Significance of Yb3+ concentration on the upconversion mechanisms in codoped Y2O3:Er3+, Yb3+ nanocrystals

The optical properties of five different nanocrystalline Y2O3:Er3+, Yb3+ samples are presented and discussed. Green and red emission was observed following excitation with 488 nm and attributed to H-2(11/2), S-4(3/2)-->I-4(15/2), and F-4(9/2)-->I-4(15/2) transitions, respectively. Striking red enhancement was observed in the upconversion spectra when exciting the Y2O3:Er3+, Yb3+ samples with 978 nm, and it became more pronounced with an increase in Yb3+ concentration. A cross relaxation mechanism (F-4(7/2)-->F-4(9/2) and F-4(9/2)<--I-4(11/2)) was responsible for directly populating the F-4(9/2) state but did not explain the difference in the magnitude of red enhancement between identically doped bulk and nanocrystalline Y2O3:Er3+, Yb3+ samples. The F-4(9/2) level was populated via a nonresonant mechanism that involved the F-4(9/2)<--I-4(13/2) transition that is more prevalent in the nanocrystals, which is due to the high energy phonons inherent in this type of material. In nanocrystalline Y2O3:Er3+, Yb3+, we observe a change in the upconversion mechanism responsible for populating the S-4(3/2) state, from a two-photon to a three-photon process with an increase in Yb3+ concentration. An explanation to account for this behavior is presented. (C) 2004 American Institute of Physics.