Optical properties and upconverted emissions of Tm 3+ in Yb 3+ doped fluorophosphate glasses

Abstract

Optical properties of Tm 3+ ions in Yb 3+ doped fluorophosphate glasses containing 27mole% PbF 2 have been studied. The absorption, luminescence and excitation spectra, and the decay patterns were obtained as function of temperature, TmF 3 and YbF 3 concentrations. For the blue emission centered at 479 nm ( 1G 4→ 3H 6 transition) the critical concentration ofTm 3+ is 0.2mole% when the concentration of Yb 3+ is kept constant at 20 mole%. The oscillator strengths of the transitions between J manifolds are calculated by using Judd-Ofelt theory at room temperature and were compared with the ones obtained experimentally. Radiative lifetimes of the excited states are determined and used to obtain nonradiative transition rates and the quantum yields. Unconverted emissions from the 1D 2 , 1G 4 and 3F 4 levels were observed when the Yb 3+ ions were excited with 966 nm light. An absolute upconversion efficiency for the blue (479 nm) and red (791 nm) emissions was obtained using a comparative method at room temperature. The efficiency of the 479 and 791 nm emissions was measured to be 5 × 10 −8 and 7 × 10 −4, respectively, for the sample containing 0.2 mole% TmF 3 and 20 mole% YbF 3 upon 966 nm excitation with an absorbed intensity of 20 mW cm −2. The emission intensity and the decay time of the 1G 4 level were found to be independent of temperature although the emission intensity and the decay time of 3F 4 level show small temperature dependence when the desired level was directly excited. However, the upconverted emission intensity of these levels depends on temperature. This behaviour may be due to a phonon-assisted energy transfer mechanism from Yb 3+ to Tm 3+.