ED Sciences Physiques et de l'Ingénieur
Ultrafast Carrier Dynamics in Quantum Materials Probed by Time- and Angle-Resolved Photoemission Spectroscopy
by Akib JABED (Centre Lasers Intenses et Applications)
The defense will take place at h00 - Salle Tudor-Johnston Institut national de la recherche scientifique (INRS), Énergie Matériaux Télécommunications Research Centre,1650 Lionel-Boulet Blvd. Varennes, Quebec J3X 1P7 (Canada)
in front of the jury composed of
- Yann MAIRESSE - Directeur de recherche - Université de Bordeaux - Directeur de these
- Cris ADRIANO - Full professor - Université de Sherbrooke - Rapporteur
- Michel COTE - Full professor - Université de Montréal - Rapporteur
- Fabio BOSCHINI - Full professor - Institut national de la recherche scientifique - Directeur de these
- Kenneth BEYERLEIN - Full professor - Institut national de la recherche scientifique - Examinateur
- Fanciulli MAURO - Associate Professor - CY Cergy Paris University - Examinateur
This thesis presents momentum-resolved studies of ultrafast electron dynamics in quantum materials. Electron scattering processes, such as electron-electron and electron-phonon scattering, underlie the macroscopic electronic, optical, and magnetic properties of materials. Furthermore, the details of the electronic band structure are intimately related to these scattering properties. Therefore, experimental probes with high energy and momentum resolutions are necessary for a comprehensive exploration of materials. In this study, we leverage time- and angle-resolved photoemission spectroscopy (TR-ARPES), the most powerful technique for accessing and tracking light-induced electron dynamics with exquisite energy, momentum, and temporal resolutions. At ALLS Laboratory, we focused our study on Bi$_2$Te$_3$, a prototypical 3D topological insulator. By employing a low-photon-energy probe (6 eV) and mid-infrared pump excitation (300 meV), we provided the first experimental evidence that temperature-induced modifications of the bulk band structure modulate electron-phonon scattering channels. Surprisingly, we show that even a 15 meV shift of the minimum of the unoccupied conduction band has a dramatic impact on electron scattering processes within the technologically relevant topological surface state. Furthermore, we observe an accumulation of carriers at the bottom of the conduction band at low temperature due to the reduction of the intervalley scattering phase space. At the CELIA laboratory, using a polarization-tunable 21.6 eV extreme ultraviolet (XUV) probe and 1.2 eV pump coupled to a time-of-flight momentum microscope, we conducted two studies. First, we investigated the role of in-gap states in SnS$_2$. Our results reveal that, unlike conventional n-type semiconductors, donor-type defect states induce downward band bending near the surface. These in-gap states enable conduction band population through below-bandgap excitation, resulting in long carrier lifetimes. We further observe a pronounced surface photovoltage effect upon pumping due to the pre-existing band bending. Second, we examined the charge density wave (CDW) compound 1T-TiSe$_2$, which has a transition temperature below 200 K. Despite longstanding debate regarding whether CDW fluctuations are driven by electron-phonon coupling or excitonic correlations, our study demonstrates that coherent CDW fluctuations persist above the transition temperature and are dominated by electron-phonon interactions. Collectively, these results advance our understanding of how carriers, defects, and collective modes shape the macroscopic properties of quantum materials. This work establishes a foundation for controlling and engineering electronic states in next-generation quantum, spintronic, electronic, and optoelectronic devices.