Mosallaeipour Marjan, Ananthamurthy Sharath, Madhusudana N V
Raman Research Institute, 560012 Bangalore, India.
Eur Phys J E Soft Matter. 2013 Aug;36(8):93. doi: 10.1140/epje/i2013-13093-0. Epub 2013 Aug 30.
There have been recent reports of continuous rotation of chiral nematic droplets in restricted ranges of diameter/pitch (d /p) values, trapped by a linearly polarized laser beam. We have developed a simple model to calculate the distortion in the helical structure of a set of flat layers, caused by the action of the strong electric field of the propagating laser beam on the dielectric anisotropy of the medium. The resulting change in the polarization state of the beam passing through the sample is then used to calculate the torque on the sample as a function of the azimuthal angle of the first layer. The main results are: i) the torque tends to zero even with circularly polarized beam for samples with thicknesses around integral multiples of 0.5p ; ii) the undistorted sample takes an equilibrium orientation for linearly polarized beam, which jumps by π/2 rad at the same sample thicknesses; iii) these samples will have a nonzero torque at all azimuthal angles of the first slice when the helical structure is distorted by the linearly polarized beam. The calculations show that a propagating accordion mode, in which the helical pitch alternately expands and contracts, gives rise to the nonzero torque. The theoretical predictions are in broad agreement with experimental results.
最近有报道称，在手性向列相液滴直径/螺距（d /p）值的有限范围内，其会被线偏振激光束捕获并持续旋转。我们开发了一个简单模型，用于计算由传播激光束的强电场作用于介质的介电各向异性而导致的一组平坦层螺旋结构的畸变。然后，利用穿过样品的光束偏振态的变化来计算样品上的扭矩，该扭矩是第一层方位角的函数。主要结果如下：i）对于厚度约为0.5p整数倍的样品，即使使用圆偏振光束，扭矩也趋于零；ii）对于线偏振光束，未畸变的样品会采取平衡取向，在相同样品厚度下，该取向会跳跃π/2弧度；iii）当线偏振光束使螺旋结构畸变时，这些样品在第一层的所有方位角处都会有非零扭矩。计算表明，一种传播的手风琴模式，即螺旋螺距交替膨胀和收缩，会产生非零扭矩。理论预测与实验结果大致相符。
