Liquid crystalline polymers for nonlinear optics : pyroelectric polymers and ferroelectric dendrimers

Sammanfattning: This thesis deals with liquid crystalline materialsincluding monomers, side-chain polymers, crosslinked polymers,and dendrimers. The materials are pyroelectric, meaning thatthere is a macroscopic electric polarization in the material,which is energetically stable in the absence of externalfields. The energetically stable, or spontaneous, polarizationmakes the material suitable for second-order nonlinear optics(NLO). Examples of second order NLO processes are secondharmonic generation (SHG), optical parametric amplification,and processes involving modulations of the refractive indices.In isotropic polymeric systems, the material is made secondorder NLO active by applying an electric field at hightemperatures, thereby inducing polar order, and by cooling toroom temperature with the field applied. The polarization isthereby "locked-in" into the polymer, and the material issecond-order NLO active. Such systems often loose polar order,and thereby second order NLO response, quite rapidly. The polarorder in pyroelectric materials is however energeticallystable, and one may therefore expect better time-dependentsecond order NLO properties in these materials. In this work,pyroelectric polymers were prepared by photopolymerization ofsurface stabilized ferroelectricliquid crystals. Thephotopolymerized material showed a clear second order NLOresponse, as measured by SHG, also in the absence of anexternal electric field. The d23-coefficient was 0.25 pm V-1. Furthermore, the pyroelectric polymers werestable at room temperature for approximately 50 days. In orderto improve the thermal stability of the polymers, a crosslinkedpolymer system was prepared, which exhibited improved thermalstability, compared to the uncrosslinked polymers. Thecrosslinked polymers were unaffected by aging at 130°C forseveral hours. The thermal stability of the polymers wasfurther analyzed by a pyroelectric technique. The largestestimated pyroelectric coefficient was 2 nC cm-2K-1. The crosslinked polymer showed a weak but clearpyroeletric response even after being heated to 260 °C.Novel liquid crystalline dendrimers of different generationwere also studied. The dendrimers were ferroelectric, meaningthat there are two stable polarization states in the material,and that the polarization may be reversed by application of anelectric field. The dendrimers showed a slightly lower value ofspontaneous polarization than their low molar mass analogue.The spontaneous polarization of the LC dendrimers wasapproximately 40 nC cm-2, and was basically independent of the generationnumber of the dendrimer. A clear, but rather small SHG responsewas recorded in these systems. By incorporating a nitro-groupin the mesogen of the LC dendrimer, the d23- coefficient of the second order NLO responsecould be improved by a factor 3, to 0.03 pm V-1. This thesis gives an introduction topyroelectricity, ferroelectricity, and linear and nonlinearoptical effects in materials. It further contains descriptionsof various techniques that were used to characterize the novelliquid crystalline materials. It discusses the results from themeasurements and the properties of the liquid crystallinematerials, with focus on second order NLO.