Laser filaments are valuable sources for standoff spectroscopy. Also known as “the non-diffracting beams beyond multiple Rayleigh ranges” , laser filaments carry extreme intensities clamped inside their cores for extended distances. This feature in addition to the fact that the position at which the laser filaments form is controllable, make them a very appealing candidate for the standoff spectroscopy. Laser filaments have opened the road to utilize the air molecules, N 2 and O 2 , to generate stimulated emission in forward and backward directions. This is technically straightforward, though, due to a plethora of energy levels the complicated nature of emission dynamics it is not fully understood, yet. This has led to exhaustive studies to understand the pumping mechanism of the molecules inside the filaments.
In this talk I will discuss a brief history of emission from laser filaments propagating in air with focus on the emission from nitrogen cations and will continue to my results. In our lab we are interested in emission from IR and UV laser filaments propagating through air. With high resolution spectrometer we traced footprints of Raman-type and resonant emissions from vibrational and rotational levels of N 2 + and their dependence on the polarization of the pump laser. To extract the time-resolved results, we use pump-probe technique to track the time evolution of spectral lines. These results shed some light on the mechanism of population inversion in nitrogen cation. Backward emission is also measured and its dependence on the laser polarization shows that the population inversion is possible in N 2 only for the circularly polarized filaments.