Abstract
The diffraction-compensated propagation of high-power laser beams in air could open up new opportunities for atmospheric applications such as remote stand-off detection, long-range projection of high-energy laser pulses and free-space communications. Here, we experimentally demonstrate that a self-guided terawatt picosecond CO2 laser beam forms in air a single centimetre-scale-diameter megafilament that, in comparison with a short-wavelength laser filament, has four orders of magnitude larger cross-section and guides many joules of pulse energy over multiple Rayleigh distances at a clamped intensity of ~1012 W cm–2. We discover that this megafilament arises from the balance between self-focusing, diffraction and defocusing caused by free carriers generated via many-body Coulomb-induced ionization that effectively decrease the molecular polarizability during the long-wavelength laser pulse. Modelling reveals that this guiding scheme may enable transport of high-power picosecond infrared pulses over many kilometres in the 8–14 μm atmospheric transmission window. A terawatt picosecond CO2 laser beam is shown to form a centimetre-scale-diameter filament in air that is capable of carrying several joules of energy.
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