  Atomically thin layer transition metal dichalcogenides have been intensively
investigated for their rich optical properties and potential applications in
nano-electronics. In this work, we study the incoherent optical phonon and
exciton population dynamics in monolayer WS2 by time-resolved spontaneous Raman
scattering spectroscopy. Upon excitation of the exciton transition, both the
Stokes and anti-Stokes optical phonon scattering strength exhibit a large
reduction. Based on the detailed balance, the optical phonon population is
retrieved, which shows an instant build-up and a relaxation lifetime of around
4 ps at an exciton density E12 cm-2. The corresponding optical phonon
temperature rises by 25 K, eventually, after some 10s of picoseconds, leading
to a lattice heating by only around 3 K. The exciton relaxation dynamics
extracted from the transient vibrational Raman response shows a strong
excitation density dependence, signaling an important bi-molecular contribution
to the decay. The exciton relaxation rate is found to be (70 ps)-1 and
exciton-exciton annihilation rate 0.1 cm2s-1. These results provide valuable
insight into the thermal dynamics after optical excitation and enhance the
understanding of the fundamental exciton dynamics in two-dimensional transition
metal materials.
