Strain dependent complex refractive index of 1L, 2L and 3L transition metal dichalcogenides.

Strain engineering offers a powerful route to modulate the optical properties of semiconducting transition metal dichalcogenides (TMDCs) by tuning their bandgap, altering exciton dynamics and breaking symmetry. Yet, the strain dependence of their complex refractive index remains largely unexplored. Here, we extract the strain-dependent complex refractive index (ñ = n + iκ) of monolayer, bilayer, and trilayer MoS2, MoSe2, WS2, and WSe2 by analyzing contrast spectra acquired from micro-reflectance measurements. A Fresnel-based multilayer model incorporating Lorentz oscillators is used to describe the dielectric response, which is directly related to the complex refractive index. Furthermore, a two-phase fitting strategy is employed to determine the high-energy dielectric background and oscillator parameters, which are required as input for the model. We observe systematic redshifts in excitonic resonances under uniaxial tensile strain. The methodology is also extended to differential reflectance data, highlighting its broader applicability. Our results provide both a dataset and a modeling framework for accurately describing the strain-tunable optical response of layered TMDCs.