Nonlinear transparency window for ultraintense femtosecond laser pulses in the atmosphere

We have found the optimum range of driver wavelengths for mid-infrared ultraintense femtosecond pulses undergoing filamentation in atmospheric air. This wavelength range between 3.1 and 3.5μm forms a nonlinear transparency window identified through a diligent scan of pulse central wavelengths in the range 2.2-4.7μm with a best resolution of 5 nm. Each of 123 wavelengths scanned corresponds to the solution of the full three-dimensional + time pulse propagation and filamentation problem on a 7-19 m path in air. Due to the discovered universal asymmetric character of the nonlinearly enhanced linear absorption in the vicinity of atmospheric molecular band, the optimum driver wavelength belongs to the long-wavelength side of the band.