  Air lasing provides a promising technique to remotely produce coherent
radiation in the atmosphere and attracts continuous attention. However, the
polarization properties of N2+ lasing with seeding has not been understood
since it was discovered ten years ago, in which the behaviors appear disordered
and confusing. Here, we performed an experimental and theoretical investigation
on the polarization properties of N2+ lasing and successfully revealed its
underlying physical mechanism. We found that the optical gain is anisotropic
owing to the permanent alignment of N2+ induced by the preferential ionization
of the pump light. As a result, the polarization of N2+ lasing tends to align
with that of the pump light after amplification, which becomes more pronounced
with increasing amplification factor. Based on the permanent alignment of N2+,
we built a theoretical model that analytically interpreted and numerically
reproduced the experimental observations, which points out the key factors for
controlling the polarization of N2+ lasing.
