  Optically active artificial structures have attracted tremendous research
attention. Such structures must meet two requirements: Lack of spatial
inversion symmetries and, a condition usually not explicitly considered, the
structure shall preserve the helicity of light, which implies that there must
be a vanishing coupling between the states of opposite polarization handedness
among incident and scattered plane waves. Here, we put forward and demonstrate
that a unit cell made from chiraly arranged electromagnetically dual scatterers
serves exactly this purpose. We prove this by demonstrating optical activity of
such unit cell in general scattering directions.
