Thickness-Dependent Second Harmonic Generation and Anisotropic Exciton Davydov Splitting in AA-Stacked ReS₂

The coexistence of multiple stacking orders in few-layer rhenium disulfide (ReS2) has led to conflicting reports of its optical properties, largely due to the difficulty in distinguishing stacking configurations caused by interlayer coupling and subtle structural variations. In this work, we identify distinctive second harmonic generation (SHG) patterns unique to AA-stacked ReS2, enabling unambiguous identification of its stacking order down to a few layers─previously thought undetectable via SHG. This structural insight allows a systematic investigation of the excitonic properties of AA-stacked ReS2, revealing layer-dependent Davydov splitting-like behavior in photoluminescence at cryogenic temperatures, consistent with enhanced interlayer coupling in the low-symmetry crystals. Polarization-resolved PL further uncovers pronounced in-plane anisotropy and valley-selective emission, reflecting its triclinic symmetry. Gate-dependent PL confirms intrinsic n-type doping and the formation of negatively charged excitons, while temperature-dependent measurements show robust excitonic stability. These results establish a direct link between the stacking order and excitonic behavior, providing a foundation for future studies and applications of anisotropic van der Waals semiconductors.