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Volume 89, Issue 3, 01 February 2001
- APPLIED BIOPHYSICS (PACS 87)
89(2001); http://dx.doi.org/10.1063/1.1336564View Description Hide Description
When film coatings are made of pigment particles embedded in a transparent resin, the optical characteristics of the resulting film are determined not only by the bulk optical properties of the constituent materials, but also by the spatial distribution of the light scattered from small particles. If the particles are separated by distances comparable to their diameter, as is the case for high particle concentrations or agglomerated systems, the near-field interactions between the radiation fields of the particles can strongly influence the resulting far-field intensity distribution. In this work we have used full-field finite-element solutions of Maxwell’sequations to calculate the near- and far-field scattering patterns for single 500 nm quinacridone spheres and for pairs of particles. We find that the scattered intensity forms a forward-directed plume that extends far beyond the particle surface, especially at short wavelengths and where the absorption is large. This results in near-field interactions between pairs of particles that can either increase or decrease the scattering (both the total scattering and the fraction of the scattered light that is directed into the backward hemisphere), depending on the orientation of the particle pair relative to the direction of the incident light. In some cases, particularly if the particles are aligned along the incident direction, the two spheres can produce a far-field scattering distribution that is approximately that of a single, larger (sometimes much larger) sphere. If the particles are aligned perpendicular to the incident direction, the strength of the scattering per particle volume is roughly the same as for a single particle, but the scattering is more forward directed. These interaction effects occur even though the surface-to-surface separation of the particles is larger than the distance for which a single particle shows significant scattered intensity. These near-field and far-field phenomena are beyond the limitations of single-scattering and independent multiple-scattering approaches, and the near-field interactions can have a significant effect on the scattering of light from films containing such particles, especially if they tend to form oriented clusters.