Cold white light emission in tellurite-zinc glasses doped with Er³⁺–Yb³⁺–Tm³⁺ under 980 nm

Tellurite-zinc glasses doped with Er³⁺, Yb³⁺ and Tm³⁺ were fabricated by conventional melt-quenching technique and characterised by absorption spectroscopy, refractive index, lifetime measurements, excitation, luminescence 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 between 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 (Ω₂, Ω₄ and Ω₆) were calculated for all samples and present the trend Ω₂>Ω₄>Ω₆. The spectroscopic parameters were computed with the Judd-Ofelt theory as well as the energy transfer micro-parameters (critical radius of interaction and energy transfer coefficient). Such results shall be used to give a comprehensive explanation for the energy transfer process between these rare earth ions.