In-Plane Anisotropy in Biaxial ReS₂Crystals Probed by Nano-Optical Imaging of Waveguide Modes

Near-field imaging has emerged as a reliable probe of the dielectric function of van der Waals crystals. In principle, analyzing the propagation patterns of subwavelength waveguide modes (WMs) allows for the extraction of the full dielectric tensor. Yet, previous studies have mostly been restricted to high-symmetry materials or narrowband probing. Here, we resolve in-plane anisotropic WMs in thin rhenium disulfide (ReS2) crystals across a wide range of near-infrared frequencies. By tracing the evolution of these modes as a function of crystallographic direction, polarization of the electric field, and sample thickness, we have determined the anisotropic dielectric tensor, including the elusive out-of-plane response. The excitonic absorption at 1.5 eV manifests itself as a clear backbending feature in the WM dispersion and a reduction of the quality factors as fully supported by numerical calculations. Our results extend the sensitivity of near-field microscopy toward biaxial anisotropy and provide key insights into the optoelectronic properties of ReS2.