# Characterisation of Holographic Recording in Environmentally Stable Photopolymerisable Glass
## 3 Results and Discussion
### Effect of the Sol Age on Holographic Recording Capability  
The effect of the sol age on the holographic recording capability was investigated by recording volume phase transmission gratings. Two batches containing ten layers (118 \(\pm\) 3 \(\upmu\)m thickness) were prepared using the sol with ages of 0 and 7 days at the time of exposure. To analyse the grating formation in PHSG and estimate its photosensitivity, diffraction efficiency growth curves were monitored in real time during exposure. Figure 3a gives a representative example of the growth curves for two layers with the same thickness (118 \(\upmu\)m) but prepared using the sol with different ages (0 and 7 days). The typical diffraction efficiency growth started after 8 s of exposure. This 8 s period was attributed to the oxygen inhibition inherent to photoinduced polymerisation of methylates [21]. After 35 s exposure, the diffraction efficiencies of 23 and 18% were achieved in layers prepared using the sol with age of 0 and 7 days, respectively (Figure 3a). Figure 3: _Cont._  
Photosensitivity (S) was defined as the ratio between the square root of the diffraction efficiency and the product of the total recording intensity (I), exposure time (t), and the visibility (V) [22]:  
\[\sqrt{\eta}=\text{SIVt} \tag{2}\]  
where visibility was defined as  
\[\text{V}=\frac{2\text{R}^{-1/2}}{(1+\text{R})} \tag{3}\]  
where R is the ratio of the recording beam intensities. In the current experimental conditions, the holographic recording was performed using two beams with equal intensity which provided R = 1. As a result, photosensitivity of layers prepared using the sol with ages of 0 and 7 days was found to be 8 \(\times\) 10\({}^{-4}\) and 7 \(\times\) 10\({}^{-4}\) mJ\({}^{-1}\) cm\({}^{2}\), respectively. Thus, an approximate 14% decrease of photosensitivity was observed within 7 days and, hence, the longer exposure was needed to achieve the required diffraction efficiency. Photosensitivity is directly linked to the polymerisation rate, which is highly dependent on the capability of the photoinitator to produce free radicals [16]. Therefore, the observed photosensitivity decrease could be explained by photodegradation of a photoinitator that might lead to the loss of the photochemical reactivity of the formulation as previously reported for similar hybrid material [23]. The diffraction efficiency was measured again 24 h after recording and an increase up to 81.5% (0-day) and 77.5% (7-days) was observed (Figure 3b). This indicated that further enhancement of the diffraction efficiency occurred during a dark process. It was found that the level of enhancement of the diffraction efficiency depended on the initial diffraction efficiency achieved after exposure and followed the same trend for samples prepared using sol of 0 and 7 days (Figure 3c). Normalized diffraction efficiency calculated as the ratio of the diffraction efficiency measured 24 h after recording and immediately after recording was used to estimate the diffraction efficiency enhancement level (Figure 3d). As seen from Figure 3d, the gratings with initial diffraction efficiency of about 10% achieved after exposure showed diffraction efficiency enhancement up to 6-fold, which corresponded to about 60% diffraction efficiency measured 24 h after exposure. Three days after exposure, the diffraction efficiency was measured again, and no further increase of the diffraction efficiency was observed. Figure 3: (**a**) Real-time diffraction efficiency growth during holographic recording on PHSG layers with thickness of 118 μm prepared using the sol with ages of 0 and 7 days; (**b**) Bragg selectivity curves of gratings recorded as shown on Figure 3a and measured in 24 h after recording; (**c**) final diffraction efficiency (DE) measured 24 h after exposure versus the initial diffraction efficiency obtained during exposure; (**d**) normalised DE versus initial DE shows the level of the enhancement during dark process. efficiency was noticed for all samples.