Optical Spectra and Gain Properties of Ho3+/Pr3+ Co-doped LiYF4 Crystal

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 Ho3+ 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 (τrad) were obtained. The radiative lifetimes of 5I6 and 5I7 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 τrad of 5I7 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 Ho3+:5I6 → 5I7 laser operation with the introduction of Pr3+ ions. It was also demonstrated that Pr3+ ion can deplete rapidly the lower laser Ho3+:5I7 level and has influence on the Ho3+:5I6 level. The Ho3+/Pr3+:LiYF4 crystal may be a potential media for 2.9 μm mid-infrared laser.