Fig. 1. Relationship of the emission power and efficiency of up-conversion phosphors, host internal temperature, and excitation density^{[35, 37, 40]}. (a) Experimental setup for measuring the efficiency of the up-conversion phosphors; (b) relationship between emission light power and excitation light density (x and y represent the concentration of doped Yb³⁺ and Er³⁺, respectively); (c) relationship between emission efficiency and excitation light density; (d) relationship between host internal temperature and excitation light density

Fig. 2. UVC up-conversion emission properties of Er³⁺ doped BaGd₂ZnO₅ under visible light excitation^[46]. (a) Up-conversion mechanism of the UV emission produced by a 532 nm solid-state laser excitation; (b) up-conversion fluorescent spectrum of Er³⁺ doped BaGd₂ZnO₅ under a 532 nm solid-state laser excitation; (c) up-conversion fluorescent spectrum of Er³⁺ doped BaGd₂ZnO₅ under a 460 nm LED excitation; (d) measured emission spectrum of blue LED; (e) UV emission spectrum of blue LED encapsulated with Er³⁺ doped BaGd₂ZnO₅

Fig. 3. UVC up-conversion emission of LiYF₄∶Pr³⁺ under the excitation of sunlight using solar-blind camera^[47]. (a) Basic working principle of solar-blind camera and ultraviolet channel imaging principle; (b)(d) UV images of LiYF₄∶Pr³⁺ excited by blue LED with different excitation density and sunlight (the upper left subscript is the excitation density, the lower right subscript is the photon count emitted in UVC); (c)(e) logarithmic relationship between excitation power and emission intensity of blue LED and sunlight

Fig. 4. UV images of Pr³⁺ doped glass and crystal samples excited by sunlight (number is the photon count emitted in UVC). (a) Tellurite glass; (b) silicate glass; (c) phosphate glass; (d) borate glass; (e)‒(k) borosilicate glass, silicon content is 20%, 30%, 40%, 50%, 60%, 70%, 80%; (l)‒(n) KLu₂F₇ crystals, the Pr³⁺ content is 1%, 2%, 3%

Fig. 5. Up-conversion emission power measurement and application of Li₂SrSiO₄∶Pr³⁺ up-conversion fluorescent materials^[48]. (a) Schematic for Up-conversion UV emission power measurement; (b) up-conversion emission power density of Li₂SrSiO₄∶Pr³⁺ excited by 450-nm laser with different power; (c) design scheme of a confocal microscope for real-time observation of the microorganisms under UVC irradiation using coverslips coated with UVC up-conversion phosphors; (d) image of Li₂SrSiO₄∶Pr³⁺ coated slides detected by the confocal microscope under 488-nm laser irradiation; (e) image of UVC photon detected by a solar-blind camera

Fig. 6. Morphology and luminescence properties of Li₂SrSiO₄∶Pr³⁺ glassy phosphor^[49]. (a) Glassy phosphor image; (b)‒(d) emission spectra of Li₂SrSiO₄∶Pr³⁺ excited by X-ray under glass, ceramic, and powder states, respectively; (e)‒(g) emission spectra of Li₂SrSiO₄∶Pr³⁺ excited by 450-nm laser under glass, ceramic, and powder states, respectively, insert shows the UV images monitored by solar blind camera