Normal modes at small cubes and rectangular particles

Abstract

The normal electromagnetic modes at small cubes and rectangular parallelepipeds are discussed. An expansion in terms of external multipoles is used; this allows an analytical evaluation of all elements of the interaction matrix. A cluster point of eigenvalues arises at the eigenvalue epsilon int( omega )+ epsilon ext( omega )=0 of a half-space. Only a few terms are needed to obtain convergence of the isolated eigenvalues which give rise to optical absorption peaks. The maximum dipole absorption peak moves from the bulk eigenvalue epsilon int( omega )=0 to the monopole eigenvalue epsilon int( omega )=- infinity with increasing extension of the parallelepiped in the direction of the dipole.