## 3 Results and Discussion
### Silver Nanoparticles Reduction on Glass Fiber Surface  
In this part of the experiment, we focused on the possibility of creating a thin layer of silver nanoparticles on the surface of fibers fabricated from SGB glass. Relating to experimental results and the literature, this phenomenon strongly depends on Ag\({}^{+}\) ions concentration [33, 37]. Therefore, we decided to fabricated glass fibers from SGB glass doped with 0.6 mol.% AgNO\({}_{3}\). Figure 5a shows the absorbance spectra of fabricated glass fibers measured perpendicular to the fiber axis. The SPR band at the maximum peak of 450 nm originating from the creation of Ag\({}^{0}\) nanoparticles has been observed for glass fiber heated by 1 h at 360 \({}^{\circ}\)C; the absorption band at 395 nm corresponding to \({}^{7}\)F\({}_{0}\)\(\rightarrow\)\({}^{5}\)L\({}_{6}\) transition in europium ions was also measured. The SPR absorption band is correlated with the brownish color over the glass fiber (Figure 5b) observed after the annealing process. The theoretical radius of silver nanoparticles can be estimated according to following formula [38]:  
\[R_{NPs}=\frac{v_{F}}{\Delta\omega} \tag{1}\]  
where \(v_{F}\) is a Fermi velocity of the silver (1.38 \(\times\) 10\({}^{8}\) cm/s) and \(\Delta\omega=2\pi\left(\frac{1}{\lambda_{1}}-\frac{1}{\lambda_{2}}\right)\) is the FWHM (full width at half maximum) of the plasmon band in units of angular frequency. The estimated value of average radius of silver nanoparticles is 5 nm. As mentioned in Section 3.1, for glass doped with 0.6 mol.% AgNO\({}_{3}\), the silver nanoparticles were formed in a bottom-up process starting with the reduction of Ag\({}^{+}\) ions inside the SGB glass, due to thermal annealing (fiber drawing and heat-treatment), followed by migration of the Ag\({}^{0}\) nanoparticles to the surface [32, 39]. This phenomenon could be useful in the construction of SERS sensors for biophotonic applications [37]. It should be noted that the self-assembling of silver nanoparticles on the surface was not observed for glass with the lower concentrations of silver ions. \begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**Sample**} & \multicolumn{5}{c}{**Asymmetry Ratio R**} \\ \cline{2-6}  & **0 h** & **1 h** & **2 h** & **3 h** & **4 h** \\ \hline SGB\_01 Ag02 Eu glass & 2.17 & 2.14 & 2.12 & 2.11 & 2.12 \\ SGB\_01 Ag02 Eu glass fiber & 2.45 & 2.19 & 2.09 & 2.10 & 2.11 \\ \hline \end{tabular}
\end{table} 
Table 1: Comparison of asymmetry ratio R for glass and glass fibers with different annealing time. Figure 4: Luminescence spectra of SGB_01 Ag02 Eu glass fiber under 394 nm laser excitation. (Inset) The dependency of the intensity changes at 616 nm and asymmetry ratio on different time of heat-treatment. In order to confirm the presence of Ag\({}^{0}\) nanoparticles on the glass fiber surface, SEM measurements were conducted. Figure 6 shows scanning electron microscope (SEM) images of SGB_06 Ag02 Eu glass fiber surface, with (a) and without (b) thermal treatment. SEM images shows clearly that the thermochemical reduction of silver nanostructures leads to growth of thin film on the surface of glass fiber during thermal treatment process (Figure 6b). According to the literature, the thickness of silver film should increase as a function of annealing time [32]. The morphology of spherical-like silver nanoparticles in the glass fiber surface without annealing is quite homogeneous, and particles are randomly distributed. After the heat-treatment process, the diameter of silver nanoparticles increases, and also were observed. This has a strong influence on the spectral transmission of light across the glass fiber surface. As can be seen, in both SEM images there are visible silver particles of a size range approx. 5-10 nm for the GF directly after drawing (Figure 6a), and 15-20 nm for GF after 1-h annealing process at 360 \({}^{\circ}\)C (Figure 6b).