“…10 shows the schematic energy level diagram of the observed upconversion emission bands with 586 nm excitation wavelength. This process was already observed in other glassy systems and vitreous ceramics, which was attributed to an energy exchange process involving a pair of Pr 3+ ions [24]. Accordingly, two ions initially excited to level 1 D 2 exchange energy in such a way that one ion is promoted to the [ 3 P J (J = 0, 1, 2) , 1 I 6 ] manifold, while the other nonradiatively decays to lower energy levels.…”