Document Type

Theses, Ph.D

Disciplines

1.3 PHYSICAL SCIENCES, Optics

Abstract

The introduction of broadband (white) LED outdoor lighting has led to significant energy savings. However, this has come at the cost of increased light pollution which has negative impacts both on ecological systems and human health. This light pollution is largely the result of the lack of control measures for the directionality of the light emitted by outdoor LED lighting. The lack of directionality also results in higher energy consumption in order to compensate for the light scattered to the atmosphere and sufficiently illuminate the target area. In this thesis holographic optical elements (HOEs) are proposed as a complementary technology to broadband LEDs. If coupled, the combination of these technologies (HOEs and LEDs) can enable outdoor lighting that is highly energy efficient and that does not contribute to light pollution. This is because HOEs can allow the functionality of traditional refractive optical elements (e.g. lenses) to be encoded within the structure of a thin film. As such they can be integrated usefully with existing LED lighting infrastructure as well as a range of other LED applications. Currently, the widespread application of HOEs has been limited by the cost of mass production and the difficulty of achieving high diffraction efficiency (η) across a broad range of angles and wavelengths. This is primarily limited by the achievable refractive index modulation (Δ n) of the medium. Photopolymerisable polymers or photopolymers are attractive materials due to their self-processing nature, low-cost

and ease of modification. The aim of the thesis was to design a photopolymer medium suitable for HOEs applications using low-cost sustainable materials that would enable high refractive index modulation and improved robustness (UV stability, water resistance, heat resistance, mechanical robustness). To begin the work an examination of the relevant modelling was conducted in order to define the operational envelope of Δn that would be useful for recording holograms suitable for HOEs for use in LED light shaping and re-directing applications. If the achieved Δn of a material were beyond the relevant range the recorded hologram would not be suitable for HOE applications as there would be more than one diffracted beam per input beam. Once this operational range was defined, a review of recently reported materials was conducted and the reported Δn’s were mapped against the calculated operational envelope. This allowed determination of the suitability of current materials on the basis of their Δn. The majority of existing materials were found to be within operational envelope in terms of Δn and therefore suitable for HOEs designed for LED applications. The limitations identified were the cost of mass production and the lack of robustness of some materials which limits their application outside of laboratory settings. Based on this conclusion, experiments were conducted with the aim of improving both the Δn and robustness of a previously reported acrylamide/polyvinyl alcohol (AA/PVA) based material that was resistant to humidity, but partially erased on exposure to UV radiation. In the context of photopolymers for holography D-Sorbitol is presented here as a novel low-toxicity plasticiser. It was added to the previously plasticiser-free AA/PVA photopolymer and its plasticising effect was demonstrated; η was preserved post UV exposure. Since D-Sorbitol has a high melting temper

ature, the effect of D-Sorbitol on the thermal stability of recorded holograms was also studied and it was found to provide superior thermal stability to alternative plasticisers (triethanol amine and glycerol) that have previously been used in other AA/PVA-based photopolymers. AA/PVA photopolymer film with D-Sorbitol were also demonstrated to have superior mechanical hardness and elastic modulus compared to AA/PVA photopolymer film with triethanol amine or glycerol, although lower than those of plasticiser-free photopolymer. In an attempt to further increase Δn of the novel photopolymer with D-Sorbitol, low refractive index (colloidal silica) and high refractive index (nanodiamond) nanoparticles were added to the composition and holograms recorded. No improvement of Δn was observed and scattering due to noise grating was observed. In addition, oil-based nanoparticles were added to an alternative version of AA/PVA based photopolymer. It was shown that holograms could be recorded in the presence of the oil nanoparticles, although significant noise gratings could be observed. In summary this thesis presents theoretical and experimental data on a novel photopolymer formulation suitable for the production of high quality, highly efficient transmission HOEs with improved hardness and temperature resistance, in a flexible film with high optical quality.

DOI

https://doi.org/10.21427/TA5E-H130

Creative Commons License

Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License
This work is licensed under a Creative Commons Attribution-NonCommercial-Share Alike 4.0 International License.

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