Up-conversion luminescence in Er/Yb-doped YF₃ thin films deposited by PLI-MOCVD

We have studied the NIR (near infrared) to visible up-conversion of Er/Yb co-doped YF₃ thin films prepared by PLI-MOCVD (pulsed liquid injection metal-organic chemical vapor deposition). Violet, strong green, as well as red emission bands corresponding to intra-4f transitions in Er³⁺ were observed under 972 nm excitation. The up-converted emission was studied as a function of the Yb concentration and in doing so, a change in the red/green emission intensity ratio was observed. We attributed this, on one hand, to an Er-to-Yb back transfer corresponding to ⁴S_3/2 → ⁴I_13/2:²F_7/2 → ²F_5/2, which starts to occur at increasing Yb concentrations. As a consequence, the red emission is shown to be enhanced through an upward energy transfer ⁴I_13/2 → ⁴F_9/2. On the other hand the depopulation of ⁴S_3/2 is also related to an upward transfer from Yb to Er corresponding to transition ²F_5/2 → ²F_7/2: ⁴S_3/2 → ⁴G_9/2 which is enhanced by an increasing Yb concentration. We performed power-dependent up-conversion luminescence measurements to confirm the proposed excitation mechanisms. Finally, lifetimes of the intermediate states ⁴S_3/2 and ⁴F_9/2 were determined, to show the influence of the Yb concentration on the relaxation processes in Er³⁺.