Characterization of Anisotropic Optical Behavior in Phase‐Change Material Sb ₂ Se ₃

Phase‐change materials with ultralow optical losses are enabling a new generation of reconfigurable photonic devices. Sb 2 Se 3 has emerged as a leading candidate owing to its negligible optical losses across the visible and infrared, yet how polycrystallinity translates into optical inhomogeneity in thin‐film form remains largely unexplored. Here, the anisotropic optical response of thermally crystallized Sb 2 Se 3 films is systematically characterized by combining imaging Mueller matrix polarimetry, spatially resolved spectroscopic ellipsometry, and many‐body perturbation theory calculations. X‐ray diffraction reveals a fiber texture with (0 k 0) planes preferentially parallel to the film surface, while (00 l ) and ( h 00) planes are randomly oriented in‐plane. Polarimetric imaging exposes pronounced linear birefringence and dichroism at the domain scale, with in‐plane contrasts of |Δ n | ≈ 0.5 and |Δ k | ≈ 0.5 at 633 nm. Spatially resolved ellipsometry quantifies a refractive‐index contrast of Δ n = 0.13 between differently oriented crystallites at telecommunication wavelengths, sufficient to induce measurable phase errors in integrated photonic circuits. First‐principles calculations reproduce the experimental dielectric functions quantitatively and establish the principal‐axis ordering n c > n a > n b . These results reveal that polycrystalline grain orientation is a critical, often overlooked source of optical inhomogeneity in Sb 2 Se 3 films, with direct implications for the design of low‐loss reconfigurable photonic devices.