Electrical and optical effects in hybrid liquid crystal cells

Abstract

The main aim of this project was to characterise electrical and optical effects in hybrid liquid crystals. Hybrid form of liquid crystals was achieved by using photoconducting alignment layers in liquid crystals cells and also by including ferroelectric, inorganic nanoparticles in the liquid crystal bulk. Such liquid crystal systems show strong electro-optic and photorefractive effects. In order to understand the optical performance of hybrid liquid crystals, it was essential that the electrical response of the system to the incident illumination levels and different amplitudes of applied electric field be understood. In order to achieve enhanced electro-optic and photorefractive response, nano size ferroelectric particles were added in of low concentration to different nematic liquid crystals; BaTiO₃ and Sn₂P₂S₆ nanoparticles were added to TL205, LC18523 and MLC6815 liquid crystals. The first liquid crystal selected was chosen because of its low ionic content and the two latter ones as they are well-known low refractive index liquid crystals that also show low birefringence. The size and shape of these nanoparticles were characterised via three different imaging techniques, namely Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM) and Nanosight TM. It was observed that the presence of the particles did not adversely affect the optical quality of the cell, but instead show increased birefringence, dielectric anisotropy and clearing temperature as well as decrease in the Freedericksz threshold. The gain coefficient, associated with the strength of beam coupling, caused by the photorefractive effect was also increased with the inclusion of ferroelectric nanoparticles. The gain coefficients were measured in two different geometries, Raman-Nath and Bragg, corresponding to large and small grating spacings, respectively. In all cases suspensions of ferroelectric nanoparticles led to the increase of the measured gain coefficients as compared to the pure liquid crystals. Beam coupling was initially measured with a DC electric field, as it is a typical requirement to observe the effect. However, there are some disadvantages of using a DC field as the movement of ions taking place on different time scales leads to complex dynamics and modelling. Therefore, the use of a low frequency AC field, instead of a DC field, was explored and strong two-beam coupling was measured. This project demonstrated the potential of nematic liquid crystals to serve as a versatile and efficient base for functionalised, tailor-made opticalmaterials, ideal for beam steering and processing

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