Highly Efficient Three‐Photon Excited Red Emission in the Yb³⁺‐Er³⁺ Upconversion System at Low Excitation Intensities for Non‐Invasive Anti‐Counterfeiting.

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 developments of non‐invasive counterfeit detection. In this study, two orders of magnitude reduction are achieved in excitation intensities for LCT in YF3:Yb/Er, accomplished by attaining an unprecedentedly efficient three‐photon excited red emission for mixing with the two‐photon excited green emission. To enable this breakthrough, deoxygenation techniques are employed during sample preparations, which surprisingly prevent concentration quenching of Yb3+ ions, 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. Furthermore, present YF3:Yb/Er phosphors demonstrate an impressive UC quantum yield of 7.8%, surpassing the popular NaYF4:Yb/Er phosphor (5.6%) 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. [ABSTRACT FROM AUTHOR]