Ab initio time-dependent optical spectroscopy applied to spin and valley dynamics in monolayer transition metal dichalcogenides

Monolayer transition metal dichalcogenides (TMDs) like MoS₂ or WSe₂ are promising semiconducting 2D materials. Not only they have a bandgap in the optical range and suitable transport properties, but also provide a stimulating arena to study fundamental physics. For instance, the valence and conduction bands are spin-split because of the strong spin-orbit interaction. Moreover, due to the lack of inversion symmetry, with circularly polarized light, one can create excitons at selected K valley. The resulting imbalance in the population of the K valleys is the basics of valleytronics.

Ultrafast pump-probe spectroscopy measures accurately the valley dynamics, starting at the photogeneration of excitons at one valley and following the subsequent valley depolarization via intervalley and intravalley scattering. For instance, exploiting the valley selective optical selection rules and using two-colour helicity resolved pump-probe lasers, one can directly measure spin and valley dynamics and to estimate the ratio between intravalley and intervalley scattering rates. In spite of the numerous experimental data, the mechanisms of photo-generation and relaxation of the spin-valley polarization in TMDs are still heavily debated. It is in this context where time-dependent ab initio calculations are helpful to understand and predict the dynamical properties of 2D materials and to reproduce ultrafast spectroscopy experiments.

In this talk I will present our predictive and parameter-free approach to calculating ultrafast carrier dynamics in 2D materials, combining density functional theory with non-equilibrium many-body perturbation theory and including spin-orbit interaction. We calculate the photo-generation of carriers by a laser pulse, taking into account relaxation mechanisms such as electron-phonon scattering or radiative recombination, and simulate time-dependent spectroscopies like Kerr rotation, transient absorption or photoluminescence.