Superradiant Coupling Effects in Transition-Metal Dichalcogenides

Optical absorption experiments of the lowest exciton are performed for transition-metal dichalcogenides with different number of atomic layers. We examine two representative materials MoSe2 and WSe2 using incoherent broadband white light. The measured absorption at the A exciton resonance does not saturate for optically thick samples consisting of hundreds of atomic layers and the absorption varies non-monotonously with the layer number. A self-consistent microscopic calculation reproduces the experimental observations, clearly identifying superradiant coupling effects as the origin of this unexpected behavior.