【作者】 潘煦；

【导师】 王长顺；

【作者基本信息】 上海交通大学 ， 光学， 2009， 博士

【摘要】 含偶氮聚合物具有出色的光偏振敏感特性,近些年来在光存储领域被广泛地研究。在可吸收波段的激光照射下,偶氮分子由于发生反复的顺反异构过程而转动;如果作用光是线偏振光,则偶氮分子会被有序排列在垂直于光偏振的方向上,从而样品薄膜表现出光学各向异性（二向色性和双折射特性）。如果采用偏振态空间调制的光场照射偶氮聚合物薄膜,则偶氮分子的取向也相应周期性变化,即在薄膜中完成了偏振全息记录。记录得到的偏振光栅可以控制光束的传播方向和偏振态,在功能性光学器件中有很大应用价值。本论文主要针对含偶氮聚合物薄膜的光致各向异性和偏振全息记录开展了一系列研究。首先,介绍了偶氮聚合物材料的化学、光学特性。其中着重介绍了非线性光学、光致异构和光致各向异性特性,这些特性对于光存储研究非常关键。进一步,通过实验具体研究了一种含偶氮离子型液晶聚合物的非线性光学和光致各向异性特性。其三阶非线性折射率由Z扫描方法测量得到,各向异性特性由光致二向色性实验作了定性分析。结果表明,该样品具有较大的三阶非线性折射率和显著的光致各向异性特性,是一种理想的光存储材料。其次,研究了三种偶氮聚合物的光致双折射性质。研究了泵浦光强对样品光致双折射值的影响,并采用理论公式对实验结果进行了拟合。另外,结果表明在室温下两种偶氮侧链液晶样品的双折射值(～10^-2)比普通偶氮侧链样品(～10^-3)大,并且液晶样品中双折射的存储非常稳定,在弛豫过程中双折射值甚至会自发增大。文中采用液晶材料的自组织特性来解释了这一现象。进一步,温度特性实验表明样品的光致双折射值与温度间存在明显的依赖关系,改变温度能有效控制光致双折射值的大小,文中对其中机制进行了讨论。再次,在光致各向异性研究的基础上,完成了两种偶氮液晶聚合物薄膜中的偏振全息记录。实验采用两束偏振相互正交的532nm线偏或圆偏光在薄膜中写入了纯偏振光栅。光栅的偏振特性通过实验和理论两方面研究,尤其是偶氮侧链液晶样品中偏振光栅独特的特性表明,该样品在线偏振记录过程中产生了圆双折射。进一步,提出了一种利用圆偏光偏振全息实现的图像存储方法,该方法中再现图像信号具有较高的衍射效率,并且图像亮度可以通过改变再现光的偏振态来有效控制。最后,液晶聚合物样品的偏振全息记录扩展到了二维记录,即采用多次曝光的方法在样品同一点上记录多个偏振光栅。实验中得到的二维偏振光栅由两个正交的一维偏振光栅元组成。光栅特性研究表明,二维光栅的衍射效率较只存在单个一维光栅时低,其偏振特性是两个一维光栅元特性的叠加。进一步,二维偏振光栅被应用到一种光斯托克斯参量测量系统中,达到了简化原系统的目的。测试结果说明改进后的系统能够提供较高的测量精确度,同时也证明了所使用的二维偏振光栅具有良好的稳定性。更多还原

【Abstract】 In the past decades, azobenzene containing polymers have attracted considerable interest in optical storage owing to their outstanding photoactive characteristics. It is well known that azo molecules undergo trans-cis-trans isomerization cycles under the irradiation at a wavelength lying in the absorption region. If the pump light is linearly polarized, the molecules are oriented perpendicular to the pump polarization direction. This photoinduced orientation results in macroscopic anisotropy, thus the dichroism and birefringence in polymer films. Since azo molecules can be photo-oriented, irradiating with a light field whose polarization is periodically modulated in space can induce the periodical orientation of molecules correspondingly, i.e. polarization holographic recording. The obtained polarization gratings are very useful for highly functionalized optical devices because they can control both the beam propagation direction and polarization state. This thesis gives a detailed description of the photoinduced anisotropy and polarization holography in azo polymer films.First of all, the chemical and optical properties of azo polymers are introduced. Especially the characteristics of nonlinear optics, photo-isomerization and photoinduced anisotropy are emphasized, because they are crucial for optical storage. Furthermore, the optical properties of an azo ionic liquid-crystalline （LQ） polymer are investigated experimentally. The third-order nonlinear refractive index is determined by Z-scan measurements, and the photoinduced anisotropy is studied through dichroism. The large value of nonlinear refractive index and pronounced photoinduced anisotropy indicates that the polymer is an ideal material for optical storage.Secondly, the photoinduced birefringence of three azo polymers is investigated. The influence of pump intensity on birefringence value is studied, and the experimental results are analyzed by theoretical fitting. The results also indicate that the two azo LQ polymers possess larger birefringence value (～10^-2) than that of the ordinary azo side-chain polymer (～10^-3) in room temperature, and the stored birefringence in LQ polymers is very stable （even exhibits self-enhancement in relaxation）. This phenomenon is attributed to the self-organization effect of the LQ molecules. Moreover, it is proved that the photoinduced birefringence has a strong dependence on the temperature, therefore the birefringence value can be controlled effectively by varying the temperature, and the mechanism is discussed.Thirdly, based on the characterization of photoinduced anisotropy, polarization holographic recordings are accomplished in two azo LQ polymers. Pure polarization gratings are recorded by two 532 nm laser beams with both orthogonal linear and circular polarizations. The polarization properties of the gratings are investigated experimentally and theoretically. Especially, the unusual diffraction properties of gratings in azo side-chain LQ polymer reveal that circular birefringence is induced in linear-polarization holographic recording. Furthermore, a method of image storage based on circular-polarization holography is presented, which provides high diffraction efficiency of the reconstructed image and an effective approach to control the image brightness by changing reconstructing polarization.At last, the polarization holography in LQ polymers is extended to two-dimensional （2D） recordings, i.e. polarization gratings overlap at the same place through multiple recordings. The 2D polarization grating in present makes up of two orthogonal 1D polarization gratings, which are written by two orthogonally polarized 532 nm beams （linearly or circularly）. The diffraction properties of the 2D gratings are investigated. It is found that the diffraction efficiency of 2D grating is lower than that of 1D grating, and the 2D grating acts as the integration of its componential 1D gratings. Furthermore, the 2D grating is applied in a stokes-meter, which simplifies the system. Test results show that the improved stokes-meter can provide a high accuracy, which also indicates the stable performance of the 2D polarization grating.更多还原