Fig. 1. Schematic of core-shell structure for orthogonal luminescence. (a) Double-emission layer design for orthogonal upconversion; (b) single-emission layer design for steady-state orthogonal upconversion; (c) single-emission layer design for non-steady-state orthogonal upconversion; (d) double-emission layer design for upconversion/downshifting dual-modal luminescence

Fig. 2. Double-emission layer design for dual-color orthogonal upconversion. (a) Schematic of NaGdF₄∶Yb/Tm@NaGdF₄@NaYbF₄∶Nd@Na(Yb,Gd)F₄∶Ho@NaGdF₄ core-shell structure for blue-green dual-color orthogonal upconversion and its upconversion emission spectra and photograph under 980/808 nm excitation^[27]; (b) upconversion emission spectra and photograph of NaGdF₄∶Yb/Er@NaYF₄∶Yb@NaGdF₄∶Yb/Nd@ NaYF₄@NaYF₄∶Yb/Tm@NaYF₄ core-shell sample^[24]; (c) upconversion decay curves of Er³⁺ and Tb³⁺ after NaGdF₄∶Yb/Tm coating with additional NaGdF₄∶Tb^[26]; (d) schematic of NaErF₄@NaYF₄@NaYbF₄∶Tm@NaYF₄ core-shell structure for blue-red dual-color orthogonal upconversion and its upconversion emission spectra and photograph under 980/800 nm excitation^[36]; (e) schematic of core-shell structure by combining various dopants for red-green/red-blue dual-color orthogonal upconversion^[29]; (f) upconversion emission photographs of NaErF₄-based multilayer orthogonal core-shell samples under 808/980/1550 nm excitation^[29]

Fig. 3. Double-emission layer design for RGB orthogonal upconversion. (a) TEM image, upconversion emission spectra and photographs of NaYF₄∶Nd/Yb@NaYF₄∶Yb/Tm@NaYF₄@NaYF₄∶Yb/Ho/Ce@NaYF₄ core-shell the above sample under various 808/980 nm excitation conditions^[28]; (b) schematic of upconversion energy transfer mechanisms of the above sample under 980 nm pulsed laser excitation^[28]; (c) multicolor upconversion emission photographs of the above sample under various 808/980 nm excitation conditions^[28]; (d) schematic of energy transfer processes for realizing full-color orthogonal upconversion in NaYF₄∶Nd/Yb@NaYF₄∶Yb/Tm@NaYF₄@NaYbF₄∶Ho@NaYF₄ core-shell structure^[29]; (e) upconversion emission intensity ratio of red and green (R/G) and photographs under 808/980 nm excitations with various pump power densities ^[29]

Fig. 4. Single-emission layer design for orthogonal upconversion. (a) TEM morphology image of NaYbF₄∶Er/Tm@NaYF₄∶Yb core-shell sample and schematic of proposed energy migration processes^[42]; (b) upconversion emission spectra and photographs of the above sample under 808/980 nm excitation^[42]; (c) upconversion emission spectra and photographs of NaErF₄@NaYbF₄ core-shell sample under 980/1530 nm excitation^[31]; (d) schematic of structure design to realize red-green orthogonal upconversion in NaErF₄∶Yb/Tm@NaYbF₄ core-shell sample^[39]; (e) upconversion emission spectra of the sample in Fig. 4(d) under 980 nm excitation at different pulse widths^[39]; (f) CIE chromatic coordinates and photographs of the sample in Fig. 4(d) under 980 nm excitation at different pulse widths^[39]

Fig. 5. Double-emission layer design for upconversion/downshifting dual-modal luminescence. (a) Schematic of core-shell structure design for dual-modal luminescence^[46]; (b) upconversion emission spectra of NaGdF₄∶20%Yb/2%Ho/12%Ce@NaYF₄∶Eu core-shell sample under 980 nm excitation^[46]; (c) down-shifting excitation and emission spectra as well as photographs of NaGdF₄∶Yb/Ho/Ce@NaYF₄∶A (A=Eu, Tb, Sm, Dy, Nd) core-shell sample^[46]; (d) schematic of dual-modal luminescence in LiYbF₄∶Y@LiGdF₄∶Yb/Tm@LiYF₄∶A@LiGdF₄∶Ce core-shell structure under 980/254 nm excitation^[40]; (e) tri-channel emission spectra and corresponding emission color photographs under single-excitation of 980, 254 nm, and dual-excitation of them, respectively^[40]

Fig. 6. Typical applications of orthogonal upconversion. (a) Three-dimensional volumetric full-color display^[28]; (b) information security^[31]; (c) anti-counterfeiting^[26]; (d) DNA nano-device for programmed tumor cell recognition and treatment^[37]

Table 1. Summary of orthogonal luminescence of multicolor output and modulation types