There are serious issues of linear diffraction and strong attenuation due to water vapor absorption when propagating broadband THz pulses for a long distance towards application of standoff THz spectroscopy. A femtosecond laser filament based THz source provided the opportunity to solve the propagation issues.

Recently, researchers at State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, reported a waveform control of enhanced THz radiation along a femtosecond laser filament in air with a high voltage technique. By applying a DC high-voltage electric field from two sharp electrodes in a direction parallel to the laser filament and scanning it along the filament, they demonstrated the longitudinal evolution of amplified THz emission. By changing the position of the pair of electrodes along the laser filament, they obtained different waveforms of THz radiation. Due to the change of the plasma density distribution at the leading and trailing ends of a laser filament, the enhanced THz waveforms could have a phase shift of ~π .

The technique is very simple. It could help to understand the THz generation process through external electric field assisted laser filamentation.
The study was published in Journal of Appl. Phys. Lett.[http://dx.doi.org/10.1063/1.4984599]

FIG. 1. (a) The image of the laser filament in air taken using a CCD camera from the side and (b) the orientation of pump polarization, filament propagation,and electrodes. The red dashed line in (a) indicates the position of the geometrical focus of the lens, which is defined as “Z？0.” The position before the geometrical focus is negative. The laser propagates from left to right along the Z-axis. The DC field was applied parallel to the laser filament.