Vibronic structure in the photoluminescence spectrum of Cr³⁺ ions in garnets

The low temperature photoluminescence from Cr³⁺ ions in Y₃Al₅O₁₂ (YAG), Y₃Ga₅O₁₂ (YGG), Gd₃Sc₂Ga₃O₁₂ (GSGG ) and Gd₃Sc₂Al₃O₁₂ (GSAG) have different characteristic lineshapes. YAG and YGG emit entirely in the R-lines with associated sidebands, whereas for GSGG and GSAG the emission is a mélange of R-line processes and broad ⁴T*_2g→⁴A_2g bands. This paper compares the experimental spectra of Cr³⁺ ions in these garnets in terms of the symmetry and energies of the vibrational models coupled to the electronic states. In GSGG, we have used fluorescence line narrowing (FLN) to probe the broadening of the vibronic structure introduced by compositional disorder.