Structural and electron-phonon interaction effects in optical spectra of Pr3+ and Sm3+ in YAG

Comparative high resolution spectroscopic investigation between 10 and 300 K of the less investigated Pr3+ and Sm3+ ions in the YAG crystals and transparent ceramics evidenced that the satellite structures around the main optical lines corresponding to the ions in the dodecahedral c-sites belong either to ions whose crystal field is perturbed by adjacent lattice irregularities or to vibronics. In case of Pr:YAG such irregularities are mainly caused by presence of doping ions in near sites (n.n. and n.n.n. pairs) in both types of materials, while in the melt-grown crystal an additional irregularity is the presence of excess Y3+ ions that occupy part of the octahedral Al3+ sites. The new data on perturbed Pr3+ pair centers remove the ambiguity in identification of several lines that were previously attributed to transitions of the isolated Pr3+ ions forbidden in the D2 symmetry of YAG ceramics and rule out the forced assumption of lowering of symmetry. In case of Sm3+:YAG ceramic the main perturbed center corresponds to n.n. pairs and plays major role in utilization of this material as ASE suppressor for the Nd:YAG laser. The presence of electron-phonon interaction was evidenced by shift or broadening of lines or by resolved weak vibronic structures. For selected optical transitions, such as the 10 K Sm3+4G5/2 → 6H5/2,7/2,9/2 emission in YAG the presence of enhanced electron-phonon effects was evidenced by strong alteration of the shape of electronic lines, such as apparition of asymmetries or even doubling of electronic lines whose Stark gap to other transitions is in quasi-resonance with Raman phonons of YAG. zero phonon lines at energies close to Raman phonons of YAG of T2g symmetry. The shape of the lines is analyzed in terms of the theory of the quasi-resonant electron-phonon coupling between a pure electronic state and vibronics.