Coherent backscattering from anisotropic scatterers

Abstract

The intensity of coherent backscattering from pointlike anisotropic scatterers is calculated. The polarized component has a peak in the backward direction, whereas the depolarized component does not exhibit a backscattering enhancement unlike the depolarized component for the isotropic scatterer case. These results agree with the measurement data on a disordered nematic liquid crystal. ͓S1063-651X͑96͒02208-8͔ PACS number͑s͒: 42.25.Ϫp Coherent backscattering ͓1-10͔ manifests itself as a sharp enhancement of light scattered backward in a narrow angular interval ϳ/l ext where is the wavelength and l ext is the extinction length. The effect is observed in highly opalescing systems in which the extinction length is significantly less than the linear size of the system. It has been studied in latex suspensions ͓1-3,5,11,12͔, ceramics ͓4͔, porous glasses ͓13͔, etc.The physical mechanism underlying coherent backscattering is quite simple. The coherent plane waves incident upon a turbid system become generally incoherent due to the multiple scattering from randomized inhomogeneities except for the waves which pass some sequence of scatterers in opposite directions. These waves can be coherent. However their interference is important only for backward direction. Such an interference is quite general for any wave process and it was first anticipated and discovered in the multiple scattering of conductance electrons in disordered metals ͓14,15͔.The backscattering problem was solved analytically for pointlike scatterers in Refs. ͓6,7,9,16͔ for a scalar field and in Refs. ͓7,8,17,18͔ for an electromagnetic field. Stephen and Cwilich ͓7͔ took into consideration the polarization effects. They showed that the backscattering peak occurs in both polarized and depolarized components. The magnitude of the peak of the polarized component is five to seven times higher than that of the depolarized component, and its form is close to triangular while the form of the peak of the depolarized component is close to Lorentzian. These results are confirmed in numerous light scattering experiments in latex suspensions ͓1-3,5,11,12͔.The coherent backscattering problem may be important not only for scalar scatterers but also for tensor ones since this effect is studied for various systems such as ceramics ͓4͔, porous glasses ͓13͔, polycrystals and liquid crystals ͓4,11͔, etc. In these systems the permittivity anisotropy may be significant. The coherent backscattering from anisotropic pointlike scatterers was briefly considered in ͓7͔. However one cannot test experimentally results obtained there because an expression for the scattered light intensity, obligatorily real, contains an imaginary part for the case of totally anisotropic fluctuations.This paper presents the study of coherent backscattering from the pointlike anisotropic scatterers. We find that the depolarized component does not practically exhibit the backscattering enhancement unlike the depolarized component for the isotropic scatterer case. The backscattering measurements performed...

Nematic liquid crystals as a new challenge for radiative transfer

“…Coherent backscattering in oriented nematics has been considered numerically . A complete analytical treatment of this effect in nematic liquid crystals has never been undertaken, and only exists for anisotropic scatterers (Kuz'min, Romanov, and Zubkov, 1996) and for anisotropic light propagation in magnetic fields (van Tiggelen, Maynard, and Nieuwenhuizen, 1996). An analytic treatment should reveal how the angular anisotropy of the line shape in coherent backscattering is affected by the optical anisotropy of the nematic, which enters in both the diffusion anisotropy and the boundary conditions.…”

Simulation of radiation transfer and coherent backscattering in nematic liquid crystals

“…where l a is the absorption length, l ext is the extinction length, cos θ is the average cosine of the scattering angle. For the phase function of the single scattering we accept 6) where N is the normalization coefficient, q = 2k sin(θ/2), θ is the scattering angle. Actually in the framework of this model it is possible to describe the multiple scattering of light in the critical region if we use the Ornstein-Zernike approximation.…”

Anisotropic enhanced backscattering induced by anisotropic diffusion

“…Measurements on nematic liquid crystals [28,29] show the absence of cone, within 1%, in the cross polarized channel, or an enhancement factor evaluated to 3% [30]. A backscatter cone in the linear orthogonal channel has been measured [31] from both 0.2-and 1-m-diameter polystyrene spheres suspensions, with enhancement of respectively 10% and 20%.…”