Superradiant coupling effects in transition-metal dichalcogenides

Cooperative effects allow for fascinating characteristics in light-matter interacting systems. Here, we study naturally occurring superradiant coupling in a class of quasi-two-dimensional, layered semiconductor systems. We perform optical absorption experiments of the lowest exciton for transition-metal dichalcogenides with different numbers of atomic layers. We examine two representative materials, MoSe$_2$ and WSe$_2$, using incoherent broadband white light. The measured transmission at the A exciton resonance does not saturate for optically thick samples consisting of hundreds of atomic layers, and the transmission varies nonmonotonously 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.