Record‐High Birefringence with Ultralow Dichroic Loss Realized by Cation‐Templated Engineering Ordered Assembly of Linear Birefringent‐Active Unit

ABSTRACT Engineered high‐birefringence materials overcome natural crystal limits (typically Δ n = 0.15−0.28), enabling advanced polarization‐sensitive photonics and quantum information processing. Here we present a tailored crystal [C(NH 2 ) 3 ]ICl 2 ( GIC ) exhibiting exceptional optical anisotropic behavior, as evidenced by its remarkable birefringence of up to 0.95 across the ultraviolet to infrared spectral range via using the Mueller matrix spectroscopic ellipsometer. Critically, the measurements confirm negligible dichroism (Δ κ ≈0 @350−1690 nm) in the material, further validating its pure birefringent character. This strict decoupling of dichroism and birefringence makes it ideal for polarization‐sensitive devices requiring minimal absorption‐induced crosstalk. It delivers an excellent laser‐induced damage threshold (LIDT) of 123.4 MW·cm −2 , making it suitable for high‐power laser systems and significantly enhancing the stability and reliability of equipment under extreme optical power conditions. First‐principles calculations demonstrate that the pronounced birefringence arises from the cooperative interplay between the inherent optical anisotropy and the a ‐axis‐oriented alignment of the linear [ICl 2 ] − anions, where the anisotropic electron density distribution amplifies polarization‐dependent refractive index splitting. This work establishes [ICl 2 ] − as a critical optical anisotropic unit (OAU). Notably, we pioneered the cation‐templated strategy to optimize linear units' orientation for achieving maximal birefringence, enabling novel high‐performance birefringent crystals.