  The measurement and manipulation of the coherent dynamics of excitonic states
constitute a forefront research challenge in semiconductor optics and in
quantum coherence-based protocols for optoelectronic technologies. Layered
semiconductors have emerged as an ideal platform for the study of exciton
dynamics with accessible and technologically relevant energy and time scales.
Here, we investigate the sub-picosecond exciton dynamics in a van-der-Waals
semiconductor upon quasi-resonant excitation, and achieve to single out an
incipient coherent excitonic state. Combining broadband transient reflectance
spectroscopy and simulations based on many-body perturbation theory, we reveal
a transient enhancement of the excitonic line intensity that originates from
the photoinduced coherent polarization that is phase-locked with the
interacting electromagnetic field. This finding allows us to define the
spectral signature of a coherent excitonic state and to experimentally track
the dynamical crossover from coherent to incoherent exciton, unlocking the
prospective optical control of an exciton population on the intrinsic
quantum-coherence timescale.
