Improving the mechanical robustness, UV curability, and humidity resistance of an acrylamide/polyvinyl alcohol-based photopolymer material for volume holography applications with LEDs by using D-sorbitol as a naturally occurring plasticiser

Scattered light from outdoor LED sources is a major contributor to blue-spectrum light pollution which negatively impacts ecology and public health. Volume holographic optical elements (HOEs) that are robust to environmental conditions and made from sustainable materials are a potential solution as they can be used to limit the amount of light that reaches objects outside of the target area for illumination. In this work, a novel photopolymer based on Acrylamide (AA) / polyvinyl alcohol (PVA), incorporating D-sorbitol as a low-toxicity plasticiser, is presented with a humidity response comparable to a previous composition without plasticiser, while reaching refractive index modulation ( "n) values capable of very high efficiency transmission HOEs. DSorbitol, a naturally occurring sugar-alcohol, was introduced to a system that uses n-phenyl glycine (NPG) as the co-initiator, in lieu of triethanol amine (TEA), which has a dual function of plasticiser and co-initiator but high viscosity and high hydrophilicity. Here, the plasticising effect of D-Sorbitol increases diffusion during holographic recording, improving refractive index modulation. This is demonstrated by the fact that there is no loss of diffraction efficiency (η) upon UV exposure of 500 lines/mm HOEs at the optimal concentration of D-Sorbitol compared to samples with no D-Sorbitol. In addition, it is shown that while there is a real-time loss of η in samples with D-Sorbitol on exposure to 80-90 %, the response was largely reversible in all samples up to maximum concentration of 21 wt. %.