Fig. 1. Characterization of NaYF4@NaYbF4:1%Tm3+@NaYF4 nanoparticles. (a) TEM image of the as-synthesized nanoparticles. The inset at the bottom shows the size distribution of the core, core–shell, and core–shell–shell nanoparticles together with Gaussian fitting. (b) HAADF-STEM image of the nanoparticles and corresponding elemental mappings of Y, Yb, and Tm. (c) HRTEM image of the nanoparticles. (d) SAED pattern of the nanoparticles. (e) EDS elemental line scan of a single nanoparticle for Y and Yb elements along the yellow arrow in the inset. (f) XRD pattern of the nanoparticles.

Fig. 2. Optical characterization of NaYF4@NaYbF4:1%Tm3+@NaYF4 nanoparticles. (a) Schematic of photon upconverting and cross-relaxation processes of Tm3+ using Yb3+ as sensitizer under continuous-wave 980 nm excitation. (b) Upconversion emission spectrum of the nanoparticles under continuous-wave 980 nm excitation. The inset is the fluorescent photograph of the nanoparticles in cyclohexane. (c) Time decay curves of Tm3+ in the nanoparticles for the D21→F43, G41→H63, and H43→H63 transitions.

Fig. 3. Characterization of nanoparticle-coated microspheres. (a) Schematic of nanoparticle-coated microspheres achieved by immersion in cyclohexane solution containing dispersed nanoparticles. SEM images of the nanoparticle-coated microspheres and their magnified surface structures after different immersion times: (b), (e) 20 s; (c), (f) 60 s; and (d), (g) 120 s.

Fig. 4. Optical characterization of nanoparticle-coated microspheres. (a) Schematic of the setup for microsphere lasing using a tapered fiber, with an enlarged coupling diagram. The inset is a micrograph of the coupling process. The mark “c.w.” stands for continuous-wave. D21→F43 emission spectra of the microspheres and micrographs with different immersion times under 2 mW pump power: (b) 20 s, (c) 60 s, and (d) 120 s. The insets are the micrographs of the nanoparticle coating microspheres under pumping.

Fig. 5. Upconversion lasing spectrum of a uniformly nanoparticle-coated microsphere under 100 μW pump power, with spectrum in the range of 430–525 nm amplified by a factor of 3.

Fig. 6. Characterization of multi-wavelength upconversion lasing. Pump power dependence of emission intensity and FWHM for various transitions: (a) 451 nm (D21→F43), (b) 479 nm (G41→H63), (c) 508 nm (D21→H53), (d) 647 nm (G41→F43), (e) 746 nm (D21→F33), and (f) 801 nm (H43→H63).

Fig. 7. (a) Normalized emission spectra of microspheres with different diameters at D21→F43 and G41→H63 under a pump power of 100 μW. (b) Pump power dependence of emission intensity for the microspheres with different diameters at D21→F43.

Fig. 8. The intensity and wavelength variations of a uniformly nanoparticle-coated microsphere under 1 mW excitation over a period of 180 min for various transitions: (a) 451 nm (D21→F43), (b) 479 nm (G41→H63), (c) 508 nm (D21→H53), (d) 647 nm (G41→F43), (e) 746 nm (D21→F33), and (f) 801 nm (H43→H63).

Fig. 9. Characterization of Er3+ upconversion lasing. (a) Upconversion lasing spectrum of an Er3+/Yb3+ co-doped nanoparticle coating microsphere under a 200 μW pump power, with spectra in the range of 390–580 nm amplified by a factor of 4. The inset is the micrograph of the nanoparticle coating microsphere under pumping. Pump power dependence of emission intensity and FWHM for transitions: (b) 409 nm (H9/22→I15/24), (c) 521 nm (H11/22→I15/24), (d) 543 nm (S3/24→I15/24), and (e) 658 nm (F9/24→I15/24).

Fig. 10. Characterization of Ho3+ upconversion lasing. (a) Upconversion lasing spectrum of a Ho3+/Yb3+ co-doped nanoparticle coating microsphere under a 100 μW pump power. The inset is the micrograph of the nanoparticle coating microsphere under pumping. Pump power dependence of emission intensity and FWHM for transitions: (b) 542 nm (S25→I85) and (c) 643 nm (F55→I85).