Near-Infrared Optical Constants and Guided-Mode Benchmarking of High-Index MoSe2 for Nanophotonics.

The integration density of photonic integrated circuits is fundamentally limited by evanescent field overlap and subsequent inter-channel crosstalk. Layered transition metal dichalcogenides (TMDCs) bypass these confinement constraints through intrinsic optical birefringence and high refractive indices. Here, we report the near-infrared optical constants and waveguide dispersion of molybdenum diselenide (MoSe2). Ellipsometry performed on centimeter-scale crystals yields an in-plane refractive index of 4.1-4.7 over 1000-2000 nm, with an extinction coefficient close to the sensitivity limit of the fit away from strong excitonic resonances. To validate the anisotropic dielectric tensor at the device scale, scattering-type scanning near-field optical microscopy (s-SNOM) was utilized to map the propagation of transverse-magnetic modes in 235 nm thick exfoliated flakes. Spatial Fourier analysis of the edge-scattered near-field interference yields effective mode indices that precisely match the modeled dispersion. Using the verified dielectric tensor, finite-element simulations demonstrate that single-mode MoSe2 waveguides optically outperform equivalent tungsten disulfide (WS2) benchmarks. The enhanced evanescent field suppression in the claddings of MoSe2 waveguide increases the coupling length by a factor of 3.5, reducing the required routing pitch and enabling a 12.5% direct increase in on-chip integration density. The results identify MoSe2 as a high-index anisotropic platform for compact waveguiding in the near-infrared.