Control of terahertz pulse polarization by two crossing DC fields during femtosecond laser filamentation in air

Abstract

Terahertz (THz) radiation induced by the femtosecond laser filament in air can be strongly enhanced through applying an external DC field. In the present work, this technique has been extended to two-dimensional biasing; namely, two pairs of electrodes, which are placed perpendicular to each other, are applied to provide two crossing DC fields. The experimental results have demonstrated that the amplitudes of two orthogonal components of the polarized THz pulse could be modulated by the electric field parallel to it, in an in situ and real-time manner, while the phase delay between them remains constant. Hence, the polarization of the output THz pulse could be freely controlled by adjusting the voltages of the two crossing DC fields, achieving arbitrary ellipticity. It might be the simplest method to manipulate the THz polarization during femtosecond laser filamentation in air.