Effect of excitation condition and Mn2+ doping on the red-to-green emission ratio in NaYF4:Er3+/Yb3+ phosphors

Rare earth ions doped up-conversion luminescent materials have attracted massive attention owing to their unique nonlinear optical properties of converting multiple low-energy photons into high-energy photons and potential application in many fields, especially in information storage and anti-counterfeiting. In this paper, the red-to-green emission ratio of NaYF₄:Er³⁺/Yb³⁺ phosphors was adjusted through power density, pulse width and frequency. The results displayed that the red-to-green emission ratio enhanced with power density, pulse width and frequency increasing. The increase was not obvious with the increase of power density because of the combined effect of two-photon up-conversion processes and the increase rate of nonradiative relaxation process ²H_11/2/⁴S_3/2 → ⁴F_9/2 (Er³⁺). The ratio changes with pulse width and frequency were originated to different non-steady-state up-conversion processes of red and green emissions. The ratio can also be changed from 0.75 to 0.98 by inducing Mn²⁺ ions in NaYF₄:Er³⁺/Yb³⁺ phosphors, which is cause by the energy transfers among the energy levels ²H_11/2/⁴S_3/2 (Er³⁺), ⁴T₁ (Mn²⁺), and ⁴F_9/2 (Er³⁺). All methods of tuning the ratio of red-to-green emissions can be used in fields of information storage and optical encryption.