Luminescence upconversion mechanisms in Yb3+–Tb3+ systems

Tb3+(5D4→7FJ) luminescence has been observed in single crystals of SrCl2:Tb3+(1%):Yb3+(1%) and Cs3Tb2Br9:Yb3+(1%) under excitation energies in the region of Yb3+ 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=100 K process (1) is the dominant mechanism, in agreement with previous assignments. However, for T<100 K in Cs3Tb2Br9:Yb3+(1%), the upconversion occurs through a GSA/ESA sequence, which is shown here in a clear manner for the first time. The efficiency of the Yb3+–Tb3+ upconversion process for SrCl2:Tb3+(1%):Yb3+(1%), at room temperature, under 2.4(10)4W/cm2, is on the order of 10−4 and decreases with decreasing temperature by four orders of magnitude.