Multi-ion cooperative processes in Yb³⁺ clusters

Cooperative luminescence (CL) occurs in spectral regions in which single ions do not have energy levels. It was first observed more than 40 years ago, and all results reported so far are from a pair of ions. In this work, upconverted CL of three Yb³⁺ ions was observed in the ultraviolet (UV) region under near-infrared (NIR) excitation. The UV CL intensity showed a cubic dependence on the NIR pump power, whereas the luminescence lifetime was nearly one-third the luminescence lifetime of single Yb³⁺ ions. The triplet CL (TCL) has a clear spectral structure, in which most emission peaks are consistent with the self-convoluted spectra from single Yb³⁺ ions. Blue shifts were observed for certain peaks, indicating complex interactions among the excited Yb³⁺ ions. The probability of the TCL process versus the average distances among three Yb³⁺ ions was derived via the first- and second-order corrections to the wave functions of lanthanide ions, indicating that the formation of Yb³⁺ clusters containing closely spaced ions favors the occurrence of the multi-ion interaction processes. Furthermore, the cooperative sensitization of one Gd ³⁺ ion by four excited Yb³⁺ ions (Yb³⁺ -tetramer) was demonstrated experimentally, which exhibited a novel upconversion mechanism - cluster sensitization. Our results are intriguing for further exploring quantum transitions that simultaneously involve multiple ions.