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.
Photosensitive cholesteric polysiloxanes, which contain an azo dye, were irradiated with linearly polarized light. The cholesteric samples were oriented in the Grandjean texture. Before irradiation they reflected circularly polarized light in the near infrared region. For perpendicular incidence, only one order of reflection was observed. Upon irradiation with linearly polarized light, which is absorbed by the azo dye, additional reflection bands appeared in the visible part of the spectrum. It turned out that the additional reflection is caused by a new Bragg type grating which shows higher reflection orders. The formation of the grating is based on the periodic deformation of the helical ordering of the molecules. The deformation is periodic, as due to photoselection, only dye molecules in equidistant layers with a suitable orientation absorb radiation. For low exposure, the grating reflects linearly polarized light. After continued irradiation, the reflection bands disappear almost completely. High birefringence, strongly dichroic dye absorption and the loss of the reflecting properties prove that a planar nematic texture has developed. The formation of this texture from the Grandjean texture is a new example for photoinduced rotational diffusion.
Much research has been done toward efficient nonlinear optical materials and their application in second harmonic generation (SHG) and in electro-optical devices." -31 Organic polymers have been of special interest because of their improved processability, their potentially large nonlinear susceptibility, their good mechanical and thermal properties and the low cost of production. Therefore, these materials are most promising for potential technical applications.Since nonlinear optical properties like SHG only appear in materials with a noncentrosymmetric structure, poling of the polymeric films in an external field is necessary. Poling can be done in a DC electrical field or by the more efficient corona poling." -41 For the poling in the DC field, the polymer has to be heated above the glass-transition temperature T, in order to enable the dipolar chromophores to orient parallel with respect to each other and to the external field. On cooling of the samples with the electrical field still on, the polar structure is frozen-in in the glassy material. The longterm stability of the resulting structure is a critical factor in view of possible technical applications.In early experiments, the NLO chromophores were only physically mixed with the polymer to form guest-host systems." -61 Doped polymers are easily produced, but generally the concentration of the chromophore is limited to rather low values and the poled, noncentrosymmetric structures suffer from relaxation phenomena which destroy the polar alignment of the chromophores. However, only recently a guest-host system showed excellent thermal stability due to an increase of T, above 200 "C. ['] A higher density of the chromophoric group and samples with improved thermal stability also result when the NLO chroniophore is chemically linked to the polymeric backbone, thus forming a copolymer.[" '* ' . '] The long-term thermal stability of some copolymers was increased via chemical cross-linking during the poling in an electric field. ["-12] Compared to organic crystals and to Langmuir-Blodgett [**I Financial support from the BMFT, pmject no. 03M4059. and from the films,", 21 the polar ordering is principally reduced in copolymers and polymeric guest-host systems due to the amorphous structure of polymeric materials. Liquid-crystalline side-chain polymers and main-chain polymers, however, combine the advantages of polymers with the favorable properties of anisotropic crystals and seem to be well suited as matrices for the polar orientation of chromophores. Both in anisotropic guest-host systems['-3. 'I and in copolym e r~[ '~. more efficient SHG was possible, which proved the advantages of anisotropic matrices.A different way to reorient the side chains was discovered when azo-dye-containing liquid-crystalline side-chain polymers were investigated with regard to their potential in optical information storage." ', 16] In this new method, continued photochemical trans-cis isomerization of the azo dye results in a reorientation of the dye molecules and the surround...
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