In-Plane Birefringence in Bulk, Few-Layer to Monolayer CrSBr Crystals: A First-Principles Investigation

Anisotropic materials that exhibit large birefringence in the mid-infrared (mid-IR) spectral range, particularly those that exhibit large in-plane birefringence, are of considerable interest for optical devices (e.g., waveplates and polarizers), yet the availability of such materials is limited in the infrared (IR) spectral range. Theoretical exploration is thus keenly sought to both identify layered 2D and bulk materials with large birefringence and to better understand the mechanisms underlying the response. In this work, we investigate anisotropic CrSBr by first-principles calculations, where following validation of our computational methods by analysis of the optical absorption and of Raman and infrared spectra, we predict giant birefringence values, with Δn exceeding 5 in the near-IR and over 2 in the mid-IR in layered semiconductor bulk CrSBr, the highest, to our knowledge, reported to date. We employed the structure of our CrSBr grown crystalline material as the starting structure for optimization. Thorough analyses of the electronic structure reveal the origin of the birefringence. The in-plane giant birefringence persists in the ultrathin monolayer (<1 nm thickness), still covering a broad portion of the IR spectral range. Our results, predicting a very large birefringence in CrSBr, may offer a promising material solution for optical devices in the mid-IR.