Our previous work first reported the cooperative sensitized luminescence from Cu2＋ or Pb2＋ by three clustered Yb^3＋ ions, in which three NIR photons can be converted into a high energy photon. Could a reverse process happen that a high energy photon is cut into three NIR photons？ This work demonstrated an example of three-photon quantum cutting （QC） phosphor, CaF2：Ce^3＋,Yb^3＋, in which three clustered Yb^3＋ ions （Yb^3＋-trimer） cooperatively and indirectly received a 306 nm ultraviolet （UV） photon energy transferred from a Ce^3＋ ion in 5d excited state and emitted three 975 nm near-infrared （NIR） photons. The cluster destruction experiments were designed to verify the necessity of the presence of Yb^3＋-trimers for QC. The dynamical analysis on luminescence of Ce^3＋ ions confirmed the energy transfer from Ce^3＋ ions to Yb^3＋-trimers. The doping concentration effect on luminescence was investigated. Furthermore, the maximum energy transfer （ET） efficiency and the corresponding QC efficiency were estimated to be 61% and 222%, respectively. Therefore, the reported three-photon QC phosphor has an attractive prospect in efficiently harvesting solar energy for silicon solar cells.
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.
