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Chirality as a bulk property retrieved from the dispersion relation

Metamaterials (MM) attracted an ever increasing research interest in the past. Their main fascination derives from the possibility to attribute conceptually effective properties to them, if their characteristic length scale (being the size of the metaatoms) is much smaller than the wavelength of light. Most notably, a negative effective negative index became possible. It implies that for a certain eigenmode that is allowed to propagate inside such a MM, the real part of the wave vector and its imaginary part have opposite signs. Such negative indices were obtained, thus far, in most cases by relying on plasmonic resonances that induce an artificial magnetization. However, by taking the definition above, it is naturally anticipated that such negative refraction is a feature that can be observed in various physical systems. Indeed, similar observations were made in, e.g. plasmonic waveguides or photonic crystals. Also recently, first experimental results were presented on a negative refractive index by relying on a chirality of the metaatoms. Interesting enough, the idea to achieve negative refraction by chirality was introduced by Pendry already in 2004 [1]. There his argumentation bases nearly entirely on the dispersion relation. Nevertheless, for the observation of negative refraction the chirality has to be strongly pronounced. To induce strong chirality one may resort to appropriately shaped three-dimensional metaatoms [2] instead of quasiplanar chiral metamaterials [3] or to rely on twisted unit cells. The latter would cause an ambi-chiral media.