Population inversion of G14 excited state of Tm3+ investigated by means of numerical solutions of the rate equations system in Yb:Tm:Nd:LiYF4 crystal

Abstract

In this work we present the spectroscopic properties of LiYF4 (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 G14(Tm3+) 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 Nd3+ to Yb3+ 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 G14 excited level is mainly populated by a sequence of two nonradiative energy transfers that start well after the Nd3+ and Tm3+ excitations at 797 nm according to Nd3+(F43/2)→Yb3+(F27/2), followed by Yb3+(F25/2)→Tm(H34)→Tm3+(G14) . 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 (Yb3+) pumping.