Abstract
Cs 2NaYbBr 6 doped with 1% Re 4+ was studied by optical absorption and luminescence spectroscopy. In the cubic host lattice the Re 4+ ions substitute for the octahedrally coordinated Yb 3+. Visible by eye red Re 4+ [ 2 T 2g ] upconversion (UC) luminescence centered around 14 000 cm −1 is observed upon Yb 3+[ 2F 5/2] near infrared (NIR) excitation around 10 805 cm −1 between 10 and 300 K. An energy transfer (ET) step from Yb 3+[ 2F 5/2] to Re 4+ [ 2 E g / 2 T 1g ] followed by an energy transfer upconversion (ETU) process on the Re 4+ is shown to be the mechanism responsible for this phenomenon. For Yb 3+ excitation at 10 805 cm −1 with a power density of 1.3 kW/cm 2 the VIS/NIR emitted photon ratio is 5% at 10 K.
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