Orange and yellow emissions through Sm3+ and Tb3+/Sm3+ doped potassium-zinc phosphate glasses for WLED applications

A spectroscopic analysis of potassium-zinc phosphate glasses activated with Sm3+ and Tb3+/Sm3+ is presented. Both glasses showed high transparency in the visible region with direct and indirect band gap energy values of 3.70 and 3.96 eV, and 3.85 and 4.22 eV for the Sm3+ doped and Tb3+/Sm3+ co-doped glasses, respectively. The Urbach energies estimated for the single doped and codoped glasses resulted to be 0.26 and 0.24 eV, respectively. Upon 408 nm excitation, the Sm3+ singly doped glass exhibited a 592 nm orange emission with correlated color temperature (CCT) and color purity (CP) of 1677 K and 99%, respectively. The Judd-Ofelt (JO) theory was applied to value the radiative parameters of the Sm3+ doped glass. It was found JO parameter values of Ω2 = 1.93 × 10−20 cm2, Ω4 = 3.72 × 10−20 cm2 and Ω6 = 2.85 × 10−20 cm2, which lead to peak stimulated emission cross-section (σpeak), gain bandwidth (σpeak × Δλeff) and optical gain (σpeak × τR) values of 4.49 × 10−22 cm2, 9.70 × 10−28 cm3 and 14.14 × 10−25 cm2s for the main samarium 4G5/2 → 6H7/2 emission. Such values resulted to be comparable or higher than those reported for sodium-tellurite and borate-silicate-based glasses. The emission in the Tb3+/Sm3+ co-doped glass under Tb3+ and Sm3+ co-excitation displayed yellow (λex = 304, 318 and 377 nm) and orange (λex = 344 and 360 nm) emission tonalities with CP values up to 88 and 97%, respectively. The overlapping of Tb3+ emission bands with those of Sm3+ excitation suggested the existence of energy transfer from terbium to samarium, which according to the Tb3+ emission decay behavior is non-radiative. Such a process arises from the Tb3+: 5D4 energy level with an efficiency of 0.41 within Tb3+-Sm3+ clusters.