Influence of Yb 3+ concentration on upconversion luminescence of Ho 3+

Abstract

Upconversion (UC) emissions at 360 (( 5 F, 3 F, 5 G) 2 → 5 I 8), 392 ( 3 K 7/ 5 G 4 → 5 I 8), 428 ( 5 G 5 → 5 I 8), 554 ( 5 S 2/ 5 F 4 → 5 I 8), 667 ( 5 F 5 → 5 I 8) and 754 ( 5 S 2/ 5 F 4 → 5 I 7) nm were obtained in 0.1 mol% Ho 3+/ x mol% Yb 3+:Y 2O 3 ( x = 2, 5, 8, 11, 15) bulk ceramics under infrared (IR) excitation at 976 nm. The intensity of the UC luminescence centered at 554 and 754 nm increased with Yb 3+ concentration from 2 to 5 mol% and decreased from 5 to 15 mol%, while the UC luminescence centered at 392, 428 and 667 nm increased with Yb 3+ concentration from 2 to 11 mol%, then started to reduce with Yb 3+ concentration until 15 mol%. This comes from the competition between the energy back transfer (EBT) process [ 5 S 2/ 5 F 4(Ho) + 2 F 7/2(Yb) → 5 I 6(Ho) + 2 F 5/2(Yb) as well as 5 F 5(Ho) + 2 F 7/2(Yb) → 5 I 7(Ho) + 2 F 5/2(Yb)] and spontaneous radiation process. The intensity of the UC luminescence centered at 360 nm always increases with Yb 3+ concentration from 2 to 15 mol%. We believe that it may come from the cooperation of energy transfer process from Yb 3+ ions in the 2 F 5/2 state and the cross energy transfer process 5 S 2/ 5 F 4 + 5 I 6 → ( 5 F, 3 F, 5 G) 2 + 5 I 8.