# Characterisation of Holographic Recording in Environmentally Stable Photopolymerisable Glass
## 3 Results and Discussion
### Holographic Grating Formation
After 24 h, no further increase of the diffraction efficiency is observed and, as shown previously in [14], its stability has been confirmed for 27 months. To identify the origin of the dark process, FTIR spectra (obtained using a Perkin Elmer GX instrument) of the PHSG layers before and after exposure were compared (Figure 4c). This approach allows mimicking of the dark and bright fringe areas of the grating and analysing changes induces by exposure. For this experiment, the layers were exposed to a uniform 532 nm wavelength beam with the intensity of 40 mW cm\({}^{-2}\) for 20 s. FTIR spectra were obtained before exposure and 2 min and 24 h after exposure (Figure 4c). It can be seen that following the exposure, the siloxane (900, 1030, and 1100 cm\({}^{-1}\)) and silicon-zirconium oxide (930 cm\({}^{-1}\)) bands constituting the inorganic network of the hybrid sol-gel material are increased, confirming the enhancement of the condensation of the material. This can only be explained by the occurrence of polycondensation reactions between the residual unreacted silanol and zirconium hydroxide groups, forming more condensed and robust  
Figure 4: (**a**) Real time diffraction efficiency growth curves of transmission gratings monitored during holographic exposure for 25, 30, and 35 s; arrows show the end of laser exposure; (**b**) diffraction efficiency growth curves of samples shown in Figure 4a monitored during both holographic recording and dark process for 25 min; (**c**) FTIR spectra of PHSG layers measured before exposure to 532 nm uniform beam and 2 min and 24 h after exposure (during dark process); (**d**) hardness test results (number of oscillations) for layers exposed to uniform beam for 20 to 160 s show that within 24 h the layers reach similar hardness. inorganic chemical bonds. Interestingly, this pure inorganic polycondensation process has been enabled by the radical organic polymerisation reactions, which increased proximity between the inorganic reactive sites. Therefore, the dominant effect on the grating formation during the dark process is proposed to be the inorganic polycondensation process which causes an increase of the density in bright fringe areas, leading to the higher refractive index contrast and as a result the diffraction efficiency enhancement. This concept is further supported by the hardness testing data of polymerised layers as shown in Figure 4d. In this experiment, layers were exposed to a uniform laser beam with the intensity of 40 mW cm\({}^{-2}\) for 20 to 160 s, to induce different extents of initial polymerisation. Hardness testing was then performed within 24 h to identify the changes in hardness caused by the dark process. As seen from Figure 4d, after exposure the layers illuminated for 20 to 160 s showed oscillation number from 52 to 80, respectively. After 24 h, these layers reached similar robustness and the oscillation number was in the range from 82 to 89. This increase in hardness supports the hypothesis that the dark process occurring after recording can be attributed to further polycondensation, leading to lower pore diameters and microporosity [25; 26].