Fig. 1. Schematic of upconversion micro/nano laser in different modes. (a) Energy level diagram of photon upconversion inside 4f orbitals of rare earth ions. (b)‒(e) Obtaining upconversion micro/nano laser: (b) random laser ^[34]; (c) Fabry‒Perot (F‒P) laser^[35]; (d) whispering gallery mode (WGM) laser^[36]; (e) plasmonic modulated laser^[37]

Fig. 2. [in Chinese]

Fig. 3. Upconversion lasing based on NaYF₄∶Yb³⁺, Er³⁺@NaYF₄ nanocrystals^[35]. (a) Laser output is realized by F‒P cavity using NaYF₄∶Yb³⁺, Er³⁺@NaYF₄ nanocrystals as the gain medium; (b) reflection spectra of distributed Bragg reflector and Al mirror; (c) normalized photoluminescent spectra of the NaYF₄∶Yb³⁺, Er³⁺@NaYF₄ nanocrystals at different excitation powers based on 3-pulse excitation scheme

Fig. 4. Upconversion micro-bottle cavity lasing based on whispering gallery modes. (a)(b) Upconversion lasing of the micro-bottle cavity based on NaYF₄∶Yb³⁺, Er³⁺@NaYF₄ nanocrystals^[35]: (a) experimental setup of a 980 nm 3-pulse excitation system; (b) lasing spectra and images of the micro-bottle cavity at different excitation powers. (c)‒(f) Upconversion lasing of the micro-bottle cavity based on NaYF₄@NaYbF₄∶30%Gd³⁺, 1%Tm³⁺@NaYF₄ nanocrystals^[50]: (c) simplified energy level diagram and energy transfer process of Tm³⁺ and Gd³⁺; (d) an upconversion emission spectrum of the nanocrystals under 980 nm continuous-wave (CW) laser excitation; (e) a 980 nm 5-pulse excitation system; (f) lasing spectra and an image of the micro-bottle cavity pumped by the 5-pulse excitation system. (g) Upconversion lasing of the cylindrical microcavity based on NaYF₄∶40%Yb³⁺, 1%Tm³⁺@NaYF₄ nanocrystals: spectra and an image of the microcavity^[51]

Fig. 5. Upconversion lasing based on rare earth ions-doped single NaYF₄ microrod. (a) Emission spectra and fluorescence images of three individual β-NaYF₄ microrods doped with 100%Yb³⁺/1%Er³⁺ (left), 20%Yb³⁺/1%Er³⁺ (middle), and 40%Yb³⁺/2%Tm³⁺ (right) ^[52]; (b) emission spectrum and optical image of a single NaYF₄∶40%Yb³⁺,2%Tm³⁺,0.5%Er³⁺ microrod, and numerical simulation of the optical field distribution at 654 nm inside it ^[52]; (c) schematic diagram of the plasma spectra test system^[53]; (d) relationship between light intensity in different wavelength bands and excitation power for the microrods with and without Ag film^[53]; (e) emission spectra of the microrods with and without Ag film at the pump power of 3.5 mJ/cm^2[53]

Fig. 6. Upconversion lasing based on Tm³⁺-doped nanocrystals coated polystyrene microsphere pumped by 1064 nm CW laser^[54]. (a) Energy transfer process and emission mechanism of Tm³⁺ ions; (b) SEM image of a coated microsphere and TEM image of its cross-section; (c) wide-field image (left) and simulated field distributions in the x-y plane (right) of a microsphere during resonance; (d) representation in 3D of the numerical simulation of E2for the transverse magnetic WGM in microsphere at 807 nm; (e) near-infrared and blue emission spectra of a coated microsphere at different excitation powers

Fig. 7. Low-threshold upconversion lasing based on Tm³⁺-doped nanocrystals coated polystyrene microsphere. (a)‒(c) Upconversion lasing based on NaYF₄∶Tm³⁺ nanocrystals coated polystyrene (PS) microsphere^[55]: (a) fabrication process of the nanocrystals coated PS microsphere cavities, and WGM resonances generated in a microsphere cavity under 1064 nm pumping; (b) SEM images of a microsphere cavity; (c) emission spectrum of a microsphere, showing sharp WGMs (blue) and spontaneous emission (red). (d)‒(f) Upconversion lasing based on a single layer of self-assembled NaYF₄∶20%Yb³⁺, 2%Tm³⁺ nanocrystals coated on PS microspheres^[56]: (d) SEM images of a coated microsphere and TEM image of its cross-section; (e) measurement scheme of upconversion lasing; (f) SEM images and spectra of microspheres coated with different sizes of nanocrystals

Fig. 8. Upconversion lasing based on liquid-quenched NaYF₄∶20%Yb³⁺, 2%Er³⁺ nanocrystals microsphere pumped by 980 nm continuous-wave laser^[57]. (a) SEM images of the microsphere and its magnified surface image; (b) schematic of upconversion lasing in a microsphere; (c) electron transition paths for upconversion lasing via energy transfer, where short dashed lines, dotted lines, colored solid lines, long dashed lines, and dashed lines represent absorption, energy transfer, spontaneous emission, stimulated emission, and multi-phonon relaxation, respectively; (d) red and green emission spectra around the lasing threshold

Fig. 9. Upconversion lasing based on rare earth ions-doped glass microsphere cavities. (a) Schematic illustration of the preparation method for rare earth-doped nanocrystal composite glass microspheres and the realization of upconversion laser output^[58]; (b) microstructure of Yb³⁺/Ho³⁺ co-doped oxyfluoride glass ceramics (GCs)^[36]; (c) schematic diagram of tapered fiber near-field coupling GC microsphere^[36]; (d) upconversion lasing spectra of the glass precursor microsphere and GC microsphere under 980 nm continuous-wave laser pumping^[36], where the inset shows a dark-field fluorescence image of the GC microsphere

Fig. 10. Upconversion lasing based on microdisk array with LiYbF₄∶1%Tm³⁺@LiYbF₄@LiLuF₄ nanocrystals^[59]. (a) Schematic illustration showing the typical fabrication procedures; (b)(c) lasing spectra of laser arrays with different microdisk sizes at varying pump powers

Fig. 11. Plasmonic cavity-modulated upconversion micro/nano laser. (a) Plasmonic cavity composed of a gold thin film, a silver nanocube and a monolayer of NaYF₄∶18%Yb³⁺, 2%Er³⁺ nanoparticles^[64]; (b) simulated plasmonic gap mode with a maximum electric field enhancement E/E0 of ~40^[64]; (c) emission spectrum of the nanoparticles and simulated scattering spectrum of a single nanocavity^[64]; (d) emission spectra of nanocrystals deposited on the glass substrate (black), gold film (green), and the nanocavity (red)^[64]; (e) schematic diagram of coupling between NaYF₄∶20%Yb³⁺, 20%Er³⁺@NaYF₄ nanocrystals and Ag nanopillars^[37]; (f) SEM image of the Ag nanopillar array coated with nanoparticles^[37]; (g) representative near-field plot for the 450 nm spaced Ag nanopillars at resonance from a FDTD method simulation^[37]; (h) power-dependent lasing spectra of nanocrystals in Ag nanopillar arrays^[37]