Light diffusion in oriented nematic liquid crystals

We study multiple light scattering from thermal fluctuations in the ordered phase of nematic liquid crystals. This state of condensed matter is characterized by long-range fluctuations of the local director, which are known to originate from bend, splay and twist deformations. This gives rise to a complicated polarization dependence of the ''single-scattering'' phase function, of which the consequences in multiple scattering will be discussed. In order to understand multiple scattering by long-range fluctuations in the dielectric constant, we first solve the Dyson and Bethe-Salpeter Equation for a scalar field amplitude in a self-consistent way. The diffusion constant is obtained numerically from the Kubo Greenwood formula. We compare the outcome with an approach based on point-like fluctuations. In the second part we take the tensorial aspects into account and solve the vector Bethe-Salpeter equation for light scattering caused by the De Gennes 1/q(2) Structure factor for the director fluctuations. Using a Kubo-Greenwood formula for vector waves, we calculate diffusion tensor and transport mean free path l* of the light. Due to uniaxial birefringence the transport mean free path is no longer given by the conventional Boltzmann formula l* = l(scal)/(1 - <cos theta>). The consequences for the angular incoherent transmission (of the universal form (l*/L) (1/2 + 3 cos theta/4) will be investigated. Finally, we discuss the time-dependence of the thermal fluctuations in the context of the recent development of Diffusing Wave Spectroscopy initiated by Wolf and Maret.