Eigenmodes, quasimodes, and quasiparticles

A general linear field supports both a discrete and continuous spectrum of modes or source-free field solutions. Although only eigenmodes are necessary for a complete representation of fields excited by sources, noneigen discrete modes (quasimodes) provide a useful and rapidly convergent alternative to the continuum eigenmode part of a field representation. Quasimode field representations, and their interpretation in terms of quasiparticles, are discussed both generally and for the special case of a vector electron plasma field, for which the complete set of eigenmodes is found via a resolvent or characteristic Green's function method.