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 mu m 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 mu m also shows a similar tendency to the emission intensity. The maximum lifetime of 1.8 mu m is measured to be 17.68 ms for the sample doped with Tm3+ of 1.28 mol%. The emission cross section of F-3(4) level is calculated. The maximum reaches 3.76 x 10(-21) cm(2) at 1909 nm. The cross relaxation (H-3(6), H-3(4) -> 3F(4), F-3(4)) between Tm3+ ions and the concentration quenching effect are mainly attributed to the change of emission with Tm3+ concentration. The largest quantum efficiency between Tm3+ ions is estimated to be similar to 147% from the measured lifetime and calculated radiative lifetime. All the results suggest that the Tm3+/LiYF4 single crystal may have potential applications in 2 mu m mid-infrared lasers.