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 ²$H11/2,$ ⁴$S3/2→4I15/2,$ and ⁴$F9/2→4I15/2$ 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 $(4F7/2→4F9/2$ and ⁴$F9/2←4I11/2)$ was responsible for directly populating the ⁴$F9/2$ state but did not explain the difference in the magnitude of red enhancement between identically doped bulk and nanocrystalline $Y2O3:Er3+,$ $Yb3+$ samples. The ⁴$F9/2$ level was populated via a nonresonant mechanism that involved the ⁴$F9/2←4I13/2$ 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 ⁴$S3/2$ 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.