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.