The optically induced birefringence in different block copolymers containing azobenzene and mesogen side groups has been studied with holographic methods. In the materials, the light-sensitive blocks carrying the azobenzene moieties are embedded in a matrix of polystyrene. This has the advantage that the reorientation of the chromophores does not lead to the formation of surface relief gratings. In addition, it is possible to reduce the macroscopic chromophore concentration to any desired value while, at the same time, maintaining the stabilization effect due to cooperative reorientation of the chromophores inside the photo-addressable phases. These short-range interactions give rise to long-term stability of the inscribed gratings. Angular multiplexing of holographic plane-wave gratings as well as of two-dimensional images has been demonstrated in samples with thicknesses up to 1.1 mm. The achievable refractive-index modulation, the photo-sensitivity, and the stability of the inscribed gratings were compared for different materials. In contrast to photopolymers, our materials do not exhibit shrinkage upon illumination. Instead, a weak light-induced volume expansion was detected and studied in detail.
The optically induced birefringence in different azobenzene and mesogen containing block copolymers has been studied with holographic methods as well as with measurements between crossed polarizers. The influence of various parameters for possible applications as holographic data storage materials was investigated. In the block copolymers, the functionalized blocks are confined in a matrix of polystyrene. This has the advantage that no surface relief
gratings appear when the chromophores are reoriented. Also liquid-crystalline stabilization due to cooperative reorientation occurs inside the functionalized blocks. These short-distance interactions in the blocks give rise to long-term stability of the inscribed gratings. So far injection-molded blends with azobenzene containing block copolymers were capable to store up to 200 holograms on one spot. On such a sample we could also demonstrate a fundamental physical effect which generally plays a role for light diffraction on thick gratings. According to the Kramers-Kronig relations, an attenuation of light is always coupled with a phase shift. In the case of a thick holographic grating, the intensity of the transmitted wave (of diffraction order zero) is a function of the angle of incidence, because part the light is diffracted into the first order when the Bragg condition is fulfilled. We measured the phase shift of the transmitted wave as function of the Bragg mismatch angle and found good agreement with analytical calculations.