Dual-function applications of photochromic BiNbO4:Er3+ ceramics based on reversible upconversion luminescence modulation†

Photochromic luminescent phosphors have attracted considerable attention owing to their excellent optical properties, but they face the problem of limited application. Herein, the reversible photochromic and photo-/thermal bleaching phenomena of a BiNbO₄ ceramic are reported, exhibiting a color change between ivory and grey upon alternating stimuli between 365/405 nm light illumination and 808 nm laser irradiation (or thermal treatment at 400 °C). Their potential coloration mechanisms are explained by the color center model, providing a comprehensive framework for understanding the original processes. Through Er³⁺ ion doping, the maximum coloration contrast decreases from 24% to 20%, while simultaneously enabling the observation of bright green upconversion luminescence. Relying on the combination of re-absorption and energy transfer processes, the upconversion luminescence intensity of the Er³⁺ ions can be effectively modulated, respectively, showing maximum regulation and recovery rates of 88.0% and 98.1%. The cycle measurements demonstrate the excellent anti-fatigue properties and reproducibility of BiNbO₄:Er³⁺ ceramics, confirming their potential dual-functional applications in anti-counterfeiting and fingerprint acquisition.