# Characterisation of Holographic Recording in Environmentally Stable Photopolymerisable Glass
## 3 Results and Discussion
### Holographic Grating Formation  
The fundamental motivation for investigation of the grating formation process is to obtain predictable diffraction efficiency and to control holographic recording capability by modifying the composition. As a result, this allows the development of HOEs with customisable specification and the addition of practical advantage in terms of their sustainable performance. In order to identify mechanisms contributing to the grating formation in PHSG, diffraction efficiency was monitored during holographic recording (exposure) and after recording (dark process when laser is off). This approach allowed studying the impact of both physicochemical reactions and physicochemical processes on the refractive index modulation formation during exposure and dark processes. Volume phase transmission gratings were recorded using the set-up presented in Figure 1. Layers with thickness of 118 \(\pm\) 3 \(\upmu\)m were prepared using the sol with the age of 0-day as described in Section 2.3. Figure 4a shows the typical real-time diffraction efficiency growth curves of gratings obtained using exposure times of 25, 30, and 35 s. As seen from Figure 4a, during exposure the growth curves for 25, 30, and 35 s recording time had the same trend and diffraction efficiencies of 14.7, 20.1, and 25.2% were reached, respectively. At this stage, the key photochemical reaction is likely to be the photopolymerisation of the methacrylate groups (MAPTMS and MAA), leading to the formation of rigidly structured nanoparticle-based species in the bright fringe areas resulting in the creation of a diffraction grating [14]. Previous research on grating formation in Class I materials shows that diffusion of photoinitiator molecules and monomers due to the concentration gradient between bright and dark regions created during exposure also plays a part in grating formation [15]. These materials represent diffusing host-guest systems where the contribution of diffusion to the final diffraction efficiency is significant. The PHSG described here is a Class II material and, thus, has a principally different structure that may affect diffusion in the volume of the material. The sol-gel process employed for PHSG development (as described in Section 2.1) enables the fabrication of nanoparticles from small hybrid molecules. In this process, all small molecules reacted via hydrolysis and condensation reactions to form oligomeric microporous silicate-based nanoparticle systems, as demonstrated by Si-NMR spectroscopy in closely related material systems [24]. Therefore, as a result of thermal curing, the PHSG can be represented as an interconnected network of microporous hybrid silicate and zirconium oxide nanoparticles, where photoinitiator molecules would be able to diffuse and further enable photoreaction processes. Thus, during exposure, photopolymerisation is expected to be the main contributor to the grating formation. Diffusion of photoinitiator may also contribute to some extent but previous work has shown that these diffusion effects are likely to be limited due to low permeability of the material [14]. The systematic study, involving the recording of structures with largely different spatial frequencies, of the diffusion effects in PHSG will be the scope of future research. After exposure, the diffraction efficiency was monitored for 25 min in dark and its increase up to 78.2, 79.5, and 79.1% was observed for gratings with the exposure of 25, 30, and 35 s, respectively (Figure 4b). These results showed that the final diffraction efficiency of gratings recorded on the layers with same thickness did not depend, in this instance, on the initial exposure. This is evidence that once grating growth is in progress, the influence of processes unrelated to the exposing interference pattern is significant. These 'dark' processes become the dominant grating growth process and continue for about 24 h leading to the increase of the diffraction efficiency (Figure 3c).