Coupled Femtoexcitons, Free Carriers, and Light

Nonequilibrium quantum matter generated by ultrafast laser light opens new pathways in fundamental condensed matter physics, as well as offering rich control possibilities in “tailoring matter by light”. Here we explore the coupling between free carriers and excitons mediated by femtosecond-scale laser pulses. Employing monolayer WSe2 and an ab initio treatment of pump–probe spectroscopy, we find that, counterintuitively, laser light resonant with the exciton can generate massive enhancement of the early-time free-carrier population. This exhibits a complex dynamical correlation to the excitons, with an oscillatory coupling between the free carrier population and exciton peak height that persists. Our results both unveil “femtoexcitons” as possessing rich femtosecond dynamics as well as, we argue, allow tailoring of early-time light–matter interaction via laser pulse design to control simultaneously excitonic and free-carrier physics at ultrafast times.