Coherent Light-matter Interaction in Monolayer Molybdenum Diselenide

The recently discovered monolayer transition metal dichalcogenides (TMDs) provide a platform to explore new coupled spin-valley physics. Engineering the quantum states of matter using light has emerged as promising and effective routes to coherently control the valley and pseudospin in condensed matters. Here, we used ultrafast pump-probe spectroscopy to investigate the coherent light-matter interactions in monolayer Molybdenum diselenide (MoSe₂). We demonstrate that optical excitation in such an atomically thin layer is strongly modified by the exciton-exciton interactions. The exciton-biexciton coupling in monolayer MoSe₂ breaks down the valley selection rules based on the non-interacting exciton picture and provides an additional route to manipulate the Floquet states in MoSe₂ that leads rich coherent phenomena. By systematically varying the driving frequency below the exciton transition, we observed the Floquet states exhibit energy redshift, splitting or blueshift. Our study reveals the crucial roles of many-body excitonic interactions in coherent light-matter interaction in low-dimensional systems.