Light Scattering in Nematics and Smectic

11 Optics and Electric Field Effects in Nematic and Smectic A Liquid Crystals 

Where does such a strong scattering anisotropy originate from It is evident that the optical anisotropy of nematic hquid crystals plays the crucial role. In fact, the scattering is caused by fluctuations of the director n, i.e. the local orientation of the order parameter tensor. The local changes in orientation of n imply local changes in orientation of the optical indicatrix. 

Let us recall the reason for the light scattering in gas or in isotropic hquid. In that case, we deal with fluctuations of the mass density. They can be represented by a sum of normal elastic vibration modes (Fourier harmonics) with wavevector q and frequency Q. When such a particular mode interacts with hght of frequency m and wavevector k the conservation laws for energy and momentum read  

It follows from the theory of elastic scattering (co = co ) that, for small modulation or fluctuation of dielectric permittivity (8e Ej ), the differential cross section for the scattered light per unit solid angle around k is given by 

The strong light scattering by nematics is due to specific properties of their dielectric tensor ( , and nj are components of the director, see Section 7.2.2)  

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