BaFS: Birefringence Enhanced by the Transformation from Optical Isotropy to Anisotropy via Interlayer Anion Substitution.

Improved birefringence, given its capacity to modulate polarized light, holds a lively role in the optoelectronic industry. Traditionally, alkaline-earth metal halides have possessed low birefringence due to their nearly optical isotropic properties. Herein, the substitution of interlayer anion with linear S─S unit that meticulously engineered by reduced valence state and strong covalent bond is integrated into the optically isotropic BaF, offering the new salt-inclusion chalcogenide BaFS. Notably, it has dramatically enhanced optical anisotropy, thereby significantly boosting birefringence of 0.238@546 nm, achieved by overall considering experimental observations with theoretical analysis. Theoretical investigation has established the significant effect of the covalent S─S bond on the birefringence index. Additionally, BaFS demonstrates a remarkable laser-induced damage threshold (LIDT, 12.0 × AgGaS@1064 nm), illuminating a promising pathway for designing materials with significant birefringence properties in laser applications.