Energy level decay and excited state absorption processes in erbium-doped tellurite glass

The fundamental excited state decay processes relating to the ⁴I_11/2 → ⁴I_13/2 transition in singly Er³⁺-doped tellurite (TZNL) glass have been investigated in detail using time-resolved fluorescence spectroscopy. Selective laser excitation of the ⁴I_11/2 energy level at 970 nm and selective laser excitation of the ⁴I_13/2 energy level at 1485 nm has established that energy transfer upconversion by way of a dipole-dipole interaction between two excited erbium ions in the ⁴I_13/2 level populates the ⁴I_11/2 upper laser level of the 3 m transition. This upconversion has been analyzed for Er₂O₃ concentrations between 0.5 mol. and 2.2 mol. . The ⁴I_13/2 and ⁴I_11/2 energy levels emit luminescence with peaks located at 1532 nm and 2734 nm, respectively, with radiative decay efficiencies of 65 and 6.8 for the higher (2.2 mol. ) concentration sample. The low 2.7 m emission efficiency is due to the non-radiative decay bridging the ⁴I_11/2 → ⁴I_13/2 transition and energy transfer to the OH^- groups in the glass. Excited state absorption was observed to occur from the ⁴I_13/2 and ⁴I_11/2 levels with peak absorptions occurring at 1550 nm and 971 nm, respectively. The decay time of the ⁴I_11/2 excited state decreased with an increase in the Er³⁺ concentration, which related to energy transfer to OH^- ions that had a measured concentration of 6.6 10¹⁸ cm^-3. Results from numerical simulations showed that a population inversion is reached at a threshold pumping intensity of ∼80 kW cm^-2 for a cw laser pump at 976 nm if [Er³⁺] ≥ 1.2 × 10²¹ cm^-3 (or [Er ₂O₃] ≥ 2.65 mol. ) without OH^- impurities being present.