Studies of ferroelectric liquid crystalline systems designed for non linear optical effects

In the rush to develop the optical communications systems of the future, electro-optic and nonlinear optical (NLO) processes, and the materials which exhibit them, have become areas of research currently attracting much interest. The ferroelectric SC*, liquid crystalline phase (FLC) possesses low symmetry properties which, under certain circumstances, permit a material demonstrating this phase to exhibit electro-optic switching, a macroscopic net dipole moment, hysteresis, bistability and second order non linear optical effects such as second harmonic generation (SHG). These characteristics can be utilised in the development of fast highly multiplexible displays or logic elements for optical computing. In addition polymer FLC (PFLC) materials offer the advantage of polymer processability thus lending themselves to the development of optical fibres, thin films and integrated components able to double the frequency of incident laser light. The second order non linear susceptibility coefficients of FLC materials, however are not usually large since the axis of maximum quadratic hyperpolarisability lies normally to the C2 symmetry axis along which both the ferroelectric dipole moment and the second harmonic signal are naturally permitted. This thesis presents the development of novel side chain PFLC systems, designed to possess enhanced quadratic hyperpolarisability along the direction of the ferroelectric dipole, with the ultimate aim of storage of the natural ferroelectric polar order in the polymer glass phase in the absence of an applied field. This enhancement was achieved by the use of donor-acceptor 7t-systems (nonlinear optic chromophores). In some cases these chromophores were included directly onto the chiral mesogenic side group of a FLC homopolymer, but in most cases a chromophore based on a 5-dimethylamino-2-nitro-benzoic acid derivative was included as a non mesogenic side chain placed next to a variety of chiral mesogenic moieties in order to form copolymeric systems. Structures based on polyacrylate and polysiloxane backbones were synthesised. The effect of the inclusion of the non mesogenic NLO chromophore comonomer on the mesophase and ferroelectric behaviour of the materials and the molecular design alterations necessary in order to produce a material which could be readily aligned, exhibit spontaneous polarisation and second harmonic generation are presented. An increasing tendency towards linear electro-optic responses is found to accompany the progressive inclusion of higher proportions of non mesogenic NLO chromophores into the ferroelectric polymer structure. 'Dilution' of the number of functionalised sites along the backbone with the use of dimethylsiloxane units, and the increased free volume this allows the side chains, is shown to be a critical factor determining the ability of the structure to respond electro-optically. Experimental apparatus, measurement procedures and cell design tailored to permit the FLC and NLO characterisation of these PFLC materials are described. Alignment problems and defect textures of relevance to these particular compounds are discussed. In preparation for the study of the more difficult polymeric systems considered within this project, a low molar mass ferroelectric liquid crystalline material designed for enhanced susceptibility is ferroelectrically characterised. The effect of the tight pitch it possesses on its electro-optic properties is discussed with reference to a potential ambiguity with characteristics of the antiferroelectric phase. Similar behaviour is seen in several of the polymeric compounds and an associated pronounced deformed helix effect is reported. Clear understanding of these effects is found to be very important to the achievement of correct phase assignment, and in determining the conditions under which the helix of the SC* phase is fully unwound, thus permitting unobscured SHG and Ps measurements. Finally the ferroelectric characterisation of a new oligomeric compound is presented. This material was expected to have qualities intermediate between those of a polymer and a low molar mass material and showed an exceptionally strong electroclinic effect. Further development of the project and future research is outlined.