Ab-initio modelling of pump-probe spectroscopy for exploring excitonic coherent phenomena in 2D materials

The study of coherent states has received much attention due to their fundamental role in quantum information. Regarding the optical properties in semiconductors, coherent excitons allow new pathways for exploring light-matter interactions. Materials with large exciton binding energies like transition metal dichalcogenides or bulk semiconductors as BiI₃ are the ideal candidates for these processes to take place. From a theoretical point of view, the use of ab-initio approaches to study femtosecond exciton dynamics is still at the early stage. A promising approach is the solution of the equation of motion for the time-dependent density matrix, accounting the excitonic effects within the Bethe-Salpeter formalism and describing the pump and probe fields at the same foot. This approach allows modelling of ultrafast pump and probe experiments during the overlapping pump-probe regime and is a promising tool for investigating coherent excitonic states. We apply our theoretical framework to model pump and probe experiments under selected conditions to describe coherent excitonic states in layered materials. First, we study the case in BiI₃, where employing a quasi-resonant pump we identify the creation of a coherent excitonic population, supported by experimental measurements. In addition, we analyse the case in a WS₂ monolayer. The spin and valley physics of WS₂ together with a suitable pump (same detuning between the A and B excitonic states), produces ultrafast oscillations due to the coherent coupling of the two states. Our results proves that our ab initio approach is a suitable tool to characterize exciton dynamics.