MECHANISM OF A POPULATION INVERSION BETWEEN THE 4I11/2 AND 4I13/2 LEVELS OF THE Er3 + ION IN Y3Al5O12 CRYSTALS.

Studies of the spectra of stimulated emission from Y//3Al//5O//1//2:Er**3** plus crystals (**4I//1//1/////2 minus **4I//1//3/////2), containing 20 wt. % activator ions, revealed that as the temperature decreased from 300 to 77 degree K, a change was observed in the emission wavelength. This could not be attributed to a thermal redistribution of the populations of the Stark components of the **4I//1//1/////2 and **4I//1//3/////2 upper and lower active levels. An analysis of the results indicated that the temperature shift of the stimulated emission spectrum was due to changes in the processes of relaxation of excitation from the upper levels fo te Er**3** plus ion which caused substantial changes in the populaitons of the **4 I//1//1/////2 and **4I//1//3/////2 multiplets.