Wavelength and Stacking Order Dependent Exciton Dynamics in Bulk ReS₂

Abstract Exciton dynamics in ReS₂ are rich and complicated due to its unique lattice and band structures. With femtosecond pump‐probe spectroscopy, the wavelength‐dependent exciton dynamics in bulk ReS₂ are studied at room temperature, and the effects from polarization and stacking order are also considered. Signatures from both bright and dark excitons are observed, where the lifetime at bright exciton levels is only several picoseconds, while it is about tens of picoseconds at dark exciton levels. The extremely long up‐/down‐going trend at selective wavelengths is attributed to the transition between different dark exciton levels. Two types of coherent dynamics are observed when the laser wavelength is scanned through different exciton levels: coherent optical stark effect and exciton‐exciton coherent interaction, both occurring within the laser pulse width. Exciton dynamics show a similar trend for the sample with AA stacking order, regardless of polarization. For AB stacking, dark excitons for Exciton II and III are missing when probe pulse polarization is perpendicular to b‐axis, which is attributed to the polarization and stacking‐order dependent band structure. Studying the rich exciton dynamics in bulk ReS₂ at room temperature not only has scientific significance, but also facilitates the development of high‐performance excitonic devices.