We report on the lasing in air and pure nitrogen gas pumped by a single 800 nm femtosecond laser pulse. Depending on gas pressure, incident laser power and beam convergence, different lasing lines are observed in the forward direction with rapid change of their relative intensities. The lines are attributed to transitions between vibrational and rotational levels of the first negative band of the singly charged nitrogen molecule-ion. We show that self-seeding plays an important role in the observed intensity changes.

We report on strong backward stimulated emission at 337 nm in nitrogen gas pumped by circularly polarized femtosecond laser pulses at 800 nm. A distinct dependence of the backward UV spectrum on pump laser polarization and intensity is observed, pointing to the occurrence of backward amplified spontaneous emission inside filaments. We attribute the population inversion to inelastic collision between the free electrons produced by the pump laser and neutral N₂ molecules. The addition of oxygen molecules is detrimental for the gain, reducing it to near threshold at atmospheric concentration.

Filaments produced in air by intense femtosecond laser pulses emit UV luminescence from excited ${\mathrm{N}}_{2}$ and ${\mathrm{N}}_{2}^{+}$ molecules. We report on a strong dependence at high intensities ($I$\ge${}1.4\ifmmode\times\else\texttimes\fi{}{10}^{14}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$) of this luminescence with the polarization state of the incident laser pulses. We attribute this effect to the onset of new impact excitation channels from energetic electrons produced with circularly polarized laser pulses above a threshold laser intensity.

We experimentally demonstrate the existence of long-lived coherent polarizations coupling simultaneously ground state X ($X^2{\mathrm{\Sigma }}_g^{+}$ ) to excited states A ($A^2{\mathrm{\Pi }}_u$ ) and B ($B^2{\mathrm{\Sigma }}_u^{+}$ ) of ${\text{N}}_2^{+}$ inside a plasma created by a short intense laser pulse at 800 nm. This three-level V-scheme arrangement is responsible for a strong optical gain without population inversion at the B–X transition at 391.4 nm. Simulations based on Maxwell-Bloch equations reproduce well the kinetics and the pressure dependence of the gain.

We report on the lasing action of atmospheric air pumped by an 800 nm femtosecond laser pulse with peak power up to 4 TW. Lasing emission at 428 nm increases rapidly over a small range of pump laser power, followed by saturation above $\sim 1.5\mathrm{TW}$. The maximum lasing pulse energy is measured at 2.6 μJ corresponding to an emission power in the MW range, while a maximum conversion efficiency of $3.5\times {10}^{-5}$ is measured at moderate pump pulse energy. The optical gain inside the filament plasma is estimated to be in excess of $0.7/\mathrm{cm}$. Lasing emission shows a doughnut profile, reflecting the spatial distribution of the pump-generated white-light continuum that acts as a seed for the lasing. We attribute the pronounced saturation to the defocusing of the seed in the plasma amplifying region and to the saturation of the seed intensity.

Recently, S. Mitryukovskiy et al. presented experimental evidence showing that backward Amplified Spontaneous Emission (ASE) at 337 nm can be obtained from plasma filaments in nitrogen gas pumped by circularly polarized 800 nm femtosecond pulses (Opt. Express, 22, 12750 (2014)). Here, we report that a seed pulse injected in the backward direction can be amplified by ~200 times inside this plasma amplifier. The amplified 337 nm radiation can be either linearly or circularly polarized, dictated by the seeding pulse, which is distinct from the non-polarized nature of the ASE. We performed comprehensive measurements of the spatial profile, optical gain dynamics, and seed pulse energy dependence of this amplification process. These measurements allow us to deduce the pulse duration of the ASE and the amplified 337 nm radiation as well as the corresponding laser intensity inside the plasma amplifier. It indicates that the amplification is largely in the unsaturated regime and that further improvement of laser energy is possible. Moreover, we observed optical gain in plasma created in ambient air. This represents an important step towards future applications exploiting backward lasing for remote atmospheric sensing.