Ultrafast charge dynamics and optical response in bilayer WSe₂: impact of excitons and electron-phonon interaction

Transition metal dichalcogenides (TMDs) are well known as semiconductors with strong intra- and inter-layer interactions between electrons and holes and between charges and phonons, with promising potential for applications in energy harvesting to charge reusable batteries. We present here results of time-dependent density-functional theory, in the density-matrix implementation, combined with many-body theory for the charge dynamics and photoluminescence in bilayer WSe₂ with AA'-stacking order that is excited by an ultrafast laser pulse. We find that because of electron- and the exciton-phonon scattering the charge dynamics occurs at the picosecond timescale in both the linear and nonlinear response regimes. Excitonic states, as the lowest-energy excited states with large transition dipole moments, play an important role in the charge dynamics and photoluminescence of the system. In particular, they contribute to the inter-valley dynamics of charges and generation of the third-order harmonic in the emission spectrum. Because of the long lifetimes of the dark inter-valley excitons, these states are especially important in the phenomena above. Obtained results may help to understand the details of the ultrafast optical response of charges in bilayers of TMDs that can be used in practical applications.