Colossal visible birefringence and ultraviolet hyperbolicity in wide bandgap semiconductor CuAlO2

Materials combining colossal birefringence with a wide bandgap are of considerable interest for miniaturized photonics, yet remain rare because strong optical transitions that enhance birefringence usually introduce optical loss. Here we show that CuAlO2 exhibits colossal ultraviolet–visible birefringence alongside a wide bandgap. Spectroscopic ellipsometry on millimeter-sized single crystals reveals a sharp, intense lowest-energy transition near 3.78 eV exclusively in the ab-plane, yielding a maximum birefringence ∣Δn∣max = 3.67, and ∣Δn∣ > 0.5 throughout the visible range, with a direct band gap of 3.71 eV. The same transition generates a Type-II hyperbolic window between two ultraviolet epsilon-near-zero points. First-principles calculations and optical selection rules attribute this anisotropy to Cu-dominated band-edge states and a dipole-allowed Cu dz2-to-px/y transition enabled by the linear O–Cu–O dumbbell structure. These results establish CuAlO2 as a natural oxide platform that bridges broadband low-loss birefringence and ultraviolet hyperbolic photonics. Materials combining large optical anisotropy with a wide band gap are desirable for broadband anisotropic photonics. Here, the authors show for CuAlO2 a colossal ultraviolet–visible optical anisotropy with a maximum birefringence of 3.67 alongside a wide direct band gap of 3.71 eV.