Biexcitonic optical Stark effects in monolayer molybdenum diselenide

Floquet states, where a periodic optical field coherently drives electrons in solids(1-3), can enable novel quantum states of matter(4-6). A prominent approach to realize Floquet states is based on the optical Stark effect. Previous studies on the optical Stark effect often treated the excited state in solids as free quasi-particles(3,7-12). However, exciton-exciton interactions can be sizeably enhanced in low-dimensional systems and may lead to light-matter interactions that are qualitatively different from those in the non-interacting picture. Here we use monolayer molybdenum diselenide (MoSe2) as a model system to demonstrate that the driving optical field can couple a hierarchy of excitonic states, and the many-body inter-valley biexciton state plays a dominant role in the optical Stark effect. Specifically, the exciton-biexciton coupling in monolayer MoSe2 breaks down the valley selection rules based on the non-interacting exciton picture. The photon-dressed excitonic states exhibit an energy redshift, splitting or blueshift as the driving photon frequency varies below the exciton transition. We determine a binding energy of 21 meV for the inter-valley biexciton and a transition dipole moment of 9.3 debye for the exciton-biexciton transition. Our study reveals the crucial role of many-body effects in coherent light-matter interaction in atomically thin two-dimensional materials.