Giant optical anisotropy and visible-frequency epsilon-near-zero in hyperbolic van der Waals MoOCl2

TL;DR

This paper experimentally determines the full dielectric tensor of hyperbolic vdW MoOCl2, revealing giant optical anisotropy and epsilon-near-zero behavior in the visible spectrum, enabling advanced nanoscale light manipulation.

Contribution

First experimental measurement of the dielectric tensor of MoOCl2, revealing its hyperbolic and ENZ properties with giant birefringence in the visible range.

Findings

MoOCl2 exhibits hyperbolic plasmon polaritons in visible and near-infrared wavelengths.

The dielectric tensor shows metallic response along one axis and dielectric along others.

Epsilon-near-zero condition occurs at approximately 512 nm, with in-plane birefringence of about 2.2.

Abstract

The realization of extreme optical anisotropy is foundational to nanoscale light manipulation. 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 ({\epsilon}_1 < 0) along the crystallographic a-axis and a dielectric response ({\epsilon}_1 > 0) along the orthogonal directions. This dichotomy drives an epsilon-near-zero (ENZ) condition at \approx 512 nm and results in giant in-plane birefringence of \delta n \approx 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.