Huge Enhancement of Green Persistent Mechanoluminescence of SrAl ₂ O ₄ :Dy ³⁺ ‐Eu ²⁺ by Nd ³⁺ for Sound‐to‐Light Conversion, Handwriting Visualization, Force Sensing, and Luminescence Thermometry

Abstract Mechanoluminescence (ML) is an appealing phenomenon in which a material emits light in response to mechanical or acoustic stimulation. The article shows a novel strategy to enhance the persistent mechanoluminescence signal in SrAl 2 O 4 :Dy 3+ , Eu 2+ phosphors by co‐doping them with Nd 3+ ions. The addition of Nd 3 ⁺ significantly improves both friction‐induced and ultrasound‐induced persistent ML. An efficient energy transfer from Eu 2+ to Nd 3+ also introduces strong, long‐lasting near‐infrared (NIR) emission. The NIR persistent luminescence extends the material's functionality to deep‐tissue bioimaging and optical thermometry based on lifetime and intensity ratio‐based readouts. The SrAl 2 O 4 :Dy 3+ ‐Eu 2+ ‐Nd 3+ phosphors, synthesized via a solid‐state method, demonstrate robust and reproducible ML and photoluminescence responses, establishing their potential for real‐world applications. As a proof of concept, the material in anti‐counterfeiting labeling is successfully applied, where invisible handwriting became visible upon mechanical stimulation. The findings establish Nd 3+ doping as an effective approach to enhance ML performance in the studied materials and open pathways for their integration into next‐generation optoelectronics, sensing, and security technologies.