  High-harmonic generation (HHG) is a coherent optical process in which the
incident photon energy is up-converted to the multiples of its initial energy.
In solids, under the influence of a strong laser field, electron-hole (e-h)
pairs are generated and subsequently driven to high energy and momentum within
a fraction of the optical cycle. These dynamics encode the band structure,
including non-trivial topological properties of the source material, through
both intraband current and interband polarization, into the high harmonic
spectrum. In the course of this process, dephasing between the driven electron
and the hole can significantly reduce the HHG efficiency. Here, we exploit this
feature and turn it into a measurement of e-h coherence in strongly driven
solids. Utilizing a pre-pump pulse, we first photodope monolayer molybdenum
disulfide and then examine the HHG induced by an intense infrared pulse. We
observe clear suppression of the HH intensity, which becomes more pronounced
with increasing order. Based on quantum simulations, we attribute this
monotonic order dependence as a signature of ultrafast electron-hole dephasing,
which leads to an exponential decay of the inter-band polarization,
proportional to the sub-cycle excursion time of the e-h pair. Our results
demonstrate the importance of many-body effects, such as density-dependent
decoherence in HHG and provide a novel platform to probe electron-hole
coherence in strongly driven systems.
