Temperature dependence and temporal dynamics of Mn2+ upconversion luminescence sensitized by Yb3+ in codoped LaMgAl11O19

Abstract

A detailed spectroscopic study of the upconversion properties in Mn2+-Yb3+ codoped LaMgAl11O19 is presented. Pulsed and continuous-wave infrared excitation in the 2F7/2--2F5/2 Yb3+ absorption peaks induces broad Mn2+ green emission at 19450 cm-1, which is assigned to the 4T1--6A1 transition in tetrahedral Mn2+ and sharp peaks associated with Yb3+-pairs luminescence. Both emissions have very different temporal evolution and can be separated by time-resolved spectroscopy. Among the different concentrations under investigation, the 2%Mn2+-5%Yb3+ codoped system presents the highest upconversion efficiency. The corresponding emission remains visible to the naked eye up to 650 K. The time dependence of the Mn2+ luminescence upon Yb3+ excitation shows distinct behaviors for different doping concentrations. The temporal evolution of the intensity for the diluted system doped with 2%Mn2+ and 5%Yb3+ together with the pure manganese compound doped with 1%Yb3+, as well as the temperature dependence of the upconversion emission intensity and lifetime are relevant to identify the underlying upconversion mechanisms. We show that the main processes responsible for upconversion in this doubly transition-metal rare-earth doped oxide are both ground-state absorption (GSA)/excited-state absorption and GSA/energy-transfer upconversion. An analysis of these processes yielding highly efficient luminescence is discussed on the basis of crystal structure and dopants.