Theory of ferromagnetic cavity polaritons

Abstract

The theory of non-retarded and retarded ferromagnetic cavity polaritons is developed. These are the excitation modes of a cavity formed between the parallel planes of two semi-infinite ferromagnets. It is shown that the cavity modes can propagate within a limited angular range. The dispersion relation of the cavity polaritons is derived first within the magnetostatic approximation, and is then extended to include retardation effects. It is shown that retardation introduces a lower cut-off on the polariton wavevector. As the cavity width is varied from zero to infinity, the polaritons are shown to develop from bulk magnons into surface magnons. For a finite cavity width, the associated polariton field configuration is non-symmetric, being larger on the wall where the propagation direction agrees with that of the allowed surface mode direction.