Energy transfer and the 2.8-'MÜ'm emission of 'Er POT.3+'- and 'Yb POT.3+'- doped low silica content calcium aluminate glasses - Sousa, D. F.; Sampaio, J. A.; Nunes, Luiz Antônio de Oliveira; Baesso, Mauro Luciano; Bento, A. C.; Miranda, L. C. M. (Physical Review B, 2000)