Millisecond lasing was observed as a result of a self-terminating 4I11/2–4I13/2 transition in erbium ions present in Lu3Al5O12:Er crystals. It was found that such lasing was due to efficient transfer of the erbium ions from a lower active level.
A study was made of various cases of a population inversion between the 4I11/2 and 4I13/2 levels in Y3Al5O12:Er3+ crystals. It is found that when nonradiative transitions from the upper levels to the active levels by the mechanism of multiphonon energy transfer are more probable, laser action occurs due to transitions between the Stark components of the 4I11/2 and 4I13/2 levels with the highest oscillator strengths. If nonradiative transitions due to ion-ion interaction are more probable, a red shift is observed in the lasing spectra.