Solid-solution Er:(Sc,Y)2O3 transparent ceramics: Optical spectroscopy, inhomogeneous line broadening, C3i sites and mid-infrared laser operation

Rare-earth-doped transparent sesquioxide laser ceramics enable engineering of their gain bandwidths via solid-solution compositions exhibiting a strong inhomogeneous spectral line broadening. We report on a detailed spectroscopic study and the first mid-infrared laser operation of Erbium-doped yttria-scandia, (Sc_xY_1−x)₂O₃, ceramics fabricated by vacuum sintering at 1750 °C from laser-ablated nanoparticles. The effect of the Sc fraction on the spectral and kinetic properties of Er³⁺ emission around 2.8 μm owing to the ⁴I_11/2 → ⁴I_13/2 is revealed. The inhomogeneous spectral line broadening leading to merging of individual Stark sub-levels of Er³⁺ multiplets is evidenced by low-temperature spectroscopy. An original method of quantifying the distribution of dopant Er³⁺ ions over C₂ and C_3i symmetry sites in the cubic bixbyite structure is suggested based on the transition probabilities derived by the Judd-Ofelt theory. In the parent Er:Y₂O₃ ceramic, the Er³⁺ ions nearly follow the ideal distribution with 3/4 of ions residing in C₂ sites, and Sc addition skews it in favor of C_3i sites. A continuous-wave Er:(Sc,Y)₂O₃ ceramic laser generated 312 mW at 2716 nm with a slope efficiency of 18.6 % and a laser threshold of 128 mW.