THz generation in an air filament recently attracts much attention. The most important reason is that THz emission can be generated close to a remote target by controlling the remote onset of the filament via controlling the initial laser parameters.As it is well known, THz emission from a laser filament can be viewed as a coherent superposition of many plasma dot sources. The plasma density along the laser filament is not uniform and very sensitive to laser parameters although the laser intensity is clamped inside it.Understanding the behavior of THz emission and measuring the plasma density along a laser filament are still challenging issues.

Researchers at State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, report on a longitudinally resolved measurement of plasma density along femtosecond laser filament in air via terahertz (THz) spectroscopy. By applying a needlelike high-voltage direct current (DC) electric field on the laser filament and scanning it along filament, the longitudinal evolution of amplified THz emission has been demonstrated. The peak frequency of the DC electric field biased THz emission is proportional to the plasma density inside the laser filament. This latter phenomenon was used to characterize the plasma density. Longitudinal distribution of plasma density of ～10¹⁵ cm^-3 along laser filament has been experimentally recorded. The technique demonstrated is very simple and helpful for understanding the THz generation process through laser filamentation. [APPLIED PHYSICS LETTERS 105, 051101 (2014)]