Scanning Near-Field Optical Microscopy Characterization of WSe2 and MoSe2 Planar Waveguides

The advancement of integrated nanophotonics requires the development and characterization of materials with high refractive indices and low optical losses. Van der Waals (vdW) materials, particularly transition metal dichalcogenides (TMDs) such as WSe2 and MoSe2, exhibit giant optical anisotropy and high refractive indices (n > 4) in the near-infrared range, making them promising candidates for next-generation photonic circuits. However, accurately determining their anisotropic optical constants, especially the out-of-plane component, is challenging due to the limited size of exfoliated flakes and the limitations of far-field techniques. This study details the application of scattering-type scanning near-field optical microscopy (s‑SNOM) for the nanoscale characterization of these materials. The principles of s-SNOM, including the excitation and detection of waveguide modes in planar vdW structures, are discussed. By analyzing the interference fringes generated by propagating modes, the effective mode index is determined. We demonstrate this methodology using WSe2 and MoSe2 flakes, where the experimental effective indices are compared with theoretical models. This comparison confirms the necessity of accounting for giant optical anisotropy and validates the out-of-plane dielectric constants, illustrating the efficacy of s-SNOM for the comprehensive optical characterization of vdW materials.