Orientation dependence in near-field scattering from TiO(2) particles.

This scattering of light by small particles embedded in a continuous transparent medium is influenced not only by the bulk optical properties of the particles and the medium but also by the size, shape, and spatial arrangement of the particles-that is, by the microstructure. If the particles are close together, as in agglomerated coatings or stereolithographic suspensions, interactions between the radiation fields of adjacent particles can lead to variations in the magnitude and spatial arrangement of the scattered light in the near and the far field, which can affect the color and hiding power of a coating, the cure depth and homogeneity in stereolithography, and the threshold intensity for stimulated emission in random lasers. Our calculations of the near- and the far-field scattering distribution for 200-nm TiO(2) spheres in pairs of various orientations and in an ordered array of five particles show that, depending on the orientation of the particles with respect to the incident light, these interactions can either increase or decrease the scattering efficiency, the isotropy of the scattering, and the magnitude of the electric field strength within the matrix and the particles. In the mid-visible range, two particles in line increase the backscattering fraction by 28% and the scattering strength by 38% over that of a single particle, whereas if the particles are in the diagonal configuration the backscattering fraction and scattering strength are actually reduced by addition of the second particle. At shorter or longer wavelengths the backscattering fraction is reduced regardless of the location of the second particle, by as much as 60% when five particles are arranged in the zigzag configuration. These results are surprising in that it is generally assumed that multiple scattering enhances backscattering. Simple models of multiple scattering or scattering of two particles as a single, larger particle are inadequate to explain these results.