Using liquid crystal phase textures for various optical applications ; Exploitation des textures de phases cristal-liquides pour diverses applications optiques

Liquid crystals have been used for their electro-optical properties since the 1970s, both in LCD screens and for less widespread applications such as smart glass. The glass industry is particularly interested in the latter. The technology currently on the market uses a polymer matrix in which droplets of liquid crystal in the nematic phase are dispersed. Under the effect of an electrical voltage, the glass switches from a scattering to a transparent state. However, the transparency of this second state is not optimal, which has prompted the search for other technical solutions. It has recently been shown that the smectic A phase can also be used to design a smart glass prototype: a type of topological defect (focal conic domains) is generated in the smectic A phase and polymerised to be maintained in the nematic phase. Thanks to this step, the sample reversibly changes from a scattering state to a transparent state when a voltage is applied. This system belongs to the family of PSLCs (Polymer Stabilised Liquid Crystals). In this thesis, we optimised the polymerisation parameters (monomer concentration, photoinitiator, UV light intensity) to maximise the contrast between the transparent and scattering states. The relationship between the electro-optical properties and the microstructure of the samples was also studied. The principle was then extended to another liquid crystal phase that is formally very close to the smectic A phase: the twist-bend nematic (NTB) phase. This phase exhibits a wide variety of topological defects, enabling us not only to show that the NTB phase can also be used for smart glass applications, but also to develop an electrically tunable diffraction grating. For the latter application, the ‘rope-like texture' of the NTB phase was polymerised to be maintained in the nematic phase, which is known to align reversibly along the electric field. We then revisited the smectic A phase, which has already been extensively studied, but whose properties have not yet been fully investigated. In ...