Blue upconversion emission of Tm 3+–Yb 3+ in ZrO 2 nanocrystals: Role of Yb 3+ ions

Abstract

The effect of Yb 3+ ions on the blue upconversion (UPC) emission of Yb 3+–Tm 3+ co-doped ZrO 2 nanocrystals is reported. The blue (490 nm) UPC emission is due to 1G 4– 3H 6 transition of Tm 3+ and the pump power dependence of this UPC emission band are cubic indicating that three excitation photons are involved in the UPC process. An additional UPC emission band at 505 nm is observed when the Yb 3+ concentration is above 1.25 mol%. The pump power dependence of the emission at 505 nm is quadratic, indicating that this emission may have its origin from excited Yb 3+ pairs producing cooperative UPC.