Revealing the In-Plane Dielectric Tensor of Triclinic ReSe2 via Nano-imaging of Anisotropic Waveguide Modes and Tailored Surface Polaritons.

In-plane anisotropic light propagation at the nanoscale enables directional, subwavelength control, yet its behavior in low-symmetry triclinic crystals remains underexplored because accessing the full dielectric tensor is challenging. Here, we use scattering-type scanning near-field optical microscopy to image mid-infrared waveguide modes in triclinic ReSe2 and directly reconstruct the in-plane dielectric tensor. We reveal principal axes rotated by ∼15° relative to the crystallographic axes and ultralow-loss guided modes propagating beyond 50 μm. Our approach is broadly applicable to other spectral ranges and low-symmetry van der Waals crystals. Integrating ReSe2 thin film with a SiC substrate, we engineer anisotropic, dielectric-tailored surface phonon polaritons exhibiting highly confined, directional fields. Nano-imaging provides a stringent validation of the reconstructed tensor. These findings establish a versatile platform for characterizing low-symmetry crystals and position ReSe2 as a promising component for anisotropic nanophotonic devices based on van der Waals materials.