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

Abstract

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.