Fig. 1. Experimental setup of a 980 nm 3-pulse excitation system^[37]

Fig. 2. Lasing spectra of the microcavity with a bottle-like geometry of diameter equal to 80 μm under 3-pulse excitation scheme at room temperature. The insets show the images of the microcavity under different excitation powers^[37]

Fig. 3. Energy-level diagrams of Er³⁺/Yb³⁺ codoped Ba₂LaF₇ nanocrystals. (a) Simplified model; (b) population inversion via phonon-assisted process at temperature T (i) <300 K, (ii)=300 K, and (iii) >300 K between ⁴S_3/2 and ²H_11/2 states; (c) optical gain versus T of the glass-ceramic at emission peak wavelength λ of 523 and 540 nm at PCW=65 mW·cm^-2; laser spectra at temperature (d) 200 K and

Fig. 4. (a) Experimental setup for the PL and lasing spectra measurement of a NaYF₄ microrod; (b) light-light curves of the hexagonal microrods with and without deposition on the Ag-coated substrate for different wavelengths; (c) corresponding emission spectra of the hexagonal microrods with and without deposition on the Ag coated substrate at the pumped power of 3.5 mJ/cm^{2 [39]}

Fig. 5. Schematic diagram of upconverting nanolasing on Ag nanopillar arrays at room temperature^[40]

Fig. 6. (a) Emission spectra of plasma at different pump powers; (b) relation between pump power, full width at half maximum of emission peak and luminous intensity^[40]

Fig. 7. (a) Upconversion emission intensity versus inner shell thickness (1-17 nm); (b) simplifified energy level diagram showing the energy gaps in Tm³⁺ and Gd³⁺activators, respectively; (c) gain spectra of nanoparticles at different excitation powers (pulsed lasers). The inset gives the corresponding optical gain versus pump power at a wavelength of 310.5 nm. The straight line is the linear regression of the measured data; (d) single mode lasing spectra measured from a microreson

Fig. 8. (a) Upconversion emission spectra of the α -NaYbF₄∶Gd/Tm (40/1%)@NaGdF₄@CaF₂ and the α -NaYbF₄∶Gd/Tm (40/1%)@NaGdF₄@CaF₂∶Ce (15%) nanoparticles. The spectra were obtained from water dispersions of the nanoparticles by excitation at 980 nm. Inset: Time decay curves of Gd³⁺ in the corresponding samples; (b) schematic illustration of the optical setup for the measurement of lasing emissions; (c) emission spectra of the NaY

Fig. 9. Schematic illustration showing the typical fabrication procedures of the proposed microlaser array^[44]

Fig. 10. (a) Lasing spectra as a function of diameter ranging from 10 to 100 μm, and the corresponding SEM images of each microdisk on the right of the image; (b) laser emission diagram of UCNCs on a 300 nm micro-laser and (c) relationship between the corresponding output intensity and pump power; (d) laser emission diagram of UCNCs on a 130 nm micro-laser and (e) relationship between the corresponding output intensity and pump power^[44]

Fig. 11. (a) Schematic diagram of the wet chemical annealing process of KLu₂F₇∶38%Yb³⁺, 2%Er³⁺ UCNPs; HAADF-STEM images of KLu₂F₇∶38%Yb³⁺, 2%Er³⁺ UCNPs (b) before and (c) after annealing at 240 ℃; intensity profiles recorded by scanning along the directions of the (d) orange and (e) green arrows of the UCNPs as shown in Fig. (b) and (c), respectively; enlarged crystal edge structure images (f) before and (g) after ann

Fig. 12. (a) Scanning electron micrograph of a 5 μm-diameter polystyrene bead coated with ELNPs; (b) transmission electron micrograph of a cross-section of the microsphere cavity; (c) schematic of excitation and lasing in microsphere. Inset: schematic of laser movement in microspheres; (d) influence of NaGdF₄ nanocrystal shell thickness on laser threshold^[47]

Fig. 13. (a) Proposed energy transfer mechanisms showing the bluepumped upconversion process in Pr³⁺/Gd³⁺codoped Lu₆O₅F₈ nanocrystals; (b) upconversion emission spectrum of the Lu₆O₅F₈∶Pr/Gd (1/5%)@Lu₆O₅F₈ nanocrystals under excitation of 450 nm diode laser; (c) output intensity versus excitation power in microresonator of various diameters; (d) corresponding optical gain versus pump p