Energy levels of the laser active Er³⁺ ion in each of the two crystallographic sites of yttrium orthosilicate

There are two non-equivalent sites of yttrium in the lattice of Y₂SiO₅. Both of them may be occupied substitutionally by Er³⁺, resulting in a laser active material. We measured absorption spectra of Y₂SiO₅:Er³⁺ at several low temperatures and we studied its emission spectrum at 4.2 K following selective excitation of individual Stark components of the ⁴I_13/2 to ⁴ I_15/2 transition by the light of a tunable colour centre laser. The results of these two experiments allow classification of the absorption and emission lines into two independent systems, thus yielding two energy level schemes 1 and 2. Moreover, simple crystal field calculations strongly suggest from which Y₂SiO₅ crystallographic site spectrum 1 arises, and from which one spectrum 2.