ИСТИНА

We discuss percularities of the THz generation from a filament created by the loosely focused TW laser pulse if transverse electrostatic field is applied along its full length. In the experiments filaments were formed in air by a plano-convex lens with a focal length in a wide range of lengths (from 50cm to 10m) by a laser pulse from the 10 Hz terawatt Ti:Sa femtosecond laser system (805 nm, 55 fs, 1-50 mJ, beam diameter 10 mm or 5mm FWHM). Transverse electron current caused by the DC-biasing of the filament plasma channel by the 5.5 cm–47 cm long electrodes with field strength of 10 kV/cm generate THz radiation. We have measured THz pulse energy, its angular pattern, autocorrelation function and spectrum. A wide band acoustic technique has been used additionally to estimate the volumetric and linear absorbed power density and diameter of the filament plasma channel. Efficient summation of the THz emission along the biased plasma channel greatly enhances the THz yield almost proportionally to the electrodes length. The THz radiation is emitted into the full cone with open angle which decreases with an increase of the electrostatic field application length. The THz spectrum measured by the Michelson interferometer lies in the 0.05 - 0.3 THz range. It is also shown that the laser beam numerical aperture has a significant effect on the efficiency of THz radiation generation and its spectrum. In the case of small numerical apertures the self-focusing leads to early (before the focus point) splitting of the beam into multiple filaments and limits the electron density in individual plasma channels and THz pulse energy due to intensity clamping. In the case of large numerical apertures the efficient generation of THz radiation is limited by the small length of the filament, as well as the THz radiation refraction on the plasma. Numerical simulations within the framework of UPPE explain the experimental findings.