# Characterisation of Holographic Recording in Environmentally Stable Photopolymerisable Glass
## 1 Introduction  
The interest in new holographic recording materials with significantly improved durability and environmental stability and/or additional functionality [1, 2] is driven by rapid development of Augmented Reality displays [3, 4, 5], sustainable and energy-saving LED lighting [6, 7], and solar energy collection technology [8, 9]. As shown previously, any variations of the diffraction efficiency, the diffraction angle, and/or the diffraction spectrum cause undesirable effects on the device performance [5]. Thus, resistance of the holographic recording material to the environment provides stable performance of HOEs at different environmental conditions and ensures their successful implementations in real-world products. Photopolymerisable glasses as holographic recording materials are good candidates due to their high recording capability [10, 11], improved dimensional stability [12], andnegligible shrinkage [13]. In addition, photopolymerisable glasses overcome the thickness limitations inherent to holographic photopolymers and allow achieving high density recording [11]. Recently, a novel photopolymerisable hybrid sol-gel (PHSG) for holographic recording of both transmission and reflection volume gratings with refractive index modulation up to \(3\times 10^{-3}\) has been reported [14]. The PHSG has significantly improved gelation time in comparison with other photopolymerisable glasses and high water resistance, both of which make it an attractive material for mass production of HOEs with no sensitivity to ambient humidity. In order to achieve full control over the performance of the material and further improve its properties, understanding of the processes of grating formation under holographic patterning is essential. Research on the mechanism of grating formation has been recently reported for a Class I photopolymerisable hybrid organic-inorganic material [15]. The material consists of a silicate sol-gel matrix that incorporates organic monomers, inorganic high refractive index species, and a photoinitiator. According to classification based on the nature of the links between the inorganic and organic components, Class I represents systems with no covalent bonds between the organic and inorganic species [16]. As found in [15], the grating formation involves two stages: exposure and dark processes. During exposure, photopolymerisation occurs in illuminated areas leading to the refractive index modulation and, as a result, grating formation. Further enhancement of the refractive index modulation is observed in darkness, and it is attributed to the diffusion of two components: monomers and high refractive index species [15]. Diffusion of mobile components in Class I photopolymerisable glasses is feasible due to the absence of covalent bonds between the sol-gel matrix and polymerisable (monomers) and diffusible (photoinitiator, dopants) species. This paper focuses on the investigation of the mechanism of grating formation in a recently developed PHSG. Similarly to the above-mentioned research, the approach is based on the analysis of grating parameters during exposure and post-recording dark processes. It should be noted that this PHSG has a principal difference in its structure, as it is the first example of a Class II photopolymerisable glass where the organic and inorganic components have strong chemical bonds [16]. Thus, it is expected that the difference in the structure of Class I and Class II materials may lead to the domination of different processes during exposure and post-recording dark process. Along with a study of grating formation, this paper reports on the investigation of other aspects of the PHSG which have an effect on the performance of the HOEs. This includes the aging effect of the sol on the holographic recording capability of PHSG and its sensitivity to temperature and environmental humidity.