# Characterisation of Holographic Recording in Environmentally Stable Photopolymerisable Glass
## 3 Results and Discussion  
### Aging of the Sol  
The developed sol, as described in Section 2.1, represents a stable dispersion of colloidal amorphous particles in an alcoholic solvent (composed of methanol and propanol).  
Figure 1: Experimental set-up for holographic recording of transmission gratings using a 532 nm laser. Monitoring of the diffraction efficiency growth curve is performed by probing with a 633 nm laser and collecting beam intensity data with a photodetector positioned at the diffracted beam path. S—electronic shutter, HWP—half-wave plate, PBS—polarising beam splitter, SF—spatial filter, CL—collimator, VA—variable aperture, PL—PHSG layer, M—mirror, PD—photodetector.  
The sol particles may interact by van der Waals forces or hydrogen bonds leading to their agglomeration and as a result higher viscosity of the sol that affects sol coatability and the morphology of the final coatings [19; 20]. As the layer uniformity is one of the crucial parameters required for recording of HOEs with sustainable performance, the optimal age of the sol for achieving uniform layers needs to be identified. In this study, the aging effect of the sol is systematically investigated in terms of the size and homogeneity of the sol particles and viscosity of the sol.  
Characterisation of the size of nanoparticles composing the sol with age up to 30 days was carried out by means of DLS analysis. A monomodal distribution curve with a peak of intensity of approximately 4.5 nm was obtained for the sol immediately after completing the synthesis (Figure 2a). An increase of the average nanoparticle size was observed with time, reaching 10.7 and 13.5 nm after 15 and 30 days, respectively (Figure 2b). To define the dispersion of nanoparticles, the full width at half maximum of the size distribution (Figure 2a) was calculated. This was found to be 4.2, 23.3, and 30.3 nm for the sol with the age of 0, 15, and 30 days, respectively. These data demonstrate that one of the main effects of sol aging was agglomeration of particles and higher dispersion of nanoparticle population.  
Figure 2: (**a**) Particle size distribution of the sol with age of 0, 15, and 30 days obtained by DLS analysis; (**b**) particle size diameter of the sol obtained from the peak of intensity of the size distribution curve (Figure 2a) versus the age of the sol; (**c**) dynamic viscosity of the sol with different age; (**d**) SEM image of the cross-section of PHSG layer prepared by use of sol with age of 7 days.  
Simultaneously with DLS analysis, the dynamic viscosity of the sol was measured in order to identify the limit period when the sol could produce uniform thick layers. Dynamic viscosity was determined by means of an AND Viscometer (SV-10). As expected, growth in nanoparticle size caused the substantial increase of dynamic viscosity from 85 to 250 mPa s within 30 days (Figure 2c). Experimental results showed that the sol with dynamic viscosity of up to 250 mPa s, corresponding to an age of up to 30 days, was suitable for coating. The layer homogeneity and thickness was then investigated by imaging a cross-section of samples via SEM. SEM images were recorded by means of a Hitachi SU-70 SEM using an accelerating voltage of 5 keV. Three measurements were done at different points to evaluate the thickness uniformity. It was found that layers prepared with sol aged up to 7 days had homogenous coatings with thicknesses of 88.8 \(\pm\) 0.5 \(\upmu\)m (Figure 2d). More aged sol led to non-uniformity of the coating, with thickness variation reaching up to 15 \(\upmu\)m within the glass slide area. Thus, it was proposed that the sol age of up to 7 days could be considered as the optimum for obtaining layers with uniform thickness. The effect of the sol age on the holographic recording capability was also investigated and the results are presented in the section below.