Laser crystals of LiYF4 (LYF) singly doped with Er3+ in 2.0% and co-doped with Er3+/Yb3+ in about 2.0%/1.0% molar fraction in the raw composition are grown by a vertical Bridgman method. X-ray diffraction (XRD), absorption spectra, fluorescence spectra and decay curves are measured to investigate the structural and luminescent properties of the crystals. Compared with the Er3+ singly doped sample, obviously enhanced emission at 1.5 μm wavelength and green and red up-conversion emissions from Er3+/Yb3+ co-doped crystal are observed under the excitation of 980 nm laser diode. Meanwhile, the emission at 2.7 μm wavelength from Er3+ singly doped crystal is reduced. The fluorescence decay time ranging from 18.60 ms for Er3+ singly doped crystal to 23.01 ms for Er3+/Yb3+ co-doped crystal depends on the ionic concentration. The luminescent mechanisms for the Er3+/Yb3+ co-doped crystals are analyzed, and the possible energy transfer processes from Yb3+ to Er3+ are proposed.
The use of Pr3+co-doping for great enhancement of mid-infrared(mid-IR) emissions at 2.9 μm and 2.4 μm is investigated in the Ho3+/Pr3+co-doped LiYF4 crystals.With the introduction of Pr3+ions,the fluorescence lifetime of Ho3+:5I7 level is 2.15 ms for Ho3+/Pr3+co-doped crystal,and the lifetime for Ho3+singly doped crystal is 17.70 ms,while the lifetime of Ho3+:5I6 level decreases slightly from 2.11 ms for Ho3+:LiYF4 to 1.83 ms for Ho3+/Pr3+:LiYF4.It is also demonstrated that the introduction of Pr3+greatly increases the mid-infrared emission of Ho3+:5I6 →5I7 which depopulates the Ho3+:5I7 level,while it has little influence on the Ho3+:5I6 level,which is beneficial for greater population inversion and laser operation.The analysis on the decay curves of the 2.0 μm emissions in the framework of the Inokuti-Hirayama model indicates that the energy transfer from Ho3+to Pr3+is mainly from electric dipole-dipole interaction.The calculated efficiency of energy transfer from Ho3+:5I7 to Pr3+:3F2 level is 87.53% for Ho3+/Pr3+(1.02%/0.22%) co-doped sample.Our results suggest that the Ho3+/Pr3+co-doped LiYF4 single crystals may have potential applications in mid-IR lasers.
The LiYF4 single crystals singly doped Ho3＋ and co-doped Ho3＋, Pr3＋ ions were grown by a modified Bridgman method. The Judd-Ofelt strength parameters （Ω2, Ω4, Ω6） of No3＋ were calculated according to the absorption spectra and the Judd-Ofelt theory, by which the radiative transition probabilities （A）, fluorescence branching ratios （β） and radiative lifetime （τ rad） were obtained. The radiative lifetimes of 5/6 and 5/7 levels in Ho3＋ （1 mol%）：LiYF4 are 10.89 and 20.19 ms, respectively, while 9.77 and 18.50 ms in Ho3＋/pr3＋ doped crystals. Hence, the τ rad of 5/7 level decreases significantly by introduction of Pr3＋ into Ho3＋：LiYF4 crystal which is beneficial to the emission of 2.9 μm. The maximum emission cross section of Ho3＋：LiYF4 crystal located at 2.05 μm calculated by McCumber theory is 0.51 ×10-20 cm2 which is compared with other crystals. The maximum emission cross section at 2948 nm in Ho3＋/pr3＋ co-doped LiYF4 crystal obtained by Fuchtbauer- Ladenburg theory is 0.68 × 10-20 cm2, and is larger than the value of 0.53 × 10-20 cm2 in Ho3＋ singly doped LiYF4 crystal. Based on the absorption and emission cross section spectra, the gain cross section spectra were calculated. In the Ho3- ions singly doped LiYF4 crystal, the gain cross sections for 2.05 μm infrared emission becomes positive once the population inversion level reaches 30%. It means that the pump threshold for obtaining 2.05 μm laser is probably lower which is an advantage for Ho3＋-doped LiYF4 2.05 μm infrared lasers. The calculated gain cross section for 2.9 μm mid-infrared emission does not become positive until the population inversion level reaches 40% in Ho3＋/pr3＋：LiYF4 crystal, but 50% in Ho3＋ singly doped LiYF4 crystal, indicating that a low pumping threshold is achieved for the H03＋：5/6 → 5/7 laser operation with the introduction of Pr3＋ ions. It was also demonstrated that Pr3＋ ion can deplete rapidly the lower laser Ho3＋：5/7 level and has influence on t
