Thickness Dependence of Linear and Nonlinear Optical Properties of Multilayer 3R‐MoS ₂

Abstract 3R‐MoS ₂ , a MoS ₂ polytype with broken inversion symmetry, enables unique light‐matter interactions and is promising for linear and nonlinear integrated photonics beyond the monolayer limit. Yet, systematic studies of its thickness‐dependent reflectivity and its impact on harmonic generation are still lacking. While AFM can offer atomic‐scale resolution, measuring 3R‐MoS ₂ on non‐solid substrates like PDMS remains challenging. To address this, a fast, non‐destructive optical method is introduced to determine the thickness of 3R‐MoS ₂ flakes from reflectivity measurements with a mean bias of less than 2 nm in the 3–200 nm range. Nonlinear characterization further reveals distinct thickness‐dependent maxima in second‐ and third‐harmonic generation (SHG/THG), with the first clear peak at ≈200 nm. These maxima arise from Fabry–Pérot‐type phase matching conditions mediated by the film thickness and can further be shaped by absorption. This work thus provides both a practical thickness metrology and new insights for exploiting thickness‐dependent 3R‐MoS ₂ nonlinearities in scalable photonic technologies.