Contents lists available at ScienceDirect Optik journal homepage: www.elsevier.com/locate/ijleo Original research article Upconversion and 1.53 μm near-infrared luminescence study of the Er 3+ -Yb 3+ co-doped novel phosphate glasses R.J. Amjad a , Abdul Sattar b , M. Reza Dousti b, ⁎ a Department of Physics, COMSATS University Islamabad, Lahore 54000, Pakistan b Unidade Acadêmica do Cabo de Santo Agostinho, Universidade Federal Rural de Pernambuco, Rua Cento e Sessenta e Três, 300, Garapu, Cabo de Santo Agostinho, PE, Brazil ARTICLE INFO Keywords: Glasses Co-doping Lifetime Quantum efficiency Luminescence Upconversion ABSTRACT Er 3+ -doped glasses are important materials to develop upconvertors, sensors, optical fibers and green solid state lasers. The upconversion and near-infrared transitions in Er 3+ -Yb 3+ co-doped phosphate glasses are studied in this work and an improvement of 45% in the quantum efficiency of the glasses is observed by the addition of 1 mol% of Yb 2 O 3 . Judd-Ofelt analysis, lifetime of some excited states of Er 3+ ions and energy transfer mechanisms are discussed. 1. Introduction The upconversion (UC) emissions of the rare earth (RE) doped materials excited at near-infrared (NIR) region is an important phenomenon which could be used to develop devices for biolabels, sensors, displays etc [1,2]. The energy transfer upconversion (ETU) in the Er 3+ -doped glasses [3,4] and Er 3+ -Yb 3+ co-doped glasses [1,5,6], are studied extensively. NIR to visible UC lumi- nescence in Er 3+ -Yb 3+ co-doped glasses is also well-known. Such system can be excited at around 975–980 nm, where the oscillator strength of the Yb 3+ ions is too large and is able to absorb the excitation light efficiently. The advantages of adding Yb 3+ ions to Er 3+ doped materials is 1) higher efficiency which facilitate the optical refrigeration, 2) increased efficiency by bombardment at the 980 nm resonance, and 3) reduction of the deleterious effects such as UC. The energy transfer from Yb 3+ to Er 3+ ions can excite the activator from which the UC emissions at around 525, 550 and 650 nm could originate. A schematic partial energy level diagram of the Er 3+ ions, as activator, and a sensitizing Yb 3+ ions in its vicinity is shown in Fig. 1. The mechanisms behind the UC phenomenon are also depicted in this figure. Excited state absorption (ESA1, ESA2), and the (ETU) possess the main contributions in an Er 3+ -Yb 3+ co-doped system. It is worthy to mention that other mechanism such as energy migration, cooperative UC and cross-relaxations may increase the interactions between ions, however, they are not shown here to simplify the diagram. Er 3+ -doped glasses benefits also from a fundamental emission at NIR region, which has applications in optical fibers. The variation of intensity, lifetime and band- width of this transition are mainly discussed in literature by changing the environment of the ion sites (choice of glass composition), rare earth (RE) concentration, co-doping with secondary RE ion, incorporation of noble metallic nanoparticle etc [4,7–9]. In this work, we aim to study the UC luminescence and NIR emissions of an Er 3+ -Yb 3+ co-doped tungsten-sodium-phosphate glass, in order to study the effect of Yb 2 O 3 concentration on electronic transitions of Er 3+ ions, targeting the improvement in quantum efficiency of the system. The glass composition is chosen based on the recent studies on the structural and optical properties of some binary WO 3 -NaPO 3 and ternary WO 3 -NaPO 3 -PbF 2 glass samples [10–12]. These glasses show relatively high refractive index, https://doi.org/10.1016/j.ijleo.2019.163426 Received 17 April 2019; Received in revised form 4 September 2019; Accepted 13 September 2019 ⁎ Corresponding author. E-mail address: mohammad.rezadousti@ufrpe.br (M.R. Dousti). Optik - International Journal for Light and Electron Optics 200 (2020) 163426 0030-4026/ © 2019 Published by Elsevier GmbH. T