Driving Floquet physics with excitonic fields

Floquet engineering, in which an intense optical field modifies the electronic structure of a material, offers a route to the control of quantum and topological properties. However, it is challenging to realize this in experiments due to relatively weak light-matter coupling and the dominance of detrimental effects, such as multi-photon absorption and sample heating. Here we use time- and angle-resolved photoemission spectroscopy to show that in a monolayer semiconductor, Floquet effects caused by an excitonic field-the time-periodic oscillations of the self-energy of an electron bound to a hole-are two orders of magnitude stronger and persist longer than optically driven counterparts. Our measurements directly capture the hybridization between the exciton-dressed conduction band and the valence band in two-dimensional semiconductors, in agreement with first-principles calculations. The onset of this hybridization with increasing exciton density also correlates with the Bose-Einstein condensation to Bardeen-Cooper-Schrieffer crossover, extensively discussed in theory for non-equilibrium excitonic insulators. These results establish exciton-driven Floquet engineering as a means for studying correlated electronic phases.