Upconversion phosphors of CaLaAl3O7:RE3+/Yb3+ (RE = Tm, Ho) and their multifunctional applications for multi-color anti-counterfeiting and laser displays†

Abstract

Upconversion emission materials have many advantages, such as narrowband sharp line emission, excellent photochemical stability, and long lifespan. In particular, under 980 nm invisible laser-beam irradiation, materials with visible light upconversion luminescence have vital application value in optical anti-counterfeiting and laser displays. In this study, a library of upconversion phosphors of CaLaAl₃O₇:RE³⁺/Yb³⁺ (RE = Tm, Ho) was produced through a high-temperature solid-state reaction method. The powders exhibited different color emissions at different excited wavelengths. Under 980 nm laser excitation, Tm³⁺/Yb³⁺ co-doped phosphors exhibited an upconversion emission of 474 nm blue light, which was attributed to the ¹G₄ → ³H₆ transition of Tm³⁺, and the fitting slope of the upconversion luminescence intensity vs. laser-pump power density was n_450–500 = 2.83, which indicated the three-photon absorption upconversion process. Additionally, Ho³⁺/Yb³⁺ co-doped phosphors had the strongest red emission at 660 nm, originating from the ⁵F₄/⁵S₂ → ⁵I₈ transition of Ho³⁺, which belongs to the two-photon absorption process. Subsequently, the changes in chromaticity coordinates of the prepared samples were analyzed under different light excitations, and different light color emissions were observed. The color purities of Ho³⁺/Yb³⁺ and Tm³⁺/Yb³⁺ co-doped samples were 99.9% and 65.4%, respectively. Finally, we verified the practical application value of the prepared powder in the fields of multi-color luminescence optical anti-counterfeiting and laser displays. The results show that the produced phosphors have vital application potential in information security protection, optical data storage, and 3D laser displays.