Upconversion in Er<SUP>3+</SUP>-doped ZrO<SUB>2</SUB> nanocrystals pumped at 1.426μm

The effect of frequency upconversion (UC) was investigated in 0.05 and 2.00 mol % Er2O3 doped in ZrO2 nanocrystals prepared by a sol-emulsion-gel process. Laser excitation at 1.426 μm produced UC luminescence at ∼ 0.800, ∼ 0.525, ∼ 0.550, and ∼ 0.660 μm corresponding to Er3+ transitions 4I9/2→ 4I15/2, 4H11/2→ 4I15/2, 4S3/2→ 4I15/2, and 4F9/2→4I15/2 respectively. The role played by excited state absorption and energy transfer mechanisms in the UC process was analyzed using rate equations for Er3+ energy level populations. The energy transfer rate between pairs of Er3+ ions (4I9/2+4I13/2→4I15/2+4S3/2) was determined from comparison between theoretical predictions and experimental data