Journal of Luminescence 228 (2020) 117538 Available online 21 August 2020 0022-2313/© 2020 Elsevier B.V. All rights reserved. Cold white light emission in tellurite-zinc glasses doped with Er 3+ –Yb 3+ –Tm 3+ under 980 nm G. Lozano a, * , O.B. Silva b , W. Faria a , A.S.S. de Camargo a , R.R. Gonçalves c , D. Manzani d , R. Bruna e , V.A.G. Rivera f , E. Marega Jr. a a Instituto de Física de S˜ ao Carlos, Universidade de S˜ ao Paulo, Caixa Postal 369, 13560–970, S˜ ao Carlos, SP, Brazil b Universidade Estadual do Piauí, 64260–000, Piripiri, PI, Brazil c Laborat´ orio de Materiais Luminescentes Micro e Nanoestruturados (Mater Lumen), Departamento de Química, Faculdade de Filosofa, Ciˆ encias e Letras de Ribeir˜ ao Preto, Universidade de S˜ ao Paulo, Av. Bandeirantes, 3900, 14040–901, Ribeir˜ ao Preto, SP, Brazil d Instituto de Química de S˜ ao Carlos, Universidade de S˜ ao Paulo, S˜ ao Carlos, SP, Brazil e Escuela Profesional de Física, Universidad Nacional Federico Villarreal, Lima, Peru f Facultad de Ciencias Físicas, Universidad Nacional Mayor de San Marcos, Lima, Peru A R T I C L E INFO Keywords: White light Tellurite Rare-earths Up-conversion Judd-Ofelt Energy transfer ABSTRACT Tellurite-zinc glasses doped with Er 3+ , Yb 3+ and Tm 3+ were fabricated by conventional melt-quenching tech- nique and characterised by absorption spectroscopy, refractive index, lifetime measurements, excitation, lumi- nescence and up-conversion spectroscopy. Tunable blue to cold white light emission based on the colour mixing of red, green and blue light was obtained via up-conversion under 980 nm and by adjusting the laser excitation intensity. The balancing of the relative intensity of each colour is provided by the energy transfer process be- tween the rare-earth ions. Moreover, luminescence spectroscopy was performed with a 405 nm laser to further understand the energy transfer mechanism that produce the white light. The Judd-Ofelt intensity parameters (Ω 2 , Ω 4 and Ω 6 ) were calculated for all samples and present the trend Ω 2 >Ω 4 >Ω 6 . The spectroscopic parameters were computed with the Judd-Ofelt theory as well as the energy transfer micro-parameters (critical radius of inter- action and energy transfer coeffcient). Such results shall be used to give a comprehensive explanation for the energy transfer process between these rare earth ions. 1. Introduction In recent years, rare-earth ions (REI) doped glasses have been heavily studied because of their luminescence properties, such as the near- infrared (NIR) to visible light emission via up-conversion (UC), which occurs when two or more low-energy photons generate the emission of photon energies higher than the initial light due to its non-linear dependence on incident light intensity [1]. Particularly, oxide glasses (e.g., TeO 2 , SiO 2 and B 2 O 3 ) are optically transparent from UV to NIR spectral range and their capability to incorporate high concentrations of REI [2] makes them candidates for a wide variety of applications such as the development of luminescent material, temperature sensors, non-linear optics, telecommunications and lasers [3–6]. The choice of a host material for REI is critical when we are dealing with visible emis- sion through UC, because the relative intensities of the emission bands depend on the vitreous environment, which is related to the static crystal feld potential that a hosted REI experiments, losses, transmittance and the doping concentration. For example, UC process in silicate or borate glasses exhibits high phonon-energy, which suppresses the transitions with small energy level difference [7]. In contrast, tellurite glasses present low phonon-energy (~780 cm 1 ), high refractive index (1.9–2.3) [8], low melting temperature (between 600 and 800 ◦ C) [9] and excellent transmission in the visible and NIR wavelength regions [10]. The addition of ZnO into the tellurite glass increases its refractive index, the density and the absorption edge shift to high energies [11,12]. Therefore, tellurite-zinc glass system is an excellent candidate for gen- eration of visible light through UC. For instance, there is evidence of NIR-to-visible light with Er 3+ doped tellurite glasses [13], where green and blue light is observed. Furthermore, glasses doped with Yb 3+ Er 3+ are an effcient system for UC due to the energy transfer (ET) process under 980 nm diode laser as an excitation source [14], where Yb 3+ acts as a donor to the Er 3+ . On the other hand, Yb 3+ Tm 3+ generates red and * Corresponding author. E-mail address: glozano@ifsc.usp.br (G. Lozano). Contents lists available at ScienceDirect Journal of Luminescence journal homepage: http://www.elsevier.com/locate/jlumin https://doi.org/10.1016/j.jlumin.2020.117538 Received 20 May 2020; Received in revised form 27 June 2020; Accepted 18 July 2020