Giant Optical Anisotropy in the Edge Surfaces of Layered Crystals: Unveiled by Direct Measurements of Out-of-Plane Optical Constants
Large optical anisotropy is paramount for efficient light manipulation in optoelectronic devices. Van der Waals layered materials, exhibiting large structural contrast between in-plane and out-of-plane directions, are inherently anisotropic in 3D space. However, measurements of their optical constants have been limited to 2D planes. Here, we directly measure reflectance spectra from the edge and basal surfaces of layered MoS2, NbOCl2, and WTe2 crystals to compare out-of-plane and in-plane optical constants in the 500–1000 nm range. Results show that out-of-plane refractive indices are smaller than in-plane values. Out-of-plane extinction coefficients are zero for MoS2 and NbOCl2 but nonzero for WTe2, confirmed by transient reflection spectroscopy. The nonzero extinction in WTe2 arises from symmetry of transition dipole moments and density of states dictated by crystal structure. Out-of-plane optical constants of MoS2 and NbOCl2 exhibit less dispersion than in-plane, whereas WTe2 shows enhanced out-of-plane dispersion around 2.14 eV, attributed to increased optical transition probability from larger density of states. These parameters indicate giant birefringence (>1.8 for MoS2, >0.6 for NbOCl2, >0.5 for WTe2) and linear dichroism (up to 100% for MoS2 and NbOCl2, 40.7% for WTe2) on edge surfaces. Results enable prediction of optical response at arbitrary incidence angles, aiding polarization-related optoelectronic devices.