The measurement and manipulation of the coherentdynamics of excitonic states constitute a forefront research challenge insemiconductor optics and quantum-coherence-based protocols foroptoelectronic technologies. Layered semiconductors have emerged asan ideal platform for the study of exciton dynamics with accessible andtechnologically relevant energy and time scales. Here, we investigatethe subpicosecond exciton dynamics in a van der Waals semiconductorupon quasi-resonant excitation and achieve to single out an incipientcoherent excitonic state. Combining broad-band transient reflectancespectroscopy and simulations based on many-body perturbation theory,we reveal a transient enhancement of the excitonic line intensity thatoriginates from photoinduced coherent polarization that is phase-locked with the interacting electromagnetic field. This finding allows usto define the spectral signature of a coherent excitonic state and to experimentally track the dynamical crossover from coherent toincoherent exciton, unlocking the prospective optical control of an exciton population on the intrinsic quantum-coherence timescale.
