Multifunctional Bi₂TeO₅: Er³⁺ with Dual-Stimuli Photochromism and Switchable Upconversion for Optical Security

Abstract

Photochromic luminescent materials are increasingly vital in optical data storage, anti-counterfeiting, and forensic technologies due to their reversible color change and emission tunability. However, most suffer from limited color contrast, single-mode transitions, and poor functional integration. Here, we report a dual-stimuli responsive Bi₂TeO₅: Er³⁺ ceramic that overcomes these limitations by combining dual-mode photochromism with wavelength-tunable upconversion luminescence (UCL) in a single system. Upon 365 nm UV exposure, the material exhibits a two-stage color change from light pink to yellow (3 min) and black (10 min) with a high contrast of 55.2%. X-ray irradiation induces a distinct light pink-to-yellow shift (33.3%) over 15 minutes. Both transitions are fully reversible under 532 nm laser bleaching with ~99% recovery. The ceramic also displays efficient UCL: green emission under 980 nm (96.4% modulation). Furthermore, the material enables rapid latent fingerprint visualization within 3 minutes and demonstrates excellent performance in powder-based forensic testing. This integrated system of dual-mode photochromism and reversible luminescence presents a versatile platform for multi-modal optical security technologies.