Giant Optical Anisotropy in 2D Metal–Organic Chalcogenates

Optical anisotropy is a fundamental attribute of some crystalline materials and is quantified via birefringence. A birefringent crystal gives rise to not only asymmetrical light propagation but also attenuation along two distinct polarizations, a phenomenon called linear dichroism (LD). Two-dimensional (2D) layered materials with high in-plane and out-of-plane anisotropy have garnered interest in this regard. Mithrene, a 2D metal-organic chalcogenate (MOCHA) compound, exhibits strong excitonic resonances due to its naturally occurring multiquantum well (MQW) structure and in-plane anisotropic response in the blue wavelength (∼400-500 nm) regime. The MQW structure and the large refractive indices of mithrene allow the hybridization of the excitons with photons to form self-hybridized exciton-polaritons in mithrene crystals with appropriate thicknesses. Here, we report the giant birefringence (∼1.01) and the tunable in-plane anisotropic response of mithrene, which stem from its low symmetry crystal structure and strong excitonic properties. We show that the LD in mithrene can be tuned by leveraging the anisotropic exciton-polariton formation via the cavity coupling effect, exhibiting giant in-plane LD (∼77.1%) at room temperature. Our results indicate that mithrene is a polaritonic birefringent material for polarization-sensitive nanophotonic applications in the short wavelength regime.