Terahertz Radiation from a Longitudinal Electric Field Biased Femtosecond Filament in Air**Supported by the National Natural Science Foundation of China (Grant No. 11574213) and the Innovation Program of Shanghai Municipal Education Commission (Grant No. 2017-01-07-00-07-E00007).

Abstract

The terahertz (THz) temporal waveform and spectrum from a longitudinal electrically biased femtosecond filament is studied experimentally. The initial direction of the electron motion inside the unbiased filament plasma is deduced from the transformation of the THz temporal waveform with applied fields of opposite polarities. Furthermore, a spectrum shift to lower frequency of the THz spectrum is observed in the presence of a biased field. It agrees well with theoretical predictions.