Thermal‐Robust and Glass‐Compatible K _1.3 Al ₉ Ga ₂ O _17.15 :Eu for Laser‐Driven Lighting and Underwater Optical Communication

ABSTRACT For specialized lighting applications in harsh environments, such as solid‐state lighting (SSL) and underwater wireless optical communication (UWOC), phosphors must harbor both exceptional thermal stability and high luminescence efficiency. This underscores the urgent need for robust, high‐performance phosphors capable of stable operation under extreme environments. Here, we achieved tunable emission ranging from blue to green in K 0.9‐2 y Ba y Al 9 Ga 2 O 17.15 :0.2Eu 2+ (0 ≤ y ≤ 0.4) phosphors via site engineering, delivering maximum internal/external quantum efficiencies (IQE/EQE) of 97.97% and 52.08% under 400 nm excitation, along with superior color stability compared to commercial counterparts. Furthermore, we developed a composite glass, K 0.9 Al 9 Ga 2 O 17.15 :0.2Eu 2+ (KAO:2Ga)@SiO 2 (KAOG), which achieves an EQE of 49.30%, outstanding thermal stability with anti‐quenching emission (109.4% at 150 °C), and remarkable seawater resistance, maintaining stable emission after 30 days of immersion. The phosphor‐in‐glass (PiG) enables laser‐driven lighting device with sunlight‐like full‐spectrum emission (Ra = 88.0, Rg = 97.9) and high luminous flux (269 lm), as well as optical communication device that exhibiting high modulation bandwidth of 12.1 MHz and fast data rate of 25 Mbps. Our work highlights the significant potential of site‐engineered phosphors and PiG for SSL and UWOC applications under extreme conditions.