Giant Optical Anisotropy\nin 2D Metal–Organic\nChalcogenates

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