Room-Temperature Brightening of Momentum-Dark Excitons in Strain-Engineered Multilayer WS2

The rich multivalley band structure of transition metal dichalcogenides enables the formation of dark excitons. Owing to their long lifetimes, dark excitons are promising candidates for exciton storage and long-range transport; however, their optically inactive nature makes their access and manipulation difficult. Here, we demonstrate the brightening of the momentum-dark KΛ′ exciton in multilayer WS2 at room temperature through tensile strain engineering. Tensile strain enhances phonon-assisted intervalley scattering by strengthening the exciton–phonon coupling associated with longitudinal acoustic (LA) phonons, thereby activating the momentum-dark KΛ′ exciton. We also experimentally show that compared to monolayer WS2, multilayer WS2 exhibits much greater exciton–phonon coupling, which allows the efficient activation and thermal stability of the momentum-dark KΛ′ exciton, enabling its persistence up to ambient temperature. These findings establish strain-engineered multilayer WS2 as a promising platform for accessing dark excitonic states under ambient conditions and pave the way for practical exciton-based devices for excitonic, valleytronic, and quantum technologies.