THEORETICAL INVESTIGATION OF PARTICLES HAVING DIRECTIONALLY DEPENDENTABSORPTION CROSS-SECTION

Examined here are a class of particles that possess highly anisotropic absorption cross sections. The particles are in the form of an eccent rically stratified sphere in which a relatively small, highly absorbin g sphere is located against the interior surface of a larger, nonabsor bing sphere. For this configuration the nonabsorbing sphere can act es sentially as a lens and focus radiation onto the absorbing sphere. Usi ng an exact solution to Maxwell's wave equations, the directionally-de pendent absorption cross sections of the particle are predicted for re fractive indites of common absorbing and nonabsorbing materials and ra diative size parameters ranging from 5 to 50. Under certain conditions , the absorption cross section of the particle can be extremely sensit ive to the direction of the incident radiation. Placing the absorber o pposite the point of incidence can result in an absorption cross secti on that is two orders of magnitude greater than its isolated, single-p article value, yet for incident angles as little as 20 deg removed fro m the focused condition, the absorption becomes comparable to or small er than the isolated value, Factors that optimize the absorption aniso tropy and the implications with regard to radiative transfer are discu ssed.