Luminescence upconversion mechanisms in Yb 3 þ – Tb 3 þ systems

TbðD4-FJÞ luminescence has been observed in single crystals of SrCl2 : Tb ð1%Þ : Ybð1%Þ and Cs3Tb2Br9 : Ybð1%Þ under excitation energies in the region of Yb absorption. Previous reports of similar systems have postulated two possible upconversion mechanisms: (1) cooperative sensitization and (2) a sequence of two cooperative absorption steps, or GSA/ESA. We present results of emission, excitation and kinetic measurements at low and high temperatures. These measurements allow assignment of the mechanism responsible for upconversion in these systems. For temperatures greater than T 1⁄4 100 K process (1) is the dominant mechanism, in agreement with previous assignments. However, for To100 K in Cs3Tb2Br9 : Ybð1%Þ; the upconversion occurs through a GSA/ESA sequence, which is shown here in a clear manner for the first time. The efficiency of the Yb–Tb upconversion process for SrCl2 : Tb ð1%Þ : Ybð1%Þ; at room temperature, under 2:4ð10ÞW=cm2; is on the order of 10 4 and decreases with decreasing temperature by four orders of magnitude. r 2001 Elsevier Science B.V. All rights reserved.