## 4 Conclusions  
Glasses with the system (98-x) (0.50P\({}_{2}\)O\({}_{5}\)-0.40 SrO-0.10 Na\({}_{2}\)O) -0.5 Er\({}_{2}\)O\({}_{3}\)-1.5 Yb\({}_{2}\)O\({}_{3}\)-xZnO (in mol %) were prepared using melt-quenching technique. The physical, thermal, structural, optical and spectroscopic properties of the glasses were measured. Using IR and Raman spectroscopies,  
Figure 4: UV-Vis-NIR transmittance (**a**) and reflectance in THz range (**b**) spectra of the glass with x = 2.5 prior to and after irradiation.  
Figure 5: Emission (**a**) and normalized emission (**b**) spectra measured in non-irradiated and irradiated spots.  
the structure of the glass is suspected to be a metaphosphate structure with Q\({}^{2}\) and Q\({}^{1}\) network units. We confirm that Zn ions act as network modifiers and lead to a depolymerization of phosphate network in agreement with the decrease in T\({}_{\text{g}}\). While the progressive addition of ZnO in the phosphate network has an impact on the structure of the glass, Zn ions have no significant impact on the sites of the Er\({}^{3+}\) and Yb\({}^{3+}\) ions. Er\({}^{3+}\) and Yb\({}^{3+}\) ions are thought to be surrounded mainly by P, Na and Sr.  
In this paper, we also discuss the effect of the alpha particles irradiation on the surface, optical and micro-luminescence properties of the glass with x = 2.5. We showed that this glass is sensitive to alpha particles irradiation and exhibits a surface photo-expansion of \(\sim\)200 nm (measured 1.5 month after the irradiation). The irradiation also increased the surface roughness and is suspected to also increase the emission at \(\sim\)1.5 \(\upmu\)m due to changes in the coordination sites of Er\({}^{3+}\) and/or to changes in the OH content. The photo-response of the glass was found to be reversible. Three months after the irradiation, a noticeable reduction in the surface photo-expansion was observed although the glass was stored in the dark at room temperature.  
In summary, our investigation brought additional novelties to the field of material research operating in harsh environments (i.e., ionizing radiation), clearly showing the need to further investigate the radiation effect of glasses with different compositions in order to develop new photo-resistant glasses for space application.  
This work was supported by the Academy of Finland through the Competitive Funding to Strengthen University Research Profiles program (310359), academy project (308558) and Academy of Finland Postdoctoral research project (285170). The Romanian authors acknowledge the financial support of the Romanian Executive Agency for Higher Education, Research, Development and Innovation Funding (UEFISCDI), under the contract 24 PED/2017, project \"Photonics devices under extreme operating conditions\"--PHOENIX. The cooperation was run under the COST Action MP1401 \"Advanced Fibre Laser and Coherent Source as Tools for Society, Manufacturing and Lifescience\".  
[PERSON] and [PERSON] conceived and designed the experiments; [PERSON] performed the experiments, [PERSON] performed the surface profile measurement of the irradiated samples; [PERSON] and [PERSON] were in charge of the measurement of the luminescence (emission and lifetime) properties; the teams from Romania performed the irradiation of the glasses and THz measurements of the irradiated samples. [PERSON] wrote the paper, which was reviewed and approved by all co-authors.  
The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.