  Interactions of quantum materials with strong-laser fields can induce exotic
nonequilibrium electronic states. Monolayer transition-metal dichalcogenides, a
new class of direct-gap semiconductors with prominent quantum confinement,
offer exceptional opportunities toward Floquet engineering of quasiparticle
electron-hole states, or excitons. Strong-field driving has a potential to
achieve enhanced control of electronic band structure, thus a possibility to
open a new realm of exciton light-matter interactions. However, experimental
implementation of strongly-driven excitons has so far remained out of reach.
Here, we use mid-infrared laser pulses below the optical bandgap to excite
monolayer tungsten disulfide up to a field strength of 0.3 V/nm, and
demonstrate strong-field light dressing of excitons in the excess of a hundred
millielectronvolt. Our high-sensitivity transient absorption spectroscopy
further reveals formation of a virtual absorption feature below the 1s-exciton
resonance, which is assigned to a light-dressed sideband from the dark
2p-exciton state. Quantum-mechanical simulations substantiate the experimental
results and enable us to retrieve real-space movies of the exciton dynamics.
This study advances our understanding of the exciton dynamics in the
strong-field regime, and showcases the possibility of harnessing ultrafast,
strong-field phenomena in device applications of two-dimensional materials.
