  The electronic Lorentz theory is employed to determine the electromagnetic
response of planar split-ring metamaterials. Starting from the dynamics of
individual free carriers, the effective permittivity tensor of the metamaterial
is calculated. Whenever the split ring lacks in-plane mirror symmetry, the
corresponding permittivity tensor has a crystallographic structure of an
elliptically dichroic medium, and the metamaterial exhibits optical properties
of planar chiral structures. Its transmission spectra are different for
right-handed vs. left-handed circular polarization of the incident wave, so the
structure changes its transmittance when the direction of incidence is
reversed. The magnitude of this change is shown to be related to the geometric
parameters of the split ring. The proposed approach can be straightforwardly
generalized to a wide variety of metal-dielectric metamaterial geometries.
