Free-space nitrogen gas laser driven by a femtosecond filament

We report an experimental proof and full characterization of laser generation in molecular nitrogen in an argon-nitrogen gas mixture remotely excited at a distance above 2 m in a femtosecond laser filament. Filamentation experiments performed with near-infrared, 1-\ensuremath{\mu}m-wavelength and midinfrared, 4-\ensuremath{\mu}m-wavelength short-pulse laser sources show that mid-IR laser pulses enable radical enhancement of filamentation-assisted lasing by N${}_{2}$ molecules. Energies as high as 3.5 \ensuremath{\mu}J are achieved for the 337- and 357-nm laser pulses generated through the second-positive-band transitions of N${}_{2}$, corresponding to a 0.5$%$ total conversion efficiency from midinfrared laser energy to the energy of UV lasing.