Spheroidal harmonic expansions for the solution of Laplace's equation for a point source near a sphere

TL;DR

This paper introduces a spheroidal harmonic expansion method for solving Laplace's equation near a sphere, offering faster convergence than traditional spherical harmonics, with applications in nanophotonics.

Contribution

It presents the novel use of prolate spheroidal harmonics for Laplace's equation near spheres, improving convergence and applicability in physics problems.

Findings

Faster convergence of series expansions using spheroidal harmonics.

Enhanced calculation of light emitter decay rates near nanostructures.

Applicable to various geometries and equations in physics.

Abstract

We propose a powerful approach to solve Laplace's equation for point sources near a spherical object. The central new idea is to use prolate spheroidal solid harmonics, which are separable solutions of Laplace's equation in spheroidal coordinates, instead of the more natural spherical solid harmonics. We motivate this choice and show that the resulting series expansions converge much faster. This improvement is discussed in terms of the singularity of the solution and its analytic continuation. The benefits of this approach are illustrated for a specific example: the calculation of modified decay rates of light emitters close to nanostructures in the long-wavelength approximation. We expect the general approach to be applicable with similar benefits to a variety of other contexts, from other geometries to other equations of mathematical physics.