Mechanisms of upconversion fluorescences in Er 3+, Tm 3+ codoped fluorozircoaluminate glasses

Abstract

The upconversion of infrared radiation into green and red fluorescences has been investigated for Er 3+ and Tm 3+ codoped fluorozircoaluminate glasses under 790 nm excitation. Introduction of Tm 3+ into Er 3+ doped system preferentially quenches the green upconversion fluorescence from the 4 S 3 2 level of Er 3+ due to the efficient cross-relaxation of 4 I 13 2 → 4 I 15 2 (Er): 3H 6 → 3H 4(Tm) which can significantly reduce the upconversion efficiency from the 4 I 13 2 level to the emitting 4 S 3 2 level. However, the Tm 3+ behaves as a good sensitizer of the red upconversion fluorescence from the 4 F 9 2 level of Er 3+ which is mainly populated by the cross-relaxation of 3H 4 → 3H 6 (Tm): 4 I 11 2 → 4 F 9 2 (Er). The excitation energy stored in the 4 I 11 2 level of Er 3+ migrates through the glass with a diffusion-limited regime. Through this regime, only Er 3+ ions which have diffused their energy into the vicinity of Tm 3+ ions can transfer the energy to the 3H 5 level of Tm 3+. At lower Er 3+ concentrations (≤ 3 mol%), this diffusion-limited energy migration is not so efficient as to enable energy transfer upconversion from the 4 I 11 2 level to the 4 F 9 2 level of Er 3+.