Fig. 1. Mechanism of upconversion process. (a) ESA; (b) ETU; (c) CET; (d) PA; (e) EMU

Fig. 2. Gd-CS_YS₂S₃ core-shell nanocrystal. (a) Structure diagram; (b) HAADF-STEM and HR-TEM image; (c) UV upconversion emission spectra under 808 nm continuouswave laser^[47]

Fig. 3. Ba₂LaF₇∶Yb³⁺/Tm³⁺ nanocrystal. (a) TEM image; (b) corresponding size distribution^[22]

Fig. 4. Relationship between the multiphonon nonradiative relaxation rate of rare earth ions and the energy gap in materials with different average phonon energies^[68]

Fig. 5. Energy transfer process of Gd-CS_YS₂S₃@IR-806 under 808 nm laser excitation ^[56]

Fig. 6. Upconversion spectra changes of 1% Tm³⁺ under excitation power of 10⁴ W·cm^-2 and 2.5×10⁶ W·cm^{-2 [75]}

Fig. 7. Upconversion luminescence of Tm³⁺ under 464 nm laser excitation. (a) Upconversion energy level diagram; (b) upconversion spectrum ^[59]

Fig. 8. Upconversion luminescence behavior of Ho³⁺ ions at different pulse widths and the curves of upconversion luminescence intensity with time in a pulse at different wavelengths^[77]

Fig. 9. Upconversion emission intensity at 311 nm of the NaYF₄ @ NaYbF₄∶Tm/Gd NaYF₄ nanoparticles as a function of excitation power for different excitation schemes^[18]

Fig. 10. Experimental results. (a) Spectra of ultraviolet upconversion laser; (b) power-intensity curve of the cylindrical microcavity; (c) stability measurement of the cylindrical microcavity^[22]

Fig. 11. Microlaser array. (a) Preparation process; (b) (c) TEM images ^[19]

Fig. 12. Experimental results. (a) UV emission spectra of 300 nm WGMs; (b) P-I curve corresponding to Fig.(a); (c) computer simulation of loss of WGMs to light of different wavelengths; (d) UV emission spectra of 130 nm WGMs; (e) P-I curve corresponding to Fig. (d); (f) loss of WGMs of different thicknesses to light of different wavelengths ^[19]

Fig. 13. ZBLAN upconversion fiber laser. (a) Schematic; (b) schematic diagram of the upconversion energy level of Tm³⁺ under the excited of 1064 nm laser^[11]