Population inversion of G 1 4 excited state of Tm 3 + investigated by meansof numerical solutions of the rate equations system in Yb : Tm : Nd : LiYF 4 crystal

In this work we present the spectroscopic properties of LiYF 4 (YLF) single crystals activated with thulium and codoped with ytterbium and neodymium ions. The most important processes that lead to the thulium upconversion emissions in the blue region were identified. A time-resolved luminescence spectroscopy technique was employed to measure the luminescence decays and to determine the most important mechanisms involved in the upconversion process that populates G 1 4 ( Tm 3 + ) excited state. Analysis of the energy transfer processes dynamics using selective pulsed laser excitations in Yb:Tm:Nd, Tm:Nd, and Tm:Yb YLF crystals shows that the energy transfer from Nd 3 + to Yb 3 + ions is the mechanism responsible for the enhancement in the blue upconversion efficiency in the Yb:Tm:Nd:YLF when compared with the Yb:Tm system. A study of the energy transfer processes in YLF:Yb:Tm:Nd crystal showed that the G 1 4 excited level is mainly populated by a sequence of two nonradiative energy transfers that start well after the Nd 3 + and Tm 3 + excitations at 797 nm according to Nd 3 + ( F 4 3 / 2 ) → Yb 3 + ( F 2 7 / 2 ) , followed by Yb 3 + ( F 2 5 / 2 ) → Tm ( H 3 4 ) → Tm 3 + ( G 1 4 ) . Results of numerical simulation of the rate equations system showed that a population inversion for 481.4 nm laser emission line is attained for a pumping rate threshold of 26 s − 1 , which is equivalent to an intensity of 880 W cm − 2 for a continuous laser pumping at 797 nm. On the other hand, a population inversion was not observed for the case of 960 nm ( Yb 3 + ) pumping.