Up-conversion emission color tuning in NaLa(MoO₄)₂: Nd³⁺/Yb³⁺/Ho³⁺ excited at 808 nm

As alternatives to Yb³⁺-sensitized up-conversion (UC) materials excited at 980 nm, Nd³⁺-sensitized UC phosphors irradiated by 808 nm have been used to decrease the absorption of water and alleviate the overheating effect in vivo biological application. Intense red and green UC emissions from ⁵F₅→⁵I₈ and ⁵F₄/⁵S₂→⁵I₈ transitions of Ho³⁺ appeared in Nd³⁺/Yb³⁺/Ho³⁺ tri-doped NaLa(MoO₄)₂ through successive energy transfer Nd³⁺→Yb³⁺→Ho³⁺ under 808 nm excitation, in which Yb³⁺ ions were proven to be the energy transfer bridge between Nd³⁺ and Ho³⁺ by lifetime measurement. The variable emission color and intensity ratios of red to green emissions were realized by adjusting the doping concentration of Yb³⁺, pulse width of the excitation laser and the addition of Ce³⁺ ion, which depends on the different population pathways to the green and red emitting states of Ho³⁺. The chromaticity modulation mechanisms of these approaches were proposed, which provides a feasible strategy to tune the UC emission color.