Upconversion Luminescence Dynamics in Er³⁺Yb³⁺ Codoped Nanocrystalline Yttria

The upconversion luminescence and dynamics in Er³⁺/Yb³⁺ codoped nanocrystalline yttria (7--65nm) are studied under 980-nm pulsed laser excitation. It is found that the red emission of ⁴F_9/2--⁴I_15/2 and the green emission of ²H_11/2/⁴S_3/2 in nanoparticles with lower concentration of Yb³⁺ result from a two-photon excitation. In nanocrystals with higher Yb³⁺ concentration, the red emissions from a two-photon excitation, while the green emissions from a three-photon excitation. The luminescence dynamics indicates that as the particle size decreases, both the rise and the decay time constants become shorter. As the size decreases to several nanometres, the rise process nearly disappears, suggesting that the upconversion luminescence originates mainly from self-excitation of Er³⁺, instead of the energy transfer of Yb³⁺ to Er³⁺.