Photoinduced Reorientation of Cholesteric Liquid Crystalline Polysiloxanes and Applications in Optical Information Storage and Second Harmonic Generation

When liquid crystalline polymers, which contain an azo dye as a side chain, are irradiated with light which is absorbed by the azo dye, both the index of refraction and the absorbance are changed. It was proven that the modulation of the optical properties mainly rests upon a reorientation of the long molecular axis of the chromophores and of the mesogenic units. This change of the alignment of the side chains occurs after numerous trans‐cis‐isomerization cycles of the azo dye. Spectroscopic investigations show that by irradiating the samples completely new and well ordered textures appear. The characteristic features of the new textures depend on the structure of the liquid‐crystalline polymer before irradiation and on the polarization state of the actinic light. We investigated cholesteric polysiloxanes which were either irradiated with linearly or with circularly polarized light (unpolarized light respectively). The characterization of the new textures was done with different spectroscopic methods. The photochemically induced changes of the texture are usable in applications like optical information storage, e.g. for the generation of holograms or for the production of displays. We further investigated samples with dipolar azo dyes which were reoriented with light under the influence of an electrical field. The cooperative effect of light and field results in a polar alignment of the azo dye which was proven by second harmonic generation (SHG). We compare material properties like longterm stability and efficiency of the SHG in samples which were prepared by this new procedure, by contact poling and by corona poling.