New NIR emission from Sm3+ in Yb3+-Sm3+ co-doped tellurite glass

Highlights: • Spectroscopy of co-doped Yb³⁺ donor-Sm³⁺ acceptor tellurite glass was carried out. • New NIR emissions from ⁶F_11/2 and ⁶F_9/2 in Sm³⁺ in the co-doped glass were seen using an ASE experiment. • The intensity of these emissions was limited by high non-radiative decay rates. • Non-resonant phonon-assisted transfer processes contributed significantly to the efficient Yb³⁺ to Sm³⁺ energy transfer. • Migration of excitation energy between donor ions was important in describing the fluorescence decay of Yb³⁺ in the glass. Yb³⁺, Sm³⁺ and Yb³⁺-Sm³⁺ doped tellurium oxide glasses were prepared. Their spectroscopic properties in the near-infrared region were studied. In the singly doped Sm³⁺ glass, excited by 403 nm, emission transitions occurred from the usual level ⁴G_5/2 leading to strong fluorescence mainly in the visible range. In the co-doped Yb³⁺-Sm³⁺ glass Yb³⁺ donor ions were excited by 976 nm and transferred their energy by non-radiative processes to Sm³⁺ acceptor ions, promoting them to ⁶F_11/2 and ⁶F_9/2. Using an amplified spontaneous emission experiment, new emissions in Sm³⁺ from ⁶F_11/2 and ⁶F_9/2 were observed at 1080, 1240, 1435 and 1457 nm. These are the first reported emissions from these levels in Sm³⁺. The Yb³⁺ to Sm³⁺ energy transfer process was studied by measurement of the fluorescence decay curve of the Yb³⁺ donor. A diffusion model taking into account the migration of excitation amongst the Yb³⁺ donor ions was used to fit the decay curve and subsequently determine the donor-acceptor energy transfer microparameter $C_{DA}$ and the transfer parameter $\gamma$. The experimental values of these parameters were higher than those calculated by Dexter's model for resonant non-radiative transfer between donor and acceptor ions. These results demonstrate the importance of the non-resonant phonon-assisted Yb³⁺ to Sm³⁺ energy transfer processes in the tellurite glass. Energy transfer was efficient as evidenced by a large reduction in the Yb³⁺ lifetime in the Yb³⁺-Sm³⁺ glass compared to in the singly doped Yb³⁺ glass. The new emissions from Sm³⁺ were though of low fluorescent intensity. This was due to the high non-radiative decay rates of levels ⁶F_11/2 and ⁶F_9/2 by multi-phonon relaxation.