G. M. Salley, R. Valiente and H. U. Güdel
Cooperative Yb³⁺-Tb³⁺ dimer excitations and upconversion in Cs₃Tb₂Br₉:Yb³⁺
Phys. Rev. B 67, 134111/1-9 (2003)                 

Abstract: Green Tb^{3+ 5}D₄  ⁷F_J luminescence visible by eye is observed under near-infrared laser excitation. Optical spectroscopic techniques including absorption, luminescence, and excitation spectroscopy are used to characterize this upconversion (UC) luminescence. The Tb³⁺ UC luminescence is present for all temperatures within a range from 10 to 300 K, and gains intensity by three orders of magnitude between 10 and 300 K. For T  100 K the dominant upconversion mechanism is the cooperative sensitization of Tb³⁺ by two Yb³⁺ ions. In this temperature regime the Tb³⁺ UC luminescence dominates the visible (VIS) spectrum for all near-infrared (NIR) excitations, resulting in the characteristic green luminescence. At 10 K, the color of the luminescence changes from green to blue, depending on the excitation wavelength corresponding to the dominance of Tb³⁺ UC luminescence or the Yb³⁺-Yb³⁺ cooperative pair luminescence. Two color excitation spectroscopy is performed to directly observe an excited state absorption (ESA) step in the Tb³⁺ UC luminescence excitation spectrum at 10 K. This allows the unambiguous assignment of a type of ground state absorption/excited state absorption (GSA/ESA) mechanism responsible for the upconversion in this system at 10 K. We explain this cooperative interaction in the framework of an Yb³⁺-Tb³⁺ exchange-coupled dimer. An energy level diagram for this dimer is presented. Excitation into dimer levels around 12000–14500 cm^–1, where neither Yb³⁺ nor Tb³⁺ single ions have levels, leads to Yb³⁺ luminescence at 10 K. For laser excitation, 53 W/mm², resonant with an ESA transition a VIS/NIR photon ratio of 2.7(10)^–5 is found at 10 K.