fig4

Figure 4. (A) Optical photographs of the as-fabricated four-mode anticounterfeiting film based on LiNbO₃:RE³⁺ (RE³⁺ = Pr³⁺, Tm³⁺, Er³⁺, Yb³⁺), recorded under daylight, 365 nm excitation, 980 nm excitation, and after heat treatment^[74]; (B) UCL spectra of LiNbO₃:0.1%Er³⁺ samples synthesized at various reaction temperatures^[74]; (C) I_red/I_green variation in the UCL of LiNbO₃:0.1%Er³⁺ dependent on the reaction temperature and doping concentration^[74]; (D) Static-dynamic luminescence evolution of the fabricated anticounterfeiting device based on UCL and PSL^[74]; (E) UCL spectra of LiNbO₃:1.5%Tm³⁺ and LiNbO₃:1.5%Tm³⁺, 2.5%Yb^3+[74]. Reproduced from Ref.^[74] with permission from the American Chemical Society; (F and G) Emission spectra of LiNbO₃:4%Yb³⁺ at 12 K^[93]. Reproduced from Ref.^[93] with permission from the Elsevier. LiNbO₃: Lithium niobate; RE³⁺: rare earth ions; UCL: upconversion luminescence; PSL: photostimulated luminescence; Tm: thulium; Yb: ytterbium.