Persistent Spectral Hole-Burning and Zeeman Effect of X-ray-Induced Sm²⁺ in CaF₂:Sm³⁺ Nanocrystals

We report photoluminescence properties as a function of temperature, Zeeman effects of 4f55d → 4f6 transitions, and persistent spectral hole-burning measurements in the 4f6 7F0 (A1g) → 4f55d (T1u) transition of Sm2+ generated by X-irradiation in CaF2:Sm3+ nanocrystals prepared via a coprecipitation method. In the absence of a magnetic field (2–293 K), the luminescence of the dominant electric dipole allowed 4f55d (A1u) → 4f6 7F1 (T1g) transition at 708.2 nm and the two weak transitions T1u → A1g and T1u → T1g at 690.2 and 703 nm, respectively, with some vibronic sidebands were observed. The Zeeman splitting of the Zeeman effect of the 7F1 level in a magnetic field of 9 T led to an observable, though nonresolved, splitting (5.68 ± 0.11 cm–1) into its MJ = −1, 0, 1 components yielding a g-factor of 1.35 ± 0.02. The intensity of the symmetry-forbidden A1u → A1g -transition at 695.8 nm could be drastically increased by an external magnetic field above 3 T at 2 K, with a quadratic magnetic flux dependence. Spectral holes burned into the A1g → T1u transition displayed a Lorentzian line shape and first-order dispersive burning kinetics. The hole-burning rate decreased with an increase of the X-irradiation dose, while an increase was observed with an increase of the Sm concentration. The direct one-phonon relaxation between the T1u and A1u levels and the two-phonon Raman scattering process contributed to the hole width. Interestingly, a nonmonotonous temperature dependence was observed for the frequency shift. The hole was relatively stable with less than a 20% reduction in hole area after 1 h and thermal cycling to 25 K. The present results highlight the significance of the Sm3+ electron traps (type I) in spectral hole-burning of CaF2:Sm3+ nanocrystals.