Upconversion emissions from high-energy states of Eu³⁺ sensitized by Yb³⁺ and Ho³⁺ in β-NaYF₄ microcrystals under 980 nm excitation

Under 980 nm excitation, multiple ultraviolet and visible upconversion luminescence from Ho³⁺ and Eu³⁺ ions were observed in Yb³⁺/Ho³⁺/Eu³⁺ tri-doped NaYF₄ microcrystals (MCs). The high-energy states (⁵H_3-7, ⁵L₆, ⁵D₃ and ⁵D ₂) of Eu³⁺ ions could be efficiently populated by two-step energy transfer (ET) processes of Yb → Ho → Eu. Four-, three-, two-photon UC processes of Eu³⁺ ions were confirmed by the dependence of ⁵H_3-7, ⁵L₆ and ⁵D₀ levels emission intensities on the pumping power.