Layer-dependent modulation of optical anisotropy in MoS ₂/ReS ₂ van der Waals heterostructures

van der Waals (vdW) heterostructures, composed of stacked materials with varying symmetries, offer exceptional opportunities in electronics and optics due to their unique anisotropic properties. However, the influence of low-symmetry layer thickness on modulating anisotropic optical responses remains elusive. Here, we fabricate heterostructures by combining monolayer (1L) MoS₂ with ReS₂ layers of varying thickness, uncovering tunable optical anisotropy. The degree of excitonic line polarization increases with ReS₂ thickness, reaching saturation due to lattice relaxation at the heterostructure interface. Density functional (DFT) theory calculations confirm that the lattice reconstruction of MoS₂ is influenced by the number of low-symmetry ReS₂ layers, providing direct evidence of interlayer coupling effects. Remarkably, we observe anisotropy ratios as high as 2.01 and 2.12 for charged and neutral excitons, respectively, underscoring robust anisotropic optical behavior. Additionally, we demonstrate that external magnetic fields can effectively modulate this anisotropy, whereas temperature variations have a negligible impact on line polarization. These findings advance our understanding of the interplay between thickness, symmetry, and external stimuli in heterostructures, paving the way for designing advanced optical devices with precise polarization control.