  Transition metal dichalcogenides (TMDs) are emerging as promising
two-dimensional (2d) semiconductors for optoelectronic and flexible devices.
However, a microscopic explanation of their photophysics -- of pivotal
importance for the understanding and optimization of device operation -- is
still lacking. Here we use femtosecond transient absorption spectroscopy, with
pump pulse tunability and broadband probing, to monitor the relaxation dynamics
of single-layer MoS2 over the entire visible range, upon photoexcitation of
different excitonic transitions. We find that, irrespective of excitation
photon energy, the transient absorption spectrum shows the simultaneous
bleaching of all excitonic transitions and corresponding red-shifted
photoinduced absorption bands. First-principle modeling of the ultrafast
optical response reveals that a transient bandgap renormalization, caused by
the presence of photo-excited carriers, is primarily responsible for the
observed features. Our results demonstrate the strong impact of many-body
effects in the transient optical response of TMDs even in the
low-excitation-density regime.
