  The interaction between off-resonant laser pulses and excitons in monolayer
transition metal dichalcogenides is attracting increasing interest as a route
for the valley-selective coherent control of the exciton properties. Here, we
extend the classification of the known off-resonant phenomena by unveiling the
impact of a strong THz field on the excitonic resonances of monolayer MoS$_2$.
We observe that the THz pump pulse causes a selective modification of the
coherence lifetime of the excitons, while keeping their oscillator strength and
peak energy unchanged. We rationalize these results theoretically by invoking a
hitherto unobserved manifestation of the Franz-Keldysh effect on an exciton
resonance. As the modulation depth of the optical absorption reaches values as
large as 0.05 dB/nm at room temperature, our findings open the way to the use
of semiconducting transition metal dichalcogenides as compact and efficient
platforms for high-speed electroabsorption devices.
