  A bilayered chiral metamaterial (CMM) is proposed to realize a 90 degree
polarization rotator, whose giant optical activity is due to the transverse
magnetic dipole coupling among the metallic wire pairs of enantiomeric
patterns. By transmission through this thin bilayered structure of less than
\lambda/30 thick, a linearly polarized wave is converted to its cross
polarization with a resonant polarization conversion efficiency (PCE) of over
90%. Meanwhile, the axial ratio of the transmitted wave is better than 40 dB.
It is demonstrated that the chirality in the propagation direction makes this
efficient cross-polarization conversion possible. The transversely isotropic
property of this polarization rotator is also experimentally verified. The
optical activity of the present structure is about 2700 degree/\lambda, which
is the largest optical activity that can be found in literature.
