RESUMO
At the selected frequencies from 0.3 to 10 THz we measured the two-dimensional (2D) distributions of fluence and polarization of terahertz (THz) emission from a single-color femtosecond filament. At the majority of frequencies studied, the THz beam has a donut-like shape with azimuthal modulations and radial polarization. At the maximal modulation, THz beam takes the form of the two lobes and polarization of the THz field degenerates into orthogonal to the laser pulse polarization direction. Violation of the radially polarized donut beam shape is due to destructive interference of THz waves driven by light pressure directed along the laser beam propagation axis and ponderomotive force parallel to the laser polarization.
RESUMO
The terahertz (THz) radiation emitted by an air-based femtosecond filament biased by a static electric field is known to have on-axis shape and relatively low frequency spectrum in contrast to the unbiased single-color and two-color schemes. Here, we measure the THz emission of a 15-kV/cm-biased filament in air produced by a 740-nm, 1.8-mJ, 90-fs pulse and demonstrate that a flat-top on-axis THz angular distribution of the emission at 0.5-1 THz transforms into a contrast ring-shaped one at 10 THz.
Assuntos
Eletricidade , Radiação Terahertz , Frequência CardíacaRESUMO
In the experiment, the laser pulse (744 nm, 0.5 mJ, 90 fs) focused into the air gap between the plane electrodes biased by a 10 kV/cm field (DC-biased filament) produced terahertz (THz) radiation. At the selected frequencies of ν=0.3, 0.5, 1 THz, a wide flat-top angular distribution was measured by a bolometer rotating in the plane of the electrodes. The simulations based on the unidirectional pulse propagation equation with fine 0.01 THz resolution and 3 PHz frequency domain showed the transition of the THz directional diagram from the flat-top at νâ²1THz to the conical one at ν>8THz due to the destructive interference of THz waves from the ionization front propagating with the superluminal velocity. Refraction on the plasma is not the major factor in ring formation.