Department of Physics and Astronomy, Macquarie University, Sydney, NSW 2109, Australia.
J Chem Phys. 2013 Jun 7;138(21):214311. doi: 10.1063/1.4808158.
Two conditions on symmetries are identified as necessary for a linear scattering system to be able to rotate the linear polarization of light: Lack of at least one mirror plane of symmetry and electromagnetic duality symmetry. Duality symmetry is equivalent to the conservation of the helicity of light in the same way that rotational symmetry is equivalent to the conservation of angular momentum. When the system is a solution of a single species of particles, the lack of at least one mirror plane of symmetry leads to the familiar requirement of chirality of the individual particle. With respect to helicity preservation, according to the analytical and numerical evidence presented in this paper, the solution preserves helicity if and only if the individual particle itself preserves helicity. However, only in the particular case of forward scattering the helicity preservation condition on the particle is relaxed: We show that the random orientation of the molecules endows the solution with an effective rotational symmetry; at its turn, this leads to helicity preservation in the forward scattering direction independently of any property of the particle. This is not the case for a general scattering direction. These results advance the current understanding of the phenomena of molecular optical activity and provide insight for the design of polarization control devices at the nanoscale.
为了使线性散射系统能够旋转光的线性偏振，需要满足两个条件：缺乏至少一个镜面反射对称和电磁对偶对称性。对偶对称性等效于光的螺旋度守恒，就像旋转对称性等效于角动量守恒一样。当系统是单一粒子种类的解时，缺乏至少一个镜面反射对称会导致个体粒子的手性的常见要求。关于螺旋度守恒，根据本文提出的分析和数值证据，解保持螺旋度的条件是并且仅仅是个体粒子本身保持螺旋度。然而，只有在向前散射的特殊情况下，粒子的螺旋度守恒条件才会放宽：我们表明，分子的随机取向赋予了解一个有效的旋转对称性；反过来，这导致在向前散射方向上的螺旋度守恒，而与粒子的任何性质无关。对于一般的散射方向则不是这样。这些结果推进了对分子旋光现象的理解，并为纳米尺度的偏振控制器件的设计提供了思路。
