Giant Anisotropy and High Second-Order Nonlinearity of 3R-MoS₂ for Multifunctional Photonics

The miniaturization of photonic circuits is impeded by the difficulty of combining active nonlinear frequency conversion with passive subdiffractional light guiding. Here, we establish a rhombohedral 3R-polytype of MoS₂ (3R-MoS₂) as a multifunctional platform resolving this challenge. Unlike its centrosymmetric 2H counterpart, 3R-MoS₂ preserves broken inversion symmetry in the bulk, enabling scalable second-order nonlinearity. Challenging the assumption of identical linear properties between polytypes, we reveal that 3R-MoS₂ not only possesses a giant optical anisotropy similar to 2H, but also exceeds it. By determining the full dielectric tensor, we demonstrate that this anisotropy enables extreme skin-depth (e-skid) waveguides capable of subdiffractional light confinement, achieving mode sizes nearly 20% below the diffraction limit. Furthermore, we harness the combination of our measured giant in-plane refractive index and high optical nonlinearity to tune the Fabry-Pérot (FP) resonator and enhance second-harmonic (SH) generation at a given wavelength, yielding an effective nonlinear response of 1.2 nm/V. These findings bridge the gap between active and passive functionalities, positioning 3R-MoS₂ as a solution for high-density multifunctional nanophotonic integration.