Power dependence of upconversion luminescence of Er³⁺ doped Yttria nanocrystals and their bulk counterpart

We demonstrate upconversion spectra of Er³⁺ doped Y ₂O₃ nanocrystals under different heat treating processes. After re-calcination, the intensity is enhanced by a factor of 18-56 at different Er³⁺ doping concentrations. This enhancement is attributed to the suppression of energy transfer from Er³⁺ to OH^-. The ln-ln pump power dependences of UC-PL intensity are separated by two regions with different slopes when Er³⁺ concentration exceeds to 0.5 mol%. In addition, the slopes are substantially unchanged by re-calcination. These phenomena are discussed by the rate equations model which mainly considers the effects of cross relaxation processes of Er³⁺, nonradiative decay processes between the neighbor energy levels, and variation of the incident pump power.