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 Yb^(3+) 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 Yb^(3+) ions.The triplet CL(TCL)has a clear spectral structure,in which most emission peaks are consistent with the self-convoluted spectra from single Yb^(3+) ions.Blue shifts were observed for certain peaks,indicating complex interactions among the excited Yb^(3+) ions.The probability of the TCL process versus the average distances among three Yb^(3+) ions was derived via the first-and second-order corrections to the wave functions of lanthanide ions,indicating that the formation of Yb^(3+) 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 Yb^(3+) ions(Yb^(3+)-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.
