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Phase‐Separation Engineering of Glass for Drastic Enhancement of Upconversion Luminescence

Optical gain materials are of fundamental importance for various applications, such as lasers, lighting, optical communication, microscopy, and spectroscopy. However, the requirements for high luminescence efficiency and excellent thermodynamic stability of materials are always contradictory. As a result, wide applications of optical materials in high‐temperature, high‐humidity, and high‐power laser‐irradiated environments are restricted. Here, a facile approach based on phase‐separation engineering is proposed to modulate the thermodynamic stability and enhance the luminescence efficiency of optical gain materials. It is shown that the thermodynamic stability and luminescence efficiency of the phase‐separated fluorosilicate (FS) gain glass are both enhanced dramatically when the SiO2 concentration is optimized. Owing to the confinement effect of phase‐separation network structure on active ions, the upconversion (UC) luminescence efficiency of the designed glass is 150 times higher than that of traditional FS glasses and even seven times higher than that of ZBLAN (ZrF4‐BaF2‐LaF3‐AlF3‐NaF) glass, which is the most commonly used material for UC fiber lasing applications. These intriguing properties of the glass indicate that phase‐separation engineering not only provides a powerful solution to conquer the conventional contradiction between thermodynamic stability and luminescence efficiency but also offers significant opportunities for manufacturing a wide range of optical composites with multiple functions.