Yaroshchuk O V, Kiselev A D, Zakrevskyy Yu, Bidna T, Kelly J, Chien L-C, Lindau J
Institute of Physics of NASU, prospekt Nauki 46, 03028 Kyïv, Ukraine.
Phys Rev E Stat Nonlin Soft Matter Phys. 2003 Jul;68(1 Pt 1):011803. doi: 10.1103/PhysRevE.68.011803. Epub 2003 Jul 24.
We apply experimental technique based on the combination of methods dealing with principal refractive indices and absorption coefficients to study the photoinduced three-dimensional (3D) orientational order in the films of liquid crystalline (LC) azopolymers. The technique is used to identify 3D orientational configurations of trans azobenzene chromophores and to characterize the degree of ordering in terms of order parameters. We study two types of LC azopolymers which form structures with preferred in-plane and out-of-plane alignment of azochromophores, respectively. Using irradiation with the polarized light of two different wavelengths, we find that the kinetics of photoinduced anisotropy can be dominated by either photoreorientation (angular redistribution of trans chromophores) or photoselection (angular selective trans-cis isomerization) mechanisms depending on the wavelength. At the early stages of irradiation, the films of both azopolymers are biaxial. This biaxiality disappears on reaching a state of photosaturation. In the regime of photoselection, the photosaturated state of the film is optically isotropic. But, in the case of the photoreorientation mechanism, anisotropy of this state is uniaxial with the optical axis dependent on the preferential alignment of azochromophores. We formulate the phenomenological model describing the kinetics of photoinduced anisotropy in terms of the isomer concentrations and the order parameter tensor. We present the numerical results for absorption coefficients that are found to be in good agreement with the experimental data. The model is also used to interpret the effect of changing the mechanism with the wavelength of the pumping light.
我们应用基于处理主折射率和吸收系数方法相结合的实验技术，来研究液晶偶氮聚合物薄膜中的光致三维（3D）取向有序性。该技术用于识别反式偶氮苯发色团的3D取向构型，并根据序参量来表征有序程度。我们研究了两种类型的液晶偶氮聚合物，它们分别形成了偶氮发色团在面内和面外优先排列的结构。通过用两种不同波长的偏振光进行照射，我们发现光致各向异性的动力学可以由光取向（反式发色团的角度重新分布）或光选择（角度选择性的反 - 顺异构化）机制主导，这取决于波长。在照射的早期阶段，两种偶氮聚合物的薄膜都是双轴的。这种双轴性在达到光饱和状态时消失。在光选择机制下，薄膜的光饱和状态是光学各向同性的。但是，在光取向机制的情况下，该状态的各向异性是单轴的，其光轴取决于偶氮发色团的优先排列。我们根据异构体浓度和序参量张量建立了描述光致各向异性动力学的唯象模型。我们给出了吸收系数的数值结果，发现其与实验数据吻合良好。该模型还用于解释泵浦光波长改变机制所产生的影响。
