Spatial dispersion, polaritons, and negative refraction

Abstract

Negative refraction occurs at an interface as a natural consequence of negative group velocity waves in one of the interfacing media. We briefly comment on the history of this understanding of the phenomenon. Several physical systems are discussed that may be capable of exhibiting normal electromagnetic waves (polaritons) with negative group velocities at optical frequencies. These systems are analyzed in a unified way on the basis of a framework provided by spatial dispersion. This framework utilizes the notion of the generalized dielectric tensor εij(ω, k) representing the electromagnetic response of the medium to perturbations of frequency ω and wave vector k. Polaritons with negative group velocity can occur in the medium (whether in natural materials or in artificial metamaterials) when spatial dispersion is strong enough. Our examples include both chiral and nonchiral systems, and bulk and surface polariton waves. We also discuss the relationship between the spatial dispersion approach and the more familiar description based on the dielectric permittivity ε(ω) and magnetic permeability μ(ω).