Layer-independent lifetime and coherence of anisotropic excitons in ReS2

Group-VII transition metal dichalcogenides like ReS₂ holds novel in-plane anisotropic excitons, owing to their reduced lattice symmetry. Despite their potential, the coherent dynamics of these anisotropic excitons remain unexplored yet. To address their coherent properties, here, we perform polarization sensitive, ultrafast non-linear optical measurements on ReS₂. By implementing four-wave mixing spectroscopy along with spectral heterodyning detection at the microscopic limit, we measure the ultrafast coherence and population dynamics of the anisotropic excitonic system in layered ReS₂. We attribute their dephasing times (T2) and radiative lifetime (T1) in a sub-picosecond range for both the anisotropic excitons. We observed the robustness of the homogeneous broadening with respect to the flake thickness, excitation powers, and temperatures. Such homogeneous broadening features suggest a low excitonic disorder level, setting a unique characteristic of ReS₂ among two-dimensional semiconductor systems. Additionally, the layer-independent measured lifetime and exciton coherence times can be fundamental for understanding the exact electronic band structure of ReS₂.