Fig. 1. Mechanism of persistent luminescence and its applications in optical information storage. (a) Schematic of energy-level model of room-temperature persistent luminescent materials; (b) schematic of energy-level model of deep-trap persistent luminescence materials; (c) schematics of optical information storage by using deep-trap persistent luminescent materials

Fig. 2. Multidimensional optical information storage applications of BaFCl∶Sm³⁺/Sm²⁺ nanocrystals. (a) Information write-in and readout by using ultraviolet light (λ=185 nm, t>10 min, P=200 µW/cm²)^[72]; (b) information erasure of point C₃ by using high-power blue light (λ=405 nm, P=220 µW)^[72]; (c) schematic of write-read-erase mechanism for BaFCl∶Sm³⁺/Sm²⁺ and reversible transition diagram^[72]; (d) dependence of Sm²⁺ emission intensity at 697 nm on power of ultraviolet light^[73]; (e) write-in and readout of multi-dimensional information (10 order grayscale value of intensity)^[73]

Fig. 3. Multicolor persistent luminescence from fluoride nanoparticles and their applications in multidimensional optical information storage^[80]. (a) Persistent luminescence spectra of NaYF₄∶Ln³⁺@NaYF₄ nanoparticles (Ln@Y) with core-shell structure. Ln³⁺ includes Tb³⁺, Er³⁺, Dy³⁺, Ho³⁺, Tb³⁺@Eu³⁺,and Nd³⁺; (b) pictures of persistent luminescence of NaYF₄∶Tb³⁺@NaYF₄, NaYF₄∶Dy³⁺@NaYF₄, and NaYF₄∶Ho³⁺@NaYF₄ nanoparticles dispersed in water; (c) chromaticity coordinate of persistent luminescence of three kinds of nanoparticles;^{(d) thermoluminescence spectrum of NaYF₄∶Tb3+@NaYF₄ nanoparticles; (e) schematic illustration of applications in multidimensional optical information storage based on trichromatic persistent luminescence nanoparticles; (f) (g) three groups of image information on the same glass substrate obtained by ink-jet printing, and three groups of images analyzed by wavelength filtering}

Fig. 4. Optical information storage applications of sulfide deep-trap persistent luminescent materials^[139]. (a) 16 possible parallel Boolean logic operations performed on (Ca_xSr_1－x)S∶Eu²⁺,Ce³⁺,Sm³⁺ thin film (blue and NIR lights were used as write-in and read-out beams, respectively); (b) energy-level diagram for blue light excitation storage and NIR photo-stimulated luminescence; (c) photo-stimulated luminescence image with resolution of ~80 lp/mm

Fig. 5. Optical storage performance of Lu₂O₃:Tb³⁺ and Lu₂O₃:Pr³⁺,Hf ⁴⁺. (a) Dependence of photo-stimulated luminescence spectra of Lu₂O₃:Tb³⁺ on stimulation time^[89]; (b) changes in photo-stimulated luminescence intensity of Lu₂O₃:Tb³⁺ in subsequent cycles of UV-IR-UV excitation;^[89] (c) thermoluminescence curves of Lu₂O₃:Pr³⁺,Hf ⁴⁺ excited by X-ray after different decay time^[90]; (d) thermoluminescence curves measured after 30 min excitation with 980, 780, and 400 nm laser shortly after X-ray irradiation^[90]

Fig. 6. Optical information storage application of LiGa₅O₈∶Cr^{3+ [48]}. (a) Thermoluminescence curves of LiGa₅O₈∶Cr³⁺ phosphor disc under different conditions; (b) photo-stimulated persistent luminescence (PSPL) decay curves of LiGa₅O₈∶Cr³⁺ phosphor disc under different conditions. Before the thermoluminescence tests, the phosphor disc was excited with UV light and delayed for 10 s, 120 h, and 120 h followed by 400 nm photo-stimulation. Before the PSPL test, the phosphor disc was excited with UV light, delayed for 120 h, and photo-stimulated with 400 nm light for 100 s

