## 3 Results and Discussion  
The GTS glass is transparent, and the Eu\({}^{3+}\) and Tb\({}^{3+}\) dopants do not cause sample coloration (Figure 1a top). Under UV light (Figure 1a bottom), the single-doped samples doped with Eu\({}^{3+}\) and Tb\({}^{3+}\) exhibit orange, red-orange and blue-green emission, respectively. The co-doped sample exhibits purple-pink emission. All the perceived colors are a result of the mixing of original Eu\({}^{3+}\) and Tb\({}^{3+}\) emissions with the scattered blue light introduced by the excitation source. The amorphous structure of the samples was confirmed using XRD (Figure 1b). No Eu-, Tb-, and Eu, Tb co-doped samples exhibit sharp diffraction peaks, indicating no crystalline phase in the volume of the sample. The Raman, IR and absorption spectra were aimed to investigate whether the doping with Eu\({}^{3+}\), Tb\({}^{3+}\) or both has an impact on the structural properties of the studied GTS glasses. The IR and Raman spectra measured for Eu-doped, Tb-doped and Eu/Tb-co-doped samples (Figure 1c) reveal no significant differences between each other and are very similar to the ones reported for Nd\({}^{3+}\)-doped germanate-tellurite glasses [21]. The bands on the Raman spectrum can be attributed to the stretching vibrations of Ge-O-Ge bridges in GeO\({}_{4}\) tetrahedra (788 cm\({}^{-1}\)), the stretching vibrations of TeO\({}_{3}\) trigonal pyramid and TeO\({}_{4}\)/TeO\({}_{3+1}\) trigonal bipyramids (735 cm\({}^{-1}\)), and the bending vibrations of Ge-O-Ge and Te-O-Te bridges (446 cm\({}^{-1}\)) [8; 29; 30]. The IR spectra (Figure 1c) comprise mainly the broad band with two components located at 722 and 822 cm\({}^{-1}\), which can be assigned to o the Te-O-Te vibrations and Ge-O-Ge vibrations, respectively. The other broad band at 525 cm\({}^{-1}\) is characteristic of GeO\({}_{6}\) octahedra [31]. The absorption spectra (Figure 1d) revealed that besides the appearance of weak, narrow lines originating from the _4f-4f_ transition of lanthanides, the dopants do not alter the absorption edge of the GTS glass. The excitation and emission spectra of Eu\({}^{3+}\)-doped glass samples are characteristic of Eu\({}^{3+}\) in amorphous material [32], with the maximum excitation at 394 nm and the maximum Eu\({}^{3+}\) emission at 612 nm (Figure 2a). The wide band at the excitation spectrum can be a result of the host-Eu\({}^{3+}\) energy transfer. The large bandgap and low phonon energies  
Figure 1: (**a**) Photographs of the single- and co-doped glass samples in ambient light and under UV excitation; (**b**) the XRD results; (**c**) IR and Raman spectra; (**d**) absorption spectra of the Eu\({}^{3+}\)-, Tb\({}^{3+}\)- and co-doped glasses. allow for observing the emission from the higher-lying \({}^{5}\)D\({}_{1,2,3}\) levels [33] (Figure 2a inset, Figure S1). The integral intensity was the highest for the sample doped with 2% Eu\({}_{2}\)O\({}_{3}\) (Figure 2b). Above that concentration, the quenching mechanism caused the intensity to decrease [34]. The Tb\({}^{3+}\)-doped samples exhibit efficient excitation bands in the 340-380 nm region, and predominantly green emission is a result of four narrow emission lines (Figure 3a). The maxima of excitation and emission are located at 378 nm and 542.2 nm, respectively. As in the case of Eu\({}^{3+}\), it is possible to observe the weak emission from the higher-lying \({}^{5}\)D\({}_{3}\) level [35] (Figure 3a inset). The sample doped with 4% Tb\({}_{4}\)O\({}_{7}\) exhibits the strongest luminescence (Figure 3b); however, to ensure the stability of the glass, for the co-doped samples, the concentration of 1% was chosen for both Eu\({}^{3+}\) and Tb\({}^{3+}\). The co-doped sample exhibits both Eu\({}^{3+}\) and Tb\({}^{3+}\) emission when excited at 375 nm (Figure 4a). Despite the excitation wavelength being a better fit for Tb\({}^{3+}\), the Eu\({}^{3+}\) emission is more intense than that of Tb\({}^{3+}\) due to the aforementioned higher concentration quenching rate for Tb\({}^{3+}\) ions.