Giant Anisotropy and High Second‐Order Nonlinearity of 3R‐MoS ₂ 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 MoS 2 (3R‐MoS 2 ) as a multifunctional platform resolving this challenge. Unlike its centrosymmetric 2H counterpart, 3R‐MoS 2 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‐MoS 2 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‐MoS 2 as a solution for high‐density multifunctional nanophotonic integration.