Optimum fluorescence emission around 1.8 μm for LiYF₄ single crystals of various Tm³⁺-doping concentrations

In this paper, optical spectra of LiYF₄ single crystals doped with Tm³⁺ ions of various concentrations are reported. The emission intensity at 1.8 μm first increases with increasing Tm³⁺ concentration, and reaches a maximum value when the concentration of Tm³⁺ is about 1.28 mol%, then it decreases rapidly as the concentration of Tm³⁺ further increases to 3.49 mol%. The emission lifetime at 1.8 μm also shows a similar tendency to the emission intensity. The maximum lifetime of 1.8 μm is measured to be 17.68 ms for the sample doped with Tm³⁺ of 1.28 mol%. The emission cross section of ³F₄ level is calculated. The maximum reaches 3.76×10^-21 cm² at 1909 nm. The cross relaxation (³H₆, ³H₄→³F₄,³F₄