# Characterisation of Holographic Recording in Environmentally Stable Photopolymerisable Glass
## 4 Conclusions  
The process of grating formation in a recently developed photopolymerisable glass (PHSG) was investigated. It was found that the grating formation had two stages: exposure to laser light and post-recording dark processes. During exposure, photopolymerisation of the methacrylate groups was likely to be the main contributor to refractive index modulation formation. During the dark process, further enhancement of the diffraction efficiency was detected. This was attributed to the inorganic polycondensation process which is thought to have the dominant effect on the grating formation at this stage.  
Enhancement of the diffraction efficiency during the dark process was found to be dependent on the initial diffraction efficiency achieved after exposure and reached a 6-fold increase. This trend was observed for samples prepared using sol aged for 0 and 7 days. To prevent dark enhancement of the diffraction efficiency and fix the grating, UV-curing was successfully applied.  
Thus, highly efficient gratings (up to 90%) can be developed in 118 \(\upmu\)m thick PHSG layers prepared with the sol aged up to 7 days. The obtained 6-fold increase of the diffraction efficiency during the dark process is beneficial as it allows for the achievement of high diffraction efficiency by using low exposure. In applications where the precise diffraction  
Figure 6: (**a**) Photograph of the experimental set-up for monitoring the impact of humidity on the diffraction efficiency of transmission gratings recorded on PHSG layers; (**b**) Bragg selectivity curves of the transmission grating at different levels of relative humidity; (**c**) Bragg selectivity curves of the transmission grating measured before and after curing at 130 \({}^{\circ}\)C for 60 min; (**d**) Photograph of the PHSG layer after curing at 130 \({}^{\circ}\)C for 60 min where the clear part corresponds to the area of the grating; (**e**) image of the grating (same sample as shown in Figure 3d) under ambient light illumination after curing at 130 \({}^{\circ}\)C for 60 min.  
efficiency is necessary, the final diffraction efficiency can be controlled by utilising an appropriate exposure and subsequently UV-curving. Insensitivity of the PHSG to humidity (up to 95%) and temperature (up to 130 \({}^{\circ}\)C) broadens the range of potential applications of PHSG for the development of holographic optical elements with improved stability at different environmental conditions.  
Conceptualization, T.M., S.M. and I.N.; methodology, T.M., B.R., P.S., M.O. and A.K.; validation, T.M., P.S., A.K. and B.R.; formal analysis, T.M. and M.O.; investigation, T.M., B.R, P.S. and M.O.; writing--original draft preparation, T.M., M.O., D.C., S.M. and I.N.; writing--review and editing, T.M., D.C. and I.N. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
Data Availability StatementData sharing is not applicable to this article.  
The authors would like to acknowledge the FOCAS Research Institute for providing research facilities.  
The authors declare no conflict of interest.