Development of Photopolymer Material with Improved Dynamic Range and Sensitivity

In this study the effect of the concentration of acrylamide and the influence of different initiators in a photopolymer composition for holographic recording of diffraction gratings is investigated. Light manipulating Holographic Optical Elements (HOEs) have a number of characteristics which can be optimised for different roles. However, at the core of these devices is the refractive index modulation that has been created in the material during recording. Typically higher refractive index modulation will enable greater diffraction efficiency. Solar concentrating HOEs can particularly benefit from material that experiences higher refractive index modulation. For a solar concentrator to have a high acceptance angle thinner photopolymer layers are preferable. Thinner layers can lead to a reduction of the device’s diffraction efficiency unless the refractive index modulation increases to compensate. This paper presents an optimisation of a photopolymer formulation for improved refractive index modulation and sensitivity of the layer. An increase in the acrylamide concentration of 66% resulted in 50% higher refractive index modulation with values reaching 5 × 10−3 in 40 micron layers. Faster recording times are an important consideration for the commercialization and mass production of photopolymer devices [1]. Higher production rates and lower costs are some of the main advantages. Altering the initiator is expected to have an effect on the material’s sensitivity and thus on recording time. Several initiators were compared, triethanolamine (TEA), methyldiethanolamine (MDEA) and dimethylethanolamine (DMEA). It was found that holograms recorded with MDEA as the initiator recorded 58% faster over TEA based photopolymer at larger layer thicknesses. The stability of the photopolymer was also tested with different protective coatings when subjected to UV light. The properties exhibited by this photopolymer composition make it a promising candidate for development of solar concentrating applications; however improvements to the durability in conditions of UV radiation will have to be made before it is suitable for solar concentration