Optical Measurement of Narrow Band Rare-Earth $4\mathit{f}$ Levels with Energies Greater than the Band Gap of the Host

Excited-state absorption spectra of erbium and thulium doped gallium lanthanum sulfide glasses reveal transitions whose final $4f$ states do not appear in the ground state absorption spectra due to resonance with the fundamental absorption of the glass host. The results give direct evidence for the model of isolated rare-earth ions with highly shielded $4f$ levels showing no noticeable interaction with the host material despite resonance. Only the energy splitting of the $4f$ levels is relevant for energy transfer processes between rare-earth ions and the host material, whereas the absolute energetic position is unimportant.