Nonadiabaticity of cavity-free neutral nitrogen lasing

We report on a theoretical study of cavity-free lasing of neutral nitrogen molecules in femtosecond laser filaments with a nonadiabatic Maxwell-Bloch model, compared with recent pump-seed experiments. The nonadiabaticity of the lasing process is revealed and it is found that electron-neutral collisions dominate the dipole dephasing rate. Moreover, we show that the asymmetry between forward and backward lasing not only depends on the different amplification lengths but also on the temporal dynamics of electron-neutral collisions. The comparison of the nonadiabatic model with simulations based on the adiabatic approximation (such as radiative transfer equations) explicitly sets a bound on the validity of the latter model for cavity-free nitrogen lasing phenomenon, which holds a unique potential in optical remote sensing applications.