Future optoelectronic devices will rely on the ability to understand and control how light interacts with matter on ultrafast timescales. In atomically thin semiconductors, this interaction is governed by excitons: bound electron-hole pairs that dominate the optical response of the material. Some excitonic states are “dark”, meaning that they cannot be directly accessed by ordinary optical transitions. This makes their dynamics particularly difficult to observe, even though they strongly influence how the material absorbs and stores energy.
In their recent work, Angela Montanaro, Francesco Valiera and collaborators demonstrate a way to follow in real time the ultrafast evolution of a dark excitonic state in monolayer WS₂. The study was supervised by our FAU LMQ member Daniele Fausti and carried out in collaboration with Politecnico di Milano and Martin Eckstein, former professor at FAU and now at the University of Hamburg. Using a three-pulse experiment inspired by light-induced transparency in atomic systems, the team couples the dark 2p state to the bright 1s exciton in WS₂ with a resonant mid-infrared field. This coupling creates a Autler-Townes splitting of the optically bright excitonic resonance, serving as a direct spectroscopic fingerprint of the time-evolution of the dark state.
This method provides the first direct view of the ultrafast dynamics of a dark exciton in a two-dimensional semiconductor and offers a general strategy to probe otherwise inaccessible states in a wide range of nanostructured and photonic materials.
For further information, read the full paper on Physical Review Letters:
Tracking the Photoinduced Dynamics of a Dark Excitonic State in Single-Layer WS₂ via Resonant Autler-Townes Splitting
Angela Montanaro, Francesco Valiera, Francesca Giusti, Francesca Fassioli, Chiara Trovatello, Giacomo Jarc, Enrico Maria Rigoni, Fang Liu, Xiaoyang Zhu, Stefano Dal Conte, Giulio Cerullo, Martin Eckstein, Daniele Fausti
Phys. Rev. Lett. 136, 016902 (2026)