LAUSR

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. Terahertz radiation from plasma in air induced by laser pulses was first demonstrated by Hamster and co-workers about 25 years ago [1]. In recent years, the THz field from a long plasma channel, a filament, generated by an intense femtosecond laser has attracted much attention [2-11]. This air plasma THz source holds several unique advantages comparing to the traditional technique including photoconductive antenna and optical rectification with optical-electrical crystal. With femtosecond filament, the THz source can be positioned far away from the laser system, bypassing the detrimental absorption of the THz field by the water vapor during propagation in ambient air [12, 13]. At the same time, high power laser pulses up to several Terawatts can be employed for THz generation without damage of emitter, which is not possible with the traditional techniques [12]. However, the conversion efficiency of this single-color filamentation method has been measured to be low, on the order of 10-9 [14, 15]. Several methods have been demonstrated to enhance the THz field, such as application of transverse or longitudinal electric field [4, 16-17], bifilamentation technique [18], and two-colors or multiple color excitation [2, 3, 5-11, 19-