Mid-infrared emissions from Er³⁺ in Ga₂S₃–GeS₂–Sb₂S₃ glasses

Emission properties of Er³⁺ in Ga₂S₃–GeS₂–Sb₂S₃ glasses at the mid-infrared region were investigated from the viewpoint of their dependence on the concentration of the active ion and the glass composition. In the Judd–Ofelt analysis, no variation in omega parameters were observed when GeS₂ was replaced by Ga₂S₃, while Ω₂ increased as Sb₂S₃ was replaced by Ga₂S₃. This is due to the structural similarity and difference between the glass network units, GaS₄ and GeS₄ tetrahedra, and SbS₃ pyramid. Clear mid-infrared emissions were observed at 2750 and 4300 nm assigned to the ⁴I_11/2 → ⁴I_13/2 and ⁴I_9/2 → ⁴I_11/2 transitions, respectively. The lifetime of the initial level of the 4.3 μm emission, ⁴I_9/2, rapidly decreased with the Er³⁺ concentration because of the cross relaxation of this level, which can take place even at considerably low Er³⁺ concentration. The cross-relaxation processes were suppressed by the increase in the content of Ga₂S₃ because the solubility of Er³⁺ ions in the glasses increases with the Ga₂S₃ content.