Negative refractive index in gyrotropically magnetoelectric media

The possibility to realize a negative refractive index in gyrotropically magnetoelectric materials is studied. Since the Tellegen (nonreciprocity) parameter may probably dramatically reduce the refractive indices, the negative refractive indices of a gyrotropic chiral material with positive permittivity and permeability can be achieved. By using the concept of equivalent isotropic medium, the respective negative equivalent permittivity and permeability corresponding to the eigenmodes inside the anisotropic material can be derived. This is a scheme to realize the negative refractive index (and hence the backward wave propagation) in artificial composite materials. Besides this scheme, we consider an alternative way to realize negative refraction by means of a magnetoelectrically anisotropic material.