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

Abstract

The optical properties of five different nanocrystalline Y 2 O 3 : Er 3+ , Yb 3+ samples are presented and discussed. Green and red emission was observed following excitation with 488 nm and attributed to 2 H 11/2 , 4 S 3/2 → 4 I 15/2 , and 4 F 9/2 → 4 I 15/2 transitions, respectively. Striking red enhancement was observed in the upconversion spectra when exciting the Y 2 O 3 : Er 3+ , Yb 3+ samples with 978 nm, and it became more pronounced with an increase in Yb 3+ concentration. A cross relaxation mechanism ( 4 F 7/2 → 4 F 9/2 and 4 F 9/2 ← 4 I 11/2 ) was responsible for directly populating the 4 F 9/2 state but did not explain the difference in the magnitude of red enhancement between identically doped bulk and nanocrystalline Y 2 O 3 : Er 3+ , Yb 3+ samples. The 4 F 9/2 level was populated via a nonresonant mechanism that involved the 4 F 9/2 ← 4 I 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 Y 2 O 3 : Er 3+ , Yb 3+ , we observe a change in the upconversion mechanism responsible for populating the 4 S 3/2 state, from a two-photon to a three-photon process with an increase in Yb 3+ concentration. An explanation to account for this behavior is presented.