Development of Photopolymer Material with Improved Dynamic Range and Sensitivity

摘要

In this study the effect of the concentration of acrylamide and the influence of different initiators in a photopolymercomposition for holographic recording of diffraction gratings is investigated. Light manipulating HolographicOptical Elements (HOEs) have a number of characteristics which can be optimised for different roles. However, atthe 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 HOEscan particularly benefit from material that experiences higher refractive index modulation. For a solar concentratorto have a high acceptance angle thinner photopolymer layers are preferable. Thinner layers can lead to a reduction ofthe 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 andsensitivity of the layer. An increase in the acrylamide concentration of 66% resulted in 50% higher refractive indexmodulation with values reaching 5 × 10~(?3) in 40 micron layers.Faster recording times are an important consideration for the commercialization and mass production ofphotopolymer devices [1]. Higher production rates and lower costs are some of the main advantages. Altering theinitiator is expected to have an effect on the material’s sensitivity and thus on recording time.Several initiators were compared, triethanolamine (TEA), methyldiethanolamine (MDEA) anddimethylethanolamine (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. Theproperties exhibited by this photopolymer composition make it a promising candidate for development of solarconcentrating applications; however improvements to the durability in conditions of UV radiation will have to bemade before it is suitable for solar concentration.