Optical Waveplates Based on Birefringence of Anisotropic Two-Dimensional Layered Materials

Birefringence is an inherent optical property of anisotropic materials introduced by the anisotropic confinement in their crystal structures. It enables manipulation of light propagation properties (e.g., phase velocity, reflection, and refraction) for various photonic and optoelectronic applications, including waveplates and liquid crystal displays. Two-dimensional (2D) layered materials with high anisotropy are currently gaining an increasing interest for polarization-integrated nanodevice applications, which advances the research on birefringent materials. In this article, we investigate the optical birefringence of three anisotropic 2D layered materials (black phosphorus (BP), rhenium disulfide (ReS₂), and rhenium diselenide (ReSe₂)). We demonstrate that the birefringence in BP (∼0.245) is ∼6× larger than that of ReS₂ (∼0.037) and ReSe₂ (∼0.047) at 520 nm and is compared to that of the current state of the art bulk materials (e.g., CaCO₃). We use these 2D materials to fabricate atomically thin optical waveplates and investigate their performance. In particular, for BP, we observe a polarization-plane rotation of ∼0.05° per atomic layer at 520 nm. Our results show that the relatively large birefringence of anisotropic 2D layered materials can enable accurate manipulation of light polarization with atomically controlled device thickness for various applications where integrated, nanoscale polarization-controllers are required.