Ultrafast Carrier Thermalization and Cooling Dynamics in Few-Layer MoS2

Femtosecond optical pump–probe spectroscopy with 10 fs visible pulses is employed to elucidate the ultrafast carrier dynamics of few-layer MoS2. A nonthermal carrier distribution is observed immediately following the photoexcitation of the A and B excitonic transitions by the ultrashort, broadband laser pulse. Carrier thermalization occurs within 20 fs and proceeds via both carrier–carrier and carrier–phonon scattering, as evidenced by the observed dependence of the thermalization time on the carrier density and the sample temperature. The n–0.37±0.03 scaling of the thermalization time with carrier density suggests that equilibration of the nonthermal carrier distribution occurs via non-Markovian quantum kinetics. Subsequent cooling of the hot Fermi–Dirac carrier distribution occurs on the ∼0.6 ps time scale via carrier–phonon scattering. Temperature- and fluence-dependence studies reveal the involvement of hot phonons in the carrier cooling process. Nonadiabatic ab initio molecular dynamics simulations, which predict carrier–carrier and carrier–phonon scattering time scales of 40 fs and 0.5 ps, respectively, lend support to the assignment of the observed carrier dynamics.