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

TL;DR

This study investigates mid-IR filamentation in ambient air near molecular resonances, revealing how focusing strength influences plasma density, spectral shifts, and energy losses, especially around CO2 resonances.

Contribution

It provides experimental insights into filamentation dynamics at 3.9 μm, highlighting the role of molecular resonances and nonlinear effects in filament behavior.

Findings

Looser focusing reduces plasma density in filaments.

Spectral dynamics show Raman downshift near CO2 resonance.

Energy losses are dominated by CO2 absorption in low plasma filaments.

Abstract

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.