  The optical properties of semiconducting transition metal dichalcogenides are
dominated by both neutral excitons (electron-hole pairs) and charged excitons
(trions) that are stable even at room temperature. While trions directly
influence charge transport properties in optoelectronic devices, excitons may
be relevant through exciton-trion coupling and conversion phenomena. In this
work, we reveal the coherent and incoherent nature of exciton-trion coupling
and the relevant timescales in monolayer MoSe2 using optical two-dimensional
coherent spectroscopy. Coherent interaction between excitons and trions is
definitively identified as quantum beating of cross-coupling peaks that
persists for a few hundred femtoseconds. For longer times up to 10 ps,
surprisingly, the relative intensity of the cross-coupling peaks increases,
which is attributed to incoherent energy transfer likely due to phonon-assisted
up-conversion and down-conversion processes that are efficient even at
cryogenic temperature.
