Atmospheric Aerosol Clearing by Femtosecond Filaments

Atmospheric aerosols, such as water droplets in fog, interfere with laser propagation through scattering and absorption. Femtosecond optical filaments are shown to clear foggy regions, improving the transmission of subsequent pulses. However, the detailed fog-clearing mechanism had yet to be determined. Here, we directly measure and simulate the dynamics of water droplets with a radius of about 5 μ m, typical of fog, under the influence of optical and acoustic interactions that are characteristic of femtosecond filaments. We find that, for filaments generated by the collapse of collimated near-infrared femtosecond pulses, the main droplet-clearing mechanism is optical shattering by laser light. For such filaments, the single-cycle acoustic wave launched by filament-energy deposition in air leaves droplets intact and drives negligible transverse displacement, and therefore, negligible fog clearing. Only for tightly focused nonfilamentary pulses, where local energy deposition greatly exceeds that of a filament, do acoustic waves significantly displace aerosols.