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 (Δn) 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 (<i>η</i>) 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 <i>η</i> 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. %.