  The influence of the orbital symmetry on the ellipticity of the high-order
harmonics is investigated. It is found that the ellipticity maps have distinct
shapes for the molecular orbital with different symmetry. Our analysis shows
that the feature of the harmonic ellipticity map is essentially determined by
the nodal structure of the nonsymmetric orbital. The results indicate that the
molecular-orbital geometry is imprinted on the ellipticity of the high-order
harmonics, which invites the use of ellipticity measurements as a probe of the
orbital structure for polar molecules.
