Ultrafast nonequilibrium dynamic process of separate electrons and holes during exciton formation in few-layer tungsten disulfide.

Femtosecond transient absorption spectroscopy has been employed to unravel separate initial nonequilibrium dynamic processes of photo-injected electrons and holes during the formation process of the lowest excitons at the K-valley in few-layer tungsten disulfide. Charge carrier thermalization and cooling, as well as concomitant many-body effects on the exciton resonances, are distinguished. The thermalization of holes is observed to be faster than that of electrons. Both of them proceed predominantly via carrier-carrier scattering, as evidenced by the observed dependence of the thermalization time on pump fluences. The fluence dependent time constants also suggest that the subsequent cooling for electrons is probably dominated by acoustic phonons, whereas for holes it is mostly controlled by LO phonons. An extremely fast red- and blue-shift crossover followed by a slow blue-shift of exciton resonance was observed in the temporal evolution of exciton resonances by resonant exciton A excitation. The rapid red-shift could be due to the strong screening of the Coulomb interaction between quasi-free charge carriers in electron-hole plasma. The subsequent slow blue-shift is the net result of the competition among many-body effects in the hot-exciton cooling process. Our findings elucidate the carrier-selective ultrafast dynamics and their many-body effects, underpinning new possibilities for developing optoelectronic devices based on transport properties of a single type of carrier.