Fig. 1. Schematic diagram of the UCL polarization spectrum test system

Fig. 2. Morphology，structure and elemental mappings picture of β-NaYF₄：Yb³⁺/Er³⁺/Tm³⁺ microcrystal

Fig. 3. UCL spectra of single β-NaYF₄：20%Yb³⁺/x%Er³⁺/0.2%Tm³⁺ and β-NaYF₄：20%Yb³⁺/0.2%Er³⁺/y%Tm³⁺ （x，y=0.2，0.4，0.6，0.8，1.0）microcrystal under the excitation of 980 nm laser

Fig. 4. Upconversion emission spectra and energy level diagram of electron transition in a single β-NaYF₄：Yb³⁺/Er³⁺/Tm³⁺ microcrystal

Fig. 5. UCL spectra and luminescence images of a single β-NaYF₄：20%Yb³⁺/0.2%Er³⁺/0.2%Tm³⁺ microcrystal under different pump-power of 980 nm laser excitation

Fig. 6. UCL polarization spectra of a single β-NaYF₄：20%Yb³⁺/0.2%Er³⁺/0.2%Tm³⁺ microcrystal

Fig. 7. UCL polarization spectra of a single β-NaYF₄：20%Yb³⁺/0.2%Er³⁺/0.2%Tm³⁺ microcrystal when 980 nm laser excited at points A， B and C，respectively