  We investigate the polarization properties of high harmonic generation from
polar molecules with a linearly polarized field. It is found that elliptically
polarized harmonics are observed in a wide spectral range from the plateau to
the cutoff. Further analyses show that the nonsymmetric structure of the
highest occupied molecular orbital is the origin of ellipticity of the
harmonics. The results provide a method for generation of large-ellipticity XUV
pulses, which will benefit the application of HHG as a tool of detection in
materials and biology science.
