Geometric Dilution and Electronic Confinement: Origin and Design of Giant Optical Anisotropy in Crystals with Layered Features

Crystals with layered features, by virtue of their intrinsic structural asymmetry, exhibit pronounced optical anisotropy, positioning them as leading candidates for high‐performance birefringent materials. Although numerous crystals with layered features exhibiting giant birefringence (Δ n ) have been experimentally reported, a comprehensive understanding of their theoretical limits, modulation mechanisms, and structure‐property relationships remains lacking. Herein, we integrate high‐throughput screening with first‐principles calculations to investigate 131 experimentally stable candidates, constructing an empirical band gap‐birefringence Pareto frontier. Our analysis reveals that giant birefringence originates from synergistic macroscopic and microscopic mechanisms. Macroscopically, a universal “volcano‐type” dependence on the packing factor ( η ) identifies an optimal geometric dilution at η ≈ 0.5. Microscopically, “covalent locking” enforces strict electron confinement within the two‐dimensional plane, thereby maximizing the polarization difference. This work establishes universal design principles, transitioning the exploration of birefringent crystals from trial‐and‐error to rational geometric and electronic engineering.