Thickness Dependence of Linear and Nonlinear Optical Properties of Multilayer 3R-MoS2

3R-MoS2, a MoS2 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-MoS2 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-MoS2 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-MoS2 nonlinearities in scalable photonic technologies.