In-Plane Anisotropy in Biaxial ReS₂ Crystals Probed by Nano-Optical Imaging of Waveguide Modes

Near-field imaging has emerged as a reliable probe of the dielectric function van der Waals crystals. In principle, analyzing propagation patterns subwavelength waveguide modes (WMs) allows for extraction full 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 wide range near-infrared frequencies. By tracing evolution these crystallographic direction, polarization electric field and sample thickness, determined tensor including elusive out-of-plane response. The excitonic absorption at ~1.5 eV manifests itself clear backbending feature WM dispersion reduction quality factors fully supported by numerical calculations. Our results extend sensitivity near-field microscopy towards biaxial anisotropy provide key insights into optoelectronic properties ReS2.