  We studied the optical properties of a dielectric photonic crystal structure
with spirals arranged in a hexagonal lattice. The dielectric constant of the
material is 9 and the filling ratio is 15.2%. We found that this kind of
structure exhibits a significant polarization gap for light incident along the
axis of the spirals. The eigenmodes inside the polarization gap are right-hand
(left-hand) circularly polarized depending on the whether the spirals are
left-handed (right-handed). The transmission spectrum of a slab of such a
structure has been calculated and matches well with the analysis of the
eigenmodes.
