Propagation effects of seeded collective emission by two-photon excited oxygen atoms

A strong ultraviolet pumping laser propagating through the atmosphere could activate the medium and produce the forward and backward coherent air lasing. In this work, we present the theoretical analyses of forward and backward lasing dynamics in the long gain medium consisting of oxygen atoms. By numerically solving the semiclassical Maxwell-Bloch equations, we demonstrate that the atom density, the gain medium length, and the collisional decoherence rate substantially regulate the forward and backward air laser radiation at 845 nm. Especially, our study points out that lower atom density and moderate collisional decoherence rate are favorable for the backward radiation amplification in a long medium, since such conditions balance the competing processes of forward and backward radiation fields. The simulation shows that long medium backward air lasing becomes challenging in the oxygen atom system via double-photon pump, because the amplification can only occur around z=0, and the amplified backward radiation does not carry the spectral information of atoms throughout the medium. On the other hand, certain mechanisms such as variation of backward-seeding delay could possibly alleviate the suppression of backward amplification.