Multistage sensitization of 4F9/2→4I11/2 and 4F9/2→4I13/2 lasing transitions in Er3+ ions in a BaYb2F8 crystal

A new method was developed for determining the mechanism of anti-Stokes filling of luminescent levels of an activator in a rare-earth crystal. An investigation was made of the dependences of the absolute populations of the ⁴I_13/2, ⁴I_11/2, ⁴F_9/2, ⁴S_3/2, H_9/2, ⁴G_11/2, and ²P_3/2 levels of Er³⁺ in a BaYb₂F₈ crystal on the energy absorbed within the Yb³⁺ sensitizer band. The mechanisms of anti-Stokes filling of the ⁴I_13/2, ⁴F_9/2, ⁴S_3/2, and ²H_9/2 levels of Er³⁺ were determined under conditions of weak (1 mJ/cm³) and strong (~10 J/cm³) pumping.