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

Properties of filaments ignited by multi-millijoule, 90 fs mid-infrared pulses centered at 3.9 μm are examined experimentally by monitoring plasma density, losses, spectral dynamics and beam profile evolution at different focusing strengths. By changing from strong (f=0.25  m) to loose (f=7  m) focusing, 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 nonlinear Raman frequency downshift, which leads to the overlap with the CO2 resonance in the vicinity of 4.2 μm. The dynamic CO2 absorption in the case of 3.9 μm 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.