Highly efficient three-photon excited red emission in the Yb3+-Er3+ upconversion system at low excitation intensities for non-invasive anti-counterfeiting

Abstract

Abstract Upconversion (UC) phosphors exhibiting luminescence color tuning (LCT) through variations in infrared excitation intensity offer great potential for high-security anti-counterfeiting applications. However, the current LCT capability is limited to high excitation intensities, hindering the development of non-invasive counterfeit detection methods. In this study, we achieve a remarkable two orders of magnitude reduction in excitation intensities for LCT in Yb3+ and Er3+ codoped orthorhombic YF3, accomplished by attaining an unprecedentedly efficient three-photon excited red emission for mixing with the two-photon excited green emission. To enable this breakthrough, we employ deoxygenation techniques during sample preparation, which surprisingly prevented concentration quenching of the sensitizer Yb3+, facilitating efficient three-photon excitation of the red emission for Yb3+ concentrations ≥ 30% even at excitation intensities as low as 10 mW cm-2. At excitation intensities of 100 mW cm-2, the three-photon excitation contributes to 91~94% of the red emission, resulting in an 11~17-fold increase in the red-to-green intensity ratio. This low-excitation-induced LCT, shifting from green to orange, showcases its potential for anti-counterfeiting applications. Furthermore, the present YF3:Yb/Er phosphors demonstrate an impressive UC quantum yield of 7.8%, surpassing the 5.6% quantum yield of the popular hexagonal NaYF4:Yb/Er phosphor under the same excitation intensity of 31.8 W cm-2. These findings represent a significant advancement in highly efficient UC fluoride phosphors, promising diverse applications across various fields.