applied Physics at Università degli Studi di Milano, that has been carried out since October 2022 at the Istituto Officina dei Materiali of the Consiglio Nazionale delle Ricerche (IOM-CNR) and within the framework of Nanoscale Foundries and Fine Analysis (NFFA) facility. My research activity has been devoted to the study of phonon dynamics in photoexcited quantum materials, employing time-resolved spontaneous Raman spectroscopy (TRRS) as the primary experimental approach. Among ultrafast spectroscopies, TRRS provides unique access to phonons and other low-energy excitations in systems impulsively driven into transient non-equilibrium states, allowing mode-specific monitoring of their population dynamics. This makes TRRS particularly powerful for investigating electron-phonon and phonon-phonon couplings, as well as the evolution of crystal symmetries after photoexcitation. Motivated by the broad span of applications of this technique and by its complementarity with other ultrafast spectroscopies targeting electronic, lattice or spin degrees of freedom, a dedicated TRRS setup was developed at the NFFA-SPRINT laboratory, hosted in the premises of the FERMI@Elettra facility (Elettra-Sincrotrone Trieste). This booklet thus summarizes my work for the commissioning and characterization of the new setup, and presents the results from three case studies addressing the properties of semiconductors, 2D chalcogenides and a strong correlated oxide. In a first experiment, the phonon relaxation dynamics of silicon is investigated. The possibility of tuning the phonon-phonon anharmonic interaction is explored by varying the external temperature and the excitation density. Furthermore, the photodoping dynamics is directly monitored via a Fano interference effect between electronic and phononic Raman scattering. A second experiment, targeting bulk and single-layer MoS2, reveals a photodoping-induced phonon frequency renormalization and a long-lived excitation anisotropy in the phonon subsystem, dictated by mode symmetries and inaccessible under equilibrium conditions. In a third experiment, TRRS is employed to monitor the evolution of the lattice symmetry of magnetite and the phonon-specific energy flow while triggering a photoinduced Verwey phase transition.