Institute of Industrial Science, University of Tokyo, Tokyo 153-8505, Japan.
J Chem Phys. 2012 Feb 14;136(6):064509. doi: 10.1063/1.3682469.
Depolarized light scattering is widely used to probe the spatial correlation of optical anisotropy in crystals, liquid crystals, and viscoelastic materials under stress, and a powerful means to study a non-equilibrium pattern evolution process of such a system. To follow the temporal change in the diagonal and off-diagonal contributions of the dielectric tensor, it is highly desirable to measure two-dimensional (2D) polarized (HH: horizontally transmitted, horizontally received) and depolarized (VH: vertically transmitted, horizontally received) scattering patterns simultaneously in a time-resolved manner. We develop a light scattering system with a video-rate time resolution as well as very high sensitivity to optical anisotropy. To detect extremely weak VH scattering from a sample without suffering from residual birefringence of the optical system itself and leakage of strong HH scattering signals, we use an objective lens specially designed for polarizing microscopy and Glan-laser prisms, respectively. This system enables us to experimentally elucidate the origin of VH scattering: we use the ratio of the VH and HH scattering intensity as a fingerprint for whether a 2D VH scattering pattern is caused by (i) optical anisotropy (intrinsic birefringence) or merely by (ii) spatial inhomogeneity of optically isotropic materials. We verify the validity of this method for a process of phase separation in a binary mixture of isotropic liquids. The simultaneous HH and VH measurement allows us to directly estimate the ratio of VH and HH scattering intensity accurately. The careful comparison of this ratio with a simple theory unambiguously demonstrates that the 2D VH scattering pattern is caused by the scattering angle dependence of the diffraction efficiency of light with the two polarization directions. That is, the origin of VH scattering is due to geometrical effects of the inhomogeneous distribution of the refractive index and not due to optical birefringence, as it should be for the optically isotropic sample. This method using the ratio of VH and HH scattering intensity may be widely used for distinguishing the two types of origins for a VH scattering pattern in an unambiguous manner.
去极化光散射广泛用于探测晶体、液晶和粘弹性材料在应力下光学各向异性的空间相关性，是研究此类系统非平衡模式演化过程的有力手段。为了跟踪介电张量对角和非对角分量的时间变化，非常希望以时间分辨的方式同时测量二维（2D）偏振（HH：水平传输，水平接收）和去极化（VH：垂直传输，水平接收）散射图案。我们开发了一种具有视频速率时间分辨率和非常高光学各向异性灵敏度的光散射系统。为了检测来自样品的极其微弱的 VH 散射，而不会受到光学系统本身残余双折射和强 HH 散射信号泄漏的影响，我们分别使用了专门设计用于偏振显微镜的物镜和格兰激光棱镜。该系统使我们能够通过实验阐明 VH 散射的起源：我们使用 VH 和 HH 散射强度的比值作为 2D VH 散射图案是否由（i）光学各向异性（固有双折射）或仅仅由（ii）各向同性材料的空间不均匀性引起的指纹。我们验证了这种方法在各向同性液体二元混合物相分离过程中的有效性。HH 和 VH 的同时测量使我们能够直接准确地估计 VH 和 HH 散射强度的比值。仔细比较该比值与简单理论，明确表明 2D VH 散射图案是由两个偏振方向的光的衍射效率随散射角的变化引起的。也就是说，VH 散射的起源是由于折射率不均匀分布的几何效应，而不是由于光学各向异性，因为对于各向同性样品，VH 散射应该归因于光学双折射。这种使用 VH 和 HH 散射强度比值的方法可以广泛用于明确区分 VH 散射图案的两种起源类型。
