Modelling and characterisation of nematic and photo–addressable liquid crystal devices

Abstract

The aim of this thesis is to design, fabricate and characterise new nematic and photo-addressable liquid crystal devices. This is done by modelling, collecting and analysing the optical response of standard planar and twist liquid crystal cells. Then, photovoltaic liquid crystal lenses are designed, modelled, fabricated, tested and characterised. The thesis introduces the cross-polarised intensity method for planar nematic liquid crystal cell characterisation on which the Optical Multi-Parameter Analyser (OMPA) is based. OMPA is a combined hardware and software tool that I used to study the temperature dependence of nematic properties and the limits of the cross-polarised intensity method. The results from this study show that the splay and bend elastic constants of the E7 liquid crystal mixture decrease with temperature, as expected, in a relatively good agreement with the theoretical models. Moreover, OMPA was successfully used to determine the liquid crystal parameters of optically thin cells from cross-polarised intensity data with a limit of the total phase lag of about π. Further analysis of the cross-polarised intensity method shows that it reliably extracts the planar liquid crystal cell parameters including the splay and bend elastic constants even if there is uncertainty on the refractive indices. However, uncertainty on the dielectric coefficients is much more critical for the accurate measurement of the elastic constants. The cross-polarised intensity method is then extended to twist liquid crystal cells for the measurement of the twist elastic constant. This new characterisation method uses a non-standard configuration of crossed polarisers and a twist cell for the collection of experimental cross-polarised intensity data. In order to fit the data, the response and the transmitted intensity of a standard twist liquid crystal cell are modelled as a function of the applied voltage. The model uses two non-coaxial spherical coordinate systems and an efficient beam propagation method. When cross-polarised intensity measurements are fitted with the numerical model, the twist elastic constant, the twist-cell pretilt and the twist-cell thickness are determined from a single experiment. The twist elastic constant of three commonly used liquid crystals is thus measured and compared to their literature values. These results, and careful analysis of the new method, suggest that it is simple, quick and reliable, but sensitive to misalignment in the experimental setup.

More information

Identifiers

ORCID for Denitsa Ognyanova Bankova: orcid.org/0000-0002-2511-5133

ORCID for Giampaolo D'alessandro: orcid.org/0000-0001-9166-9356

Catalogue record

Export record