Light Scattering by a Spheroidal Particle

TLDR

Electromagnetic scattering of spheroidal particles involves decomposing oblique incident waves into TM and TE modes, with incidence along the rotation axis reducing to a Mie‑like form, and depends on size, eccentricity, refractive index, incident angle, and scattering direction. The authors solve Maxwell’s equations by separating vector wave equations in spheroidal coordinates, expanding in spheroidal wavefunctions, and determining expansion coefficients from boundary‑condition continuity equations. The resulting scattering solutions for prolate and oblate spheroids share the same form, with oblate equations derivable from prolate ones by exchanging the corresponding spheroidal wavefunctions.

Abstract

The solution of electromagnetic scattering by a homogeneous prolate (or oblate) spheroidal particle with an arbitrary size and refractive index is obtained for any angle of incidence by solving Maxwell's equations under given boundary conditions. The method used is that of separating the vector wave equations in the spheroidal coordinates and expanding them in terms of the spheroidal wavefunctions. The unknown coefficients for the expansion are determined by a system of equations derived from the boundary conditions regarding the continuity of tangential components of the electric and magnetic vectors across the surface of the spheroid. The solutions both in the prolate and oblate spheroidal coordinate systems result in a same form, and the equations for the oblate spheroidal system can be obtained from those for the prolate one by replacing the prolate spheroidal wavefunctions with the oblate ones and vice versa. For an oblique incidence, the polarized incident wave is resolved into two components, the TM mode for which the magnetic vector vibrates perpendicularly to the incident plane and the TE mode for which the electric vector vibrates perpendicularly to this plane. For the incidence along the rotation axis the resultant equations are given in the form similar to the one for a sphere given by the Mie theory. The physical parameters involved are the following five quantities: the size parameter defined by the product of the semifocal distance of the spheroid and the propagation constant of the incident wave, the eccentricity, the refractive index of the spheroid relative to the surrounding medium, the incident angle between the direction of the incident wave and the rotation axis, and the angles that specify the direction of the scattered wave.