Controlling the Polarization of Nitrogen Ion Lasing

Air lasing provides a promising technique to remotely produce coherent radiation in the atmosphere and has attracted continuous attention. However, the polarization properties of N₂⁺ lasing with seeding have not been understood since it was discovered 10 years ago, in which the polarization behaviors appear disordered and confusing. Here, we performed an experimental and theoretical investigation of 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 the N₂⁺ lasing tends to align with that of the pump light after amplification, which becomes more pronounced as the amplification factor increases. 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.