Highly efficient cooperative up-conversion of Yb3+ in NaYF4

Strong blue cooperative up-conversion emission around 475 nm has been observed in Yb3+-doped hexagonal NaYF4. The influence of concentration of the Yb3+ ion on the luminescence intensity is investigated. It is found that the sample shows the strongest cooperative luminescence when the Yb3+ ion concentration is 75%. The investigation shows that hexagonal NaYbF4 is an efficient blue up-converted phosphor, which might be potentially applicable in three-dimensional solid-state fluorescence display.