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.

With their long lifetime and protection against decoherence, dark excitons in monolayer semiconductors offer a promising route for quantum technologies. Optical techniques have previously observed dark excitons with a long-lived valley polarization. However, several aspects remain unknown, such as the populations and time evolution of the different valley-polarized dark excitons and the role of excitation conditions. Here, using time- and angle-resolved photoemission spectroscopy, we obtain a holistic view of the dynamics after valley-selective photoexcitation. By varying experimental conditions, we reconcile between the rapid valley depolarization previously reported in TR-ARPES, and the observation of long-lived valley polarized dark excitons in optical studies. For the latter, we find that momentum-dark excitons largely dominate at early times sustaining a 40% degree of valley polarization, while valley-polarized spin-dark states dominate at longer times. Our measurements provide the timescales and how the different dark excitons contribute to the previously observed long-lived valley polarization in optics. © 2025. The Author(s).

Semiconductor interfaces are at the heart of the functionality of many devices for opto-electronic applications. At these interfaces, the importance of ultrafast dynamics – processes that occur on sub-nanosecond timescales – has been long understood. While these ultrafast spectroscopic studies have revealed important information, there remains a rich array of physics that is hidden within sub-micrometer length scales when using spatially-averaged techniques. However, powerful tools that could access material dynamics in semiconductors simultaneously at ultrafast time- and sub-micrometer length scales are challenging to implement. Here, we review recent developments in time-resolved photoemission electron microscopy as a technique to study ultrafast electron dynamics at semiconductor interfaces at the nanoscale. In particular, we review recent work in traditional semiconductor interfaces and heterojunctions, low-dimensional materials, and semiconductors for photovoltaic applications. © 2024 The Authors