## 3 Results and Discussion
### Spectroscopic Properties of SGB Glass and Glass Fibers  
In order to determine the effect of silver ion concentration on the spectroscopic properties of fabricated glasses, the shape of excitation and emission bands were analyzed. Figure 2a shows the excitation spectra of europium ions in the SGB glasses as a function of Ag\({}^{+}\) ions content. The spectra monitored at the wavelength of 616 nm (\({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)F\({}_{2}\) transition) have shown a range from 350 nm to 500 nm; two main intense bands centered at the wavelengths of 395 nm and 464 nm originated form \({}^{7}\)F\({}_{0}\)\(\rightarrow\)\({}^{5}\)L\({}_{6}\) and \({}^{7}\)F\({}_{0}\)\(\rightarrow\)\({}^{5}\)D\({}_{2}\) transitions respectively. Analysis of the intensity level of \({}^{7}\)F\({}_{0}\)\(\rightarrow\)\({}^{5}\)D\({}_{2}\) transitions, in comparison to an un-doped sample, showed that a 20 % enhancement was obtained in glass doped with 0.1 mol.% AgNO\({}_{3}\). This phenomenon can be explained by the local field effect of Ag\({}^{0}\) nanoparticles created by partial thermo-reduction of silver ions [27; 28]. Further increases of the Ag content leads to the quenching of the excitation band (inset Figure 2a). In all samples, a band at 395 nm is characterized by an intense transition in the UV-VIS spectral range; hence, it is suitable for the effective excitation of the Eu\({}^{3+}\) ions by laser radiation at 394 nm. In the next step, the influence of Ag ions concentration on the luminescence spectra of europium ions was determined (Figure 2b). In that case, similar behavior of emission intensity as a function of silver ions concentration was observed (inset Figure 2b). However, in comparison to a reference sample, 8% enhancement of luminescence signal at 616 nm was obtained for glass doped with 0.1 mol%AgNO\({}_{3}\). Enhancement is attributed to the localized surface plasmon resonance (LSPR) from nano-metal particles to Eu\({}^{3+}\) ions while also quenching takes place as a result of energy transfer from Eu\({}^{3+}\)\(\rightarrow\)Ag\({}^{0}\) due to Ag\({}^{0}\) SPR. Minimum intensity was observed for glasses doped with 0.3 mol.% AgNO\({}_{3}\). Moreover, in glass co-doped with 0.6 mol.% AgNO\({}_{3}\)/0.2 mol.% Eu\({}_{2}\)O\({}_{3}\), both signals--excitation and emission--slightly increased. This could be explained by the mirror effect corresponding with self-assembling of silver nanoparticles on the glass surface [32]. Antimony-germanate-borate SGB glass characterized by highest emission intensity at the wavelength of 616 nm (SGB_01 Ag02 Eu) was selected to analyze the influence of additional annealing on luminescence properties of europium ions. Five uniform glass samples (10 mm diameter and  
Figure 2: Excitation spectra (**a**) and luminescence spectra (**b**) of antimony-germanate-borate glass doped with 0.2 mol%Eu\({}_{2}\)O\({}_{3}\) and different content of silver: (1) 0 Ag; (2) 0.1 Ag; (3) 0.3 Ag and (4) 0.6 Ag, respectively. (inset) Effect of Ag content on excitation band at 395 nm (**a**) and emission at 616 nm (**b**). 3 mm thickness) were prepared in a single melting process. Four of them were heated at 360 \({}^{\circ}\)C in a tube furnace for 1 to 4 h. The observed changes in the luminescence spectra of Eu\({}^{3+}\) doped SGB glasses as a function of different heating time is shown in Figure 3. It is worth noting that there was a dynamic increase in the luminescence signal after the first 2 h annealing process (inset Figure 3), which confirmed that the surface plasmon resonance of Ag\({}^{0}\) nanoparticles occurs in the vicinity of Eu\({}^{3+}\) ions. Longer annealing time leads to a higher volume fraction of Ag\({}^{0}\) NPs; in consequence, a slight reduction of the luminescence intensity was observed. This phenomenon is related to the Eu\({}^{3+}\)\(\rightarrow\)Ag\({}^{0}\) energy transfer. However, for all measured glasses, the intensity of the \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)F\({}_{2}\) transition was higher than in the reference glass sample.