Multiphonon relaxation and infrared-to-visible conversion of Er3+ and Yb3+ ions in barium-thorium fluoride glass

Abstract

The upconversion of infrared radiation into green and red fluorescence has been studied for Er³⁺ and Yb³⁺ ions in BaF₂/ThF₄ fluoride glass over a wide temperature range and several dopant concentrations. At room temperature an upconversion efficiency of 3.3×10⁻⁵ has been obtained for the green emission from the glass with 1 mol % ErF₃ and 19 mol % YbF₃ pumped by 973‐nm radiation with intensity of 16.5 mW/cm². For an absorbed intensity of 10 W/cm² the efficiency for green upconversion emission is predicted to be 2%. Radiative transition rates for the excited states of Er³⁺ were calculated using Judd–Ofelt theory and intensity parameters obtained from measured integrated absorption coefficients. The calculated values of the multiphonon relaxation and the upconversion fluorescence intensity, obtained from the rate equation model of Wright, are discussed in light of the experimental results.