Mechanism of a population inversion between the ⁴I_11/2 and ⁴I_13/2 levels of the Er³⁺ ion in Y₃AI₅O₁₂ crystals

Studies of the spectra of stimulated emission from Y₃AI₅O₁₂:Er³⁺ crystals (⁴I_11/2–⁴I_13/2) containing 20 wt.% activator ions, showed that, as the temperature decreased from 300 to 77°K, a change was observed in the emission wavelength (at T = 300°K it was found that λ₁ = 2.9364 μλ₂ = 2.8310μ and at T = 77°K it was found that λ₁ = 2.6930 μ, λ₂ = 2.8970 μ). This could not be attributed to a thermal redistribution of the populations of the Stark components of the ⁴I_11/2 and ⁴I_13/2 upper and lower active levels. An analysis of the results showed that the temperature shift of the stimulated emission spectrum was due to changes in the processes of relaxation of excitation from the upper levels of the Er³⁺ ion which caused substantial changes in the populations of the ⁴I_11/2 and ⁴I_13/2 multiplets.