Spectroscopy and enhanced frequency upconversion in Nd³⁺–Yb³⁺ codoped TPO glasses: energy transfer and NIR to visible upconverter

TeO₂–Pb₃O₄ (TPO) glasses codoped with Nd³⁺ and Yb³⁺ ions have been fabricated by conventional melting technique. The absorption, emission and excitation spectra of the samples have been recorded. The optical band gap in both the doped/codoped glasses is found to be ∼3.31 eV. Judd–Ofelt analysis has been carried out by using the absorption spectrum of 0.8 mol% Nd³⁺ doped glass to determine the radiative properties viz radiative transition probabilities, branching ratios, radiative lifetimes, quality factor and emission cross sections of some emitting levels for Nd³⁺ ions. The radiative transition probability for the ⁴G_7/2 → ⁴I_9/2 transition (∼1926 Hz) is found to be maximum compared to other ⁴G_5/2 → ⁴I_9/2 (∼1622 Hz) and ⁴F_5/2 → ⁴I_9/2 (∼865 Hz) transitions. Upconversion (UC) luminescence of the samples has been examined by the 980 nm CW diode laser excitation. Effect of addition of Yb³⁺ ions in the Nd³⁺ doped glasses on UC emission intensity has been discussed. The UC emission intensity corresponding to the green, red and NIR bands in the codoped glass has been enhanced by ∼17, ∼12 and ∼42 times as compared to that of the Nd³⁺ singly doped glass. The quantum efficiency for the ⁴G_7/2 level is found to be ∼32%. The nephelauxetic ratio, bonding parameter and covalency of Nd³⁺ ions have been found positive which represents the covalent bonding between Nd³⁺ ion and oxygen atom. The colour tunability from yellowish-green to dominant green region has been obtained in the optimized codoped TPO glass. (paper)