Abstract
Room-temperature blue and violet upconversion (UC) emissions of Er 3+ at 409 nm ( 2H 9/2 → 4I 15/2) and 390 nm ( 4G 11/2 → 4I 15/2) were observed in Y 2O 3:Yb 3+–Er 3+ nanocrystals by diode laser excitation of 980 nm. At low Yb 3+ concentration both wavelengths result from three-photon UC processes. Unexpectedly, a four-photon and a decreased three-photon UC processes were observed at high Yb 3+ concentration for the violet and the blue emissions, respectively. The four-photon process supposably results from the cross relaxation 4D 7/2 + 4I 15/2 → 4G 11/2 + 4I 13/2 of Er 3+ ions with the 4D 7/2 state populated by the 2 2F 5/2(Yb) + 4F 9/2(Er) → 2 2F 7/2(Yb) + 4D 7/2(Er) process, whereas the decreased photon process arises from the larger UC rate at high Yb 3+ concentration.
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