Fig. 1. Luminescent polarization properties of rare-earth-doped upconverting materials. Scanning electron microscopy (SEM) images of β-NaYF₄∶Yb,Tm microrods, microdisks and β-NaYF₄∶Gd,Yb,Tm nanorod (left). Polar plots of luminescence intensity as function of emission polarization angle, which are corresponding to transitions of Tm³⁺ from three different upconverting microcrystals^[10]

Fig. 2. Excitation polarization properties of rare-earth-doped upconverting materials. (a) Normalized upconverting luminescent spectra of single β-NaYF₄∶Yb,Pr microcrystal in π- and σ-configuration; (b) site symmetry illustration of Y³⁺/Yb³⁺/Pr³⁺ ion in hexagonal NaYF₄ structure; (c), (d) luminescent spectra of single β-NaYF₄∶Yb,Pr microcrystal in π- and σ-configuration, recorded at excitation polarization angles varying from 0° to 360°, without polarizer in detection part; (e), (f) polar plots of integrated luminescence intensity as function of excitation polarization angle for blue emission of β-NaYF₄∶Yb,Pr microcrystal in π- and σ-configuration^[12]

Fig. 3. Optical torques on upconverting microcrystal. (a) Schematic representation of experimental setup used for both luminescence characterization and optical trapping experiments; (b) characteristic emission spectra obtained when single microcrystal is incorporated to optical trap obtained for polarization parallel (purple) and perpendicular (green) to linear polarization of 980 nm trapping laser; (c) diagram of orientation of trapped microcrystal in respect to linear polarization of trapping beam; (d) polar diagrams of ratio of 656 nm peak to 664 nm peak as function of polarization angle obtained for two perpendicular trapping laser polarizations^[17]

Fig. 4. Upconversion-STED super-resolution imaging of NaYF₄∶Yb,Tm nanocrystals. (a) Confocal (left) and super-resolution (right) images of 13-nm NaYF₄∶Yb,8%Tm nanocrystals; (b) intensity profiles between arrows across two nanocrystals in (a), showing FWHM of 32 nm after Gaussian fitting; (c) energy level diagram of Yb³⁺/Tm³⁺ co-doped upconverting nanocrystals including typical cross-relaxation pathways among Tm³⁺ emitters, where solid arrows indicate excitation and emission, curved arrows mean non-radiative relaxation, dashed arrows connected by dotted lines represent energy transfer processes; (d) transient response of upconverted emission measured at 800 nm from 8% and 1% Tm³⁺-doped nanocrystals after 980 nm excitation is switched on at time  0 ms^[22]

Fig. 5. One-scan FED nanoscopy imaging of excitation orthogonalized NaYF₄∶Er@NaYF₄@NaYF₄∶Yb,Tm nanocrystals. (a) Design layout of upconverting nanocrystals with orthogonalized dual excitation-emission. Er³⁺-doped NaYF₄ core can emit green emission under the first NIR laser excitation (808 nm, doughnut-shape laser beam) and blue emission can be generated from Yb³⁺/Tm³⁺-doped NaYF₄ shell through excitation of the second NIR laser (940 nm, Gaussian-shape beam). Resolution-enhanced image can be obtained by implementing subtraction of acquired solid and doughnut confocal images. (b) Blue channel image under Gaussian-shape 940 nm laser excitation. (c) Green channel image under doughnut-shape 808 nm laser excitation. (d) FED image with improved resolution. Scale bar of (b)-(d) is 1 μm. (e), (f) Magnified region indicated by white boxes in (b) and (d), respectively. Scale bar of (e) and (f) is 200 nm. (g) Corresponding normalized intensity plots along white dashed line in (e) and (f). (h) Normalized intensity profile plot (blue, green and red curves) along white dashed lines in (b)-(d), respectively^[28]

Fig. 6. Migrating photon avalanche (MPA) in core/shell upconverting nanoparticles. (a) MPA mechanism with different lanthanide emitters (X³⁺). Through long-range propagation, a fraction of avalanching energy from Yb³⁺/Pr³⁺ co-doped system can be migrated to X³⁺ via Yb³⁺ sublattice. (b) S-shaped emission intensity curve of NaYF₄∶Yb,Pr nanoparticles plotted versus excitation intensity under 852 nm CW Gaussian beam. Inset shows emission spectrum of Yb³⁺ with peak at 977 nm. (c) Emission intensity from Ho³⁺ at 541 and 646 nm and Pr³⁺ at 609 nm versus excitation intensity. (d) Emission intensity from Tm³⁺ at 452 nm and Pr³⁺ at 484 nm versus excitation intensity^[31]

Fig. 7. Whispering galley mode (WGM) lanthanide upconverted microlasers spanning visible spectrum to near-infrared. (a), (b) SEM (a) and transmission electron microscopy (TEM) (b) images of polystyrene (PS) microspheres absorbed with β-NaGdF₄∶Yb,Tm@NaGdF₄ upconverting nanoparticles (20-30 nm in thickness). (c) Pump power-dependent emission spectra of β-NaGdF₄∶Yb,Tm@NaGdF₄-absorbed PS microspheres. (d)-(f) Pump power-dependent upconverting emission spectra of Tm³⁺ (blue bands) (d), Er³⁺ (green bands) (e), and Ho³⁺ (red bands) (f) from microsphere WGM microlasers^[53]