Giant Optical Anisotropy and Visible-Frequency Epsilon-near-Zero in Hyperbolic van der Waals MoOCl2.

The realization of extreme optical anisotropy is foundational for nanoscale light manipulation. The van der Waals (vdW) crystal MoOCl2 has emerged as a promising candidate for this quest, hosting hyperbolic plasmon polaritons in the visible and near-infrared wavelengths. However, the fundamental anisotropic dielectric tensor governing this behavior has remained elusive. Here, we resolve this problem by providing the first experimental determination of the full dielectric tensor of hyperbolic vdW MoOCl2. Via spectroscopic ellipsometry, Mueller matrix, and reflectance measurements, we quantify the material's optical duality: a metallic optical response (ε1 < 0) along the crystallographic a-axis and a dielectric response (ε1 > 0) along the orthogonal directions. This dichotomy drives an epsilon-near-zero (ENZ) condition at ≈512 nm and results in a giant in-plane birefringence of Δn ≈ 2.2 for MoOCl2. As a result, our work provides the critical missing experimental parameters for MoOCl2, establishing it as a benchmark hyperbolic and ENZ material.