Flexoelectricity and multi-dimensional structures for liquid crystal photonics

The investigations carried out in this thesis involve the characterization and the manipulation of dielectric and flexoelectric phenomena in liquid crystalline systems. The first half of the thesis considers the flexoelectric effect in achiral and chiral nematic liquid crystals (LCs). These chapters demonstrate a new approach to measure the sum of the flexoelectric coefficients in achiral nematic LCs confined to hybrid aligned and planar-aligned geometries. The importance of ionic effects was considered, and the experimental and theoretical findings highlight the covariance between the ionic concentration and the flexoelectric coefficients. Observations of the flexoelectro-optic (FEO) effect in chiral nematic LCs were carried out by first inducing a uniform lying helix (ULH) alignment so that an electric field could be applied perpendicular to the helical axis. A polymer templating method was implemented initially, which was found to enhance the FEO tilt angle compared with that observed following a bulk polymerization technique, but was inferior to the electro-optic response measured prior to the polymerization process. Using an alternative approach, an in-situ direct laser writing (DLW) technique was employed to confine the polymer network to localised regions (polymer walls) within the device. Encouragingly, the ULH alignment was spontaneously formed when periodic polymer walls were written in the nematic phase (induced by applying a relatively large electric field to the device so as to unwind the helical structure) and spaced at the order of the device thickness. Fabrication of polymer walls in the chiral nematic phase was also investigated where it was found that the quality of the alignment was lower than that observed for polymer walls written in the nematic phase. The second half of the thesis demonstrates the manipulation of electro-optic phenomena by exploiting the dynamic aberration correction element inherent in the DLW facility. It is shown that electrically tunable defects can be formed from two-dimensional polymer structures written in topologically discontinuous states. Three-dimensional polymer structures were also created in the form of arrays of pillars, polymer helices and polymer knots. In the final experimental chapter, results are presented that demonstrate the creation of reconfigurable optical elements based upon these polymerizable LC devices, whereby a simple emoticon is used to show how different features can be made to appear and disappear at specific applied voltages.