Longitudinally resolved measurement of plasma density along femtosecond laser filament via terahertz spectroscopy

Abstract

We 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 ∼1015cm -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. © 2014 AIP Publishing LLC.