Abstract:Femtosecond laser filament could produce THz wave at forward direction. In our experiment, THz pulse emitted from a femtosecond laser filament has been investigated. It is found that the polarization of the studied THz pulse mainly appears as elliptical. This observation supplements the previous conclusion obtained by C. D'Amico et al. that THz wave emitted by a filament is radially polarized. The mechanism of generating elliptically polarized THz wave has been interpreted by either four-wave optical rectification or second order optical rectification inside the filament zone where centro-symmetry of the air is broken by the femtosecond laser pulse.

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A systematic investigation of external focusing effect on terahertz (THz) emission from a two-color femtosecond laser-induced filament in air is demonstrated. By collecting the THz emission precisely under different external focusing conditions, an optimum external focal length around 50 cm was observed to generate stronger THz emission. The optimum dependence on the filament volume and the filament's core intensity with external focusing could be responsible for this stronger THz emission.