Quantum cutting by cooperative energy transfer in YbxY1-xPO4 : Tb3+

We present experimental evidence for cooperative energy transfer from Tb3+ to two Yb3+ ions and a determination of the energy-transfer rate. Energy transfer from Tb3+ to Yb3+ was investigated by luminescence measurements on (YbxY1-x)PO4 doped with 1% Tb3+. Time-resolved luminescence experiments were analyzed using Monte Carlo simulations based on theories for phonon-assisted, cooperative, and accretive energy transfer. The luminescence decay curves of the D-5(4) emission from Tb3+ show an excellent agreement with simulations based on cooperative energy transfer via dipole-dipole interaction, while a phonon-assisted or an accretive energy-transfer mechanism cannot explain the experimental results. The energy-transfer rate to two nearest-neighbor Yb3+ ions is 0.26 ms(-1). This corresponds to an upper limit of the energy-transfer efficiency of 88% in YbPO4. Application of cooperative energy transfer has prospects for increasing the energy efficiency of crystalline Si solar cells by photon doubling of the high energy part of the solar spectrum.