ИСТИНА

The mid-infrared OPCPA-based laser facilities have recently reached the critical power for self-focusing in air. This ensures the demonstration of the major difference between the mid- and near-infrared filamentation in air: the odd optical harmonics, harshly suppressed by the phase-mismatch in the near-infrared (800 nm), gain reliable energies in the mid-infrared (3.9 µm) filament [Mitrofanov et al., Sci. Rep. 5, 8368 (2015); Panagiotopoulos et al., Nat. Photonics 9, 543 (2015)]. Another issue that makes mid-infrared filamentation different from the near-infrared one is a lot of molecular vibrational lines belonging to atmospheric constituents and located in the mid-infrared range [Mathar, Appl. Opt. 43, 928 (2004)]. We simulate the 3.9-µm filamentation using Forward Maxwell equation. The tunnel ionization and transient photocurrent are taken as the collapse arresting mechanism, and the complex refractive index accounting the absorption and refraction in the numerous molecular vibrational bands allows us to quantify the linear losses in atmosphere. For the Gaussian 3.9-µm 80-fs (FWHM) pulse the energy loss in the carbon dioxide (CO_2) absorption band at 4.3 µm is about 1% in the linear propagation regime. But when we take the 80-mJ pulse (about 3 critical powers for self-focusing), the Kerr-induced spectral broadening develops significantly before the clamping level of intensity is reached. In the collimated beam geometry about 2% of the initial pulse energy is absorbed on the CO_2 band before the filament is formed. In the developed filament all the partial losses due to plasma, harmonic generation and absorption on vibrational lines grow up rapidly with the propagation distance, and the latter overwhelms all the rest ones. Indeed the new mechanism is revealed – the linear absorption is enhanced by the nonlinear spectral broadening. This nonlinearly enhanced linear absorption (NELA) provides the major optical losses in the atmosphere as compared with plasma and high-frequency conversion.