Ultrafast Carrier Thermalization and Cooling Dynamics in Few-Layer MoS<sub>2</sub>

Femtosecond optical pump–probe spectroscopy with 10 fs visible pulses is employed to elucidate the ultrafast carrier dynamics of few-layer MoS<sub>2</sub>. 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 <i>via</i> 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 <i>n</i><sup>–0.37±0.03</sup> scaling of the thermalization time with carrier density suggests that equilibration of the nonthermal carrier distribution occurs <i>via</i> non-Markovian quantum kinetics. Subsequent cooling of the hot Fermi–Dirac carrier distribution occurs on the ∼0.6 ps time scale <i>via</i> carrier–phonon scattering. Temperature- and fluence-dependence studies reveal the involvement of hot phonons in the carrier cooling process. Nonadiabatic <i>ab initio</i> 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.