## 3 Results and Discussion
We confirm the absence of variation in the absorption cross-section at 975 nm and at 1.5 \(\upmu\)m with an increase in x up to 10 mol %, indicating that the sites of the Er\({}^{3+}\) and Yb\({}^{3+}\) ions are not strongly affected by the change in the glass composition. Although the progressive addition of ZnO has a noticeable impact on the structure, we confirm that up to 10 mol %, Zn ions do not enter in the second coordination shell around the Er\({}^{3+}\) and Yb\({}^{3+}\) ions. Therefore, these Er\({}^{3+}\) and Yb\({}^{3+}\) ions are thought to be surrounded mainly by P, Na and Sr. The emission spectra of the glasses were measured using a 975 nm pumping. No noticeable increase in the intensity of emission at 1.5 \(\upmu\)m was observed in agreement with the absorption cross-section at 975 nm presented in Table 2. The normalized emission spectra of the prepared glasses under 975 nm excitation wavelength are displayed in Figure 2b. The spectra of the glasses exhibit an emission band, which is typical of Er\({}^{3+}\) emission (\({}^{4}\)I\({}_{13/2}\rightarrow{}^{4}\)I\({}_{15/2}\)) in glass, with a bandwidth of 42 nm. An increase in the concentration of ZnO leads to small changes in the shape of the emission band confirming that Er\({}^{3+}\) and Yb\({}^{3+}\) ions' sites are not strongly affected by the changes in the glass composition. The lifetime values of Er\({}^{3+}\):\({}^{4}\)I\({}_{13/2}\) level in the investigated glasses were measured and are reported in Table 2. Within the accuracy of the measurement, the lifetime values do not depend on x. Surprisingly, it should be pointed out that the lifetime values in the investigated glasses are smaller than those reported in [5]. The glass with x = 2.5 was irradiated using alpha particles. No craters or cracks were formed during the irradiation of the glasses. The alpha particles irradiation leads to the appearance of spot which could be seen with naked eyes and also with a 3D interference microscope as shown in Figure 3a. \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Glass Label**} & \multirow{2}{*}{**x**} & **Er\({}^{3+}\) Ions-cm\({}^{-3}\)** & **Yb\({}^{3+}\) Ions-cm\({}^{-3}\)** & **\(\sigma_{\textbf{Abs}}\) at 980 nm** & **\(\sigma_{\textbf{Abs}}\) at 1550 nm** & **Er\({}^{3+}\):\({}^{4}\)I\({}_{13/2}\)** \\  & & **(10\({}^{20}\))** & **(10\({}^{20}\))** & **(10\({}^{-21}\) cm\({}^{2}\))** & **(10\({}^{-21}\) cm\({}^{2}\))** & **\(\tau\) (ms)** \\  & & \(\pm\)**5\%** & \(\pm\)**5\%** & \(\pm\)**10\%** & \(\pm\)**10\%** & \(\pm\)**0.20 ms** \\ \hline REF & 0 & 1.58 & 4.75 & 8.30 & 5.79 & 3.47 \\
2.5 Zn & 2.5 & 1.56 & 4.68 & 8.28 & 6.32 & 3.19 \\
5 Zn & 5 & 1.60 & 4.81 & 9.12 & 6.23 & 2.98 \\
10 Zn & 10 & 1.66 & 4.97 & 9.06 & 6.63 & 3.27 \\ \hline \end{tabular}
\end{table} 
Table 2: Optical and luminescence properties of the glasses. Figure 2: UV-Vis-NIR absorption (**a**) and normalized emission (**b**) spectra of the investigated glasses. The emission spectra were obtained using a 976 nm excitation. The spot corresponds to a surface photo-expansion of \(\sim\)200 nm (Figure 3b). As reported in [20], this surface photo-expansion could be attributed to macroscopic and microscopic structural changes in the glass due to defects such as oxygen vacancies, trapped electron and hole centers or to structural deformations. Analysis of the changes in the glass structure using micro-Raman spectroscopy is in progress, and it will be reported separately. Associated with the surface photo-expansion, the surface roughness was found to increase after irradiation as reported in Table 3.  
We noticed that the increase in the surface roughness is a reversible process as it decreases progressively over a period of 3 months, although the glass was stored in the dark at room temperature. However, no significant change in the surface expansion could be detected during these 3 months.