Filamentation of Mid-IR pulses in ambient air in the vicinity of molecular resonances

Properties of filaments ignited by multi-millijoule, 90-fs mid-IR pulses centered at 3.9 {\mu}m are examined experimentally by monitoring plasma density and losses as well as spectral dynamics and beam profile evolution at different focusing strengths. By softening the focusing from strong (f=0.25 m) to loose (f=7 m) we observe a shift from plasma assisted filamentation to filaments with low plasma density. In the latter case, filamentation manifests itself by beam self-symmetrization and spatial self-channeling. Spectral dynamics in the case of loose focusing is dominated by the non-linear Raman frequency downshift, which leads to the overlap with the CO2 resonance in the vicinity of 4.2 {\mu}m. The dynamic CO2 absorption in the case of 3.9-{\mu}m filaments with their low plasma content is the main mechanism of energy losses and either alone or together with other nonlinear processes contributes to the arrest of intensity.