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 Gd3+ 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. Scientists in China have observed ultraviolet light emission from a cluster of rare-earth Yb3+ ions in a CaF2 matrix. Zhen-Yu Liu and co-workers from Jilin University excited the polycrystalline powders with 978 nm near-infrared laser light. The resulting up-converted emission in the ultraviolet is believed to be due to a phenomenon called co-operative luminescence, whereby multiple Yb3+ ions emit a single shorter-wavelength photon by simultaneous depopulation from their excited states. In this particular case, three Yb3+ ions are thought to be involved — a hypothesis confirmed by low-temperature laser spectroscopy of the sample. This process is interesting because it could lead to light emission at wavelengths that lie outside the absorption and emission bands of single ions.