950 publications from this institution
Nature is abundant in material platforms with anisotropic permittivities arising from symmetry reduction that feature a variety of extraordinary optical effects. Principal optical axes are essential characteristics for these effects that define light-matter interaction. Their orientation – an orthogonal Cartesian basis that diagonalizes the permittivity tensor, is often assumed stationary. Here, we show that the low-symmetry triclinic crystalline structure of van der Waals rhenium disulfide and rhenium diselenide is characterized by wandering principal optical axes in the space-wavelength domain with above π/2 degree of rotation for in-plane components. In turn, this leads to wavelength-switchable propagation directions of their waveguide modes. The physical origin of wandering principal optical axes is explained using a multi-exciton phenomenological model and ab initio calculations. We envision that the wandering principal optical axes of the investigated low-symmetry triclinic van der Waals crystals offer a platform for unexplored anisotropic phenomena and nanophotonic applications.
The realization of extreme optical anisotropy is foundational for nanoscale light manipulation. The van der Waals (vdW) crystal MoOCl<sub>2</sub> 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 MoOCl<sub>2</sub>. Via spectroscopic ellipsometry, Mueller matrix, and reflectance measurements, we quantify the material's optical duality: a metallic optical response (<i>ε</i><sub>1</sub> < 0) along the crystallographic <i>a</i>-axis and a dielectric response (<i>ε</i><sub>1</sub> > 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 Δ<i>n</i> ≈ 2.2 for MoOCl<sub>2</sub>. As a result, our work provides the critical missing experimental parameters for MoOCl<sub>2</sub>, establishing it as a benchmark hyperbolic and ENZ material.
