Bright Chromium-Sensitized Lanthanide NIR-II Mechanoluminescence in a Piezoelectric Oxide

Near-infrared (NIR) luminescent materials are critical components of high-performance optical devices for frontier applications in anti-counterfeiting, non-destructive analysis, and deep-tissue imaging. In this work, a bright Cr³⁺-sensitized lanthanide NIR-II mechanoluminescence (ML) is developed in piezoelectric β-Ga₂O₃. Specifically, a nearly complete energy transfer (ET) from highly doped Cr³⁺ ions to Yb³⁺ and Er³⁺ acceptors is observed, resulting in pronounced ML at 1002 and 1542 nm. The NIR-II luminescence intensity is further improved by cation alloying due to the promotion of octahedral distortion, culminating in 5.9- and 3.0-fold stronger ML emissions compared to the well-established CaZnOS:Yb³⁺ and CaZnOS:Er³⁺, respectively. Furthermore, mechanistic investigations revealed that the ML behavior with high dopant concentration is enabled by strain-induced piezoelectric potential without the involvement of trapping states, thus exhibiting exceptional cycling stability. Finally, an innovative multi-level encryption framework is established for information protection with wavelength-selective readout capabilities by optical or mechanical excitation.