Fig. 7. 3D optical information storage based on transparent glass ceramics^[49]. (a) Schematic illustration of multilayer transparent glass ceramics-configured optical information storage medium and write-in/readout process for optical information; (b) 3D optical image of multilayer transparent glass ceramics obtained by high-temperature thermal stimulation; (c) photographic images of transparent glass ceramics with different heat treatment conditions (the top, middle, and bottom images are those under natural light, UV light, and after UV irradiation, respectively); (d) photoluminescence spectra of parent glass and transparent glass-ceramics at room temperature

Fig. 8. Tuning trap depth in persistent luminescent materials by band-gap engineering strategy. (a) Thermoluminescence curves of Zn(Ga_1－xAl_x)₂O₄:Cr³⁺,Bi³⁺, in which the molar mass ratio of Al in the samples of 0Al, 2Al, 4Al,and 33Al are 0%, 2%, 4%, and 33%, respectively^[116]; (b) photoluminescence excitation spectra and (c) energy-level model diagram of Zn(Ga_1－xAl_x)₂O₄:Cr³⁺,Bi^3+[116]; (d) thermoluminescence curves of Y₃Al_5－xGa_xO₁₂:Ce³⁺,V^3+[119]; (e) energy-level model diagram and (f) photographic images of Y₃Al_5－xGa_xO₁₂:Ce³⁺,V³⁺( NL, UV, and TSL are the images took under natural light, UV light, and persistent luminescence at room temperature)^[119]

Fig. 9. Persistent luminescence and photo-stimulated luminescence in oxide glass. (a) Persistent luminescence images of Ca-Al-Si-O glass samples 5 min after the removal of the 800 nm femtosecond laser, in which the green, blue, and red images were took from the glass doped with Tb³⁺, Ce³⁺, and Eu³⁺, respectively^[124]; (b) excitation, photoluminescence, and persistent luminescence spectra of the Ca-Al-Si-O glass samples^[124]; (c) absorption spectra of the Ca-Al-Si-O glass before and after the laser irradiation^[124]; (d) thermoluminescence curves of Zn-Si-B-O∶Mn²⁺ glass samples after different UV light exposure^[125]; (e) photos of Zn-Si-B-O∶Mn²⁺ glass under natural light, and photos of persistent luminescence and photo-stimulated luminescence^[125]; (f) mechanisms of the persistent luminescence and photo-stimulated luminescence in Zn-Si-B-O∶Mn²⁺ glass^[125]

Fig. 10. Photo-stimulated luminescence in nitrides. Thermoluminescence curves of (a) SrCaSi₅N₈∶Eu²⁺,Tm^{3+ [128]}, (c) SrLiAl₃N₄∶Eu^2+[130],and (e) CaSi₁₀Al₂N₁₆∶Eu^2+[131]. Room-temperature persistent luminescence decay curves (when laser is off) and photo-stimulated luminescence (when laser is on) of (b) SrCaSi₅N₈∶Eu²⁺,Tm^3+[128], (d) SrLiAl₃N₄∶Eu^2+[130], and (f) CaSi₁₀Al₂N₁₆∶Eu^2+[131]

Fig. 11. Energy-level engineering, luminescence control, and optical information storage applications in oxynitrides. (a) HRBE energy-level model of SrSi₂O₂N₂^[132]; (b) thermoluminescence curves in SrSi₂O₂N₂∶Eu²⁺,Ln³⁺ (SSON:Eu,Ln) and SrSi₂O₂N₂∶Yb²⁺,Ln³⁺ (SSON∶Eu,Ln)^[132]; (c) photographic images and persistent luminescence spectra of flexible films containing deep-trap persistent luminescent phosphors (from left to right: BaSi₂O₂N₂∶Eu²⁺,Dy³⁺, SrSi₂O₂N₂∶Eu²⁺,Dy³⁺, Sr_0.5Ba_0.5Si₂O₂N₂∶Eu²⁺,Dy³⁺, and SrSi₂O₂N₂∶Yb²⁺,Dy³⁺)^[20]; (d) information readout from the flexible films by high-temperature thermal stimulation^[20]

Table 1. Representative deep-trap persistent luminescent materials and their major properties