Absorption and emission spectra of thulium and erbium in borate and phosphate glasses

Absorption and emission spectra of Tm³⁺ and Er³⁺ were measured in phosphate and borate glasses. The oscillator strengths of the transitions were calculated from the experimental data and compared with those of aqueous solutions. It was found that the oscillator strengths in the glasses are higher than in the solutions. The measured bands were attributed to forced electric transitions which occur when the ion is situated in glass at C₂ symmetry. A model of the average site is proposed. From the spectral shift of the hypersensitive transitions conclusions were drawn as to the influence of the crystal field in glass. From the width of the inhomogeneous broadening it was concluded that the number of rare earth sites is higher in phosphate than in borate glass. The relation between the nonradiative transitions and the phonon energy of the glasses is discussed. The high luminescence efficiency arising from transitions to levels which are higher than the ground state of thulium indicate a possibility of population inversion.