Designing Photochromic Transparent Ceramics With Large Photochromic Contrast and High Transmission for Rewritable Information Displays and 3D Optical Information Storage

Three‐dimensional (3D) optical storage enables volumetric data recording with enhanced storage capacity, yet its application in transparent ceramics (TCs) remains hindered by the trade‐off between high optical transmittance and large photochromic contrast. Here, we introduce a hydroxyl‐centered lattice design in Ca 2+ ‐doped Y 2 O 3 TCs that enables the incorporation of an exceptionally high concentration of structurally integrated ─OH groups without compromising near‐theoretical in‐line transmittance. Under 248 nm irradiation, these Y‐OH units act as photoactive, metastable intermediates that are cyclically cleaved and reformed, initiating and sustaining a cascaded generation of oxygen vacancies and associated F + color centers. This hydroxyl‐assisted reaction affords giant yet reversible photochromic modulation while preserving high transparency: optimized Y 2 O 3 :0.02%Ca 2+ TCs exhibit >80% in‐line transmittance at 600 nm together with a photochromic contrast up to 92.3%. Leveraging this unique combination of hydroxyl‐rich composition and chain‐amplified light‐induced defect formation, we further demonstrate rewritable 3D optical information storage in a transparent ceramic medium, achieving micro‐level patterning with ∼50 µm spatial resolution and reliable erasure by thermal stimulate. This work establishes hydroxyl‐engineered TCs with feedback‐assisted oxygen‐vacancy generation as a powerful platform for programmable defect photonics and provides a general design blueprint for next‐generation transparent photochromic materials.