Stanley, Maurice; (1993) An investigation of the second order nonlinear optical properties of self assembled organic materials. Doctoral thesis (Ph.D.), University College London (United Kingdom).

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Abstract

The requirement for efficient second order nonlinear optical materials which are easy to produce and form into devices is paramount for the successful application of nonlinear optics to optical data storage and signal processing. Mature technologies such as lithium niobate are making some progress, but the immense potential of organic materials cannot be ignored. The combination of molecular engineering and computer modelling can control many parameters including absorption bands, dipole moments and conformation of the molecules, to tailor their properties to the specific application. The self assembling nature of liquid crystal materials has been suggested to offer several potential benefits for the engineering of useful bulk materials for nonlinear optical applications. This thesis describes the investigation and exploitation of self assembled systems, both in monomeric and polymeric form, for such applications, and critically assesses the benefits and drawbacks of these materials. Two cases have been studied in detail. The first is the use of poled nematic liquid crystal side chain polymers for use in electro-optic and second harmonic generation applications. The second is an investigation into the inherent non-centrosymmetry of chiral smectic C liquid crystals. The potential advantages of using poled liquid crystal side chain polymers over amorphous polymers are a five times theoretical improvement in the nonlinear coefficient and improved orientational stability to increase device lifetimes. In practice the improvement in the nonlinear coefficient was found not significant, and cross-linking the polymer whilst being poled offers even greater stability, even at elevated temperatures. A simple experiment was developed to monitor the electro-optic coefficient of the polymer as the material was poled. The form of the response gave insight into the reorientation mechanisms both in poling and relaxing after the poling field had been removed. Ferroelectric liquid crystals can form a noncentrosymmetric symmetry if their helix can be suppressed using surface alignments and/or electric fields. The second order nonlinear coefficients of these materials have been measured using second harmonic generation (SHG). Commercial materials had relatively small coefficients, but molecular engineering has developed new materials with improved nonlinear coefficients by over one order of magnitude, and is continuing to improve. A standard material was used to investigate a number of potential device alignment configurations. SHG was also used to measure liquid crystal material parameters for application in display technologies.

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