不同类型偶氮材料光致双折射的温度特性研究

Temperature dependent characteristics of photo-induced birefringence in different types of azo materials

在不同温度下,分别测量了掺杂偶氮材料、偶氮聚合物和偶氮液晶聚合物的光致双折射行为,并利用双e指数模型对光致双折射的动力学过程进行了拟合.实验结果表明,偶氮材料的光致双折射源于偶氮分子的光致异构和光致分子取向,光致双折射大小随温度的升高表现出先增大后减小的趋势.在抽运光的作用下,含偶氮材料的光致双折射包含一个由偶氮分子取向引起的快过程和一个由偶氮分子带动大分子取向引起的慢过程.关闭抽运光后,掺杂偶氮材料和偶氮聚合物表现为可逆的弛豫,而偶氮液晶聚合物则展现出长久光存储特性.

At different temperatures, a semiconductor laser with a wavelength of 650 nm is used as probe light, and an Nd：YAG continuous laser with a wavelength of 532 nm is selected as pump light. The azo samples are placed between a pair of orthogonal polarizers with the vertical direction clockwise and counterclockwise 45 degrees, respectively. The polarization direction of the pump light is set to be the vertical direction. In order to reduce the effect of the stray light, a chopper is placed in the optical path of the probe light. The signal of photo-induced birefringence is recorded by a phase-locked amplifier（NF-LI5640）. The photo-induced birefringences of the doped azo material, the azo polymer and the azo liquid crystal polymer are measured respectively, and the dynamic processes of photo-induced birefringence are fitted by a double e-index model. The experimental results show that with the influence of the pump light, photoinduced birefringences of the three types of azo materials rise rapidly at first and then gradually tend to reach their own saturation state because of the photo-induced cis and trans isomerism and the photo-induced molecular orientation properties of azo molecules. The photo-induced birefringence shows a tendency to increase at first and then decrease with the temperature increasing, which can be understood as a competitive mechanism. The photo-induced birefringence depends on the photo-induced orientation and irregular thermal motions of azo groups. In the range below the glass transition temperature of the samples, the increase of the temperature of samples contributes to the rearrangement of the azo molecules due to the influence of the pump light. When the temperature of the samples is higher than the glass transition temperature, molecular chains begin to move. The irregular thermal motions of azo components and polymer molecules are aggravated. This destroys the orientations of the polymer molecules and results in the drop of the photo-induced birefringence. Comparing the doped azo material with the azo polymer sample, the azo liquid crystal polymer sample exhibits not only a larger photo-birefringence, but also the photo-induced birefringence that does not change obviously after the pump light has been turned off, which means that the azo liquid crystal polymer sample has long optical storage properties. This shows that the azo liquid crystal polymer material is an ideal polarization-sensitive optical recording medium, which is expected to be used in the fields of optical storage, polarization holography and optical information processing.