Berry Curvature Induces Exciton Fine-structure and Valley-dependent Autler-Townes Doublet in Molybdenum Diselenide Monolayer

The geometry phase of Bloch states in the momentum space, characterized by the Berry curvature, can strongly modify the electron dynamics and lead to novel transport phenomena, such as the anomalous Hall effect. Recently it was predicted that the nontrivial Bloch band geometry can also modify the collective optical excitations in transition metal dichalcogenide monolayers, and lift the energy degeneracy of exciton states with opposite angular momentum through an effective valley-orbital effect. Here we report the first experimental observation of the Berry curvature signature in the exciton spectrum of MoSe₂ monolayer using novel techniques based on intraexciton optical Stark spectroscopy. We demonstrate the time-reversal-symmetric analog of the orbital Zeeman effect resulting from the valley-dependent Berry curvature, which leads to energy difference of +14 (-14) meV between the 2p+ and 2p- exciton fine-structure in the K (K’) valley. The coherent light-matter coupling between intraexciton states are remarkably strong, leading to a prominent valley-dependent Autler-Townes doublet. Our study opens up new pathways to manipulate the quantum states with infrared radiation and suggests the possibility to control the light-matter interaction via topological quantum phase engineering.