Fig. 1. Fluorite structure of MF₂ crystal^[26]

Fig. 2. Structure of rare earth clusters in fluorite crystal^[26]

Fig. 3. Clusters and clustering features of RE³⁺ in MF₂ crystal^[26]

Fig. 4. Neodymium cluster structure evolution in SrF₂ crystal^[42]. (a) Linear combination fitting of XANES; (b) fitted structural evolution; (c) comparison of the absorption and the fitted results^[42]

Fig. 5. Number of codopant ions dependent formation energy in SrF₂ crystal^[19,47]. (a) [Nd³⁺-La³⁺] clusters; (b) [Nd³⁺-Gd³⁺] clusters; (c) [Nd³⁺-Y³⁺] mixed clusters

Fig. 6. The [Nd³⁺-Y³⁺] mixed cluster structure evolution in SrF₂ crystal^[42]. (a) Linear combination fitting of XANES; (b) fitting results

Fig. 7. Structure and spectral properties of Yb,Na∶CaF₂^[57,60]. (a) Local structures of Yb ions; (b)‒(e) influence of Na⁺ on the transmission, absorption, emission, and decay curves of Yb∶CaF₂, respectively A，B，and C represent the 2%Yb∶CaF₂ codoped with 0，3%，and 20% Na ions，respectively

Fig. 8. Low temperature spectral properties of Yb,Na∶CaF₂^[60-61]. (a) Absorption spectra; (b) emission cross section; (c) gain cross section

Fig. 9. Laser operations realized with Yb,Na∶CaF₂ crystal^[60-61,66]. (a) CW laser operation at room temperature; (b)(c) temperature dependent slope efficiency and threshold, respectively; (d) CPA femtosecond laser; (e) 1 TW femtosecond pulse laser realized in POLARIS

Fig. 10. Absorption and emission spectra of NYSF crystal^[19]. (a) Concentration of Nd³⁺ dependent peak absorption cross section; (b) normalized emission intensity at 1057 nm; (c) concentration of Y³⁺ dependent peak absorption cross section; (d) emission spectra

Fig. 11. Absorption and emission spectra of NRCF and NRSF crystals^[72]. (a)(b) Absorption and emission spectra of NRSF; (c) emission spectra of NRSF and NRCF (cubic and square antiprism represent the second stage and third stage of cubic and square antiprism clusters, respectively, the red arrows denote the movement directions of anions in the first shell of rare earth ions)

Fig. 12. Continuously tunable photoluminescence parameters of NRSF and NRCF crystal^[42]. (a) Emission cross section and bandwidth; (b) fluorescence lifetime and emission wavelength

Fig. 13. Mode-locked pulses with NYCF and NYSF crystal. (a) (b) Autocorrelation trace and spectrum of the mode-locked pulses in NYCF^[23]; (c) (d) autocorrelation trace and spectrum of the mode-locked pulses in NYSF^[22]

Fig. 14. CPA-based repetitive pulse amplification in NYSF crystal^[24]

Fig. 15. Comparison of the emission spectra of crystals codoped with one and two kinds of regulators^[105]. (a) Crystals doped with Y³⁺/La³⁺; (b) Y³⁺/Gd³⁺; (c) Lu³⁺/La³⁺; (d)Lu³⁺/Gd³⁺

Fig. 16. Cluster structure and the energy transfer processes^[108]. (a) Aggregating of rare earth in CaF₂/SrF₂; (b)(c) energy transfer processes among the Tm³⁺ and among the Er³⁺ ions, respectively

Fig. 17. Emission spectra and laser performances of Tm∶CaF₂ crystal^[110-111]. (a) Normalized emission spectra; (b)(c) continuous and widely tuned laser output curves of crystals

Fig. 18. Laser operation realized in Er-doped fluorite crystals^[119,122]. (a) CW; (b) tunable laser; (c) (d) self Q-switched and Q-switched mode locked lasers